Understanding the Apparent Non-Reliability in the Sensing Characteristics of MnO2 Self-Assembled Hierarchical Nanostructure

In this paper, the self-assembled hierarchical nanostructure of MnO
₂
, consisting of 3-D nanoflowers and 1-D nanorods, was synthesized by employing a hydrothermal technique. The gas sensing property of the same was investigated and the apparent anomaly concerning the n-type material characteristics and p-type sensing behavior was explained using the underlying mechanistic framework. The morphological (field emission scanning electron microscopy, transmission electron microscopy), structural (X-ray diffraction), and surface compositional (X-ray photoelectron spectroscopy) characterizations were carried out in detail, to establish the formation of
$\delta$
-MnO
₂
(nanosheets having thicknesses ~ 5–18 nm) and
$\alpha$
-MnO
₂
(nanorods having diameters ~ 20–24 nm) in the synthesized hierarchical nanostructure. The self-assembled MnO
₂
nanostructure offered pulse-like gas sensing performance with very fast response time and recovery time at room temperature. A possible sensing mechanism has also been furnished considering the effect of Mn interstitials (Mn
${_{i}}$
) reduction from Mn
⁴⁺
/Mn
³⁺
states to Mn
³⁺
/Mn
²⁺
states (in the presence of methanol), to explain the apparent anomaly in the sensing characteristics.