In a previous post, we covered three ways to prepare metal layers for ATR-SEIRAS. We showed that this requires some means of depositing a metal thin film on an infrared-transparent internal reflection element, either through metal sputtering, chemical (“electroless”) deposition, or electrodeposition on a support layer. We discussed some of the pros and cons of each methods.
With the exception of electrodeposited Au layers, freshly prepared Au layers require a conditioning process to increase the SEIRAS enhancement, resulting in higher vibrational signals. We’ll discuss that process in this post and suggest a useful probe system for following the process.
What does “Conditioning” mean?
The empirical observation is that prolonged potential cycling of a Au thin film electrode for ATR-SEIRAS results in higher vibrational signals. The exact mechanism of this process is still unclear, but is often referred to as electrochemical annealing, electropolishing, or electrochemical conditioning of the film. Au electrodes are known to undergo surface reconstruction at oxidizing potentials. It is suggested that by repeatedly cycling the potential into Au oxidation, the mobility of surface Au atoms is increased, resulting in the slow growth of Au islands. Since the enhancement effect is known to depend on the morphology of the metal film, the conditioning process is thought to improve the morphology to provide better enhancement. There is likely also a cleaning effect at play – by cycling the potential, contaminants may be forced off the surface.
Conditioning in Practice
A variety of aqueous electrolytes have been used for film conditioning, including 0.1 M sulfuric acid, 0.1 M perchloric acid, 0.1 NaF, and 0.1 M KClO4. Although some authors have reported using prolonged (many hours) cycling in the double layer region in aqueous sulfuric acid, we find that careful excursions into Au oxidation are beneficial.
Before describing a typical conditioning procedure and best practices from our lab, we will note that Au layers are notoriously unstable and great care should be taken when working with them. Au oxidation and gas evolution both cause large stresses in the Au thin film which can cause it to buckle and delaminate from the internal reflection element. A Au film which fails in this way is not recoverable. Therefore, while following this procedure, we recommend occasional visual checks of the film to verify its integrity. As you grow more comfortable with the process, cues from ATR-SEIRAS spectra collected during the process will help determine whether the layer is still usable.
In the process below, we’ll often suggest that you collect a spectrum. We refer to a “single beam” spectrum; that is, the power spectrum that results from averaging many interferograms and computing the Fourier transform. This should not be confused with an absorbance or transmission curve, which are calculated from the ratio of two such spectra, e.g. A = -log(I/I0). Collecting individual spectra is useful because absorbance or transmission curves can be calculated using spectra collected at the beginning of the process. If your data collection software is not already set up to easily collect single beam spectra and perform calculations with them, we suggest you set it up to do so before attempting the conditioning process for the first time.
- With the Au layer at the open circuit potential (OCP), collect a reference spectrum. (You can use this to compare changes in the SEIRAS response as the conditioning process progresses.)
- Starting from OCP, cycle the potential in a window ±200 mV around the OCP. Complete three cycles in this window.
- Increase the potential bounds by 100 mV, and complete three cycles in this new window.
- Repeat step 3, under the following constraints:
- Do not enter hydrogen evolution! Set the cathodic potential bound to be just positive of hydrogen evolution.
- When the anodic potential bound reaches Au oxidation, proceed to step 5
- In small increments (typically 50 mV), increase the anodic potential bound into Au oxidation. Complete three cycles, and, if necessary, increase the upper bound by another 50 mV.
- As a rule of thumb, we very rarely push the window past the peak of Au oxidation (roughly 100 – 200 mV beyond the onset), as the stress induced by oxidation can delaminate the layer. The goal is to strike a delicate balance between “just enough” gold oxidation and “too little”. We find this is often dependent on layer prepration method and electrolyte composition and encourage end-users to experiment.
- We recommend collecting spectra to compare against the spectrum which was collected in step 1. Overall, a more active Au layer should show increased absorbance in liquid water bands, indicating improved enhancement.
Acetate Adsorption as a Probe System
Completing the conditioning process can be challenging without clear evidence of increasing SEIRAS activity. We’ve found that using 50 – 100 mM aqueous acetate buffer (pH 3.6 – 5.6) is an excellent tool for conditioning Au layers, particularly for novice users. Acetate adsorbs on Au at anodic potentials (around +600 mV vs. Ag/AgCl) but is replaced by at lower potentials (0 mV vs Ag/AgCl), and the asymmetric carboxylate stretch at 1400 cm-1 is a great diagnostic for the changes in the SEIRAS enhancement.
You can find out more about this system in the application note here.