After using oxygen plasma for functionalization of surface, we have hydroxyl on the surface, but we have used only oxygen. Where is hydrogen coming from?
The presence of the OH group, atomic and molecular O is likely to originate from the contamination of atmospheric air and water vapor in the plasma processing environment typically in air or Atmospheric plasmas. If possible take a Optical emission measurement, which will also show the trace of atomic H (Halfa, Hbeta lines) along with OH radicals.