Hydrothermal Synthesis, Physico-Chemical Characterization and Electrochemical Behavior of β-MnO2 Nanorods
Afef Bayoudh1, Noureddine Etteyeb1, Faouzi Sediri1, 2, *
1Unité de recherche de Nanomatériaux et Environnement (UR15ES01), IPEIT, Université de Tunis, Tunisie, Tunisia
2Département de Chimie, Faculté des Sciences de Tunis, Université Tunis El Manar, Tunisie, Tunisia
To cite this article:
Afef Bayoudh, Noureddine Etteyeb, Faouzi Sediri. Hydrothermal Synthesis, Physico-Chemical Characterization and Electrochemical Behavior of β-MnO2 Nanorods. American Journal of Nanosciences. Vol. 2, No. 1, 2016, pp. 1-7. doi: 10.11648/j.ajn.20160201.11
Received: August 30, 2016; Accepted: October 7, 2016; Published October 17, 2016
Abstract: A simple hydrothermal method was developed for the synthesis ofpyrolosite β-MnO2 nanorods, using potassium permanganate (KMnO4), manganese sulfate (MnSO4.H2O) and oxalic acid (H2C2O4). The effects of the reaction time, the hydrothermal temperature and the amount of H2C2O4 on the structure and the morphology of the final products were studied. The β-MnO2 nanorods are up to several micrometers in length and about 37 nm in average diameter. The samples were analyzed through X-ray diffraction (DRX), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. Electrochemical measurements of thin film of β-MnO2 nanorods have revealed reversible redox behavior with charge-discharge cycling processes corresponding to reversible cations intercalation/deintercalation into the crystal lattice. This process is easier for the small Li+ to the larger Na+ one and to the largest K+ cation.
Keywords: Nanostructures, Hydrothermal Synthesis, X-ray Diffraction, Electrochemical Properties
In recent years, construction of nano-sized transition metals oxides (ZnO, VO2, MoO2,WO3, MnO2, …) with different shapes including needles, rods, wires, spindles, tubes,… have attracted much attention [1-5].These materials were widely applied in the fields of catalysis , supercapacitors , lithium ion-batteries  and magnetic materials . Among these materials, the manganese oxide MnO2, as one of the most important transition metals oxides, has drawn particular attention owing to its high activity, low cost and nontoxicity [10-16]. It has various polymorphic structures, such as α-, β-, γ- and δ- types [17-20], which are made of the basic unit [MnO6] octahedral with different connectivity.β-MnO2 (pyrolosite) has a rutile type-tetragonal symmetry (P42/mnm) with (1 × 1) tunnel structure constructed of single type chain with edge-sharing [MnO6] octahedral as illustrated in figure 1 . Owing to its particular properties, β-MnO2 is a traditionally attractive material for various industrial applications. It is an important battery cathode material used for lithium ion- batteries , sodium ion- batteries  and supercapacitors .
There are many reports on the usage of β-MnO2 nanostructures as catalyst. For example, it was found that β-MnO2nanomaterials had catalytic performance on H2O2 decomposition [25-27] and on the degradation of organic polluants in waste water . Recently, this variety of β-MnO2 nanostructured, with different morphologies, such as nanowires , nanopincers like , nanowhiskers , nanotubes , nanorods  and hollow bipyramid  have been synthesized. A variety of strategies have been designed to prepared desirableβ-MnO2 with different shapes. β-MnO2 nanorods were successfully prepared by a refluxing route using manganese sulfate (MnSO4, H2O), sodium persulfate (Na2S2O8) and sodium hydroxide (NaOH) as the raw materials . A simple hydrothermal method was developed for the synthesis of uniform single-crystal β-MnO2nanorods only using potassium permanganate and sodium nitrite in acidic solution . G. Xi et al. have prepared β-MnO2 structures through a two-step process including the hydrothermal synthesis of γ-MnOOH nanorods precursors followed by calcinations of the fabricated precursor . Y. Wu et al. have synthesized β-MnO2 nanowires using KMnO4 and H2C2O4 via aqueous and hydrothermal routes .
In this paper, we described a simple one-pot hydrothermal synthesis of β-MnO2 nanorods. Compared to other soft chemical routes, this technique could precisely control the morphology of the product only by adjusting the experimental conditions (such as reaction temperature, reaction times and reactant concentration). As far as we know the synthesis of β-MnO2 nanorods using KMnO4, MnSO4. H2O and H2C2O4 under hydrothermal conditions had not been reported. We have investigated the effects of the hydrothermal temperature, reaction time and the amount of H2C2O4 on the crystallographic structure and morphology of products. The optimal conditions for the preparation of homogeneous and high crystallized β-MnO2 nanorods were described. The electrochemical properties of the β-MnO2 nanorodsfilms deposited on indium tin-oxide (ITO)-coated glass have been investigated using cyclic voltammetry in the presence of Li+, Na+, and K+ in propylene carbonate as electrolytic solution.
2. Experimental Section
2.1. Preparation of β-MnO2 Nanorods
β-MnO2 nanorods were synthesized through a hydrothermal process. Potassium permanganate KMnO4 (≥ 99 %), manganese sulfate (II) monohydrate MnSO4.H2O (98 +%) and oxalic acid anhydrous H2C2O4 (98%) were purchased from Across Organics and are used without further purification.
The detailed synthesizing process was as follows. In a typical synthesis, 79 mg of KMnO4and 86 mg of MnSO4.H2O were dissolved in 10 mL of distilled water, then 45 mg of H2C2O4 was added into the above solution under continuous stirring. The resulting suspension was transferred into a Parr Teflon-lined digestion bomb with a capacity of 45 mL which was sealed and maintained in an electric oven at different temperatures and various reaction times. After the bomb cooled down to room temperature, a black precipitate was obtained. The resulting precipitate was filtered and washed with distilled water and ethanol for several times and then dried in an electric oven at 60°C. Comparative experiments were carried out to investigate the influence of reaction time and temperature on the structure and the crystallinity of the samples, for that many experiments were carried out at 180°C during different times (1 hour, 2 hours, 4 hours, 6 hours and 24 hours). The other experiments we performed took place under varied reaction temperature, 120°C and 220°C, with a 24 hour -reaction time.
2.2. Characterization Techniques
X-ray powder diffraction data (XRD) were obtained on a X’Pert Pro Panalytical diffractometer with CuKα radiation (λ = 1.5418Å) and graphite monochromator. The XRD measurements were carried out by applying a step scanning method (2θ range from 3° to 70°), with a scanning rate of 0.017° s-1 and a step time of1s. Scanning electron microscopy (SEM) studies were recorded with a JEOL JSM-5400 mocroscope. Fourier-transform infrared spectroscopy (FTIR) was recorded from 4000 to 400 cm-1 on a Nicolet 380 spectrometer in pellets of samples dispersed in KBr. Raman spectroscopy was performed using a Jobin Yvon T 64000 spectrometer(blue laser excitation with 488 nm wavelength and <55 mW power at the sample). The electrochemical measurements were carried out using one compartment cell and a BioLogic SP150 potentiostat/galvanostat apparatus. Ag/AgCl electrode and a stainless grid were used as reference and counter-electrode electrode, respectively. The working electrode is a film of β-MnO2 nanorods deposited on a plate of indium tin oxide (ITO). The electrode was prepared thus: an aqueous suspension of 3 mg/mL of β-MnO2 nanorods was prepared then 100 µL of this suspension were dropped onto a 1 cm2 surface ITO coated glass (plate of indium tin oxide).The surface modified ITO coated glass was then dried and covered with 10 µL of a Nafion solution (obtained by dissolving in ethanol a commercial Nafion solution 9/1 V/V) used as working electrode. Lithium perchlorate 1M (LiClO4), sodium perchlorate 1M (NaClO4) and potassium perchlorate 1M (KClO4) dissolving in propylene carbonate (PC) were used as electrolyte solutions. The operating voltage was controlled between -1.0 V and 1.0 V. All measurements were performed at different scan rates at room temperature.
3. Results and Discussion
3.1. X-ray Diffraction
The figure 2 contains the X-ray diffractgrams of the as-synthesized samples atdifferent temperatures: 120°C (a), 180°C (b) and 220°C (c) after 24 hours of hydrothermal treatment. Indeed, the diffraction pattern shows the presence of amorphous material, when the synthesis was carried at 120°C (figure 2a). However, when the synthesis was carried out at 180°C, the XRD pattern shown in figure 2b indicates that the diffraction peaks are identical to those of the tetragonal crystalline phase β-MnO2 (P42/mnm) with lattice constant a= b = 4.397 Å and c = 2.867Å, according to the literature values (JCPDS#24-0735).No peak of any other phase or impurity was detected from the XRD pattern. Increasing the temperature to 220°C (figure 2c), the diffraction pattern shows the presence of β-MnO2 (JCPDS #24-0735) with the presence of peaks which are characteristic of α-MnO2 (JCPDS # 44-0141). It could be concluded that β-MnO2 with high purity can be obtained via the hydrothermal treatment at 180°C for 24 hours.
The phase structure of the resulting samples prepared at 180°C for different reaction times: 1 hour (a), 2 hours (b), 4 hours(c), 6 hours (d) and 24 hours (e) were determined by X-ray powder diffraction (XRD), as shown in figure 3. Indeed, figure 3a shows the XRD pattern of the as-synthesized sample after 1 hour of hydrothermal treatment, it’s an amorphous material. It was found that the crystallization starts after 2 hours (figure 2b) and improves when the reaction time increases, above 4, 6 and 24 hours at 180°C as shown in the figures 2c, 2d and 2e. In fact, strong and sharp diffraction peaks indicate good crystallinity of the hydrothermal product prepared after 24 hours at 180°C. The reaction temperature and time have a significant effect on the crystalline phase of tetragonal β-MnO2.
To fully understand the effect of the introduction of H2C2O4 on the crystallinity of β-MnO2 nanostructures, we carried out an experiment of the hydrothermal process without oxalic acid (H2C2O4), using only potassium permanganate (KMnO4) and manganese sulfate (MnSO4.H2O) and the others experiments with the introduction of 45 mg and 150 mg of H2C2O4. XRD patterns of the resulting samples synthesized without oxalic acid (figure 4b) and with 45 mg of oxalic acid (figure 4a), can be indexed as the tetragonal phase of β-MnO2, while the sample prepared using 150 mg of H2C2O4 (figure 5) can be identified to be a pure manganite γ-MnOOH phase with lattice parameters a= 5.300 Å, b=5.278 Å, c= 5.300 Å, α = γ = 90° and β = 114.36° according to JCPDS# 41-1379.
3.2. Scanning Electronic Microscopy
The morphology of the as-synthesized materials was investigated by using the scanning electron microscopy (SEM). Figure 6 shows SEM images of the β-MnO2 synthesized at 180°C for 4 hours (a), 6 hours (b) and 24 hours (c). In fact, the reaction time plays an important role in controlling the morphologies of theMnO2 nanostructures. When the hydrothermal reaction time is 4hours nanoparticules are obtained. The sample is homogenous and displays rod like morphology with a diameter of 88 nm and about 4 µm in length when it reaches 6 hours. Furthermore, with the increase of the reaction time to 24 hours, the product is mainly nanorods, but the nanorods became longer and thinner. They are 37 nm in diameter and about 6 µm in length.
To substantially study the influence of the amount of oxalic acid on the morphology of the samples prepared at 180°C for 24 hours, observations by scanning electron microscopy were performed. The observation by SEM of the as-synthesized sample without oxalic acid shows that the material is made of micro rods (figure 7a). Whereas, when the synthesis was carried out with 150 mg of oxalic acid, figure 7b reveals that pure crystalline manganite γ-MnOOH was obtained in a large scale with a high uniformity, the as-prepared γ-MnOOH nanorods are up to 2 µm in length and about 30 nm in diameter.
3.3. Vibrational Spectroscopic Study
The vibrational spectroscopic methods, which include IR and Raman diffusion spectroscopy, afford useful alternatives and/or supplements to X-ray diffraction for structural characterizations of materials. Since they are sensitive to local structures of materials, IR and Raman diffusion spectroscopy can yield a more complete and reliable description of materials such as MnO2-related compounds, where crystalline disorders as well as different local structural properties can be expected . Thereby, figure 8 shows the FTIR spectrum of the β-MnO2 nanorods prepared at 180°C for 24 hours recorded in the region 400-4000 cm-1. All bands located at 417 cm-1, 604 cm-1 and 720 cm-1can be ascribed to the Mn-O vibrations of the rutile-type MnO6 octahedral framework which confirms the formation of β-MnO2 nanocrystals [39-41].
Raman spectroscopy is known to be an appropriate technique for the investigation of the short-range order, phase structure. The Raman bands of MnO2 are determined by the [MnO6] octahedral environment; hence the use of Raman spectroscopy to identify the structure of the different crystalline phases. The Raman spectrum of β-MnO2 nanorods (figure 9) displays 5 three sharp peaks at 538, 661 and 750 cm-1 assigned to the characteristic MnO2 vibration modes. Indeed, the peak observed at 661 cm-1 is assigned to the stretching mode Mn-O vibrations. Nevertheless, the two peaks located at 538 and 750 cm-1 can be assigned to the deformation modes of the metal-oxygen chain of Mn-O-Mn in the MnO2 octahedral lattice [38,42,43].
3.4. Electrochemical Characterization
It was reported that the electrochemical properties of the electrode materials are influenced by many factors such as instinctive structure, morphology, and preparation processes . Cyclic voltammetry curves shown in figure 10 were recorded from the 1st to the 50th cycles at room temperature with a scan rate of 50 mV/s in the range [-1.0 to 1.0 V] in 1 MLiClO4 in propylene carbonate solution. The CV data are consistent with good stability of the as-prepared crystalline β-MnO2nanorods coated film. The curves showed a single oxidation and reduction process. After several voltammetry cycles (50 cycles), the voltammetry responses remain unchanged (figure 10).The absence of even a minimal deviation from the anodic and cathodic peaks positions upon extending cycling confirms the absence of degradation problems which can affect the β-MnO2 electrode. The voltammograms show the presence of a single redox process. The cathodic and anodic peaks at -0.70 V and 0.70 V can be attributed to intercalation/deintercalation process of Li+ ions into the tunnel cavities of the sample, which mainly corresponds to the reduction of Mn4+ to Mn3+ and the oxidation of Mn3+ to Mn4+, respectively. The cathodic and anodic peaks currents increased as the scan rate does (figure 11). This phenomenon indicates the dynamics of the lithium intercalation/deintercalation occurring in the material to provide electroneutrality. The electrochemical reaction can be written thus:
MnO2 + xe- + xLi+ = LixMnO2
The electron transfer during Mn4+/Mn3+ couple redox processes are accompanied by the insertion of Li+ cation at reduction and by its release at oxidation of the material in order to equilibrate the charge.
The quantity of charges Q exchanged during the experiment can be deduced from the surface of cyclic voltammetry curves and the coulombic efficiency during the insertion/desinsertion release process can be expressed by the ratio Qred/Qox. Figure 12shows that the value of the coulombic efficiency after several cycles is 87%. This indicates that the Li+ intercalation-deintercalation is quasi-reversible at 50 mV/s scan rate. When the films of β-MnO2 nanorods have been studied in propylene carbonate containing NaClO4 or KClO4 as electrolytes instead of LiClO4 (figure 13), the reduction current peaks have increased upon medium transfer from K, Na+ to Li+ containing electrolyte. This observation indicates that intercalation/deintercalation processes involving K+ and Na+ are less effective and more difficult in comparison to those with Li+ cations. It should be noted that the Vander Waals radius of K+ and Na+ are larger than that of Li+ cation. In addition the diffusion process of cations is mainly controlled by the size of the tunnel. The tunnel size of β-MnO2 (1.89 Å) is considerably larger than the radius of Li+ (0.76 Å) and Na+ (1.02 Å), but it is smaller than that of K+ (1.33Å) . The excellent stability of β-MnO2 nanorods makes it an interesting possible future electrode for rechargeable lithium cell.
β-MnO2 nanorods were synthesized hydrothermally via a simple and elegant route from the mixture of KMnO4, MnSO4.H2O and H2C2O4 under soft conditions. In fact the presence of H2C2O4, the temperature and reaction times played an important role in the structure and the morphology of the final products. Electrochemical measurements carried out on thin film of β-MnO2 nanorodsdeposited on ITO-modified glass electrodes have revealed quasi-reversible redox behaviors corresponding to partial reduction of manganese oxide, with charge-discharge cycling corresponding to the intercalation/deintercalation of Li+ into the tunnel cavities of β-MnO2 nanorods. The CV curves recorded in propylene carbonate medium (LiClO4, NaClO4 and KClO4) show the presence of a single redox process. In fact, the CV curves indicate that the diffusion process involving Na+ and K+ is less effective and difficult. Thus, the redox process is more difficult for larger cations than for small ones. The presence of only one redox wave for each cation indicates that the dynamics of intercalation/deintercalation is the same and occurs mainly in the cavities. All these results indicate that the resulting β-MnO2 nanorods are promising cathode materials in lithium-ion batteries.