The basics of in-situ Electrochemcial FTIR spectroscopy

•Molecular composition and symmetry

•Bond lengths and force constants

•Identity and orientation of adsorbed intermediates/poisons/products

•Mechanism

Actually-most important by far is simply the ability to identify adsorbed poisons and solution and adsorbed products and intermediates and so elucidate mechanism………

Why (in-situ) infra red spectroscopy

Problems with the application of IR Spectroscopy in-situ to the study of the electrode/electrolyte

interface

1. All common solvents, and especially water absorb IR radiation very strongly. Water, 1640 = 20 mol-1 dm3 cm-1; to keep water absorption in this region down to 0.6, optical pathlength must be 5 m, and preferably

c. 2 m.

2. High sensitivity and stability are required to be able to ‘pick out’ the very weak absorptions of the near electrode species, (M – mM, and/or monolayer), from the intense background absorptions and noise.

These two problems are addressed thus:

1 In external reflectance spectroscopy by trapping a thin layer ( a few microns) of solution between the reflective WE and the cell window. This minimises the solvent absorption which is then anulled using differential data collection methodology.

2 Early in-situ IR systems used lock-in detection techniques which suffer from a number of problems including: complicated hardware and data collection, long measurement times, complicated data collection protocols. The advent of Fourier Transform InfraRed (FTIR) spectrometers rendered such techniques obselete.

A area above spectrometer, S spectrometer sample compartment. (a) IR beam in, (b) UV laser beam, (c) cooling/heating water in, (d) IR-transparent prism, (e) glass cell body, (f) water jacket, (g) Teflon cap and cell body, (h) thermo-couple leads, (i) contact wire, (j) electrolyte, (k) reflective working electrode.

The thin-layer configuration:External reflectance in-situ FTIRS

The in-situ FTIR cell

Data manipulation

The reference spectrum, (R0, 8 cm-1 resolution, 16 or 100 co-added and averaged scans, 3s or 16s, respectively, per scanset), was collected at 1100 mV. The potential of the electrode was stepped up from -200mV and a series of spectra, Rn, collected as as a function of potential, or the potential was stepped and held, and spectra, Rn, collected as a function of time.

The spectra are presented as:

(Rn/R0) vs /cm-1

Peaks pointing up, to +(Rn/R0), arise from the gain of absorbing species in Rn with respect to R0, and peaks pointing down, to -(Rn/R0), to the loss of absorbing species.

(1) The in-situ electrochemical FTIR cell; (2) BioRad FTS 6000 Spectrometer; [(3) 325nm

Laser]; (4) Heated block controller; (5) Control PC; (6) Heating/ cooling unit; (7)

Cooling/heating tubes to cell.