Ultrasonic synthesis of α-MnO2 nanorods: An efficient catalytic conversion of refractory pollutant, methylene blue

Journal Pre-proofs Ultrasonic synthesis of α-MnO2 nanorods: An efficient catalytic conversion of refractory pollutant, methylene blue Vellaichamy Balakumar, Ji Won Ryu, Hyungjoo Kim, Ramalingam Manivannan, Young-A Son PII: DOI: Reference:

S1350-4177(19)31103-4 https://doi.org/10.1016/j.ultsonch.2019.104870 ULTSON 104870

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Ultrasonics Sonochemistry

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16 July 2019 9 November 2019 10 November 2019

Please cite this article as: V. Balakumar, J. Won Ryu, H. Kim, R. Manivannan, Y-A. Son, Ultrasonic synthesis of α-MnO2 nanorods: An efficient catalytic conversion of refractory pollutant, methylene blue, Ultrasonics Sonochemistry (2019), doi: https://doi.org/10.1016/j.ultsonch.2019.104870

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Ultrasonic synthesis of α-MnO2 nanorods: An efficient catalytic conversion of refractory pollutant, methylene blue Vellaichamy Balakumar, Ji Won Ryu, Hyungjoo Kim, Ramalingam Manivannan and Young-A Son* Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, South Korea *Corresponding author: E-mail address: [email protected] (Y.-A. Son) Abstarct In this work, uniform α-MnO2 nanorods were synthesized via a simple hydrothermal followed by ultrasonication method using ultrasonic bath (20 kHz, 100 W) without using any surfactant and template. The crystallographic phases and surface morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transition elelctron microscopy (TEM) analysis, respectively. Functional group identification and chemical states of α -MnO2 nanorods were confirmed by Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The as-syntehsized uniform nanorods of α -MnO2 exhibit excellent catalytic conversion of toxic organic contaminant (methylene blue (MB)) in the presence of NaBH4 as reductant. The α -MnO2 exhibits excellent stability up to four repeated catalytic cycles with nearly 92% conversion. The kinetic rate constant (k), and turnover frequency (TOF) were 0.736 min-1 and 0.02 mmol mg-1 min-1, respectively. In addition, the fast electron transfer mechanism were investigated and discussed. These results open a new avenue 1

for developing various metal oxide catalysts, which are expected to be very useful catalytic conversions. Keywords: Ultrasonic syntehsis, α-MnO2 nanorods, catalytic conversion, MB dye, resuable

1.Introduction Multiple valence states of manganese ions favor the existence of MnO2 (manganese dioxide) in many polymorphic forms as α, β, γ, λ and ƹ depending on the linkage of fundamental MnO6 octahedron units. MnO2 is one of the promising green material because of its low cost, high abundance and high environment compatibility and good electrocatalytic activities [1]. Due to its unique nature, they have been investigated in various potential applications such as catalytic, photocatalytic, solar cells and energy storage [2-5]. Numerous methodologies have been employed to prepare MnO2 such as sol-gel [6], electrodeposition [7], hydrothermal synthesis [4], sonochemical reduction [8], etc. Among all the preparation methods, hydrothermal synthesis is the mostly adopted one to prepare MnO2 with different morphologies, including nanoparticles [5, 8], nanorods [4, 6], nanotubes [9], nanoflakes [10], nanowires [6, 7] and nanoflowers [11]. Therefore, MnO2 can be utilized for many promising applications with good surface area. For example, B. Bai et al., [12] have prepared three-dimentional ordered MnO2 mesoporous structure for catalytic oxidation of HCHO. Catalytic oxidation of resorcinol and formaldehyde nanomaterials were reported by using MnO2 (Zhao et al.,) [13]. Chen et al., [14] have reported different crystal structures of MnO2 apllied for catalytic oxidation of NO. F. Nawaz et al., [15] have investigated the catalytic ozonation of 4-nitrophenol by using MnO2. Tert-butyl

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hydroperoxide as an oxidant and α-MnO2 as catalyst were also introduced for the oxidative cyclization of anthranilamides or aminobenzylamines with alcohols by Z. Zhang et al. [16]. Nowadays, the ultra-sonication method has been sucessfully utilized for the synthesis of various metal, metal oxide and carbon based nanomaterials [17], [18]. Ultrasound has attracted considerable attention due to its promise in the development of novel nanomaterials and improve the quality size and shape. Ultrasonic method is simple, eco- and environmental- friendly compared to the conventional methods, which makes their utility to develop the metal oxides with controlled phase purity, size, shape and particles distribution [19-22]. The recognized toxic organic contaminant, MB has been widely employed for various areas such as textile, paper, printing, pharmaceutical and pulp industries are global concern [23]. A nonbiodegradable organic dyes are great threat to aquatic life and other living organisms [24]. Among them, MB and their products have carcinogenic effects on human and animals even less than 1 ppm levels in water. Up to now various analytical technologies, including adsorption [25, 26], chemical oxidation [27], membrane separation [28] and phtocatalytic degradation [23] have been employed to removal of dyestuffs in waste water. But these conventional processes are usually insufficient for purifying the wastewater. Among them, catalytic conversion of toxic organic contaminants are simple, low cost and eco-friendly method. Based on the literature, first time uniform α -MnO2 nanorods were synthesized via a simple hydrothermal followed by ultrasonication method is reported herein. The as-synthesized α-MnO2 nanorods were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transition elelctron microscopy (TEM) Fourier-transform infrared spectroscopy (FI-IR) and

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X-ray photoelectron spectroscopy (XPS) analysis. According to the obtained results, they exhibit excellent catalytic conversion of toxic organic contaminants, MB in the presence of NaBH4 as reductant. Importantly, the fast electron transfer mechanism were investigated and discussed.

2. Experimental section 2.1 Materials and methods Potassium permanganate (KMnO4), sodium borohydride (NaBH4) and methylene blue (MB) dye were purchased from Sigma Alrich and used in this study without further purification. The asprepared α-MnO2 nanorods were characterized by XRD measurements and performed on a Cu Kα radiation (λ = 1.54056 Å) using (PHILIPS, X'Pert-MPD System, Max P/N: 3 kW/40 kV, 45 Ma, Netherlands). Infrared spectra experiments were collected on ALPHA-P spectrometer. The surface structure was obtained using scanning electron microscope (LYRA3 XMU) and transmission electron microscope (TEM, Tecnai G2 F30 S-Twin) operating at 100 kV. JASCO FT-IR 460 Plus spectrophotometer were used to identify the functional groups present in the αMnO2 nanorods. Chemical states of α-MnO2 nanorods were measured by XPS experiments using a Multilab 2000 spectrometer.

2.2 Synthesis of α-MnO2 nanorods

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In a typical procedure, 2 mg of KMnO4 was disolved in 10 ml distilled water and concentrated HCl (5 ml) were added to the above solution slowly. Then, the mixture of solution was ultrasonicated until a transparent purple solution was formed. Afterwards, a transparent purple solution was kept on to the oil-bath and refluxed for 24 h at 200 °C. The precipitates were cooled and ultrasonicated for 1h using ultrasonic bath (20 kHz, 100 W). Finally, the precipitates were washed and filtered with distilled water several times and dried at 110 °C in a vacuum oven at 24 h. The detailed synthesize process of α-MnO2 nanorods as shown in Scheme. 1.

2.3 Experimental procedures for conversions of MB The catalytic activity of as-prepared catalyst, α -MnO2 nanorods (1.0 mg) and 3.0 % platinum nanoparticles were added into the 2 mL of MB aqueous solution (0.2 mM) at pH = 7 was taken in a cuvette followed by adding 1 mL (0.5 M) of freshly-prepared NaBH4 solution and the solution were measured UV–vis spectra in the wavelength range of 400–800 nm at different time intervals. Simultaneously, the color of the solution was taken in photograph using digital camera at different time intervals. The dye catalytic conversion efficiency was calculated by formula; % = C0-C/C0 x 100

(1)

where C and C0 represent the absorbance intensity before and after catalytic reaction, respectively. The kinetic rate constant was calculated using the equation; -lnC/C0 = kt

(2)

5

where k is the kinetic rate constant and t is the conversion time. C0 and C are the MB concentration of initial and at time t, respectively.

3. Results and discussions 3.1 XRD XRD was analyzed to confirm the synthesized α-MnO2 nanorods crystallinity and phase purity in the 2θ range of 10-80° as shown in Fig. 1. The well-defined diffraction peaks were observed at 12.71°, 21.60°, 25.36°, 28.35°, 37.22°, 41.74°, 49.42°, 56.32°, 59.95°, 65.21°, 69.12°, 72.57° and 78.44corresponded to the (1 1 0), (2 0 0), (2 2 0), (3 1 0), (2 1 1), (3 0 1), (4 1 1), (6 0 0), (5 2 1), (0 0 2), (5 4 1), (3 1 2) and (3 3 2) planes, respectively [4, 5, 8]. These diffraction peaks showed good agreements with reported crystalline α -MnO2 nanorods from JCPDS data (card no. 440141). The average crystallite size of α-MnO2 nanorods were calculated using Debye-Scherrer equation [29] and found to be 20 nm. D = Kλ/β cos θ where K denotes the Scherrer’s constant (K = 0.94), λ is the X-ray wavelength, β is the fullwidth at half-maximum (FWHM) of the diffraction line in radians and θ is half the diffraction angle. 3.2 Surface analysis

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Both size and shape of the α -MnO2 nanorods were investigated by using SEM and TEM. As shown in Fig. 2A-C, the different magnification SEM images of synthesized α-MnO2 nanorods with uniform size are showed clearly. Further, these results were also analyzed by HRTEM and the different magnification results shown in Fig. 3A-D. The result clearly revealed that the average diameter width of α-MnO2 nanorods was 20 nm and the high crystalline nanorods, with inter-planar distances d = 0.25 nm (Fig. 3D) corresponding to the plane (2 1 1). These results are well agreements with XRD and SEM. The purity of the samples was further investigated by EDX (Fig. 2D) and mapping analysis (Fig. 4A-D). The presence of Mn and O are clearly indicated the formation of α -MnO2 nanorods without any impurities. These nanorods may represent an advantage for applications requiring rapid catalytic processes to enhanced efficiency.

3.3 FT-IR Fig. 5 shows the FT-IR spectrum of the prepared α-MnO2 nanorods. The major peaks observed at 452, 509 and 709 cm−1 are attributed to the Mn-O-Mn vibrations of MnO6 octahedra in αMnO2 nanorods. The obtained results were consistent with the previous reported results [30]. The two broad bands were also detected at 3183 and 1626 cm−1 corresponding to the O-H stretching and bending vibration modes of surface adsorbed interlayer water molecules [11]. In addition, the peak at 1034 cm-1 is attributed to the Mn atoms combined with the O-H bending vibrations [31].

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3.4 XPS To determine the chemical state of as-prepared α -MnO2 nanorods, XPS analysis was investigated. The typical XPS survey spectrum of the α-MnO2 nanorods clearly demonstrates the existence of Mn and O elements as shown in Fig. 6. Notably, the three peaks at 531.8 eV, 533.2 eV and 534.6 eV are attributed to the O1s [32]. The Mn 2p3/2 and Mn 2p1/2 electrons peaks were detected at 641.8 eV and 653.5 eV respectively [9]. This result is in accordance with XRD, FT-IR and HRTEM. Based on the above results and discussion, it can be concluded that the synthesized α-MnO2 nanorods was pure and well crystalline nature.

3.5 Catalytic conversion of MB The catalytic activity of the as-synthesized α-MnO2 nanorods catalysts was investigated through the catalytic reduction of dyes and nitro-aromatics as a model reaction [21, 29, 33]. As shown in Fig. 7A (blue line), UV-Vis spectrum of MB the strong absorption peak at 663 nm is corresponding to conjugated π bond of the molecular structure MB [34]. In addition, after adding NaBH4 the color and absorbance spectra of MB showed no obvious changes, indicating that the conversion of MB does not take place without catalyst. Interestingly, under the same experimental conditions after adding of α -MnO2 nanorod catalysts the absorbance peak at 663 nm decreased as shown Fig. 7A. At the same time, the color of MB was also faded and 8

finally disappeared as a function of reaction time as shown in Fig. 7B. Without NaBH4 nothing happens to the reduction process. This result indicates the decomposition of MB dye molecules through catalytic reduction. The catalytic conversion percentage (Fig. 8A) and kinetic rate constant (Fig. 8B) of MB was found to be 98 % and 0.736 min-1 respectively. Turnover frequency (TOF) was calculated by dividing turnover number (TON) by the reaction time (min). TOF of MB reduction was found to be 0.02 mmol mg-1 min-1. The catalytic reduction was compared with various reported nano catalysts are shown in Table. 1. [35-42]

3.6 Catalytic conversion mechanism According to our results and literature works, the BH4- (act as nucleophile) and MB dyes (act as electrophile) were adsorbed on to the surface of α-MnO2 nanorods as shown in Scheme. 2. Then, the surface adsorbed hydrogen is responsible to conversions of adsorbed MB dye molecules [43, 44]. Concomitantly, the conversion products desorbed on to the surface of MnO2 nanorods and the catalyst was reusable. These results are well agreements with earlier reported results [29, 33]. Then, the free surface of α -MnO2 nanorods was used to next catalytic conversion cycles. The stability of the catalyst is a great important for real practical applications. After each run catalytic cycle, the α-MnO2 nanorods catalyst was washed with ethanol followed by an excess of doubledistilled water and used for next catalytic reaction cycle. Even after four consecutive cycles the catalyst exhibits good activity and stability as shown in Fig. 9A. Further, the reusable catalyst surface morphology was analyzed by SEM and result are shown in Fig. 9B. These result

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indicated that the α-MnO2 nanorods catalyst shows good stability towards catalytic conversion of organic contaminants.

4. Conclusion In summary, uniform α-MnO2 nanorods were synthesized via a simple hydrothermal followed by ultrasonication method. The synthesiszed α-MnO2 nanorods were characterized by XRD, SEM, TEM, FT-IR and XPS spectral analysis. The obtained results clearly confirm the formation of uniform α-MnO2 nanorods with 20 nm width. The as-syntehsiszed uniform nanorods of α-MnO2 exhibits excellent catalytic conversion of toxic organic contaminants and exhibits excellent stability up to four repeated catalytic cycles. The catalytic conversion percentage, kinetics rate constant and TOF of MB was found to be 98 %, 0.736 min-1 and 0.02 mmol mg-1 min-1 respectively. In addition, the fast electron transfer mechanism were also investigated and discussed. These results open a new avenue for developing various metal oxide catalysts, which are expected to be very useful catalytic conversions.

Acknowledgements This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant No. NRF-2017R1E1A1A01074266). This work was supported by the industrial Fundamental

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Technology Development Program (10076350) funded by the Ministry of Trade, Industry and Energy (MOTIE) of Korea.

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Figure, Scheme and Table Captions Scheme 1. Illustration of synthesis process of α-MnO2 nanorods

Fig. 1 XRD pattern of α-MnO2 nanorods

Fig. 2(A-C) SEM images of α-MnO2 nanorods with different mangnifications and (D) EDX spectrum of α-MnO2 nanorods

Fig. 3 (A-D) HR-EM images of α-MnO2 nanorods with different mangnifications

Fig. 4 (A-D) SEM mapping analysis of α-MnO2 nanorods for elements of C, O and Mn

Fig. 5 FI-IR spectrum of α-MnO2 nanorods

Fig. 6 XPS survey spectrum of α-MnO2 nanorods (A), XPS spectra of O 1s (B) and Mn 2p (C)

Fig. 7 (A) UV-Vis spectra of MB (MaBH4) with α-MnO2 nanorods at different time intervals and (B) Corresponding photographic images at different time intervals Fig. 8 (A) The plot of conversion % vs. reaction time (min) and (B) The kinetics plot of ln(C/C0) against the reaction time Scheme. 2 The catalytic conversion mechanism of MB using α-MnO2 nanorods

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Fig. 9 Reusability of MB catalytic conversion by α-MnO2 nanorods (A) and SEM image αMnO2 nanorods after being used for four cycles (B) Table. 1 Comparison of various nano catalysts in the reduction of MB dyes

18

Scheme 1.

19

Fig. 1

20

Fig. 2

21

Fig. 3

22

Fig. 4

23

Fig. 5

24

Fig. 6 25

Fig. 7

26

Fig. 8

27

Scheme. 2

28

Fig. 9

29

Table. 1 Catalyst

Catalyst

Conversion

Rate

Reusability

(weight)

time

constant (k)

(cycles)

3.0 mg

90 s

0.015  s−1

6

35

Ag NPs@Mt

2 mg

9 min

1.70 min− 1

20

36

ZBD@Ag

2 mg

6 min

0.0061 s−1

5

37

3D-

5 mg

15 min

0.2523 min-1

-

38

Au@TA-GH

2 mg

9 min

0.31 min−1

5

39

Ag/RGO/TiO2

5.0 mg

Immediately

-

-

40

SiNWAs–Cu

-

14 min

0.529 s−1

5

41

Fe3O4@C@Au 5 mg

10 min

0.331 min−1

6

42

α-MnO2

10 min

0.736 min-1

5

This work

Fe3O4@COF-

References

Au

graphene/Ag

1 mg

nanorods

30

Graphical Abstract Ultrasonic synthesis of α-MnO2 nanorods: An efficient catalytic conversion of refractory pollutant, methylene blue Vellaichamy Balakumar, Ji Won Ryu, Hyungjoo Kim, Ramalingam Manivannan and Young-A Son* Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, South Korea *Corresponding author: E-mail address: [email protected] (Y.-A. Son) Graphical abstract Uniform α-MnO2 nanorods were synthesized via hydrothermal followed by ultra-sonication method. They exhibited high efficient catalytic conversion of refractory pollutant, methylene blue.

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Highlights  α-MnO2 nanorods were synthesized via a simple hydrothermal followed by ultrasonication method

 α-MnO2 nanorods exhibit excellent catalytic conversions toward refractory pollutant, methylene blue  99.7 % was achieved for the catalytic conversion efficiency  The nanorods showed better resuability and improved catalytic cycling stability

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