I passivated stainless steel in sulphuric acid and calculated the thickness to be 0.6 nm which seems inaccurate. The thickness values calculated from XPS is around 5 nm. Is the thickness values calculated from EIS accurate? I used Randles circuit to fit the data.
You are dealing with very small thickness in nm. Your impedance measurements are giving you thickness based on certain calculation which requires area of cross section, conductance values. The accuracy of impedance measurements will decide the thickness. I think your calculation based on impedance measurements may be wrong as compared to XPS. Impedance measurements should be done in vacuum with proper electromagnetic shielding, shielded connections to the impedance meter having suitable conductance range, then only the measurements can be reliable.
Dear Subash,
I would like to know, how did you calculated the thickness of your film using XPS. Can you please tell it. ?
Electrochemical impedance spectroscopy (EIS) is an indirect method, you do not measure the thickness of your passive layer directly. In contrast, you always have to rely on several assumptions for the EIS data evaluation - those assumptions are necessary to develop an equivalent circuit for the system under consideration. So far so good, if your system really meets those assumptions, everything should be fine. However, a real sample always looks different from what you assume in your equivalent circuit, especially in the case of (very) thin films it is reasonable that the thickness variation because of surface roughness plays an important role. What about grain boundaries in addition, and how to include their impact in an equivalent circuit? Suppose that you have some "holes" in your passive film, then those features will definitely dominate the flowing currents in comparison to the surfaces that are protected by an insulating passive film. In average, thus, the resistance (impedance) of your system will decisively be to small, resulting in thinner oxide layer thicknesses in the data evaluation. In XPS, you will also observe these holes (in this areas, the metal signals will be stronger), however averaged over the entire surface investigated, you will have much stronger oxidic contributions, and thus the oxide thickness will be more correct if you take e.g. the ratio between the oxide and the metal signals to derive the oxide thickness. Hope this helps, Dirk
Thank you Dr. Dirk ...your answer has been really helpful. Thank you Dr.Kumar for your kind answer.
Dear Mishra,
In depth profile measurements from the oxygen peak you can calculate the thickness. A simple search in internet should render you the the desired results. Unfortunately I don't have papers to send you..
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