Oriented Growth of α-MnO₂ Nanorods Using Natural Extracts from Grape Stems and Apple Peels

Abstract

We report on the synthesis of alpha manganese dioxide (α-MnO₂) nanorods using natural extracts from Vitis vinifera grape stems and Malus domestica 'Cortland' apple peels. We used a two-step method to produce highly crystalline α-MnO₂ nanorods: (1) reduction of KMnO₄ in the presence of natural extracts to initiate the nucleation process; and (2) a thermal treatment to enable further solid-state growth of the nuclei. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) images provided direct evidence of the morphology of the nanorods and these images were used to propose nucleation and growth mechanisms. We found that the α-MnO₂ nanorods synthesized using natural extracts exhibit structural and magnetic properties similar to those of nanoparticles synthesized via traditional chemical routes. Furthermore, Fourier transform infrared (FTIR) shows that the particle growth of the α-MnO₂ nanorods appears to be controlled by the presence of natural capping agents during the thermal treatment. We also evaluated the catalytic activity of the nanorods in the degradation of aqueous solutions of indigo carmine dye, highlighting the potential use of these materials to clean dye-polluted water.

Keywords: dye degradation; green synthesis; manganese oxide; nanorods; natural extracts; oriented attachment.

Figure 1

X-ray diffraction (XRD) patterns of the as-synthesized sample and alpha manganese dioxide (α-MnO) nanoparticles under different calcination temperatures (600 °C, 800 °C). (a–c) corresponds to experiments using the Malus domestica Cortland Apple Peels extract and (d–f) to experiments using the Vitis vinifera Grape stems extract.

Figure 2

Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of manganese oxide (α-MnO₂) nanorods at different magnifications. (a–c) Malus domestica apple peel extract, calcinated at 600 °C; (d–f) Malus domestica apple peel extract, calcinated at 800 °C; (g–i) Vitis vinifera stems extract, calcination at 600 °C; (j–l) Vitis vinifera stems extract, calcinated at 800 °C.

Figure 3

Fourier transform infrared (FTIR) spectrum of Manganese Oxide in the range from 500 to 3800 cm⁻¹, KBr pellet sampling (a–c) Vitis vinifera stems extract; (d–f) Malus domestica apple peels extract.

Figure 4

X-ray photoelectron spectroscopy (XPS) Spectra of the Manganese Oxide samples. (a–c) samples synthesized with the Vitis vinifera stems extract; (d–f) samples synthesized with the Malus domestica apple peels extract (BE values are corrected using the carbon peak at 284.6 eV as a reference).

Figure 5

Magnetization as a function of the applied field at 5 K and 300 K for samples (a) A6, A8; (b) V6, V8; and zero-field-cooling (ZFC)/field cooling (FC) magnetization curves of the α-MnO₂ nanorods under an applied field of 100 Oe for samples; (c) A6, A8; (d) V6, V8.

Figure 6

Schematic representation of the proposed nucleation and growth mechanism of α-MnO₂ nanorods using natural extracts from Malus domestica and Vitis vinifera.

Figure 7

Ultraviolet-visible (UV-Vis) spectra of indigo carmine solutions after exposure to α-MnO₂ nanorods at various concentrations. (a,b) samples synthesized with the Malus domestica apple peels extract; (c,d) samples synthesized with the Vitis vinifera grape stems extract. The right hand side shows digital photographs of the color change of the specimens.