Morphological Evolution of Two-Dimensional MnO2 Nanosheets and

Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India. J. Phys. Chem. C , 0, (),. DOI: 10.1021/jp403...
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Morphological Evolution of Two-Dimensional MnO2 Nanosheets and Their Shape Transformation to One-Dimensional Ultralong MnO2 Nanowires for Robust Catalytic Activity Arun Kumar Sinha, Mukul Pradhan, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India S Supporting Information *

ABSTRACT: This paper reports the nucleation and growth of 2D δ-MnO2 nanosheets from a simple wet chemical reaction at low temperature. The reaction between aqueous KMnO4 and pure ethyl acetate constitutes a new a biphasic platform for product evolution. Then the synthesized δ-MnO2 nanosheets undergo shape transformation to 1D α-MnO2 nanowires in the same pot via a dissolution−recrystallization mechanism. The detailed growth mechanism of nanosheet formation and the morphology transformation are described in this paper. Moreover, this low temperature technique can be used to produce ultralong α-MnO2 nanowires in industrial amounts which are difficult to synthesize by conventional high temperature methods. In this method, we have prepared 25.00 g of α-MnO2 nanowires in our nanomaterial laboratory from 18 batches. Lastly, solvent-free catalytic activity of α-MnO2 nanowires is investigated for the hydrolysis of benzonitrile to benzamide in one step.

1. INTRODUCTION Manganese dioxide nanostructures have been the subject of intensive research throughout the globe due to their potential applications in batteries,1 supercapacitors,2 gas sensors,3 and catalysis4 and electrocatalysis5 reactions. Many research groups have focused their studies on the control design/synthesis of MnO2 nanostructures because of their high surface areas, resulting in improvements in the surface related physical properties. Therefore, the nucleation or growth mechanism of nanostructure formation has important significance for the design of nanomaterial morphology in nanoscience and nanotechnology. A number of MnO2 nanomaterials in various geometrical morphologies have been produced such as rods,6 wires,7 diskettes,8 hollow tubes,9 sheets,10 flowers,11 urchins,12 thin films,13 belts,14 and dendrites.15 Therefore, there are many methods of evolution of MnO2 nanostructures. The synthetic approaches/methods are the sol−gel method, template-based method, hydrothermal method, solvothermal method, and others. Recently researchers have concentrated their efforts and energy on the synthesis of MnO2 nanomaterials of varied dimensions and morphologies with easy handling and costeffective routes. The anisotropic structure or the oriented growth of one-dimensional (1D) nanowires is difficult to obtain and usually requires solid templates (such as porous alumina, polymer nanotubes), soft templates (such as polymer, surfactant), and patterned catalysts to control the direction of nanocrystal growth.16 However, the use of templates and substrates increases the production costs for industrial or large scale production. Therefore, a self-seeded method is fascinating for the facile synthesis or growth of 1D nanomaterials. However, it is also very difficult to produce industrial amounts © 2013 American Chemical Society

of 1D MnO2 nanowires from the hydrothermal/solvothermal method because the reaction system does not allow the production of large amounts of material from this method. In the case of hydrothermal/solvothermal method, the Teflon pot or the reaction system has a total maximum volume capacity less than 100 mL and the reaction system is a closed one. Therefore, there remains a chance of a serious accident in the closed system and one cannot scale up the amount of MnO2 nanomaterial in the closed reaction system to a huge amount (say from gram to kilogram level). In a word, industrial scale production of MnO2 nanomaterial is very difficult from the hydrothermal reaction. In recent years, a huge amount of MnO2 nanomaterial has been used as cathode material in battery systems in place of bulk/conventional MnO2 material for betterment of activity owing to the surface area increment.17 Therefore, production of MnO2 nanomaterial with perfect morphology (1D or 2D) on an industrial scale is a challenging job in the energy advance material fields. The problem will be solved if the reaction system is wet chemical or a sol−gel reaction which is easily handled by unskilled operators and one can scale up the amount to meet the production cost. Li et al. reported the synthesis of 1D α-MnO2 nanorods through a liquid-phase comproportionation reaction between the KMnO4/MnSO4 system18 and 1D MnO2 nanowires by the oxidation of Mn2+ by S2O82− at low temperature19 but hydrothermal condition. Until now most of the research groups have reported on the Received: April 10, 2013 Revised: October 9, 2013 Published: November 1, 2013 23976

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Figure 1. Preparation of gram level α-MnO2 nanowires from aqueous KMnO4/ethyl acetate biphasic system and their different stages (a−f) in digital/FESEM images.

of 2D δ-MnO2 nanosheets and their morphology transformation to 1D nanowires by a simple wet chemical reaction. Finally, we have fabricated a MnO2 membrane/wheel which is successfully used as a reusable robust catalyst bed for a one-step hydrolysis of benzonitrile. Thus the material becomes technologically sound and academically foolproof.

synthesis 1D MnO2 nanorods or nanowires by the reduction of Mn7+ by Mn2+ in different environments.20 Therefore, to find out a proper wet chemical/sol−gel reaction at a relatively low temperature is a great achievement that will give an industrial amount of 1D/2D MnO2 nanostructures for its real application in the different industries. In general, one cannot produce welldefined 1D MnO2 nanostructures (nanorods, nanowires) without hydrothermal or solvothermal reaction. All the published communications/articles reported the synthesis of well-shaped 1D MnO2 nanomaterial from hydrothermal or solvothermal reaction; i.e., the reaction proceeds under autogenic pressure and temperature. There are few examples of the synthesis of MnO2 nanowires in wet chemical reaction at low temperature.21 In the case of 2D MnO2 nanosheets, prepared by a chemical exploitation mechanism, this mechanism was involved in all types of layer metal oxide materials. However, the synthesis of metal oxide nanosheets in the exploitation mechanism requires multistep processing, involving a high-temperature solid-state reaction for formation of layered material, protonation of interlayer alkali metal ions, and an acid−base reaction with an aqueous solution of quaternary ammonium cations, and finally yields negatively charged nanosheets.22 This top-down approach is costly, is timeconsuming, and is not easy to handle. Moreover, it is fairly difficult to exfoliate the protonated layer compounds completely into single-layer nanosheets. However, this problem will be solved, if the reaction becomes a single-step chemical reaction that forms single-layer nanosheets. In this article, we have reported a simple wet chemical reaction for the synthesis of micrometer-wide and only a few nanometers thick δ-MnO2 nanosheets. Then ultralong α-MnO2 nanowires are produced on an industrial scale using the lowcost chemicals KMnO4 and ethyl acetate without the use of an autoclave or hydrothermal reaction conditions. This simple wet chemical reaction gives a high quality nanosheet in a single step reaction and nanowire without the use of any stabilizer, porous 1D template, seed catalyst, or growth controlling agent. Here we have presented an in-depth mechanistic study on the growth

2. EXPERIMENTAL SECTION 2.1. Materials and Analytical Instruments. All the reagents were of AR grade. Triple distilled water was used throughout the experiment. KMnO4 was obtained from Lobachemie Indoaustranal Co. Ethyl acetate was purchased from SRL. All the reagents were used without further purification. Powder X-ray diffraction (XRD) was done in a PW1710 diffractometer, a Philips, Holland, instrument. The XRD data were analyzed by use of JCPDS software. Raman spectra were obtained with a Renishaw Raman microscope, equipped with a He−Ne laser excitation source of emitting wavelength 633 nm and a Peltier cooled (−70 °C) charge coupled device camera (CCD). Fourier transform infrared spectroscopy (FTIR) measurements of the samples were done in KBr pellets in reflectance mode with a Nexus 870 Thermo-Nicolet instrument coupled with a Thermo-Nicolet Continuum FTIR microscope. Field emission scanning electron microscopy (FESEM) was performed with a supra 40, Carl Zeiss Pvt. Ltd. instrument. Transmission electron microscopy (TEM) was performed with an H-9000 NAR instrument, Hitachi, using an accelerating voltage of 200 kV. The chemical states of the elements on the surface were analyzed by a VG Scientific ESCALAB MK II spectrometer (U.K.) equipped with a Mg Kα excitation source (1253.6 eV) and a five channeltron detection system. Nitrogen adsorption−desorption measurements were performed at 77.3 K using a Quantachrome Instruments utilizing the BET model for the calculation of surface areas. The pore size distribution was calculated from the adsorption isotherm curves using the Barrett−Joyner−Halenda (BJH) method. 23977

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2.2. Synthesis of 2D and 1D MnO2 Nanomaterial. We have described the synthesis of δ-MnO2 nanosheets and αMnO2 nanowires from the same reaction system (aqueous KMnO4/pure ethyl acetate). The reaction consists of two steps: 2.2.1. Formation of 2D δ-MnO2 Nanosheets. For the preparation of δ-MnO2 nanosheets, KMnO4 and ethyl acetate were employed as starting material. An aqueous solution of KMnO4 (750 mL and 20 mM) and 200 mL of ethyl acetate are taken together in 1 L capacity round-bottom flask. Thus a biphasic mixture is formed and kept on a water bath under refluxing condition at 80−85 °C. After the complete discharge of the pink color of KMnO4, brown MnO2 precipitate was thrown out at the bottom of the round-bottom flask. The brown product is shown in a digital image in Figure 1. The brown product of colloidal nature was separated from the upper part of ethyl acetate by a separating funnel. This brown colloid, i.e., nanosheets, was used for the shape transformation reaction. 2.2.2. Shape Transformation of 2D δ-MnO2 Nanosheets to 1D α-MnO2 Nanowires. This step involves the shape transformation of δ-MnO2 nanosheets into α-MnO2 nanowires. The synthesis of δ-MnO2 nanosheets is described in section 2.2.1. Now the synthesized brown colloidal MnO2 was kept in a round-bottom flask (having capacity 1 L) and heated in a water bath at 80−90 °C. A watch glass was used to close the open end of the round-bottom flask. After 90−120 h, the whole brown mass was changed to fluffy black α-MnO2, which looks like moistened cotton fiber. The color change indicates the formation of α-MnO2 nanowires. The black mass was separated and dried at 90 °C on a water bath and used for further characterization. All the steps of transformation reaction are shown in Figure 1d,e. During the shape transformation reaction, we have collected the transformed MnO2 material in different time gaps to establish the exact reaction mechanism. In this connection, we are also able to sign for the preparation of an industrial scale amount (about 25 g which is shown in the Supporting Information, Figure S1) of ultralong α-MnO2 nanowires using 14 L of KMnO4 and 3.6 L of ethyl acetate in 18 batches of reaction. From each batch of reaction, we have used 750 mL of aqueous KMnO4 (20 mM) and 200 mL of pure ethyl acetate, and at the end of reaction, 1.50 g of ultralong αMnO2 nanowires was obtained.

Figure 2. Powder XRD patterns of MnO2 material using Cu target having wavelength 1.545 Å. (a) δ-MnO2 nanosheets (NS); (b) αMnO2 nanowires, as prepared; (c) α-MnO2 nanowires at 200 °C; (d) α-MnO2 nanowires at 400 °C.

3. RESULTS AND DISCUSSION We have developed a process for the synthesis of 2D δ-MnO2 nanosheets and ultralong α-MnO2 nanowires by a simple wet chemical reaction. The nanowires are formed from δ-MnO2 nanosheets by the shape transformation mechanism at a relatively low temperature. The synthesized δ-MnO2 nanosheets and nanowires are characterized using different physical methods. Parts a and b of Figure 2 represent the X-ray diffraction patterns of the synthesized nanosheets and nanowires which perfectly match with the δ and α phases of MnO2, respectively.23 As shown in Figure 2b, all the diffraction peaks can be exclusively indexed as α-MnO2 with JCPDS Card No. 44-0141. On the basis of the above studies, it can be concluded that the α-MnO 2 nanowires are formed due to the crystallization of amorphous δ-MnO2 nanosheets through the shape transformation mechanism and formed 1D α-MnO2 nanowires. The synthesized α-MnO2 nanowires and heat treated α-MnO2 nanowires have phase purity akin to the corresponding α structure because there are no other phases existing in the diffraction patterns shown in Figure 2b−d.

The powder XRD experiment is also supported by Raman spectroscopic measurements, and the spectra for nanosheets and nanowires are shown in Figure 3. In the initial stage, the product δ-MnO2 nanosheets are amorphous in nature and show weak Raman peaks at 580 and 637 cm−1 due weak absorption of laser light. This is due to the reflection of laser light by the δ-MnO2 nanosheets. In the case of the as-prepared nanowires, the same two Raman bands appear distinctly. The Raman band significantly increases after the shape transformation of δ-MnO2 nanosheets to nanowires, demonstrating the increased crystallinity of α-MnO2 nanowires and that the water content in the tunnels of MnO2 nanowires decreases significantly.24 The elemental EDS analysis shows that the nanosheets and nanowires have K+ ions which are located in the tunnel structure because the material is derived from KMnO4. The EDS analyses of α-MnO2 nanowires and δ-MnO2 nanosheets are shown in the Supporting Information, Figures S2 and S3, respectively. The counter K+ ion is inserted into the tunnel of the MnO2 nanostructures and evolved the stable α phase. The Raman spectra of K+ containing α-MnO2 nanowire shows the three main peaks at 179, 574, and 637 cm−1 have

Figure 3. Raman spectra of (a) δ-MnO2 nanosheets (NS), (b) nanowires, (c) nanowires at 200 °C, and (d) α-MnO2 nanowires at 400 °C.

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Figure 4. XPS spectra for Mn 2p in (a) δ-MnO2 nanosheets and (c) α-MnO2 nanowires and O 1s spectra for (b) δ-MnO2 nanosheets and (d) αMnO2 nanowires.

Figure 5. FESEM images of δ-MnO2 nanosheets at different magnifications. Condition: the colloidal MnO2 sheet sample was spotted and dried on a glass surface.

Figure 6. FESEM images of α-MnO2 nanowires at different magnifications in different positions.

to the possible Mn2O3 or Mn3O4 takes place by the incident laser heating during Raman spectral measurement.9 The oxidation states of the δ-MnO2 nanosheets and nanowires were analyzed by X-ray photoelectron spectroscopy. Figure 4 presents the XPS spectra of δ-MnO2 nanosheets and nanowires. XPS spectra give the binding energies of Mn 2p and O 1s. The peaks at 654.42 and 642.98 eV are assigned to Mn 2p1/2 and Mn 2p3/2 binding energies, indicating the Mn(IV) state in both MnO2 nanomaterials shown in Figure 4a,c. The band at 529.50−531.5 eV is attributed to the O 1s binding energy shown in Figure 4b,d. From the shape of the O 1s levels,

been observed in our spectral window. The band of 574 cm−1 is attributed to the Mn−O lattice vibration in MnO2. The band of 637 cm−1 is assigned to Mn3O4 formation during the spectrum acquisition because of the local heating of the samples out of laser irradiation, which is consistent with the reported Mn3O4 sample (650−660 cm−1).9 In addition, the observed Raman band of Mn3O4 is slightly shifted to the low frequency region compared to the reported data, which is due to the possible K+ doping into α-MnO2 nanowires.9 The transformation of MnO2 23979

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Scheme 1. Two-Dimensional Nanosheet Formation from Interface Reaction and Their Transformation to 1D Nanowires Shown Schematically along with Photographs

Figure 7. TEM images of ultrathin δ-MnO2 nanosheets just like graphene at different magnifications.

Field emission scanning electron microscopic (FESEM) images show the morphology of the as-synthesized MnO2 products at different magnifications. Graphene-like 2D metal oxide δ-MnO2 nanosheets are presented in Figure 5. The width of the nanosheets is a few nanometers (nm), and the length is about a few micrometers (μm). Upon heat treatment at low temperature (about 80−90 °C for 120 h), the nanosheets are changed into nanowires. Large amounts of wirelike materials are shown in Figure 6. The longest nanowire observed in the sample is more than 10 μm. The product contains 100% nanowires and no other morphology has been detected, which reveals that the α-MnO 2 with nanowire structure is morphologically pure. The typical diameter of the nanowires lies in the range 20−30 nm (Supporting Information, Figure S5). To further analyze the characteristics of the nanowires, high-magnification FESEM images are given in Figure 6c,d. Typical nanowires are hairlike with a smooth surface. Figure 6d shows the tip surface of a typical nanowire, in which is roundshaped and swordlike. The α-MnO2 nanowire is composed of Mn and O, and that is also identified by a line mapping experiment. Red represents the presence of the Mn Kα1 signal, and cyan represents the presence the O Kα1 signal, shown in the Supporting Information, Figure S6. Furthermore, heat treatment under dry heating conditions (200 and 400 °C in the presence of O2) leads to a high order of crystallinity for the nanowires, i.e., makes the nanowire defect-free. Interestingly, after dry heat treatment at 200 or 400 °C, the produced α-

Figure 8. (a−c) TEM images of ultralong α-MnO2 nanowires at different magnifications and (d) HRTEM image.

it can be concluded that there are three types of O atom in both MnO2 materials and they are assigned as lattice oxygen, surface OH group, and oxygen of the absorbed water which might have been accumulated during the washing or aging in air.25 As a result, the absorbed water is detected in the prepared MnO2 by the XPS studies, which is also consistent with the FTIR results. It can be seen that the FTIR spectrum of the MnO2 shows absorption at 3452 cm−1 (Supporting Information, Figure S4), which corresponds to the stretching vibrations of water molecules or hydroxyl groups in the solid MnO2 tunnels. 23980

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Scheme 2. Schematic Representation of Hydrolysis Reaction of Benzonitrile in α-MnO2 Catalyst Bed

on does not occur or is not observed in the case of MnO2 nanosheets or nanoparticles and always black powder material resulted in α-MnO2. Figure 7 presents TEM images at different magnifications of the δ-MnO2 nanosheets which are formed from the reaction between KMnO4 and ethyl acetate but in the 2D interface of the biphasic solvent system. This shows that the nanosheets are of high quality in terms of dimension (ultrathin and micrometers long) and are just like graphene. Upon shape transformation, the nanosheets are changed to ultralong nanowires. Figure 8a−c presents the TEM images of the αMnO2 nanowire morphologies. The HRTEM image shows continuous but distinct lattice fringes in a single nanowire, indicating the crystalline nature of the nanowires. The HRTEM images reveal that the interplanar distance along the growth axis is 0.701 nm (shown in Figure 8d) which is consistent with the interplanar distance of the (110) plane of α-MnO2, thus confirming that the nanowire elongation axis is in the (110) direction. The BET surface areas of the MnO2 nanomaterials were also examined. The nanowire morphology shows a higher surface area than those of nanosheets and bulk commercial MnO2. In the dry condition, the nanosheet material remains restacked and the effective surface area becomes smaller. The BET surface area results and BHJ pore size distribution are shown in the Supporting Information, Figure S9, for MnO2 nanowire, nanosheet, and commercial bulk material.

Figure 9. Stepwise transformation of δ-MnO2 nanosheets into αMnO2 nanowires as revealed from (a) XRD and (b) Raman analysis.

MnO2 nanowires show uniform, smoothened nanocrystal morphology in the Supporting Information, Figure S7. One interesting observation is that when the as-prepared α-MnO2 nanowires were dried at 70−90 °C from aqueous solution, they appeared as a pallet having low density. The nanowires are interconnected with one another and forming a woven/ interlaced structure. The surfaces as well as the interior parts of the pallet have a porous nature which is shown in the Supporting Information , Figure S8. However, this phenomen-

Figure 10. Digital images of (a, b) MnO2 sheet of membrane from different angles and (c) MnO2 wheel. 23981

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Figure 11. FESEM images of MnO2 membrane (cross section) at different magnifications.

3.1. Growth Mechanism of 2D δ-MnO2 Nanosheets and Their Morphology Transformation toward 1D Ultralong α-MnO2 Nanowires. Aqueous KMnO4/ethyl acetate (biphasic system) in a 1 L round-bottom flask was heated at ∼95 °C on a water bath under refluxing condition. A brown mass is formed at the interface (horizontal platform) of the biphasic system and is slowly precipitated out from the interfacial reaction. The two solvents embrace each other, and the interface acts or plays as one type of planar 2D orientations or soft platform. Therefore, the horizontal planar interface acts as a soft template for the formation of δ-MnO2 nanosheets. The chemical reaction is very simple, and it occurs in two steps. At the interface (between pure ethyl acetate/aqueous KMnO4), ethyl acetate undergoes slow hydrolysis and forms acetic acid and ethanol under refluxing condition.

The reaction between KMnO4 and ethyl acetate is a slow reaction, and it requires a long time for completion. Generally, a high reaction rate results in the production of nonoriented particles with massive precipitates via homogeneous nucleation. For example, the reaction between KMnO4 and pure ethanol (or propanol) with a high reaction rate produces blackish brown MnO2 segregated particles. KMnO4 + CH3CH 2OH → K xMnO2 + RCO2 H

Reaction 3 occurred within a short time and in this case nonoriented spherical particles are grown, which is shown in Supporting Information , Figure S10. The two reactant molecules are in the same phase (which are miscible with each other, i.e., homogeneous phase) and react at a very high rate. In the present case, at the interface region, ethyl acetate molecules undergo slow hydrolysis and form ethanol as one of the products of hydrolysis. The as-produced ethanol molecule reacts with KMnO4 and forms brown δ-MnO2 nanosheets at the interfacial region of the biphasic solvent system. In contrast, a relatively slow reaction rate at the interface region between the two biphasic systems predominantly gives rise to heterogeneous nucleation and induces the formation of the highly oriented and well-shaped graphene-like δ-MnO2 2D nanosheets, which are shown in Figure 7. The δ-MnO2 nanosheets are not stable under the proposed reaction conditions. Under hydrothermolysis condition (80−85 °C on water bath), the δ-MnO2 nanosheets are changed to α-

CH3CO2 C2H5 + H 2O → CH3CO2 H + CH3CH 2OH (1)

KMnO4 + CH3CH 2OH → KMnO2 + CH3CO2 H

(3)

(2)

The as-produced ethanol reacts with KMnO4 and forms brown MnO2 colloid with a 2D layerlike structure. The presence of CH3CO2H in the aqueous part is confirmed by a NaHCO3 test. The δ-MnO2 nanosheets are stabilized by the CH3COO− group, and they remain adsorbed on the surface of the nanosheet. 23982

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Figure 12. Solventless hydrolysis of benzonitrile catalyzed by α-MnO2 nanowire fabricated bed. The produced crystalline benzamide compound upon sublimation gets deposited in the inner wall of the watch glass covering which is automatically separated.

MnO2 nanowires by the shape transformation mechanism via a series of intermediates which are characterized by XRD, Raman, FESEM, and TEM analyses. All the analytical reports give a clear idea about the crystallinity and phase transformation and all support the proposed pathway. Li et al. have established the growth mechanism of (α, β, and γ) 1D MnO2 nanowires, introducing a rolling process of δ-MnO2 nanosheets in the (NH4)2S2O8/MnSO4 reaction mixture. The phase transition takes place via curling and a tubular structure as an intermediate.26 α-MnO2 single-crystal nanorods have been reported through a low temperature liquid-phase comproportionation reaction using the KMnO4/MnSO4 reaction system, which involves no catalyst or template to prepare α-MnO2 single-crystal nanorods.18 In our reaction system KMnO4 and ethyl acetate give rise to 1D α-MnO2 nanowires in a stepwise fashion from amorphous δ-MnO2 nanosheets as is shown in Scheme 1. As the crystallization process progresses, all well-known diffraction peaks due to α-MnO2 appear. This means that, during shape/ phase transformation in water medium, there occurs dissolution of amorphous δ-MnO2 nanosheets and α-MnO2 appears but with a crystalline 1D nanowire morphology. The growth mechanism is investigated by time-dependent FESEM and TEM analysis in an ex situ fashion (the stepwise transformation is shown in Supporting Information, Figures S11 and 12). During the shape transformation, the MnO2 material was collected at different time gaps for FESEM and TEM analysis. At the initial stage, δ-MnO2 nanosheets are observed, and then they are gradually changed into 1D nanowire morphology with the progress of reaction time. The change of shape indicates that the δ-MnO2 nanodomains are gradually detached from the nanosheets under the reaction conditions and form α-MnO2 nuclei. Once the αMnO2 nuclei are formed in solution, the tiny δ-MnO2

nanodomains will diffuse into the thermodynamically stable α-MnO2 nuclei to help them grow into 1D nanowires. The nanosheets of δ-MnO2 vanish and reappear as 1D nanowires of α-MnO2 and the diameter and length of the nanowires increase with time. The whole process is summarized by the following steps. step 1: δ‐MnO2 solid (nanosheet) → MnO2 liquid dissolution of MnO2 solid nanosheets

step 2: MnO2 liquid → α ‐MnO2 solid

seed formation

step 3: α ‐MnO2 solid (seed) → α ‐MnO2 nanowire 1D growth of MnO2

In this reaction system, the growth mechanism does not follow the rolling process. No curling of the sheets or growth of tubular structure appeared in the intermediate stages examined from TEM images. The as-formed nanowires with larger diameters/smaller diameters have been obtained together, exhibiting an asymmetric growth through the shape transformation mechanism. The reason why the wire morphology is formed with different diameters is not yet completely understood. The crystallization of α-MnO2 nanowires from δ-MnO2 nanosheets is also shown by HRTEM analysis at different times. It is also clear that, as the reaction time elapsed, the lattice gaps of αMnO2 nanowire were gradually viewable, which indicates the crystallization process (shown in Supporting Information, 23983

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Figure S13). The complete crystallization from amorphous δMnO2 to crystalline MnO2 is also observed by selected area electron diffraction (SAED) analysis, wich is shown in the Supporting Information, Figure S14. The ring-type SAED pattern gradually acquired linearity, i.e., single-crystalline type. To study the crystallinity of MnO2 at different intermediates, the intermediate samples at different time lags are isolated and characterized by powder XRD and Raman analysis, which is shown in Figure 9. In the initial stage (for example, at 0 h), the product is almost amorphous, although weak reflections of MnO2 can be observed from the XRD pattern as well as low from the Raman band intensity. However, during the shape transformation, the intensity of the reflections in the powder XRD and Raman spectra significantly increases, indicating the increased crystallinity of the α-MnO2 nanowires. 3.2. Catalytic Activity of α-MnO2 Nanowires as Reusable Catalyst Bed. Manganese dioxide is used as oxidant in organic synthesis,27 and its effective usage as a catalyst depends on the method of preparation of the catalyst. We have prepared a MnO2 membrane/wheel (composed of ultralong α-MnO2 nanowires shown in Figure 10) which becomes a heterogeneous catalyst where surface area plays an important factor. The applications of MnO2 are numerous, and it is used in many kinds of organic reactions.28 We have produced α-MnO2 nanowires; the wires interlaced and appear like a fibrous cake and look like a membrane/wheel, while a water suspension of α-MnO2 wires is filtered through a Büchner funnel using No. 1 Whatman filter paper (the details of the procedure are given in the Supporting Information, Figure S15). Very recently, a similar type of inorganic membrane was reported by several groups using vacuum filtration techniques. Wang et al. prepared a membrane with MnO2 and V2O5 nanowires by vacuum filtration techniques, and the hydrogen bond network of the materials in the membrane helps to separate water and oil, especially from an emulsion.29 Jung and his groups have designed three-dimensional aerogels from the suspension of nanostructured 1D (such as Ag, Si, MnO2 nanowires and single-walled carbon nanotubes) and 2D materials (such as MoS2, graphene, and h-BN) via supercritical drying. This aerogels have high surface area, low density, and high electrical conductivity, and they have excellent opportunities for future applications.30 Suib et al. have prepared a paperlike membrane from a MnO2 nanowire suspension by drying at 85 °C on Teflon substrate. This membrane remains flexible and has been applied to trap gold nanoparticles from solution.31 In this article, we have studied the catalytic performance of αMnO2 fixed bed which looks like a membrane/wheel/cake. The cake, collected from a Büchner funnel after drying, becomes a reusable catalyst bed for the hydrolysis of a typical aromatic nitrile (C6H5-CN).32 The product of the hydrolysis of aromatic nitrile is the corresponding amide, C6H5-CONH2, which is shown in Scheme 2. The hydrolysis reaction is important in academics as well as in industrial laboratories. The reported hydration reaction of benzonitrile is without the use of any other cosolvent. A porous α-MnO2 nanowire cake, just like a wheel, taken in a watch glass, was used as a reusable heterogeneous catalyst for hydrolysis reaction. A 1 mL volume of pure benzonitrile was dropped onto the MnO2 catalyst bed taken on a watch glass. Liquid benzonitrile penetrates and spreads all around into the catalyst bed within the nanopores of the fibrous catalyst bed, as shown in Figure 11. The

benzonitrile-soaked MnO2 nanowire bed on the watch glass was covered with another watch glass of the same size. These two matched watch glasses were tied up using a Teflon tape so that the setup becomes a leakproof reactor like the assembly shown in Figure 12. The whole setup was heated at 180−200 °C by a 200 W tungsten bulb. After complete hydrolysis reaction, pure benzamide was produced (weight percentage yield ∼ 70%), sublimed, and deposited in the inner wall of the covering watch glass as crystalline material. The 1H NMR spectrum of the pure C6H5-CONH2 is given in the Supporting Information, Figure S16. The pure product from the hydrolysis reaction was scooped out from the glass covering without any separation problem. No solvent extraction or column chromatography is needed for separation of the reaction product from the starting material and catalyst bed. Therefore, the product of the one-step hydrolysis reaction is obtained as pure white crystals. Actually, there remain water molecules in the tunnel of the α-MnO2 structure.32 The water molecule was used in the solid state hydrolysis reaction. This solid state reaction strategy will be useful in industry for large scale production of pure benzamide compound.

4. CONCLUSION We have demonstrated a successful pathway for the growth of δ-MnO2 nanosheets and their shape transformation to nanowires. This low temperature synthetic route is based on a simple wet chemical biphasic reaction between KMnO4 and ethyl acetate. The 1D growth process does not need the participation of any catalyst or porous template and requires no expensive and precise equipment. The product ensures higher purity and greatly reduces the production cost, and thus offers a great opportunity for industrial scale-up preparation of 1D αMnO2 nanomaterial. Finally, we have designed a porous membrane as a reusable catalyst bed which is important academically as well as in the chemical industry. The catalysis employing α-MnO2 nanowires as a reusable catalyst bed under solvent-free condition progresses for high-yield synthesis of the hydrolyzed product of aromatic nitriles in one step.

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ASSOCIATED CONTENT

S Supporting Information *

Digital picture of α-MnO2 nanowires; EDS patterns of MnO2 nanomaterial; FTIR spectra; diameter and length of α-MnO2 nanowires; elemental mapping of α-MnO2 nanowire; FESEM image of α-MnO2 nanowires at 200 and 400 °C; surface morphology of α-MnO2 nanowire sheet of membrane; comparison of BET surface area of MnO2 nanomaterial; spherical MnO2 nanoparticle; stepwise growth mechanism; MnO2 catalyst bed preparation; 1H NMR spectrum of benzamide compound. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+)03222-255303. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to CSIR, UGC, DST, New Delhi, and IIT Kharagpur, India. 23984

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dx.doi.org/10.1021/jp403527p | J. Phys. Chem. C 2013, 117, 23976−23986