Morphological control of highly aligned manganese dioxide nanostructure formed by electrodeposition

Morphological control of highly aligned manganese dioxide nanostructure formed by electrodeposition

Materials Letters 79 (2012) 184–187 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/m...

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Materials Letters 79 (2012) 184–187

Contents lists available at SciVerse ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Morphological control of highly aligned manganese dioxide nanostructure formed by electrodeposition Won-Hee Ryu, Jun-Hyok Yoon, Hyuk-Sang Kwon ⁎ Department of Materials Science & Engineering, Korea Advanced Institute of Science & Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

a r t i c l e

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Article history: Received 10 February 2012 Accepted 24 March 2012 Available online 1 April 2012 Keywords: Manganese dioxide Anodic electrodeposition Highly aligned nanostructure Nanostructured materials Morphologies controlling method

a b s t r a c t Highly aligned MnO2 nanostructure was successfully synthesized by an anodic electrodeposition. Effects of deposition parameters such as anodic current density and chemical additive on the structural morphology of the MnO2 nanostructure were examined. The pore size of the MnO2 nanostructure was increased with increasing the concentration of (NH4)2SO4 and also with decreasing the anodic current density. The change in morphology is closely associated with the nucleation and growth rate of MnO2 that is very sensitive to the chemical additives and anodic current density. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Nanostructured manganese dioxide (MnO2) has received a great attention due to its high specific surface area, functional properties, and potential applications as molecular/ion sieves, capacitors, Li-ion batteries, and catalysts [1–4]. Manganese dioxide has many flexible crystallographic forms (such as α-, β-, γ-, δ-, ε-, and λ-MnO2) that connect [MnO6] octahedron units to each other in different ways. MnO2 also has a variety of morphologies, such as 1-, 2-, and 3-dimensional nanostructures [5]. Various synthetic strategies have been developed to prepare nanostructured MnO2, such as templating, hydrothermal reaction, and electrodeposition [6–8]. Among these methods, the electrodeposition method has remarkable advantages for the synthesis of nanostructured MnO2 because the technique is simple and templatefree. Additionally, it is a one-step process that is able to control the nanostructure morphology of MnO2 with its deposition on a conducting substrate. Consequently, the nanostructured MnO2 prepared by electrodeposition is electrically contacted with the substrate. The morphology of nanostructured MnO2, determining its functional properties, is influenced by the deposition parameters such as applied current density, deposition time, and chemical additive [9,10]. It is the research objective of the work to examine the effects of electrochemical parameters such as concentration of (NH4)2SO4 and applied current density on the morphology of highly aligned MnO2 nanostructure prepared by electrodeposition process.

Highly aligned MnO2 nanostructure was prepared by anodic electrodeposition onto a commercially pure nickel foil with an exposed surface area of 1 cm2. For pre-treatment, the Ni foil (0.05 mm thick with 99.5% purity) was degreased with acetone, rinsed, activated for 1 min in 10 wt.% H2SO4 solution, and washed with distilled water. Electrochemical cell with a working electrode of nickel foil and a platinum counter electrode was used for electrodeposition. Electrodeposition of highly aligned MnO2 nanostructure was conducted using dc current in a solution containing Mn ions, with the solution being agitated by a magnetic stirrer at 100 rpm. The solution for the deposition of MnO2 was prepared by mixing 0.1 M Mn(CH3COO)2 with various concentrations of (NH4)2SO4. To examine the effects of the additives on the surface morphologies of the highly aligned MnO2 nanostructure, different functional additives such as Na2SO4 and (NH4)2SO4 were added into the electrolytic bath. Details in the electroplating condition are presented in Table 1. The highly aligned MnO2 nanostructure was anodically electroplated for 120–1200 s at 0.005–0.05 A/cm2 at a total charge of 6 C/cm2. All experiments were performed at an ambient temperature (22±2 °C). The surface morphologies of the highly aligned MnO2 nanostructure were analyzed using scanning electron microscopy (SEM). The crystal structures and chemical composition of the samples were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectrometry (EDS). 3. Results and discussion

⁎ Corresponding author. Tel.: + 82 42 350 3326; fax: + 82 42 350 3310. E-mail address: [email protected] (H.-S. Kwon). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.091

Fig. 1a shows X-ray diffraction pattern of the MnO2 synthesized by electrodeposition. Although weak peaks at 17° and 26° indicate the

W.-H. Ryu et al. / Materials Letters 79 (2012) 184–187 Table 1 The chemical compositions of electrodeposition baths for the preparation of highly aligned MnO2 nanostructure. Bath no.

Mn(CH3COO)2 (M)

1 2 3 4

0.1 0.1 0.1 0.1

Na2SO4 (M)

(NH4)2SO4 (M)

0.1 0.1 0.5

presence of undefined crystalline MnO2, the MnO2 products are nearly amorphous, as confirmed by the broad MnO2 peak with a low intensity. However, XPS analysis revealed the formation of MnO2, as identified by the binding energy of Mn shown in Fig. 1b. The two typical MnO2 peaks corresponding to Mn 2p1/2 and Mn 2p3/2 were confirmed at 653.7 and 642.0 eV in the XPS spectrum, respectively. In addition, no impurity peaks such as sulfur originated from electrolyte were observed in the XPS peak of S 1s, indicating the high purity of MnO2. From the EDS analysis, the atomic ratio of Mn:O is approximately 1:2 without any impurity. Therefore, the MnO2 phase was successfully synthesized by the electrodeposition process. The electrodeposition of MnO2 proceeds via the following oxidation reaction: [10] 2þ

Mn

þ



þ 2H2 O→MnO2 þ 4 H þ 2e :

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The surface morphology of MnO2 electrodeposited on Ni foil can be modified by chemical additives that affect the nucleation and growth rate of the MnO2. Wu et al. electrochemically deposited manganese oxide nanowires in a mixed aqueous solution of manganous acetate and sodium sulfate [8]. Ammonium sulfate, instead of sodium sulfate, was chosen as the functional additive in the solution employed in this work. Effects of the additive on the surface morphologies of MnO2 nanostructure are shown in Fig. 2. Evidently, porosity of MnO2 nanostructure was significantly increased by adding (NH4)2SO4 to the solution. The estimated pore size of the nanostructure was approximately 50 nm. In addition, a lot of highly aligned nanowires with length of 200–500 nm on the surface, were found on the surface, and the aligned nanowires were three dimensionally connected each other. The morphology of MnO2 is more effectively modified by (NH4)2SO4 rather than by Na2SO4. It appears that the selective growth of aligned MnO2 nanostructure is accelerated with the addition of (NH4)2SO4. The selectively adsorbed NH4+ ions onto the electrochemically active surface would affect the adsorption and subsequent oxidation of Mn 2 + to MnO2. Similarly, the surface morphologies of copper deposits were dramatically changed by the addition of (NH4)2SO4 in the solution, as reported in another work [11]. The surface morphology of MnO2 nanostructure varied depending on the concentration of (NH4)2SO4 (Fig. 2d). The porosity of the MnO2 nanostructure increased with increasing the concentration of (NH4)2SO4. The formation of MnO2 nanowires on the surface is also accelerated by the high concentration of (NH4)2SO4. Effects of anodic current density on the surface morphology of the MnO2 nanostructure are shown in Fig. 3. Evidently, the surface morphology appears to be very sensitive to the anodic current density. As the current density increased from 0.005 A/cm 2 to 0.05 A/ cm 2, the pore size of the MnO2 nanostructure decreased, and the nanowires on the surface were densely formed. Generally, small deposits and rapid nucleation occur during electrodeposition with a high applied current or potential [12]. The nucleation and growth of MnO2 nanowires were slow at a low current density (0.005 A/cm 2). The slow deposition of MnO2 on the Ni current collector allows enough time to form nanostructure with highly aligned nanowires and extensive pores. On the other hand, nucleation of MnO2 was preferentially formed on to current density under high anodic current density of 0.05 A/cm 2, and consequently aligned MnO2 nanowires grew denser and shorter. Therefore, the morphologies of the highly aligned MnO2 nanostructure are easily controlled by the applied anodic current density. Additional studies related to effects of the additives and applied current density on the electrochemical properties of the MnO2 nanostructure will be continued in detail in future work.

4. Conclusions

Fig. 1. (a) XRD patterns and (b) XPS spectra of the electrodeposited MnO2 prepared in the 0.1 M Mn(CH3COO)2 + 0.1 M (NH4)2SO4 at an anodic current density of 0.01 A/cm2. The XPS spectra were fitted to manganese.

Highly aligned MnO2 nanostructure was successfully synthesized by anodic electrodeposition. The existence of MnO2 was confirmed by XPS and EDS analyses, and the products were shown to be amorphous. Morphology of the MnO2 nanostructure was successfully controlled by adding various additives (Na2SO4, (NH4)2SO4) with different concentrations of (NH4)2SO4 to the solution and also by applied anodic current density during anodic electrodeposition. The morphology of the MnO2 nanostructure was more drastically changed by the addition of (NH4)2SO4 relative to the addition of Na2SO4. Furthermore, the porosity of the MnO2 nanostructure increased with the concentration of (NH4)2SO4 and also with decreasing the anodic current density. The remarkable change in the morphology is associated with the nucleation and growth rate of MnO2 that is very sensitive to the chemical additives and anodic current density.

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Fig. 2. SEM micrographs of the surface morphologies of the MnO2 nanostructure electroplated on Ni at an anodic current density of 0.01 A/cm2 in different baths: (a) 0.1 M Mn(CH3COO)2; (b) 0.1 M Mn(CH3COO)2 + 0.1 M Na2SO4; (c) 0.1 M Mn(CH3COO)2 + 0.1 M (NH4)2SO4; and (d) 0.1 M Mn(CH3COO)2 + 0.5 M (NH4)2SO4.

Fig. 3. SEM micrographs of the surface morphologies of the MnO2 nanostructure electroplated on Ni in the 0.1 M Mn(CH3COO)2 + 0.1 M (NH4)2SO4 bath at an anodic current density of (a) 0.005 A/cm2, (b) 0.01 A/cm2, (c) 0.02 A/cm2, and (d) 0.05 A/cm2 (total charge: 6C/cm2).

Acknowledgments This work was funded by the BK21 Program of the Korea Ministry of Knowledge Economy and partially by the Global Institute for Talented Education (GIFTED).

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