According to the BOLS-NEP premise, when atoms are under-coordinated, the bond strength gain shifts the core level even deeper and shifts up the nonbonding electrons in energy. Energy band theory suggests that the O 1s level shift be proportional to the H-O bond energy, as the O:H contribution is negligible. XPS profile in Fig. shows that the O 1s level shifts from 536.6 to 538.1 and even 539.7