X-ray Photoelectron Spectroscopy with Electrical Modulation Can Be Used to Probe Electrical Properties of Liquids and Their Interfaces at Different Stages

Abstract

Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity-and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.

Lab-based operando x-ray photoelectron spectroscopy for probing low-volatile liquids and their interfaces across a variety of electrosystems

“…The critical frequency for the present system was measured as ∼170 Hz, whereas it was reported to be ∼70 Hz for the system described in Sec IV B, on a similar system, indicative of a higher capacitance for that dielectric layer. 40 This number is consistent because, therein, a much thinner (∼50 nm) hydrophobic coating with CYTOP was utilized, instead of the 1 μm Teflon coating of this work, and taking into account that the capacitance is inversely proportional to the thickness of the dielectric layer.…”

“…[76][77][78] Within the last three years, we have reintroduced XPS as a powerful analytical arsenal for probing electrical potential developments of liquids and their interfaces with dielectric substrates to extract novel information about them, under in situ and in operando conditions. [37][38][39][40] In order to probe impedance type properties of the samples under investigation, it is necessary to impose a varying frequency (AC) actuation for which the natural choice is a sine-wave. However, as we said earlier, the sine-wave distorts the form of the XP signal, see Figs.…”

“…The capacitance of the dielectric under the drop was estimated as 60 pF. 40 The results of recorded AC spectra together with the DC one are summarized in Fig. 18.…”

“…38) actuations, in order to extract chemically resolved electrical information of such systems, guided by modeling. 39,40 ILs and other low-volatile liquids have witnessed a colossal research activity due, mostly, to their potential use in battery and other energy harvesting and/or storage applications. 41 Within the scope of the present contribution, we will review exemplary cases of o-XPS experiments performed by our group, emphasizing mostly the methodology, as well as pointing out the new findings and developments to illustrate that XPS can be used as a contactless probe of local electrical potential.…”

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X‐ray Photoelectron Spectroscopy

“…Facilitated by their high vacuum compatibility, over the past decade ILs have been fairly well characterized in‐vacuo by X‐ray photoelectron spectroscopy (XPS) as a function of electrochemical potential [34–58] . XPS allows for the determination of surface composition of an IL while also probing the binding energy shifts in the ions ( U XPS ; units of eV) as a function of an external electrochemical potential ( U ; units of V).…”