Synthesis and characterization of nanostructured α-MnO₂ and its composites with SnO₂ and TiO₂ for efficient gas sensing

INTRODUCTION

Manganese dioxide exhibits polymorphism. Attributing to its unique framework, abundant oxygen species, catalytic nature, large surface area, nanostructured MnO₂ is considered a vital gas sensing material. This study focuses on the synthesis of α-MnO₂, its composites and their structural, optical, morphological analysis via X-ray diffraction, UV-Visible, FT-IR, FE-SEM respectively.

METHOD

α-MnO₂ is synthesized via a co-precipitate route. For this purpose 2M solution of KMnO₄ and 3M solution of Mn(NO₃)₂ are prepared separately in 20ml deionized water under vigorous stirring. KMnO₄ solution is added to Mn(NO₃)₂ followed by the drop wise addition of 4M solution of NaOH. The precipitate obtained is filtered, dried at 120⁰C overnight and annealed at 400⁰C for 4 hours to obtain the MnO₂ powder. Composites of α-MnO₂ is obtained via solid state reaction method wherein α-MnO₂ is grinded (1:1 ratio by weight) respectively with SnO₂ and TiO₂ using agate mortar and pestle followed by annealing them at 400⁰C for 2 hours.

RESULTS

The structural analysis reveals the formation of tetragonal α-MnO_2, SnO_2, TiO_2. The average crystallite size of α-MnO_2, α-MnO₂-TiO₂ and α-MnO₂-SnO₂ is 15.04nm, 24.25nm and 19.33nm respectively. The crystallinity increased from 81% in α-MnO₂ to 92.98% α-MnO₂-TiO₂ and 94.75% in α-MnO₂-SnO₂. The optical band gap of 5.05eV for α-MnO₂ decreased to 3.16eV and 3.4eV for α-MnO₂ –TiO₂ and α-MnO₂-SnO₂ respectively. The morphology discloses the formation of nanorods of α-MnO_2, granular structure of SnO₂ and TiO₂. The gas sensing is due to adsorption of gas on the surface of thin film and depends on the crystallite size, morphology, porosity.

CONCLUSION

The gas sensing can be enhanced by the formation of nanocomposites with small crystallite size, increased crystallinity, reduced optical band gap, porous morphology and abundant oxygen species. The thin films fabricated using the nanocomposite can be used to detect analyte gas at room temperature.