Mild aqueous synthesis of urchin-like MnOx hollow nanostructures and their properties for RhB degradation
Graphical abstract
Urchin-like MnOx (Mn3O4/MnO2) hollow nanostructures can be successfully synthesized by H2O2 oxidation of intermediate manganese carbonate (MnCO3) in basic aqueous solution. They had good degradation property for RhB under HCl stimulation. In our present system, degradation of RhB is not closely related to irradiation light. The degradation mechanism was discussed by combining cycled degradation results and different irradiation source stimulated degradation results.
Introduction
Nowadays, nanocrystals attracted great interests because of their potential technological applications. Property studies of nanocrystals based on purposeful synthesis are still the focus. Size, shape and dimensionality have strong impacts on nanomaterials properties.
Manganese oxides are of considerable importance in technological applications, including catalysis, ion-exchange, molecular adsorption, batteries and magnetism [1], [2], [3], [4], [5], [6]. Over the past decades, various MnOx nanostructures were synthesized, such as nanoparticles [7], [8], nanorods and nanowires [9], [10], [11], nanotubes [12], urchin-like and other hierarchical structures [13], [14], [15], [16], [17]. Particularly, hollow MnOx nanostructures attracted significant attention because of their high specific surface area, porosity, low density, and good permeation property. Up to now, synthetic methods for hollow MnOx nanostructures involved the use of various templates, such as silica [18], [19], manganese carbonate (MnCO3) [17], [20], [21], [22], [23] and emulsion droplet [24]. MnCO3 was the most common precursor that used as sacrificial template. For example, Cao et al. [20] prepared MnO2 hollow structures by using MnCO3 as precursor, MnCO3 was first synthesized at 150 °C for 10 h via hydrothermal method. Fei et al. [21] and Yu et al. [22] synthesized MnCO3 precursor by mixing MnSO4 and NaHCO3 for 3 h at room temperature, then the precursor transformed to MnO2 hollow structures via oxidation by KMnO4. Zhao et al. [23] synthesized MnO2 hollow microspheres by thermal treatment of the as-prepared MnCO3 precursors at 400 °C for 4 h, and the MnCO3 precursor was obtained by hydrothermal method at 160 °C for 4 h. These reaction processes all included two steps: one was the preparation of MnCO3, the other was the oxidation of MnCO3. Moreover, the preparation and/or oxidation processes were time-consuming and power-wasting procedures. Thus, it is still a challenge to develop easy and fast methods to synthesize well-defined hollow MnO2 nanostructures.
By our strategy of using intermediate MnCO3 as precursor, urchin-like MnOx structures with hollow interior were synthesized at low temperature (30 °C) for short reaction time (9 min) without involvement of any surfactant and organics. The reaction process was fast and energy-efficient. The hollow structures of 1–1.5 μm are composites of λ-MnO2 and Mn3O4 crystals. Mn3O4 nanoflakes grew at the surface of MnOx spheres. The existing time of MnCO3 particles in the reaction system is a key factor for urchin-like structures. Reaction temperature and adding sequence of reactants could control the composition ratio of MnO2 and Mn3O4 in samples.
Recently scientists paid more attention to manage dye wastes by nanomaterials [25], [26], [27], [28], [29], [30], [31], [32], [33]. Manganese oxides with hollow structure exhibited more excellent adsorption and degradation properties because of their high specific surface area and good permeation property [20], [21], [33]. The degradation reactions usually performed under UV or visible light and/or with H2O2 addition. Few literatures reported degradation without light sources or H2O2. Chowdhury et al. [34] and Wang et al. [35] reported that Mn3O4 nanocrystals could decolorize methylene blue (MB) without light sources and H2O2 addition, but the degradation mechanism in acid condition was not discussed in detail.
In this work, we performed degradation experiments of RhB with the prepared urchin-like MnOx hollow nanostructures as catalysts. The samples exhibited excellent degradation properties for RhB in acid condition without light stimulation and H2O2 addition. The pH value was a key factor in the degradation, the optimum value was 3.0. The best decolorization and degradation efficiency of RhB could reach 85% and 66% after 5 min’s degradation reaction (97% and 90% after 60 min) respectively. The whole degradation process was green and energy-efficient. The composition and amount of sample catalysts showed prominent effects on RhB degradation. The degradation mechanism was discussed in detail via UV–vis results of powders before and after degradation.
Section snippets
Preparation of MnOx samples
All chemical reagents were analytical (AR) grade and used as received. In a typical procedure, 0.01 mol of Mn(CH3COO)2·4H2O and 0.015 mol of Na2CO3 respectively dissolved in 60 mL of distilled water, to form two homogeneous solutions. Then, Na2CO3 solution was added to Mn(CH3COO)2 solution under constant stirring. After the mixed solution was stirred for 2 min, 0.015 mol of NaOH and 0.01 mol of H2O2 were added to the above solution, with strong stirring for another 4 min successively. At the end of
Structure and morphology study on MnOx powders
X-ray diffraction (XRD) was carried out to determine the structure and composition of the samples. XRD patterns of the chosen four samples (Mn30a, Mn30b, Mn90a, Mn90b) are presented in Fig. 1. Fig. 1a and b shows XRD patterns of Mn30a and Mn30b samples which were prepared at 30 °C. The two samples differed in adding sequence of H2O2 and NaOH in the preparation process. They are dominated by λ-MnO2 (ICDD card no. 44-0992) with a strong sharp (1 1 1) diffraction peak at 19°, (3 1 1), (2 2 2) and (4 4 0)
Conclusions
In our present work, a simple template method was developed to synthesize urchin-like MnOx hollow nanostructures with diameter of 1–1.5 μm in a short time. These nanostructures were obtained directly in an aqueous system without a size-selection process, and the synthetic procedure is highly reproducible. The crystal composition of the samples can be manipulated through varied reaction temperatures. The as-prepared samples were used to degrade RhB without light sources and H2O2 addition. Series
Acknowledgments
This work was supported by Hunan Provincial Natural Science Foundation of China (Grant No. 11JJ5006), the National Natural Science Foundation of China (Grant Nos. J1103312 and J1210040), Science and Technology Project of Changsha City (Grant No. k0905033-11), and also supported by Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2011B159).
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