  The adsorption behaviour of hazardous gas molecules, namely nitrogen dioxide
(NO$_2$) and ammonia (NH$_3$), on phosphorene nanosheet (PNS) was explored by
means of ab initio technique. To improve the structural solidity of pristine
PNS, we have introduced the passivation of hydrogen and fluorine at the
terminated edge. The structural solidity of both hydrogen and fluorine
passivated PNS is verified in terms of formation energy. The main objective of
this research work is to probe NO$_2$ and NH$_3$ gases using PNS as a base
sensing material. The adsorption of various preferential adsorption sites of
these gas molecules is studied in accordance with the average HOMO-LUMO gap
changes, natural-bond-orbital (NBO) charge transfer, HOMO-LUMO gap, and
adsorption energy. Notably, the negative value of adsorption energy is found
upon the adsorption of NO$_2$ and NH$_3$ on PNS and it is in the range of
$-1.36$ to $-2.45$ eV. The findings of the present research work recommend that
the hydrogenated and fluorinated PNS can be effectively used as a chemical
sensor against NO$_2$ and NH$_3$ molecules.
