Determination of the Electric Field Intensities in a Mid-Infrared Spectroelectrochemical Cell Using Attenuated Total Reflection Spectroscopy with the Otto Optical Configuration

Thickness Measurement of Nanoscale Polymer Layer on Polymer Substrates by Attenuated Total Reflection Infrared Spectroscopy

For the first time, attenuated total reflection (ATR)-Fourier transform infrared (FT-IR) spectroscopy was utilized to measure the thickness (d0) of a nanoscale polymer layer on polymer substrate with significant credibility. First, a mathematical formula, A/A0 = 1 - 2d0/ d(p), was derived based on a self-defining subsection function (where d(p) was defined as depth of penetration of ATR and A and A0 were defined as the absorption band area of the characteristic functional group only contained in bulk substrate with a thin polymer layer attachment and the same group in blank substrate, respectively). On the mathematical model, through changing incidence angles, a series of values of A (A0) and corresponding d(p) were obtained, and when plotting A/A0 versus 2/d(p), d0 was obtained as the slope. With polystyrene (coating)/olypropylene (substrate) as a model system, we obtained the relevant values (d0). Comparing the results with the values of practical coating thickness (calculation and TEM observation), we found that this method was able to characterize well the thickness of a thin polymer layer on a polymer substrate in the range from 10 to 110 nm. Errors in the measurement were given and analyzed. Furthermore, this method was well applied in the thickness measurement of a polyacrylamide graft layer on a polypropylene film surface. The effect of pressure in the ATR technique on the coating thickness measurement was also discussed. In comparison with other methods such as XPS, SEM, TEM, and AFM, this approach based on a universal ATR technique was very convenient and fast. This method is expected to widen the application of the ATR-FT-IR technique and stimulate the further development of many fields such as surface self-assembly and surface functionlization.

Digital signal processing for step-scan phase and electrochemical potential double-modulation Fourier transform infrared spectrometry

Step-scan double-modulation (phase and electrochemical potential) Fourier transform infrared (FT-IR) spectrometry provides both spectroscopic and dynamic information about faradaic reactions. Recently introduced digital signal processing (DSP) can be used, instead of two lock-in amplifiers, for the optical signal demodulation at two modulation frequencies. In order to establish the merits of double-modulation FT-IR spectrometry with DSP, spectro-electrochemical experiments are performed in the attenuated total reflection configuration and with the commonly used ferri/ferrocyanide redox couple. Because of a large potential drop associated with the uncompensated resistance, a satisfactory signal-to-noise ratio for the alternating current (ac) optical measurements is obtained only with the employment of positive feedback compensation. In this arrangement, the amplitude of electrochemical modulation is sufficiently large to convert a significant fraction of the reduced form to the oxidized form and back to the reduced form. Large amplitude ac voltammetry demonstrates that the phase of faradaic admittance at the formal potential is approximately 45 degrees at 2.00 Hz. In addition, these experiments allow for calculation of the interfacial ac potential. This variable is needed for the normalization of the in-phase and the quadrature spectra in order to overcome the problem associated with the iR(u) drop. Because of the integral relationship between the faradaic current and the electromodulation reflectance coefficient, the phases of electromodulation reflectance coefficient with respect to the interfacial ac potential are expected to be -45 degrees and 135 degrees for the reduced and oxidized forms, respectively. However, dynamic information from double-modulation FT-IR spectrometry is available only if demodulation at the electrochemical potential modulation frequency is performed with respect to a defined phase. Because of an undefined demodulation phase implemented in the current version of DSP software, step-scan double-modulation FT-IR spectrometry with DSP is suitable only to provide spectroscopic information. In order to overcome this limitation, the demodulation of the ac optical signal at the electrochemical modulation frequency must be synchronized in phase with the ac potential modulation applied to the electrochemical cell.