Introduction

Nanomaterials have captured the imagination of researchers lately due to the significant difference in their properties compared to the coarse-grained counterpart. Metal-oxide nanocrystals are expected to find useful applications in catalysis, energy storage, magnetic data storage, sensors, and ferrofluids (Zarur and Ying 2000; Majetich and Jin 1999; Nayral et al. 2000; Raj and Moskowitz 1990). In particular, colloidal metal-oxide nanocrystals are of great interest for technological applications owing to their unique size-dependent properties and excellent process ability. There have been numerous types of reactions that have been catalyzed using colloidal and supported metal nanocatalysts such as oxidations (Spiro and De Jesus 2000; Shiraishi and Toshima 1999; Launay et al. 1998; Akram et al. 2007; Altaf et al. 2009; Kabir-ud-Din et al. 2008; Akram et al. 2011), cross-coupling reactions (Li et al. 2002; Li and El-Sayed 2001; Narayanan and El-Sayed 2003; Kogan et al. 2002; Gopidas et al. 2003; Yeung and Crooks 2001), electron transfer reactions (Narayanan and El-Sayed 2004a, b; Sharma et al. 2003), hydrogenations (Ohde et al. 2004; Somorjai 1997; Boudjahem et al. 2004; Claus and Hofmeister 1999), fuel cell reactions (Anderson et al. 2002; Long et al. 2000; Moore et al. 2003), and many others. Numerous review articles have been published on the use of colloidal (Bradley 1994; Duff and Baiker 1995; Toshima 1996; Boennermann et al. 1996; Narayanan and El-Sayed 2004a; Mayer 2001; Bonnemann and Richards 2002; Moiseev and Vargaftik 2002) and supported (Toshima and Yonezawa 1998; Puddephatt 1999; Henry 2000; Kralik et al. 2000; Kralik and Biffis 2001; Thomas and Raja 2001; Thomas et al. 2003) transition metal nanoparticles as catalysts for a variety of organic and inorganic reactions.

As regards all other non-noble metals or transition metal oxides studied in modern times, MnO2 is one of the most attractive inorganic materials not only because of its physical and chemical properties and wide range of applications in catalysis, ion-exchange, molecular adsorption, biosensor and particularly energy storage but also because of its low cost and environmentally benign nature. Especially in catalysis, MnO2 becomes an obvious choice as an oxidant. Solution-based synthesis and use of metal-oxide nanoparticles, however, require special mention due to their low cost, mildness, convenience and use without additional templates and apparatus. In spite of their important applications, there have been only few reports on synthesis of monodisperse colloidal manganese oxide nanoparticles (Kabir-ud-Din and Iqubal 2009; Jana et al. 2007; Khan et al. 2010). In this paper, we report a facile one-step solution phase synthetic approach of MnO2 nanoparticles without any template or stabilizing agent.

Experimental

Materials

All chemicals were analytical grade reagents. Potassium permanganate (98.5 %, E. Merck, SA), and sodium thiosulfate (99 %, E. Merck, SA) were used as received without further purification. Ultra-pure water was used for the preparation of all reagents solutions. Permanganate solutions were stored in a dark glass bottle and standardized by titration against oxalate.

Synthesis of manganese dioxide nanoparticles

In a typical synthesis of MnO2 nanoparticles in solution phase was carried out as follows: 0.0158 g of KMnO4 was dissolved in 75 ml of ultra-pure water, and the mixture was fleetly stirred for 2 h. 0.0931 g of sodium thiosulphate dissolved in 25 ml of ultra-pure water and then it was added to the permanganate solution at room temperature. The color of the solution changed rapidly from purple to yellow–brown (indicating the onset of the formation of MnO2 nanoparticles) and finally dark brown. The resulting solution was perfectly transparent and was very stable over extended periods of several months.

Instrumental details

UV–visible absorption spectra were obtained on UV-1800 Shimadzu spectrophotometer using 10 mm quartz cell. Transmission electron microscopy (TEM) was carried out on a JEOL-2010 transmission electron microscope operated at an accelerating voltage of 80 kV. One or two drops of the solution containing the as-synthesized composites were deposited onto the amorphous carbon film supported on a copper grid and allowed to dry at room temperature in air. EDX spectrum was obtained from Oxford, Link, ISIS 300 instrument. Fourier transform infrared (FT-IR) spectra of the samples were recorded with a Perkin Elmer BX FT-IR infrared spectrometer in the range of 4,000–400 cm−1.

Results and discussion

UV–vis spectral study

In order to study the growth mechanism for the evolution of spherical MnO2 nanoparticles, a UV–visible study was carried out. The progress of the reaction or the formation of spherical MnO2 nanoparticle from aqueous KMnO4 was monitored by a UV-1800 Shimadzu spectrophotometer measuring the changes in the specified absorbance maxima. It was observed that the intensity of charge transfer transition (at 506, 525, 545, and 566 nm) decreased gradually by addition of sodium thiosulphate solution to the aqueous solution of potassium permanganate. The purple color of KMnO4 started to fade, and the color changed from purple to yellow–brown and finally to dark brown. The progress of the reaction has been accounted from a steady decrease of all the four absorbance maxima at specified band (506, 525, 545, and 566 nm) positions with increase in [Na2S2O3] as shown in Fig. 1. The purple color of KMnO4 faded away with increase in thiosulphate concentration producing a brown coloration, indicating the progress of the reduction reaction. All the spectra cross a common isosbestic point and are shown by a circle in Fig. 1, which gives valuable information about the reaction that 1 mol of reactant is converted to 1 mol of the product and a chemical equilibrium existed between the reactant and product at a particular wavelength, 360 nm.

Fig. 1
figure 1

UV–visible spectra of the mixtures containing a fixed amount of KMnO4 (= 8.0 × 10−4 mol dm−3) and varying amounts of Na2S2O3

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A broad distinctive peak was observed in the 360 nm range for a lower concentration, which is in contrast to the above-mentioned procedure for monitoring the four peaks. A significant blue shift of the adsorption edge for the as-obtained MnO2 dispersion in comparison to bulk MnO2 is found which can be explained because of the presence of very small MnO2 nanoparticles.

TEM study

The morphology and particle size of the product were determined from transmission electron microscopy. The observed results are given in Fig. 2a for freshly prepared manganese dioxide nanoparticles. Because of the random nature of aggregate formation, the synthesized whisker shape MnO2 nanoparticle aggregates have a broad distribution of sizes and shapes. This variety of sizes and shapes are apparent from the TEM images. The whiskers formed are 150–200 nm in length and 10–20 nm in diameter. The HRTEM image of the single whisker, the corresponding SAED pattern, and the lattice image of the sample under the ultrasound are shown in Fig. 2b. The observed lattice spacing of 2.6 and 2.9 Å correspond to the (103) and (112) planes of MnO2, respectively. The SAED pattern reveals the MnO2 particles are crystalline in nature. The size of nanoparticle according to the HRTEM image was found to be about 10 nm.

Fig. 2
figure 2

a TEM of MnO2 whiskers and b TEM of a single MnO2 whisker and c HRTEM image of MnO2 whisker and (binset) SEAD pattern of MnO2

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The chemical composition of the nanoparticles has been analyzed using EDX analysis as shown in Fig. 3. The Cu peaks are the signal detected from the TEM grid. This result confirms the presence of Mn and O in the sample.

Fig. 3
figure 3

EDX analysis of as-synthesized MnO2 and the elemental composition

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Fourier transforms infrared spectral study

Fourier transforms infrared (FT-IR) spectroscopy is known for its high sensitivity, especially in detecting inorganic and organic species with low content. The FT-IR spectrum is presented in Fig. 4. Two absorption bands observed at 600 and 475 cm−1 are corresponded to the characteristic stretching collision of O–Mn–O, which demonstrated the presence of the MnO2 in the sample. The typical broad absorption in the wavelength ranges between 4,000 and 3,000 cm−1 are allocated both the stretching collision of H–O–H and hydroxyl absorption, while the peak detected at 1,616 cm−1 symbolized the bending collision of adsorbed water. The simultaneous presence of both these peaks indicated the existence of adsorbed H2O molecule for this sample. Furthermore, the absorption peak at the wavelength near 1,236 and 812 cm−1 represented the surface –OH groups of Mn–OH for colloidal MnO2 nanoparticles.

Fig. 4
figure 4

FT-IR spectra of as-synthesized MnO2 nanoparticles

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Conclusions

In summary, Manganese dioxide nanoparticles have been synthesized, via a facile one-step solution phase approach, by the reduction of potassium permanganate with sodium thiosulphate at room temperature. TEM analysis revealed the size of nanoparticles as 10 nm and polycrystalline nature of particles is also observed. This low temperature synthetic route, based on the simple reaction with no participation of catalysts or templates and requiring no expensive and precise equipment, ensures higher purity of the products, greatly reduces the production cost, and thus offers great opportunity for industry scale-up preparation of nanostructure materials.