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Novel Synthesis of Birnessite-Type MnO2 Nanostructure for Water Treatment and Electrochemical Capacitor Junli Zhou, Lin Yu, Ming Sun, Shanyu Yang, Fei Ye, Jun He, and Zhifeng Hao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400577a • Publication Date (Web): 04 Jun 2013 Downloaded from http://pubs.acs.org on June 6, 2013

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Industrial & Engineering Chemistry Research

Novel Synthesis of Birnessite-Type MnO2 Nanostructure for Water Treatment and Electrochemical Capacitor Junli Zhou, Lin Yu,* Ming Sun, Shanyu Yang, Fei Ye, Jun He, Zhifeng Hao Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China

*corresponding author：Tel. +86 20 39322202; fax: +86 20 39322231. E-mail: [email protected] (Lin Yu)

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ABSTRACT:

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We report for the first time a novel and rapid (1 h) synthesis method for

birnessite-type MnO2 nanostructures via a polyol-reflux process through oxidizing MnCl2 with H2O2 under basic conditions in the presence of polyvinylpyrrolidone (PVP). Influencing factors such as the dosage of reactants and the reaction times are systematically investigated. The molar ratios of OH/Mn played an important role in the formation of birnessite-type MnO2 nanostructure with good crystallinity and ordered 3D nanostructures. A possible formation mechanism for the nanostructure was proposed. The flower-like birnessite-type MnO2 nanostructure is composed of nanosheets with an average diameter ca. 300-500 nm, and shows mesoporous characteristics with a pore diameter of 20 nm. Due to its unique mesoporous structure, the birnessite-type MnO2 exhibits excellent ability to remove organic pollutants (Ponceau 2R) and shows a potential application as electrochemical capacitor.

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1. Introduction The design and synthesis of transition metal oxide functional materials with controlled shape and desired morphology have attracted much attention because of their novel properties and potential applications1-4. In particular, the assembling fabrication of manganese oxides nanomaterials with different morphologies has been widely used for catalyst5-7, environment protection8, 9, sensors10, supercapacitor11 and so forth. Nowadays, various manganese oxide structures, such as nanowires, nanorods, spheres, etc., have been synthesized by a variety of methods. However, it still remains a great challenge to develop facile and economic synthetic methods for the construction of hierarchical architectures with controlled morphologies. Birnessite-type manganese oxide is a kind of layered manganese oxide which formed by [MnO6] octahedral sharing edges, with alkaline cations and water molecules between the layers12. Birnessite-type manganese oxide has gained much attention due to its readily preparation and functionalization. Methods to synthesize birnessite-type nanostructures, normally, involved oxidation of Mn2+, reduction of MnO4-, redox reactions between Mn2+ and MnO4- via hydrothermal process. Monodisperse Birnessite-type manganese oxide honeycomb and hollow nanospheres have been prepared at room temperature through the reactions between KMnO4 and oleic acid, which showed high catalytic activities for oxidative decomposition of formaldehyde at low temperatures8. Potassium birnessite nanowires and nanoribbons were successfully fabricated through the redox reactions between KMnO4 and MnCl2 in NaOH solution via hydrothermal process, and a small magnetoresistance

was

observed

in

this

paramagnetic

potassium

birnessite13.

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One-dimensional manganese oxide nanobelt bundles with birnessite-type structure, which exhibits good capacitive behavior and cycling stability, have been synthesized by hydrothermally treating the precursor, K-type layered manganese oxide in a NaOH solution14. The 2D birnessite-type MnO2 nanowalls have been fabricated on Si(111) substrate by a solution method and exhibit prominent magnetic anisotropy15. Birnessite-type

manganese

oxides

nanosheets

deposited

on

ITO

assisted

photoelectrochemical activity were synthesized by anodic electroplating 16. Herein, we report the synthesis of 3D birnessite-type MnO2 nanostructures via a polyol-reflux process through oxidizing MnCl2 with H2O2 under basic conditions in the presence of poly (vinyl pyrrolidone) (PVP). Our approach follows the preparation of the 2D lithium- and sodium-type birnessites by using H2O2 as an oxidizing agent for Mn2+ ions17-19. While, in our processing, the oxidation of Mn2+ was conducted in NaOH-ethylene glycol reflux system. To the best of our knowledge, the polyol-reflux method was adopted for the first time to synthesize 3D birnessite-type MnO2 nanostructures in this study. Besides H2O2, PVP was introduced as a structure directing agent as well as a stabilizer for preventing the aggregation of nanomaterials. The one-step method for rapid (1 h) preparation of manganese oxides nanosheets into 3D nanostructures was conducted under mild conditions without a high-temperature and pressure treatment. The as-prepared birnessite-type MnO2 with 3D nanostructures held the advantages of mesoporous, making it particularly attractive for environmental protection and other applications.

2. Experimental Section 2.1. Preparation of Birnessite-type MnO2 nanostructures. All the reagents are of

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analytical grade and used without further purification. In a typical experiment, 2.4 g NaOH, 0.5 g PVP and 2.97 g MnCl2·4H2O were added to ethylene glycol (90 mL) in a 250 mL round flask. H2O2 (10 mL) was then added to the obtained red solution under magnetic stirring at refluxing temperature (ca.100 °C). OH/Mn2+ molar ratio was fixed at 5.0, 4.0, 3.5, and 2.7, respectively. The reaction was stopped after refluxing for 1 h and the mixture was cooled to room temperature. The resulting black precipitate was collected by centrifugation and washed with ethanol several times to remove the impurities and PVP. 2.2. Sample Characterizations. The X-ray diffraction (XRD) patterns of the samples were analyzed by a Bruker D8 X-ray diffractometer equipped with a Cu Kα radiation source using an operation voltage and current of 40 kV and 40 mA. Raman scattering (RS) spectra were collected on a dispersive Horiva Jobin Yvon LabRam HR800 Microscope, with a 633 nm laser and 24 mW laser powers. Scanning electron microscopy/energy dispersive X-ray analysis (SEM-EDX) was performed in a Digital Scanning Microscope S-3400N operated at 15 kV. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL 2100F transmission electron microscope at 200 kV. N2 adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020 Analyzer (USA). Pore sizes were calculated by the Barrett, Joyner and Halenda (BJH) method. 2.3. Decolorization experiments. The decolorization experiment of was performed in a round bottom flask at room temperature. A solution of Ponceau 2R (20 mg L-1, 100 mL) which has been adjusted to pH=1.7 using diluted H2SO4 and 5 mg of birnessite-type MnO2

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were mixed and stirred in the reaction vessel. A small quantity of reaction mixture (e.g., 5 mL) was taken out from the reaction vessel at definite intervals until the end of the reaction and then centrifuged at 10,000 rpm for 10 min to remove the sedimentation before UV analysis. The decolorization rate was calculated based on UV–visible spectrum (T-245) by monitoring its characteristic peak at 507 nm. The birnessite-type MnO2 was collected after each run and then used repeatedly. Each cycle test lasted 100 min. Before the beginning of the next cycle, the remaining solution was replaced with 20 mg/L fresh solution. 2.4. Electrochemical test. Electrochemical performance of the birnessite-type MnO2 nanostructure materials was evaluated in 1 mol L-1 Na2SO4 electrolyte at room temperature, using a Solartron SI 1287 electrochemical interface and SI 1260 impedance/gain-phase analyzer. The test was performed in a three-electrode cell, in which MnO2 electrode was assembled as the working electrode, a platinum mesh and a saturated calomel electrode (SCE) were used as the counter and the reference electrode, respectively.

The working

electrode was fabricated by compressing a mixture of the MnO2-acetylene blackpolytetrafluoroethylene (PTFE) with a weight ratio of 0.85:0.15:0.05 on a foamed nickel with a dimension of 1cm×1cm at 0.2 MPa.

3. Results and Discussion 3.1. Characterizations of Birnessite-type MnO2 nanostructures. Figure 1a displays the

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Figure 1. (a) XRD pattern (the inset is a schematic illustration of its crystal structure) and (b) Raman spectra of the birnessite-type MnO2 nanostructures.

typical XRD pattern of the birnessite-type MnO2 and the schematic illustration of its crystal structure. The XRD pattern showed the layered structure of monoclinic birnessite type MnO2 (PDF#43-1456), with lattice constants a = 0.5175 nm, b = 0.2849 nm and c = 0.7338. The d001 value of 0.73 nm is related to the interlayer spacing. The structure of the birnessite-type MnO2 was further characterized by Raman spectroscopy (Figure 1b). Five major vibrational features corresponding to MnO2 can be identified at 281, 406, 509,581 and 639 cm−1. The Raman band at 639 cm−1 can be viewed as the symmetric stretching vibration ν2(Mn–O) of MnO6 groups, while the band located at 581 cm−1 can be attributed to the ν3 (Mn–O) stretching vibration in the basal plane of [MnO6] sheets20. From these vibrational features, it can be inferred that a birnessite-type MnO2 with layered structure was obtained. SEM and HRTEM were employed to the characterization of the birnessite-type MnO2 (Figure 2). SEM images show interconnected 7



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Figure 2. SEM image of the birnessite-type MnO2 (a) Low-magnification image (Inset: EDX spectrum) and (b) high-magnification image (Inset: N2 adsorption/desorption isotherms and pore size distribution). (c) TEM image of the birnessite-type MnO2. (d) TEM image of the birnessite-type MnO2 from the edges of nanosheets (Inset: SAED pattern taken from the edge of nanosheet). (e) HRTEM image of the birnessite-type MnO2 (Inset: the magnified HRTEM image).

flowerlike architectures with a diameter ca. 300-500 nm. These nanoflowers are composed of nanosheets with a thickness of about 10 nm which connected with each other to form the 3D flowerlike nanostructure by self-assembly (Figure2b). Ultrasonic dispersion was used in preparation of the TEM sample. However, the well 3D flowerlike architecture was still observed (Figure 2c). The solid combination between the nanosheets as building blocks makes the structure more stable. The 2D plate-like nanosheets can be observed clearly in 8


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Figure 2d. These nanosheets become thinner gradually from the center to the edge. The selected area electron diffraction (SAED) pattern (Figure 2d, inset) taken from the edge of nanosheet exhibits high polycrystalline nature of birnessite-type MnO2. The lattice fringes are clearly visible attributing to the (200), (-112) and (020) planes. The HRTEM image also confirms the birnessite-type MnO2 nanostructures are well crystallized with the interplanar spacing 0.72 nm on average (Figure 2e), which is in good agreement with XRD result. The energy dispersive X-ray (EDX) measurement (Figure 2a, inset) shows that no elements other than Mn, Na, C, and O are present. The element Si comes from the substrate where samples are dispersed. The pore size distribution of the nanoparticles was calculated from nitrogen desorption curve using the Barrett-Joyner-Halenda model. The results show a pore size distribution centered at 20 nm (Figure 2b, inset). The isotherm has a distinct type IV hysteresis loop observed in the P/P0 range of 0.6-0.99, which is a fingerprint of a mesoporous feature 21, 22. 3.2. Influence Factors on the formation of Birnessite-type MnO2 nanostructures. Layered structure manganese oxides prefer to be formed under basic conditions23. Therefore, the NaOH/MnCl2 molar ratio employed is believed to play an important role in the formation of the birnessite-type MnO2. The XRD and

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Figure 3. (a) XRD patterns and SEM images: (b) 2.7 OH/Mn; (c) 3.5 OH/Mn; (d) 4.0 OH/Mn and (e) 5.0 OH/Mn of the products obtained after refluxing at 100 °C for 1 h with varied NaOH/MnCl2 molar ratios.

SEM results clearly showed that different NaOH/MnCl2 molar ratios significantly affect the formation and ordering of layered structures (Figure 3). The XRD pattern of product obtained at lower NaOH/MnCl2 molar ratios (2.7 OH/Mn) presents no obvious peak (Figure 3a), indicating a very poorly crystallized compound. For products prepared with higher NaOH/MnCl2 molar ratios (3.5, 4.0, 5.0 OH/Mn), the diffraction peaks at 12.7° which could be indexed as (001) are evident. The crystallinity of the products increases with the rise of NaOH/MnCl2 molar ratio. This phenomenon can be explained by the charge density of manganese oxide layer19. The layered structure is predominantly held by an electrostatic attraction between the negatively charged [MnO6] octahedral sheets and the positively charged Na+ in the interlayer space. A layered manganese oxide with low charge density or low Na+ content is readily expanded by crystal water filling the interlayer space. Therefore, for products prepared with low NaOH/MnCl2 molar ratios, the intensities of the diffraction peaks of birnessite phase were lower. Products prepared at higher NaOH/MnCl2 10


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molar ratios have high Na+ contents, and the higher intensity birnessite phase is easily formed under such conditions. The structures of products obtained with varied NaOH/MnCl2 molar ratios were further characterized by Raman spectroscopy. The peaks at 281, 406, 509, 581 and 639 cm−1 corresponding to MnO2 become strong with increasing NaOH/MnCl2 molar ratios (Supporting Information, Figure S1). SEM images of the product prepared with varied NaOH/MnCl2 molar ratios were also obtained. Under a lower molar ratio of NaOH/MnCl2 (2.7), the product shows the morphology of nanoparticles (Figure 3b). Raising the NaOH/MnCl2 molar ratios to 3.5, we can obtain product composed of randomly self-assembled nanoplatelets (Figure 3c). Under a NaOH/MnCl2 molar ratio of 4.0, the well self-assembled flowerlike architectures with a diameter ca. 300-500 nm appears (Figure 3d). When the molar ratio of NaOH/MnCl2 is further increased up to 5, irregular flowerlike architecture is observed (Figure 3e). Hydrogen peroxide (H2O2) was used as an oxidizing agent for Mn2+ ions in our synthetic process. Without H2O2, the birnessite-type manganese oxide is failed to produce as proved by the amorphous and weak XRD pattern (Supporting Information, Figure S2). The role of PVP was found to be very critical. In a series of further experiments we discovered that PVP acts both as a stabilizer for preventing the aggregation of the flowerlike architectures and as a structure directing agent for preparation of the flowerlike architectures24, 25. Without the assistance of PVP, the nanosheets were self-assembled in a random manner, and no distinct flowerlike architectures could be observed (Figure 4b). In

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Figure 4. XRD patterns(a) and SEM images of the products obtained after refluxing at 100 °C for 1 h with different amount of PVP (4.0 NaOH/MnCl2): (b) free of PVP; (c) 0.1 g PVP; (d) 0.3 g PVP and (e) 0.5 g PVP.

the presence of a lower concentration of PVP (

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