Introduction

One-dimensional (1D) nanostructured materials, including nanotubes, nanorods, and nanowires, have attracted intense research interest, owing to their novel physical and chemical properties (Chen et al. 2005). Various methods have been developed to prepare various 1D nanomaterials, such as arc discharge, laser ablation, chemical vapor deposition, templating, and hydrothermal methods, etc. MnO2 is one of the most attractive materials due to its ion exchange, molecular adsorption, catalytic, electrochemical, and magnetic properties (Chen et al. 2005). They are widely used as catalysts (Chen et al. 2005; Brock et al. 1998; Shi et al. 2010) molecular-sieves, ion-sieves (Feng et al. 1999) and especially as electrode materials in Li/MnO2 batteries because of its energetic compatibility in a reversible lithium electrochemical system, eco-friendliness, and low cost. The properties of MnO2 depend not only on the manganese oxidation state, but also on the structure type of MnO2 crystal. Great effort has been made to prepare nanocrystalline MnO2 with different structures. Over the past few years, the synthesis of nanostructures with controllable size and shape has been increased attracting attention (Dong et al. 2006). Manganese compounds, such as various manganese oxides/oxyhydroxides have been noticed owing to their specialities. Manganese oxyhydroxides, MnOOH, are of considerable importance in many technological applications, e.g., electrochemical reaction, battery and electrochromic application (Dong et al. 2006). Mn3O4 is known to be an active catalyst in several oxidations and reductions, and can be used as a catalyst for the oxidation of methane and carbon monoxide of the selective reduction of nitrobenzene (Dong et al. 2006). Nanorods have potential applications in nanodevices (Xia et al. 2003; Lu and Lieber 2006). Manganese oxides have a wide range of applications such as battery materials (Hosono et al. 2009), catalysts and sensors. At present, a tremendous amount of comprehensive investigations are under way into the unique applications of nanorods and nanotubes, because they provide a great opportunity to investigate the dependence of optical/electrical properties, thermal transport, and mechanical performance on the nano-scaled dimensionality and size (Wang 2000). Nanomaterials, especially nanorods have enhanced performances in many fields such as sensors (Dietl et al. 2000), solar cells (Yu 2009) catalysts and battery materials. Of the various non-noble metals or transition-metal oxides studied, MnO2 enjoys a place of pride because of its cost-effectiveness and eco-friendly nature. Beyond these advantageous properties, MnO2 is a very promising material in a neutral electrolyte system. In the case of pseudo capacitors, various noble and transition-metal oxides such as RuO2, IrO2, NiO, CoOx, SnO2 and MnO2 were used as electrode materials (Subramanian et al. 2008). Zirconium dioxide is the most studied ceramic material. Pure ZrO2 has a monoclinic structure at room temperature and transitions to tetragonal and cubic at increasing temperature. Zirconia supports are of particular interest as they can possess relatively high surface areas; they are mechanically and thermally stable and are solids with adsorbent properties, as well as catalysts themselves. Wambach et al. (1999) have recently reviewed the preparation of metal–zirconia catalysts along with their structural and chemical characteristics. Stabilized zirconia is used in oxygen sensors, fuel cell membranes and electroceramics. It will be used as insulators in transistors in future nanoelectronic devices. In this study an attempt has been made to synthesize and characterize novel, mixed MnO2–ZrO2 nanorods.

Experiment

Materials

The precursors zirconiumoxychloride (ZrOCl2), manganese sulfate (MnSO4) and the precipitant (NaOH) were purchased from Aldrich. All solutions were made up with deionised water.

Preparation of simple metal oxide nanoparticles

Fifty milliliters of 0.1 M MnSO4 was added dropwise to an aqueous solution of NaOH (50 ml, 1.0 M), making a final volume of 100 ml. The mixture was stirred well and refluxed at an elevated temperature for 2.0 h. The sample was collected by centrifugation, washed with water and dried over for 4 days at room temperature. Similar procedure was carried out for the preparation of ZrO2 nanoparticles using ZrOCl2 as the precursor.

Preparation of MnO2–ZrO2 nanorods

MnO2–ZrO2 nanorod was prepared by wet chemical method. In this method, 25 ml of 0.45 M ZrOCl2 was added to the aqueous solution of 50 ml of 1.0 M NaOH solution and stirred well. To this mixture, 25 ml of 0.45 M MnSO4 was added making a final volume of 100 ml. The resulting mixture was stirred well and refluxed at an elevated temperature for 2.0 h. The sample was collected by centrifugation, washed with water and dried over for 4 days at room temperature. Similar procedure was carried out to prepare different concentrations of (0.05–0.35 M) MnO2–ZrO2 mixed oxides.

Characterizations

The solution of the metal oxide nanoparticles in DMSO was used for recording the ultraviolet visible spectroscopic (UV–Vis) spectra. For recording the UV–Visible absorption spectra, a computer-controlled JascoV-500 spectrophotometer was used. The FTIR spectra were recorded using a Shimadzu instrument. The X-ray diffraction (XRD) patterns were recorded for the powdered materials using a Bruker AXS (D8 Advance) X-ray diffractometer. Energy-dispersive spectroscopic (EDAX) and SEM measurements were carried out by JEOL JSM-6360F field emission scanning electron microscope. Transmission electron microscopy (TEM) images were recorded using Philips CM 200 model with the operating voltage range 20–200 and with a resolution of 2.4 Å.

The electrochemical studies were carried out in a three-electrode cell. Pt wire was used as a counter electrode, silver–silver chloride electrode as a reference electrode and metal oxide nanoparticle-coated GCE was used as working electrode. The impedance studies were carried out using electrochemical workstation (mode 650C), CH-Instrument Inc., TX, USA. The charge transfer resistance was obtained from the diameter of the semicircles of the Nyquist plots. TGA/DTA analysis was carried out using PerkinElmer model and DSC analysis using METTLER TOLEDO model.

Result and discussion

UV–Vis studies

Ultraviolet visible (UV–Vis) absorbtion spectra of nano MnO2, nano ZrO2 and mixed MnO2–ZrO2 were recorded in the range 200–800 nm as shown in (Fig. 1). The absorption wavelength appears at 348 nm for nano MnO2 (Luo et al. 2004) and 335 nm for nano ZrO2. The absorption peaks for different concentrations (0.05–0.45 M) of MnO2–ZrO2 mixed oxides have been found to be in the range 281–348 nm, and are shown in Fig. 1. The variations in the absorption peaks for simple and mixed oxides are due to the smaller size of the nanoparticles (Singh et al. 2012). The mixed oxide nanoparticles exhibited blue shift compared with that of the simple oxides. The blue shift of the absorption peaks of the metal oxide nanoparticles result from certain unique effects of nanomaterials such as nanoscale effect and the blue shift reduces absorption of longer wavelength and is thus undesirable for UV protection (Yang et al. 2004). The absorption peaks of metal oxide nanoparticles appeared at a shorter wavelength region. Thus the metal oxide nanoparticles are found to be solar UV blockers (Yang et al. 2004). With the increase in concentrations of nano MnO2–ZrO2 mixed oxides there appeared increase in absorption values.

Fig. 1
figure 1

UV-VIS spectra of (a) nano MnO2, (b) nano ZrO2, (c) 0.05 M MnO2–ZrO2, (d) 0.15 M MnO2–ZrO2, (e) 0.25 M MnO2–ZrO2, (f) 0.35 M MnO2–ZrO2, (g) 0.45 M MnO2–ZrO2

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FTIR studies

Metal oxides generally give absorption bands below 1,000 cm−1 that arise from interatomic vibrations given in Fig. 2. The frequencies observed at 511–954 and 425–964 cm−1 correspond to Mn–O (Rema Devi et al. 2007) and Zr–O (P´erez-Maquedaa and Matijevicb 1997) bond vibrations, respectively. The peaks observed at 1,336 and 1,338 cm−1 are assigned to O2 stretching frequency (Guedes et al. 2009). Symmetric frequencies of Mn–O and Zr–O were observed at 1,191 and 1,103 cm−1 (Guedes et al. 2009; Du et al. 2009). The peak observed at 3,401 cm−1 corresponds to O–H vibration of water. The characteristic peaks observed at 1,122 and 1,103 cm−1 are due to the presence of inorganic ions (Rema Devi et al. 2007).

Fig. 2
figure 2

FTIR Spectra of (a) nano ZrO2, b nano MnO2, c 0.05 M MnO2–ZrO2, d 0.15 M MnO2–ZrO2, e 0.25 M MnO2–ZrO2, f 0.35 M MnO2–ZrO2, g 0.45 M MnO2–ZrO2

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Fourier transform infrared spectroscopic (FTIR) spectra of MnO2–ZrO2 mixed oxides synthesized at five different concentrations of both MnSO4 and ZrOCl2 (0.05, 0.15, 0.25, 0.35 and 0.45) are shown in Fig. 2c–g. For mixed oxide nanorods (0.45 M) MnO2–ZrO2, the combination of both Mn–O and Zr–O bonds appear in the range 511–999 cm−1 Fig. 2g. The frequencies observed at 1,336 and 1,396 cm−1 are assigned to O2 stretching and bending frequencies, respectively (Partha Sarathi and Thilagavathi 2011). The characteristic peak at 1,122 cm−1 clearly indicates the presence of inorganic ions (Du et al. 2009). In the case of other concentrations of (0.05–0.35 M) MnO2–ZrO2 mixed oxides, the bands were almost similar to 0.45 M MnO2–ZrO2.

SEM and EDAX behaviors of nano MnO2, nano ZrO2 and MnO2–ZrO2

Scanning electron microscopy (SEM) was used to identify the morphology of the synthesized metal oxides and mixed metal oxides MnO2, ZrO2 and MnO2–ZrO2. From Fig. 3a, b the prepared MnO2 and ZrO2 display sponge-like structure, granular flakes and mixed granular appearance. When MnSO4 and ZrOCl2 were mixed, the surface morphologies and roughness of the particles are changed to rod-like structure and become homogeneous which confirmed the formation of MnO2–ZrO2 nanoparticles. SEM micrographs of nano MnO2–ZrO2 synthesized at five different concentrations of both MnSO4 and ZrOCl2 (0.05, 0.15, 0.25, 0.35, and 0.45) are shown in Fig. 3c–g. Particles synthesized in 0.05 M of MnSO4 and ZrOCl2 appear to be very uniform in spherical morphology. In the case of 0.15 M of MnSO4 and ZrOCl2 and 0.25 M of MnSO4 and ZrOCl2, the particles were modified from spherical into rod-like morphology. However, the nano MnO2–ZrO2 mixed oxides synthesized in the 0.35 M of MnSO4 and ZrOCl2; 0.45 M of MnSO4 and ZrOCl2 were completely changed into nanorods. By choosing the adequate concentration we would be able to form nanorods. EDAX analysis confirms the presence of Mn, Zr and O as shown in Fig. 3.

Fig. 3
figure 3figure 3

SEM behavior of (a) nano MnO2, b nano ZrO2 and SEM image EDAX pattern of (c) MnO2–ZrO2, d MnO2–ZrO2, e MnO2–ZrO2, f MnO2–ZrO2, g 0.45 M MnO2–ZrO2

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TEM behaviors of nano MnO2, nano ZrO2 and MnO2–ZrO2

Transmission electron microscopy (TEM)’s image of MnO2 nanoparticles shows the size of the nanoparticles to be 20 nm and Fig. 4a shows the size of the ZrO2 nanoparticles to be 100 nm. The morphology of the synthesized nano ZrO2 also appeared rod-like and that of nano MnO2, sphere-like (Fig. 4b). The morphology of mixed nano metal oxide (0.45 M of MnO2–ZrO2) was found to be a uniform rod and the size is observed to be 20 nm as shown in Fig. 4c.

Fig. 4
figure 4

TEM image of (a) nano ZrO2, b MnO2 nanoparticles, c MnO2–ZrO2 (0.45 M) nanorod

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XRD behaviors of nano MnO2, nano ZrO2 and MnO2–ZrO2

The X-ray powder diffraction patterns for the simple and mixed oxide nanoparticles as in Fig. 5 are typical of crystalline nanoparticles. The particle size was calculated using Scherrer’s equation. In the case of nano MnO2 the particle size was 45 nm and in the case of nano ZrO2 the particle size was 8.3 nm. The average crystallite sizes of nanorods, MnO2–ZrO2 is found to be in the range of 39–56 nm. With the increase in concentrations of MnO2–ZrO2 from 0.05 to 0.45 M there is corresponding increase in size. The sharp peaks indicate more crystallinity and hence it shows more conductivity because high crystallinity materials usually show higher conductivity.

Fig. 5
figure 5

XRD Pattern of (a) nano 0.05 M MnO2–ZrO2, (b) 0.15 M MnO2–ZrO2, (c) MnO2–ZrO2, (d) MnO2–ZrO2, (e) MnO2–ZrO2, (f) nano MnO2, (g) nano ZrO2

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Thermal analysis

Figure 6 shows the thermograms of nano ZrO2, nano MnO2 and different concentrations of mixed MnO2–ZrO2 oxides. Figure 6a shows the TGA/DTA behavior of MnO2. The first weight loss step from 42 to 90 °C corresponds to loss of moisture. The next step from 91 to 580 °C was due to the presence of extra bounded water molecules. The final weight loss step from 581 °C onwards corresponds to the degradation of MnO2. The first weight loss step from 30 to 100 °C (Fig. 6b) corresponds to loss of moisture. The next step from 101 to 170 °C was due to the presence of extra bounded water molecules. The final weight loss step from 170 °C onwards corresponds to the degradation of ZrO2. The DTA analysis also exhibited the same behavior.

Fig. 6
figure 6

TGA/DTA Curves of (a) nano MnO2, b nano ZrO2, c 0.05 M MnO2–ZrO2

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Thermal stability of nano MnO2–ZrO2 mixed oxides synthesized at five different concentrations of both MnSO4 and ZrOCl2 (0.05, 0.15, 0.25, 0.35, 0.45 M) is characterized by TGA/DTA analysis and the results are shown in Table 1. In these curves the first weight loss step from 42 to 170 °C corresponds to the loss of moisture. The next weight loss step from 91 to 390 °C corresponds to extra bounded water molecules. The final weight loss step from 281 to 321 °C corresponds to the degradation of mixed oxides (MnO2–ZrO2). The weight loss values at the critical temperatures such as 280, 300, 320 and 390 °C reveal that the type of simple oxides employed affects the thermal properties of the resulting mixed oxides. The thermogram of 0.05 M MnO2–ZrO2 is shown in Fig. 6c.

Table 1 TGA analysis of nano ZrO2, nano MnO2 and MnO2–ZrO2 mixed oxides
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DSC thermogram of MnO2, ZrO2 and different concentrations of mixed MnO2–ZrO2 are recorded at the heating rate 10 °C min−1. The glass transition temperature (Tg) crystallization temperature (TC) and melting point (TM) of the mixed oxide are determined from the DSC curves (Fig. 7a) shows the melting point (TM) of MnO2 at a temperature of 237 °C and the (TC) value at 220 °C. The melting point (TM) of ZrO2 was recorded at a temperature of 95 °C, the TC value recorded at 10 °C and the Tg value recorded at −70 °C. These results can be seen in Fig. 7b. The TC value of MnO2–ZrO2 mixed oxide is in the range of 29.93–36.35 °C. The mixed oxide was melted at the temperature range of 76.24–93.11 °C. According to DSC the melting temperature TM of different concentrations of mixed oxide was decreased remarkably than the simple oxides. Table 2 shows the TC and TM values of 0.05–0.45 M MnO2–ZrO2 mixed oxides. DSC thermogram of 0.05 M MnO2–ZrO2 and is shown in Fig. 7c.

Fig. 7
figure 7

DSC Thermograms of (a) nano MnO2, b nano ZrO2, c 0.05 M MnO2–ZrO2

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Table 2 TC and TM for 0.05–0.45 M MnO2–ZrO2 mixed oxides
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Cyclic voltammetric behavior of nano MnO2, nano ZrO2 and MnO2–ZrO2

Cyclic voltammetric studies of ZrO2 exhibited one oxidation peak at 0.179 V with higher peak current at pH 1.0 which led to the selection of pH at 1.0 as optimum pH for further voltammetric studies. Cyclic voltammetric behavior of MnO2 showed one anodic peak (Fig. 8a) at 0.415 V which is due to the presence of MnO2 and it is reduced at 0.352 V. Cyclic voltammetric behavior of ZrO2 showed one oxidation peak (Fig. 8b) observed at 0.179 V, which indicated the formation of ZrO2, whereas for different concentrations of mixed oxides, two oxidation peaks were observed in the range 0.127–0.3238, 0.6,535–0.6571 V (Fig. 9), respectively, which were entirely different from the behavior, those obtained from nano MnO2 and nano ZrO2, confirmed the formation of mixed nano MnO2–ZrO2 oxide. Cyclic voltammetric behavior of nano MnO2–ZrO2 mixed oxides synthesized at five different concentrations of both MnSO4 and ZrOCl2 (0.05, 0.15, 0.25, 0.35, 0.45 M) are shown in Fig. 9a–e. With the increase in concentrations (0.05–0.45 M) of nano MnO2–ZrO2 there is corresponding increase in peak potentials and peak currents also.

Fig. 8
figure 8

Cyclic Voltammetric behavior of (a) nano MnO2, b nano ZrO2

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Fig. 9
figure 9

Cyclic voltammetric behavior of (a) 0.05 M nano MnO2–ZrO2, b 0.15 M MnO2–ZrO2, c 0.25 M MnO2–ZrO2, d 0.35 M MnO2–ZrO2, e 0.45 M MnO2–ZrO2

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The plot of peak current versus different scan rate for nano MnO2–ZrO2 mixed oxide exhibits a straight line (Fig. 10) indicating a good adherent behavior on electrode surface. When peak currents of nano MnO2–ZrO2 are correlated with the square root of scan rate, a straight line is observed (Fig. 11). These facts reveal that the voltammetric redox behavior of mixed metal oxide nanoparticles is controlled by adsorption process. Thus it proves to be an anticorrosive agent for paints.

Fig. 10
figure 10

Plot of peak current versus scan rate for 0.45 M nano MnO2–ZrO2

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Fig. 11
figure 11

Plot of peak current versus square root of scan rate for 0.45 M nano MnO2–ZrO2

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When the metal salts of Mn and Zr were mixed during chemical co-precipitation, the mixed metal oxide nanoparticles were produced. The mixed metal oxide was coated on glassy carbon electrode and the cyclic voltammogram was recorded. The voltammogram also exhibited oxidation and reduction peaks with higher peak current. This might be due to the formation of mixed oxide from metal salts of Mn and Zr.

The increase in the peak current of the mixed oxide from that of simple oxides and the increase in the peak current due to increase in the metal concentration confirm the increase in the conductivity of the mixed oxide. To temper the cutting edge of electrochemistry, it will be necessary to carry out capacitance measurements, as these are closer to bulk-like techniques.

For a simple parallel plate capacitor, charge on the capacitor, Q, is proportional to the voltage drop across the capacitor V, and C is the capacitance. Therefore the equation is

$$ Q = CV $$

Capacitance is a crucial factor in electrochemical experiments because it gives rise to current during the charging of the capacitor. To calculate the magnitude of this current, with respect to time (t) and capacitance should be treated as constant recognizing that dQ/dt is an expression for current and dV/dt is the potential scan rate. The equation is

$$ \begin{aligned} {\text{d}}Q \, /{\text{dT}} = & C{\text{d}}V/{\text{dt}} \\ {\text{Therefore }}i = & C{\text{v}} \\ \end{aligned} $$

From this very simple derivation, we have an expression for the charging current at the steady state when applying a ramping voltage. If there are no possibilities for electron transfer between the solution and the electrode this is the only current that we will observe. The cyclic voltammogram of mixed oxide shows the influence of various scan rates. The capacitance was calculated from cyclic voltammograms using the above said equation. The calculated values of MnO2, ZrO2 and 0.05 M MnO2–ZrO2 mixed oxide are 13, 30 and 39 μF respectively. The capacitance of chemically synthesized mixed oxide is suggested that, they can well be used as an electronic material.

EIS studies of nano MnO2, nano ZrO2 and MnO2–ZrO2

Electrochemical impedance measurements were carried out at a frequency range say from 1,000 to 0.01 Hz at open circuit potential. The simple equivalent Randle circuit for studies is given as an insert in (Fig. 12). The chemically synthesized simple oxide and mixed oxide nanoparticles coated on glassy carbon electrode are used (0.0314 cm2) as working electrode. The cell is composed by a 1.0 cm2 Pt electrode, Ag/AgCl serves as reference electrode. The measurements were made for simple oxides such as nano MnO2, nano ZrO2 (Fig. 12a, b) and nano MnO2–ZrO2 mixed oxides synthesized at five different concentrations of both MnSO4 and ZrOCl2 (0.05, 0.15, 0.25, 0.35, 0.45 M) as shown in Fig. 12c–g. To understand the electrical properties of the electrode/interfaces clearly the Randle’s equivalent circuit was chosen to fit the obtained impedance data. In Randle’s circuit, it is assumed that the resistance to charge transfer (Rct) and the diffusion impedance (Rct) are parallel to the interfacial capacity (Cdl). The parallel combination of Rct and Cdl gives rise to a semicircle in the complex plane plot of Z″ against Z′, the semicircle diameter equals charge transfer resistance (Rct). This resistance exhibits the electron transfer kinetics of the redox probe at the electrode interface. From the Table 3, it is clear that the decrease in electrical resistivity with the increase in concentration is due to the improvement in crystallite and/or grain size, decrease in defects. This fact confirms the anticorrosive activity of the synthesized nano MnO2–ZrO2 mixed oxide samples. The lower the concentration of MnO2–ZrO2 mixed oxide nanoparticles, higher the resistance which is responsible for anti corrosive activity. The small observed variations might be from the nature of electrode conductivities (Chandrasekaran et al. 2008). More resistivity implies a physical adsorption of the corresponding electrodes during the diffusion path way whereas the lower value indicates the good conductivity behavior.

Fig. 12
figure 12

Electrochemical impedance spectra of (a) nano MnO2, b nano ZrO2, c 0.05 M MnO2–ZrO2, d 0.15 M MnO2–ZrO2, e 0.25 M MnO2–ZrO2, f 0.35 M MnO2–ZrO2, g 0.45 M MnO2–ZrO2

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Table 3 Rct and Cdl for 0.05–0.45 M MnO2–ZrO2 mixed oxides
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Conclusion

To summarize, nano MnO2, ZrO2 and MnO2–ZrO2 were synthesized by wet chemical method. The characterizations of the chemically synthesized nanoparticles were done using UV–Visible spectroscopic and FTIR studies. The sizes of the synthesized oxides were in the nm range and they were found to be thermally stable. SEM and TEM studies show a uniform rod-like morphology in the case of mixed oxide. CV studies revealed that the mixed oxide has good adherent and electrochemical activity on GC and thus it is found to be corrosive protection agent for paints formulation. The capacitance of chemically synthesized mixed oxide was suggested that, they might well be used as an electronic material.