Fig. 2

Charge density difference plots. The adsorption configurations and charge transfer for each case in a different order from Fig. 1 are plotted in a–e. The yellow isosurface indicates an electron gain, while the blue one represents an electron loss. The unit of isosurface value is e Å⁻³. Apparently, electron transfer in covalent a PG/NO and b PG/NO₂ structure are much more obvious than others