Synthesis and Characterization of Nanosized Manganese Oxyhydroxide Compounds by Sonochemical Method

Introduction

Recently, nanostructured materials such as chalcogenides, metal oxides and metals are well known for their substantially fascinating physical properties and are of great importance in both basic scientific research and potential technological applications [1, 2]. MnOOH is of considerable importance in several industrial applications, such as catalysts, rechargeable batteries, molecular sieves, etc. [3, 4]. It has been shown that the properties of these materials strongly depend on their powder dimension and morphology, crystalline structure, specific surface area, bulk density, etc. [5, 6]. Meanwhile, γ-MnOOH is an important precursor for the synthesis of intercalation compounds such as lithium manganese oxides, which are anticipant of potentially low-cost, environmentally friendly positive materials for lithium ion batteries [7, 8]. Recently, much effort has been made toward the preparation of nanostructured MnOOH with different crystallographic forms, which were expected to display better performance in their intended purposes [9, 10].

Various synthetic routes have been developed to prepare γ-MnOOH, such as precipitation [11], hydrothermal [12], electrochemical deposition [13], etc. Among them, the redox hydrothermal method is the most commonly used one. Moreover, MnOOH has been synthesized by solution-based routes via oxidation–reduction reactions, such as reactions between MnSO₄ and H₂O₂ in an alkaline solution, MnAc₂ and K₂S₂O₈ [14], KMnO₄ and PEG200 [15, 16], KMnO₄ and MnAc₂, and Mn(NO₃)₂ and PEG10000 [17]. In this study, MnOOH nanoparticles have been prepared by using [Mn(Hsal)₂] complex as a novel manganese precursor and SDS as surfactant via a simple sonochemical method. Furthermore, the effect of preparation parameters such as sonication time and type of solvent and surfactant on the morphology of MnOOH nanoparticles were investigated.

Experimental

Results and discussion

The crystal structures of the as-prepared MnOOH nanoparticles (sample no. 1) have been identified by XRD (Figure 1). According to this figure, all of the diffraction peaks can be readily indexed to a monoclinic phase of MnOOH with lattice constants of a = 8.98, b = 5.28 and c = 5.71 Å, which agree with the reported data (JCPDS card No. 18-0805). No diffraction peaks from other species could be detected, which indicates that the obtained samples are pure.

Figure 1:

XRD pattern of sample no. 1.

The FTIR spectrums of as-prepared sample no. 1 and [Mn(Hsal)₂] complex are shown in Figure 2(a) and (b), respectively. In Figure 2(a), the sharp peaks at 576 and 668 cm⁻¹ are attributed to the vibrations of the Mn–O bonds in MnOOH [18, 19]. Absorptions at 1,656 and 3,423 cm⁻¹ are assigned to traces of water and carbon dioxide because MnOOH nanoparticles exhibit a high surface-to-volume ratio [18, 20]. Figure 2(b) reveals the IR spectrum obtained from the [Mn(Hsal)₂] complex. The two strong absorption bands at 1,652 and 1,587 cm⁻¹ were attributed to the benzene rings and v_C=O, respectively. The peaks below 650 cm⁻¹ could be attributed to the Mn–O vibrations; therefore, it was found that the [Mn(Hsal)₂] complex was formed.

Figure 2:

FT-IR spectra of (a) MnOOH (sample no. 1) and (b) [Mn(Hsal)₂] complex.

To investigate the solvent effect, several experiments have been performed using different solvents, which are mentioned in Table 1. These solvents have been chosen because of their difference in vapor pressure and dissolving of reactants. Moreover, decreasing of solvent vapor pressure increases the intensity of cavitation collapse, and consequently, the rates of sonochemical reactions are increased. The vapor pressure of methanol is higher than ethanol and also vapor pressure of ethanol is higher than water. SEM images of samples 1, 2 and 3 which are obtained by using water, ethanol and methanol as solvent are shown in Figure 3(a)–(c), respectively. By comparing these figures, it was found that the size of the nanoparticles was increased. Furthermore, the product is mainly composed of agglomerated nanoparticles due to difference in vapor pressure of solvents.

Figure 3:

SEM images of MnOOH synthesized by using (a) water (sample no. 1), (b) ethanol (sample no. 2) and (c) methanol (sample no. 3) as solvent.

Figure 4 shows the SEM images of MnOOH obtained by using different amounts of SDS. In the synthesis of monoclinic phase of MnOOH nanoparticles, our present experiments, amount of SDS has influence on the morphology and size of products. It can be observed that with increasing the amount of SDS from 0.1 g (Figure 4(a)) to 0.18 g (Figure 4(b)) and then to 0.24 g (Figure 3(a)), the agglomeration of nanoparticles will be decreased. Furthermore, with more increasing of SDS from 0.32 g (Figure 4(c)) to 0.38 g (Figure 4(d)) the size and morphology of samples remain nearly constant, and only the agglomeration of nanoparticles increases a little.

Figure 4:

SEM images of MnOOH prepared in the presence of different amounts of SDS: (a) sample no. 4, (b) sample no. 5, (c) sample no. 6 and (d) sample no. 7.

To investigate the effect of reaction time on the morphology at constant power 60 W, the reaction is carried out in 10, 20, 30, 40 and 50 min. The behavior of samples 8 and 9 under shorter sonication time (10 and 20 min) might have resulted from insufficient energy absorbed by the samples; thus, the agglomerated structures are obtained (Figure 5(a) and (b), respectively). Figure 3(a) shows the SEM image of the sample no. 1 prepared after 30 min of sonication, which shows very small MnOOH nanoparticles. By increasing the reaction time to 40 min and then 50 min (sample nos 10 and 11, respectively) the products are agglomerated, because of the active surface of synthesized nanoparticles as shown in Figure 5(c) and (d), respectively.

Figure 5:

SEM images of MnOOH prepared with different sonication times: (a) 10 min (sample no. 8), (b) 20 min (sample no. 9), (c) 40 min (sample no. 10) and (d) 50 min (sample no. 11).

The morphology and size distribution of the MnOOH nanoparticles were further studied by TEM image (Figure 6). According to this figure, it was found that the morphology of MnOOH obtained from sample no. 1 is quasi-spherical shapes with particle size ~10–15 nm.

Figure 6:

TEM image of sample no. 1.

The sonochemical formation mechanism of the MnOOH nanoparticle is probably related to the radical species generated from water molecules by the absorption of the ultrasound energy. It has been known that during an aqueous sonochemical process, the elevated temperature and pressure inside the collapsing bubbles cause water to vaporize and further pyrolysis into H and OH radicals [21]:

The cationic precursor releases Mn²⁺ ions from the [Mn(Hsal)₂] complex and giving rise to MnOOH nanoparticles:

Therefore, it is clear that ultrasound energy can generate radical species from water molecules to enhance the formation of MnOOH.

The EDS spectrum of MnOOH obtained from sample no. 1 is shown in Figure 7. As shown in Figure 7, Mn and O elements are observed in the EDS spectrum. EDX data further prove the formation of pure monoclinic phase of MnOOH product.

Figure 7:

EDS pattern of MnOOH (sample no. 1).

Conclusion

The results described here suggested that MnOOH nanoparticles with particle size ~10–15 nm can be obtained by the reaction between [Mn(Hsal)₂], SDS and NaOH under ultrasound irradiation. Moreover, the effect of preparation parameters such as sonication time and type of solvent and surfactant was investigated. The obtained products were characterized by EDX, TEM, XRD, FT-IR and SEM. The advantage of using ultrasound radiation is that it yields smaller particles. To the best of our knowledge, it is the first time that [Mn(Hsal)₂] is used as Mn source for the synthesis of MnOOH nanoparticles.