**2.1 Preparation of porous ZnO nanostructures**

A facile precipitation cum calcination route was used for the preparation of porous ZnO nanoparticles [32]. Briefly, appropriate amount of zinc nitrate (4 g) was dissolved in distilled water, followed by the addition of polyethylene glycol (PEG) (3 g) and hexamine (3 g). The entire precursor solution is subjected to a vigorous stirring process using a magnetic stirrer for 30 minutes at a temperature of 60 °C. Following this, the ammonia solution was added to the precursor solution until their pH reached 9. After that, the solution is allowed to gelation process by placing them in a hot plate at the temperature of 90 °C, which results in the formation of a dark brown gel. After 4 hours, the dried brownish gel was placed in a silica crucible and calcined at a temperature of 500 °C for 2 h that finally lead to the formation of white-colored porous ZnO nanostructures.
