Nano-FTIR for solid-liquid interface
We developed a new methodology that enables studies of the molecular structure of solid-liquid interfaces with nanoscale spatial resolution. It is based on Fourier transform infrared nanospectroscopy (nano-FTIR), where the infrared (IR) field is plasmonically enhanced near the tip apex of an atomic force microscope (AFM). Single layer of graphene seals a liquid electrolyte reservoir while acting also as a working electrode. The photon transparency of graphene enables IR spectroscopy studies of its interface with liquids, including water, propylene carbonate, and aqueous ammonium sulfate electrolyte solutions.
We illustrate the method by comparing IR spectra obtained by nano-FTIR and attenuated total reflection (which has a detection depth of a few microns) demonstrating that the nano-FTIR method makes it possible to determine changes in speciation and ion concentration in the electric double and diffuse layers as a function of bias.
By parsing the contributions of various species at different locations to the dielectric function, we will be able to identify possible interface specific species. For example, the peaks near 1200 and 1300 cm-1 could be assigned to interfacial ionic species formed in the solution (e.g., bisulfate ions) or relatively enhanced combinational modes of solvated sulfate ions with librational modes of water. However, to better interpret the nano-FTIR spectra, a thorough thermodynamically based analysis of all the electrolyte component’s spatial distributions needs to be performed.
The interfacial sensitivity and nondestructive nature of nano-FTIR, coupled with the use of electrically conductive graphene membranes capping a liquid cell opens new opportunities for minimally invasive in situ and/or operando spectroscopic characterization of the graphene-electrolyte interface with full electrochemical bias control.
Our electrochemical cell with fluid input/output makes possible easy refreshing and/or changing of the electrolyte. Further, this methodology of solid-liquid interface exploration is not limited to pure graphene as graphene can be coated with a diversity of thin inorganic or organic films, or even living biological systems. Indeed, some of these platforms have already been tested successfully in our laboratory.
Other techniques, such as Kelvin-Probe Force Microscopy, Raman spectroscopy and X-ray Photoelectron Spectroscopy have also incorporated for this graphene liquid cell.
For more details, please see the published work:
- Yi-Hsien Lu, Jonathan M. Larson, Artem Baskin, Xiao Zhao, Paul Ashby, David Prendergast, Hans A. Bechtel, Robert Kostecki, and Miquel Salmeron. Infrared Nanospectroscopy at the Graphene-Electrolyte Interface. NanoLetters. 2019. 19, 8, 5388-5393. DOI: 1.10.1021/acs.nanolett.9b01897