Preparation and processing of MnO2 nanowires


3.1. Materials

For the preparation of MnO2 nanowires, Potassium permanganate (KMnO4), maleic acid (C4H4O4), citric acid (C6H8O7) and Sodium chloride (NaCl) were used. All chemicals were of analytical grade, obtained from the University of Poonch Rawalakot Azad Jammu & Kashmir.

3.2. Preparation and processing of MnO2 nanowires

            Take 8.34 g KMnO4 (potassium permanganate) and 1.32 g C4H4O4 (maleic acid), and dissolved in 240 ml and 80 ml distilled water, respectively. After that, the C4H4O4 solution was added to the KMnO4 solution dropwise to maintain the precursor’s PH 7 at room temperature. Then 80 ml of 1 molar sodium chloride solution was added in maleic acid and permanganate solution and was kept at a stable place for 24 h to obtain a black gel. The black gel was centrifuged and washed six times with double distilled water and dried at 700C overnight. Nanowires of MnO2 was obtained by annealing the dried powder at 6000C for 4 h in furnace (Ghorbani et al., 2017).

3.3. Characterizations

Some of the following characterizations were done in order to study the morphology of MnO2 nanowires.

3.3.1. UV-Vis Spectroscopy

            UV-Vis spectroscopy states to absorption spectroscopy in the ultraviolet-visible spectral region. UV-4000 spectrophotometer was used to analyze the optical absorption characteristics of MnO2 nanowires.

         Figure 3.1: Schematic diagram of UV-Vis spectrophotometer

3.3.2. X-ray diffraction analysis (XRD)

            Structural analysis of MnO2 nanowires was done by the X-ray diffraction (XRD) study. X-ray diffraction spectroscopy is very effective tool for the determination of the atomic arrangement in a crystal. XRD is a very important parameter because it gives information about the phases of the nanoparticles and can make differences between crystalline and amorphous materials. X-ray diffraction (XRD) analysis of MnO2 nanowires was carried out using powder X-ray diffractometer (Bruker D8 Advance) which was activated at a voltage of 40kV and a current of 30mA with Cu-Kα1 radiations.

                        Figure 3.2: Schematic diagram of XRD apparatus

3.3.3. Morphology of MnO2 Nanowires

            Elemental composition, morphologies, and structures of prepared manganese oxide (MnO2) nanowires were investigated by using SEM (Scanning electron microscope. Dried sample posting on the carbon coated grid and is subjected to characterization.                        

3.3.4. Fourier transform infra-red spectroscopy (FTIR) Analysis

 The chemical composition of the MnO2 nanowires was analyzed by using FT-IR spectrometer (Perkin-Elmer LS-55 luminescence spectrometer). Dried powder form of the samples was used for this characterization using KBr pallets mode at a spectral resolution of 4cm-1. The frequency range was selected from 4000 to 400 cm-1.

                     Figure 3.3: Schematic diagram of FTIR apparatus

Chapter 04


4.1. X-ray diffraction spectroscopy of MnO2 nanowires

            The structure and size of MnO2 nanowires were investigated by XRD (x-ray diffraction spectroscopy). In x-ray diffraction spectroscopy various strong peaks observed at 2θ values of 28.570, 37.290, 41.730, 45.430, 49.880, 55.110, 59.830, 64.060 and 72.740corresponding to the (hkl) planes (110), (101), (200), (210), (411), (211), (220), (002), (310) and (301)  respectively. X-ray diffraction spectra of all peaks of manganese oxide nanowires are in strong agreement with the (JCPDS No. 24-0735 & 89-0511). X-ray diffraction spectrum of MnO2 nanowires are shown in (Figure 4.1). Similarly for sample 02 various strong peaks observed at 2θ values which are in strong agreement with the (JCPDS No. 72-1982).

4.1.1. Grain size

            Debye Scherrer’s equation was used to calculate grain size of MnO2 nanowires.

Where “λ” denote x-ray wavelength “β” is FWHM (full width half maximum) “θ” denote Bragg’s diffraction angle and “D” denote grain size. The crystallite size of MnO2 nanowires was around 28.87 nm at high intensity peak of (110) for sample 01 and 14.43 nm at high intensity peak of (310) for sample 02 from Debye-Sherrer equation (Rajamanickam  et al., 2014).

Using Bragg’s law to determine the value of d-spacing.

Using the following equation to determine the value of Lattice Parameter.

                  Figure 4.1: X-ray diffraction pattern of MnO2 nanowires (Sample 01)

                  Figure 4.2: X-ray diffraction pattern of MnO2 nanowires (Sample 02)

Table 4.1: Calculations of Crystallite size, FWHM, d-spacing and MnO2 nanowires (Sample 01 & 02 of high intense peaks)

Sample codeCitric acid (Sample 01)Maleic acid (Sample 02)
FWHM (β) radians0.00490.0099
Crystallite size (nm)28.8714.43
Lattice Parameter (Å)4.4429.838
High intense peak(110)(310)

 4.2. Optical Characteristics of MnO2 nanowires

            UV-vis spectrophotometer was used to study the optical properties of MnO2 nanowires synthesized by sol-gel method. The scanning range was selected from 300 to 800 nm. Absorption spectrum of the MnO2 nanowires is shown in (Figures 4.3 & 4.4).

         Figure 4.3: UV- Vis spectroscopy of MnO2 nanowires (Sample 01)

            Optical characteristics of nanowires provides information about physical properties like band structure and optically vibrant flaws. MnO2 nanowires manifest a colour change which was first confirmation of formation of nanowires. From (Figures 4.3 & 4.4) it was observed that MnO2 nanowires showed peaks at 370 nm for sample 01 and 349 nm for sample 02 related to ultraviolet and green emission (Lee et al., 2008). The band gap of MnO2 nanowires was calculated by extrapolating the curve drawn between (h ν) and (αhν)2 where ν is the frequency and α is the optical absorption coefficient. This value for MnO2 nanowires was found to be 2.28 eV for sample 01 and 2.47 eV for sample 2. This value was slightly less as compared to reported results of MnO2 nanowires. Particles size of MnO2 increases results in energy gap will decrease. This decreases of band gap and ultimately band gap energy decreases (Sultan et al., 2017).

      Figure 4.4: UV-Vis spectroscopy of MnO2 nanowires (Sample 02)

4.3. Functional or FTIR Analysis

                 FTIR (Fourier transform infrared spectroscopy) was used to study the functional groups and chemical composition present in manganese oxide (MnO2) nanowires, prepared by the sol-gel method. In sample one of manganese oxide (MnO2) nanowires, different peaks were identified at 537, 718, 1510, 1687, 2363 and 2810 cm-1 as shown in (Figure 4.5). Similarly, for sample two of manganese oxide (MnO2) nanowires different transmission peaks were identified at 504, 740, 1523, 1628, 2360 and 2810 cm-1 as shown in (Figure 4.6). The peaks at 1510 cm-1 and 1687 cm-1 corresponding to O-H bonding of vibrations absorbed by water molecules. In structure formation, the O-H peak movement plays a vital role. The peaks at 2810 cm-1 indicating the existence of the ethanol group or C-H bending and stretching. Similarly, the peak identified at 2363 cm-1 corresponding to the O=C=O group and related to vibration of CO2 molecule in air. The standard peak of manganese oxide structure in the range of 412-709 cm-1 (Rajamanickam  et al., 2014).

                         Figure 4.5: FTIR analysis of MnO2 nanowires (Sample 01)

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