English

• 0.   Introduction

  Micro/nanomotors are a class of micro/nanoscale devices which can convert external energy or chemical substances into driving forces to realize autonomous movement. According to the propulsion mechanism^[1], they can be categorized into physical-driven (including light energy^[2], magnetic field^[3], ultrasonic wave^[4], electric field^[5], and thermal energy^[6]) and fuel-driven (e.g., H₂O₂^[7], glucose^[8], lactose^[9], and urea^[10]). Fuel - driven micro/nanomotors are powered by catalytic reactions which present high speed and efficiency. A large number of catalytic materials have been used in the design of fuel - driven micro/nanomotors. Traditional catalysts including noble metals (e.g., Pt^[11], Ag^[12]), active metals (e.g., Mg^[13], Zn^[14]), metal oxides (e.g., MnO₂^[15], Fe₂O₃^[16]), or enzymes^[17], have excellent catalytic performances. However, some of these materials have problems of high cost and complicated preparation process, which are not conducive to large-scale production and practical application. Therefore, the development of low-cost, easy-to-prepare and high catalytic activity micro/nanomotor materials still faces major scientific challenges.

  The extensive application of micromotors has been made great progress in explosives detection^[18], biomedicine^[19], environmental restoration^[20], nano engineering^[21] and other fields for the past few years. At present, the issue of environmental restoration, especially sewage treatment, is a huge challenge facing the world^[22]. Generally, organic pollutants in water are removed by physical adsorption and chemical oxidation. Micro/nanomotors have a promising potential application in this field and many works have been reported to date. Liang et al. used silver nano-colloids as the surface - enhanced Raman scattering (SERS) substrate to introduce SERS technology, and designed micro/nanomotors with water quality monitoring functions, which could identify the characteristic peaks of pollutants in the SERS spectrum of the resulting solution^[23]. The combination of micro/nanomotors and sensing technology provided a new direction for environmental monitoring. Ma et al. prepared Janus micromotor by incorporating iron ions into the zeolite framework^[24]. The micromotors used Pt to catalyze H₂O₂ to generate driving force, while iron ions catalyze H₂O₂ to continuously generate hydroxyl radicals, which degraded phenol as a model pollutant. Similarly, Shi et al. used reduced graphene oxide (rGO) to immobilize Fe₃O₄ nanoparticles (NPs) to synthesize heterogeneous Fe₃O₄- rGO/Pt microjets which could improve the degradation capability of Fenton composite microjets and effectively remove methylene blue in a short time^[25]. In response to recent oil spills, many researchers had applied micro/nanomotors to dynamic oil removal. The canned hollow MnFe₂O₄ micromotor could quickly and efficiently collect oil droplets through the hydrophobic surface - oil interaction of the pre - existing long oleic acid chains^[26]. For the first time, Luis et al. used the wax printing template-assisted rolled-up method to prepare rGO/Pt tubular micromotors^[27]. The hydrophobicity and high specific surface area to volume ratio of rGO provided favorable conditions for reversibly collecting and releasing oil in water. The removal and recycling of heavy metal from sewage are also crucial in environmental remediation issues. A multifunctional mesoporous silica-based microinjector was immobilized MnO₂ and γ-Fe₂O₃ NPs on the inner and outer surface, respectively^[28]. The interaction between Fe₂O₃ NPs and heavy metals could remove heavy metal ions such as Cd²⁺ and Pb²⁺ in wastewater. And the microjets could be easily guided and recycled by a magnetic field. Hou et al. designed a self - propelled tubular micromotor based on a copolymer of aspartic acid and cysteine (PAsp-Cys), which could quickly remove inorganic and organic heavy metals in polluted water^[29]. This was due to the strong binding force between sulfhydryl groups of PAsp - Cys and metals (especially mercury). Meanwhile, the multi - carboxyl and amino groups of PAsp were also used to adsorb various metal ions.

  Metal - organic framework (MOF) with large pore size, stable crystal lattice, and functional groups is a kind of porous material assembled by metal ions and bridged organic ligands, which has attracted a lot of attention in recent years^[30]. Therefore, the introduction of MOF materials into fuel - catalyzed micro/nanomotors, which increases the catalytically active sites and active surfaces of the micro/nanomotors, is expected to realize the efficient role of the micro/nanomotors in environmental remediation. Ikezoe et al. proposed the application of MOF materials to micromotors for the first time, which expanded the new research direction of micromotor preparation materials^[31]. Recently, we have synthesized a spherical and dodecahedron - based ZIF -67 catalytic micromotor by changing the solvent in one step^[32]. Owing to the higher surface area and hollow structure of ZIF - 67, the micromotors can dramatically adsorb the methyl blue (MB) within 5 min.

  A series of derivatives can also be obtained by using MOF as a precursor, which not only retains the original porosity and morphology of the precursor, but also exhibits stronger catalytic activity^[33]. In this work, Mn₂O₃ micromotors were prepared by using Mn - MOF as a precursor through high-temperature annealing. The size of Mn₂O₃ was about 4 μm, with a microporous structure and good crystallinity. At the same time, the movement behavior of MnMOF derivative micromotors under different conditions and the degradation properties of MB were studied. The prepared Mn₂O₃ micromotors had excellent autonomous movement ability, and their moving speed can reach 81.32 μm·s^-1 in 10% H₂O₂ solution. The experimental results illustrate that after adding H₂O₂, the Mn₂O₃ micromotors showed a strong degradation effect on MB. These Mn₂O₃ micromotors using Mn-MOF as the precursor had lower material costs and are suitable for mass production. Compared with other external stimuli, such as light, ultrasound, magnetic field and temperature, H₂O₂ and the Mn₂O₃ micromotors had a Fenton - like reaction to efficiently degrade pollutants, and H₂O₂ can drive the micromotors to achieve autonomous movement simultaneously.

  1.   Experimental

  1.1   Materials

  Manganese acetate tetrahydrate (Mn(CH₃COO)₂· 4H₂O), MB and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP), 1, 3, 5-trimellitic acid, and 2-methylimidazole are commercially available from Aladdin Inc. Hydrogen peroxide (H₂O₂, 30%) was purchased from Shanghai Chemical Reagent Co., Ltd. Triton X - 100 was purchased from Alfa Aesar Chemical Co., Ltd. The ultrapure water used in the experiment was purified by the Milli - Q system. All the reagents were analytical grade and used as received without further treatment.

  1.2   Synthesis of Mn-MOF derivatives

  Mn - MOF was prepared according to the reported method^[34]. First, 0.49 g of Mn(CH₃COO)₂·4H₂O was dissolved in 100 mL of the mixture of water and ethanol (1∶2, V/V). After it was completely dissolved, 2 g of PVP was added under vigorously stirring until completely dissolved. It was recorded as solution A. Then, 0.9 g of 1, 3, 5-trimellitic acid was dissolved in the mixture of 80 mL of water and ethanol (1∶2, V/V). It was designated as solution B. Solution B was dropwise added to solution A under vigorous stirring and it was kept with stirring for 20 min, and then, the reaction was allowed to stand at room temperature for 12 h. After the reaction was completed, the solution was divided into two layers including the supernatant and the underlying white precipitate. The solution was centrifuged and the precipitate was washed using ethanol three times. The final product was dried under vacuum at 60 ℃ for 6 h to obtain Mn-MOF. The white powder of Mn-MOF was evenly spread on the bottom of the porcelain boat. The outer layer of the porcelain boat was covered with tin foil. The porcelain boat was placed in a muffle furnace. The temperature was programmed at 700 ℃ at a heating rate of 10 ℃·min^-1 and held for 3 h at 700 ℃. After the end of the procedure, it was naturally cooled to room temperature to get black Mn-MOF derivatives.

  1.3   Characterization

  The composition of Mn-MOF and Mn-MOF derivative micromotors was characterized by X-ray diffraction (XRD, BRUKER D8 Advance, Cu Kα, λ =0.154 001 nm, 2θ =10° - 80°, 40 kV, 40 mA) and FT - IR (Bruker model VECTOR - 22 Fourier transform spectrometer). Thermogravimetric analysis (TGA) was performed in air atmosphere using a differential scanning calorimeter (DSC214 polyma). The microscopic morphology was observed by a scanning electron microscope (SEM, Hitachi S-4800, 3 kV) and a transmission electron microscope (TEM, Hitachi HT7700, 100 kV). Chemical bond analysis was performed by X - ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe), and UV - Vis absorption spectroscopy was performed by a UV - Vis spectrometer (PerkinElmer, Lambda 650S). Motion videos were recorded by an inverted fluorescence microscope (Ti - S/L100). The calculation and analysis of motor speed were done by NIS - Elements software. Before the motion experiment, Mn - MOF derivatives were coated with a 10 nm-gold layer by the E-1010 ion sputter.

  2.   Results and discussion

  The morphologies of Mn - MOF and its derivative were characterized by SEM and TEM, as shown in Fig. 1. From Fig. 1a and 1b, we can see that the Mn - MOF precursor showed a perfectly sphere shape with a smooth surface and an average diameter was about 4 μm. After annealing in the muffle furnace from 400 to 700 ℃, it still maintained relatively complete spherical morphology without cracking (Fig. 1c, 1d; Fig.S1a - S1c in Supporting information). However, the surface of the sphere became rough after annealing, which may be due to the release of gas molecules caused by the decomposition of Mn - MOF during annealing. As presented in Fig. S1d, when the annealing temperature reached 800 ℃, its spherical morphology was obviously broken. Video S1 (Supporting information) shows that the motion performance of derivatives of MnMOF obtained at annealing temperatures of 700 ℃ was better than those of the samples obtained at 400, 500, and 600 ℃. Therefore, we choose the MnMOF derivative obtained at an annealing temperature of 700 ℃ for subsequent research.

  Figure 1

  

  Figure 1.  SEM (a, c) and TEM (b, d) images of Mn-MOF (a, b) and its derivative obtained at an annealing temperature of 700 ℃ (c, d)

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  The structures of Mn-MOF and its derivative were further analyzed by XRD, as shown in Fig. 2a. The Mn-MOF precursor exhibited similar characteristic peak with the reference^[34]. After annealing, the diffraction peaks are consistent with Mn₂O₃ completely (PDF No.41-1442, lattice constant a=0.940 9 nm), indicating the perfect crystallinity and high purity of the product. As shown in the TG curve of Mn-MOF (Fig. 2b, top), the first 18.28% weight loss around 200 ℃ is attributed to the loss of lattice water. The second weight loss around 480 ℃ originates from the thermal decomposition of the sample, indicating that Mn -MOF has a stable crystal structure above 480 ℃. The TGA curve (Fig. 2b, bottom) of Mn₂O₃ derived from Mn - MOF showed no change within the temperature range illustrating it has good thermal stability. The XPS results show that there are two elements (Mn and O) in the sample. Fig. 2c shows the high-resolution XPS spectrum of Mn2p. The peaks of Mn2p_1/2 and Mn2p_3/2 were situated at 652.4 and 641.2 eV, respectively. The peaks appearing at 640.8 and 643.9 eV correspond to Mn³⁺ and Mn⁴⁺, respectively^[35]. As shown in Fig. 2d, the peaks of O1s at 529.0 and 531.0 eV correspond to crystal lattice oxygen and the surface adsorbed oxygen species in Mn₂O₃, respectively. The result is well consistent with that of the XRD analysis.

  Figure 2

  

  Figure 2.  XRD patterns of Mn-MOF and its derivative (a); TGA curves of Mn-MOF and its derivative (b); XPS spectra of Mn2p (c) and O1s (d) of the derivative of Mn-MOF

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  The Mn₂O₃ micromotors derived from Mn - MOF can move efficiently by decomposing H₂O₂. Fig. 3a and 3b display time - lapse images taken from Video S1 in a 2 s period powered by different H₂O₂ concentrations. These images illustrate a tail of oxygen bubbles generated and released from the Mn₂O₃ surface. Video S1 shows that the Mn₂O₃ micromotors display autonomous locomotion with different speeds in different H₂O₂ concentrations. The concentration of H₂O₂ fuel strongly influences the velocity of the Mn₂O₃ micromotors. Fig. 3c shows the relationship between micromotors' speed and H₂O₂ concentrations. When the H₂O₂ concentration was increased from 1% to 10%, the speed of the micromotors was correspondingly increased from 21.71 to 81.32 μm·s^-1 with an average lifetime of about 30 min.

  Figure 3

  

  Figure 3.  Time-lapse images of Mn₂O₃ micromotors in different concentrations (a: 1%, b: 7%) of H₂O₂; Relationship between micromotors' speed and H₂O₂ concentration (c)

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  In this work, MB was selected as a model pollutant to study the removal performance of Mn₂O₃ micromotors. To analyze the removal process of MB, the absorption peaks of MB in solution were measured by the UV - Vis spectrometer. The maximum absorption wavelength of MB was 598 nm. Fig. 4 shows the changes of UV absorbance of different samples with time. As shown in curves a and b, the absorbance of MB did not change without or with H₂O₂, indicating that H₂O₂ has no effect on the removal of MB. It can be seen that the absorbance of MB decreased only after adding Mn₂O₃ micromotors (curve c), which further proves that Mn₂O₃ has an adsorption effect on MB. As shown in curve d, after adding Mn₂O₃ micromotors and H₂O₂ to MB solution simultaneously, the absorbance of MB was significantly reduced, which indicates that the Fenton - like system composed of Mn₂O₃ micromotors and H₂O₂ exhibits higher degradation efficiency for MB than the adsorption performance of pure Mn₂O₃.

  Figure 4

  

  Figure 4.  Changes of absorbances of different samples with time: (a) MB, (b) MB and H₂O₂ solution, (c) MB and Mn₂O₃ micromotors, (d) MB, Mn₂O₃ micromotors and H₂O₂ solution

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  FT - IR measurements were performed to study the removal mechanism of MB. Fig. 5 shows the FT - IR spectra of Mn₂O₃ (a), MB (b), mixed Mn₂O₃ and MB solution without H₂O₂ (c) or with H₂O₂ (d). In curve a, the characteristic absorption peaks at 612 and 529 cm^-1 correspond to the asymmetric and symmetric Mn—O—Mn stretching vibrations in Mn₂O₃, respectively^[36]. Curve b shows that the characteristic absorption peaks at 1 575 and 1 496 cm^-1 correspond to the stretching vibrations of the C=C and C=N groups in MB, respectively. The peak located at 1 169 cm^-1 corresponds to the stretching vibration of the S=O group. Compared with pure Mn₂O₃ (curve a), the characteristic absorption peak of MB appears additionally in curve c, which indicates that in the absence of H₂O₂, Mn₂O₃ exhibits an adsorption effect on MB. However, the absorption peak of MB was not observed in curve d, which proves that in the presence of H₂O₂, Mn₂O₃ micromotors exhibit degradation effect rather than adsorption to MB.

  Figure 5

  

  Figure 5.  FTIR spectra of Mn₂O₃ (a), MB (b), mixed Mn₂O₃ and MB solution without H₂O₂ (c) or with H₂O₂ (d)

  Vertical dotted line indicates the bond stretching mode of MB

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  Based on the above discussions, the likely catalytic mechanism for the activation of H₂O₂ by Mn₂O₃ micromotors in the degradation of organic pollutants through the Fenton - like effect is proposed as follows. Firstly, Mn³⁺ on the surface of Mn₂O₃ is easily reduced by H₂O₂ to produce Mn²⁺, H⁺ and HO₂· under nearneutral conditions. Then, Mn²⁺ reacts with H₂O₂ to produce Mn³⁺, HO^- and ·OH. Therefore, Mn₂O₃ can decompose H₂O₂ cyclically and produce a large number of reactive oxygen species (HO₂·, ·OH), which realizes the effect of the micromotor to remove MB^[37]. Moreover, after adding Mn₂O₃ micromotors and H₂O₂ to the MB solution simultaneously, Mn₂O₃ contacts with H₂O₂ in the solution to continuously generate bubbles, which not only promotes the movement of the micromotors to achieve mixing, but also accelerates the degradation process without the need of external mechanical stirring.

  Fig. 6 presents that the amount of Mn₂O₃ micromotors and time has a direct effect on the degradation of MB. It shows that different amounts of Mn₂O₃ micromotors degraded the MB sample with the same concentration within 5 min. With the increase of the amount of Mn₂O₃ micromotors, the absorbance of MB was getting lower and lower. It indicates that the degradation efficiency is positively correlated with the amount of Mn₂O₃ micromotors. As shown in the right panel of Fig. 6, the absorption peak of MB gradually decreased with time going, which proves that the Mn₂O₃ micromotor has high degradation efficiency compared to static adsorption.

  Figure 6

  

  Figure 6.  Effect of amount of Mn-MOF derivative micromotors (left) and effect of time (right) on degradation of MB

  (a) 1 mg·mL^-1, (b) 2 mg·mL^-1, (c) 3 mg·mL^-1, (d) 4 mg·mL^-1

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  Harnessing excellent autonomous motion and catalytic performance of MOF derivative micromotors, they hold great potential for environmental applications. Table 1 lists some specific applications of the same type of Mn₂O₃ micromotors in environmental remediation. Compared with them, these Mn₂O₃ micromotors have the facile design strategy, lower material and production costs, as well as higher degradation efficiency for organic pollutants.

  Table 1

  

  Table 1.  Micro/nanomotors based on MOF derivatives in the field of environmental remediation

  下载: 导出CSV

  Type of micromotor	Morphology	Fabrication	Application	Time of removal / min	Ref.
  Fe-zeolite	Janus particles	Solidstate ion exchange and ion sputtering processes	Degradation of phenol	90	[24]
  Eu-MOF/EDTA-NiAl-CLDH/MnO2	Microtubes	Chemical coprecipitation	Removal of Fe3+	120	[38]
  Ag-ZIF	Janus particles	Solvothermal	Degradation of RhB	150	[39]
  Fe2O3	Octahedra	Decomposition	Adsorption of MB	5	[16]
  Mn2O3	Spheroidal	Decomposition	Degradation of MB	5	This work

  3.   Conclusions

  In conclusion, Mn-MOF derivative (Mn₂O₃) micromotors were successfully synthesized by thermal decomposition of Mn - MOF. The Mn - MOF derivatives present perfect spherical morphology with rough surface and pore characteristics. In addition, Mn₂O₃ micromotors show excellent catalytic activity and efficient autonomous mobility in H₂O₂. With the increase of H₂O₂ concentration, the speed of Mn₂O₃ micromotors was increased. It can reach 81.3 μm·s^-1 in 10% H₂O₂ with an average lifetime of about 30 min. At the same time, Mn₂O₃ micromotors exhibit a Fenton-like effect in H₂O₂, which can efficiently degrade MB in water.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  SEM (a, c) and TEM (b, d) images of Mn-MOF (a, b) and its derivative obtained at an annealing temperature of 700 ℃ (c, d)

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  Figure 2  XRD patterns of Mn-MOF and its derivative (a); TGA curves of Mn-MOF and its derivative (b); XPS spectra of Mn2p (c) and O1s (d) of the derivative of Mn-MOF

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  Figure 3  Time-lapse images of Mn₂O₃ micromotors in different concentrations (a: 1%, b: 7%) of H₂O₂; Relationship between micromotors' speed and H₂O₂ concentration (c)

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  Figure 4  Changes of absorbances of different samples with time: (a) MB, (b) MB and H₂O₂ solution, (c) MB and Mn₂O₃ micromotors, (d) MB, Mn₂O₃ micromotors and H₂O₂ solution

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  Figure 5  FTIR spectra of Mn₂O₃ (a), MB (b), mixed Mn₂O₃ and MB solution without H₂O₂ (c) or with H₂O₂ (d)

  Vertical dotted line indicates the bond stretching mode of MB

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  Figure 6  Effect of amount of Mn-MOF derivative micromotors (left) and effect of time (right) on degradation of MB

  (a) 1 mg·mL^-1, (b) 2 mg·mL^-1, (c) 3 mg·mL^-1, (d) 4 mg·mL^-1

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  Table 1.  Micro/nanomotors based on MOF derivatives in the field of environmental remediation

  Type of micromotor	Morphology	Fabrication	Application	Time of removal / min	Ref.
  Fe-zeolite	Janus particles	Solidstate ion exchange and ion sputtering processes	Degradation of phenol	90	[24]
  Eu-MOF/EDTA-NiAl-CLDH/MnO2	Microtubes	Chemical coprecipitation	Removal of Fe3+	120	[38]
  Ag-ZIF	Janus particles	Solvothermal	Degradation of RhB	150	[39]
  Fe2O3	Octahedra	Decomposition	Adsorption of MB	5	[16]
  Mn2O3	Spheroidal	Decomposition	Degradation of MB	5	This work

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