(Invited) Development of a New Experimental Procedure for Investigating the Dielectric Behaviour of Water Confined within Nanocavities
Authors: Whitt Daniel P , Bouillard Jean-Sébastien G , Kemp Neil T 
Published: December 2024

Understanding the behaviour of liquids when confined in nanoscale environments is becoming increasingly important for numerous technological advancements, including batteries, fuel cells, electrolysis, and the synthesis and deposition of advanced materials. While significant knowledge exists regarding the macroscopic properties of liquids, little is known about their behaviour when confined in nanoscale environments where the presence of strong surface interactions or large electric fields can have a profound effect on both the molecular structure of the liquid and its chemical reactivity. We have used nanocavities of varying heights and electrochemical impedance spectroscopy to investigate how the molecular behaviour of liquids changes when the nanocavity height is below the electrical screening length (Debye Length). In the case of de-ionised water, preliminary results show unusual behaviour when the nanocavity size decreases below 100 nm.

Aside from measurements on nanocavities with a fixed size, this paper also presents a novel and unique means of performing impedance spectroscopy measurements on nanocavities of variable height. Fixed height devices were fabricated using titanium pillars of the desired height to support an additional glass substrate containing a patterned top electrode. Photolithography methods were used to create the top and bottom electrodes (5nm Ti / 50nm Au). Variable height devices were constructed with the use of piezoelectric-controlled x, y, z stages and with levelling monitored using capacitive sensors. Impedance spectroscopy was performed on the devices with a Solartron 1260 with a 1296A Dielectric interface and current-voltage sweeps on a HP 4140B.

For static height devices an unusual change in the dielectric frequency response occurs when the cavity size is reduced below 100 nm. With a large nanocavity height the capacitance measured is large and has a highly non-linear frequency response due to the presence of two electrical double layers. These are located in close proximity to both electrodes and acts as large parasitic capacitances. When the nanocavity height is reduced, the capacitance significantly reduces (by orders of magnitude) and begins to show signs of a flat-response. This is consistent with the formation of a constant and uniform electric field across the nanocavity which occurs when the electrical double layers from each electrode start to overlap. To study this effect in more detail, a new and innovative setup has been developed that allows dynamic control of the cavity height whilst performing impedance spectroscopy measurements. Piezoelectric transducers are used to control the distance between the electrodes with nanometre resolution. Inbuilt capacitor sensors with a feedback loop ensure the consistency of the nanocavity size across the device geometry and throughout the measurement duration. This allows for continuous analysis of the same water sample over a range of nanocavity dimensions, measurement frequencies and electric field strengths.

To conclude, preliminary data shows interesting trends that indicate how electrical double layers behave in confined nanoscale cavities. A unique new measurement system will be demonstrated that gives greater control and precision of the nanocavity height. We expect this will provide new insight into the molecular behaviour of liquids in nanocavities with heights much less than those previously measured and where strong surface interactions begin to dominate.