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Basic knowledge of the spectroelectrochemical method

In the early 1960s, Professor RM. Adam, a renowned American electrochemist, was supervising graduate student T. Kuwana, to observe the electrode reaction during the electrochemical oxidation of o-phenylenediamine derivatives, accompanied by color changes, so he proposed whether to design an electrode that can be seen through and use spectroscopy to identify all the colored substances formed. This new method was realized by T. Kuwana, in 1964. The first light transmitting electrode (OTE) he used was a thin layer of SnO2 doped on a sheet of glass. This conductive glass is called Nesa glass, which can be measured as an electrode at the same time. The absorption of light by the concentration of electroactive substances in the electrolytic cell opened the field of spectroelectrochemical research. Spectroelectrochemistry is one of the most popular research areas for the development of electrochemistry and electroanalytical chemistry, both in the past and in the future.

  • Introduction

Part 1 - Introduction

Spectroelectrochemistry (SEC) is a general term for electrochemical methods that apply in situ or ex-suit spectroscopy technology to study the interface of the electrode solution.
Spectroelectrochemical method (SEC) is characterized by the use of a spectroelectrochemical cell to perform electrochemical and spectroscopic measurements simultaneously, to understand the reaction on the electrode surface and the interface between the electrode and the solution and the electronic state of the molecule.
Spectroelectrochemical measurement method is classified according to the spectrum measurement cell, and can be divided into ex-situ and in-situ.The first is a method of measuring electrodes by spectroscopic measurement outside the electrolytic cell, such as low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, etc. The disadvantage of this off-site measurement is that it is impossible to accurately observe the status of some unstable electrochemical products or intermediates, and it is difficult to meet the needs of electrochemical mechanism research. The latter one refers to the spectroscopic measurement performed in the electrolytic cell, the method of observing the inside of the electrolytic cell, especially the electrode/solution interface state and process during the electrochemical operation is called the on-site method. For example: on-site infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, polarized spectroscopy, ultraviolet-visible spectroscopy, paramagnetic resonance spectroscopy, circular dichroism, etc.1-1). This series will focus on UV-Vis spectroelectrochemistry, the most studied and theoretically most solid method for a comprehensive understanding of this. Spectroelectrochemical techniques are of great interest both for the widespread application of this method and for the future development of other spectral wave electrochemical techniques.
Spectroelectrochemical methods can be divided into light transmission method, reflection method and parallel incidence method according to the light incidence method.

Fig. 1-1. Spectroelectrochemical methods are divided according to the method of light incidence.
Fig. 1-1. Spectroelectrochemical methods are divided according to the method of light incidence.

Fig. 1-1 Spectroelectrochemical method, according to the light incident method can be divided into light transmission method, reflection method and parallel incidence method.
Transmittance is the method by which an incident beam of light is transmitted vertically across an electrode and its adjoining solution. As shown in Fig. (a) and (b), the reflection method includes both internal reflection (c) and specular reflection (d). In the internal reflection method, the speed of incident light passes through the back of the light-transmitting electrode and penetrates into the electrode solution interface, so that the incident angle is just larger than the reflection angle, and the light is totally reflected. The specular reflection rule is to let light enter from the solution side, reach the electrode surface, and be reflected by the electrode surface. The parallel incidence method (e) and (f) a light beam is emitted between an electrode and a solution near the electrode surface.
There is also a classification method, which can be divided into thin layer spectroelectrochemical method (b) and (f) and semi-infinite diffusion spectroelectrochemical method (a), (d) and (e) according to the relative thickness of the solution layer near the electrode.
The thin layer spectroelectrochemical method involves the exhaustive electrolysis of the active material in the electrolytic cell.
Therefore, in general thin-layer spectroelectrochemical experiments, longer excitation time is often used, such as longer electrolysis time in potential step experiments and slower potential scan rate in cyclic voltammetry experiments.
Semi-infinite diffusion spectroscopy electrochemical experiments generally use a shorter excitation time. Commonly used electrical signals are single-potential step, dual-potential step, linear potential sweep and constant current.
In this series we will introduce a basic principles of a transmission-thin layer spectroelectrochemistry and their applications in electrochemical analysis.
The basic measurement system requires a spectrometer, light source, transmissive electrolysis cell, potentiostat, and computer. You also need a circuit that can trigger the control, to carry out spectroelectrochemical synchronized measurement control.
For a simple transmission experiments, a standart laboratory spectrometers can usually be used.
There are many types of spectrometers. When combined with electrochemical measurement, the electrode connection for electrochemical measurement, the laboratory space and the difficulty of system installation should be considered. It is recommended to use a small spectrometer with a CCD array detector.
For thin-layer spectroelectrochemical measurements, the electrolyte is typically injected into a thin quartz cell. The thickness of the liquid layer is about 50-200 µm. A transparent light-transmitting electrode or grid-type light-transmitting electrode is built as the working electrode. The light is irradiated in the direction of the electrode, the transmitted light is detected by the light receiving element, and the absorbance is measured.
The thin layer of solution is used to quickly achieve complete electrolysis of the active reactants in the electrolysis cell, minimizing the effect of the active reactants in solution on the product absorbance measurements.
By changing the absorption wavelength and absorbance in the spectroscopic spectrum, the chemical species near the electrode can be identified.
Spectroelectrochemical methods have unique advantages over conventional electrochemical methods in the description of reaction mechanisms and kinetics. Parameter determination, based on the measurement of current and potential, e.g. current versus sweep speed, concentration, time or electrode rotation speed, among other parameters. The main disadvantage is that this pure electrochemical measurement lacks the characteristics of the electrode reaction molecules, the current only represents the total rate of all processes occurring on the electrode surface, and there is no useful direct information about the reaction products or intermediates. Similarly, most of the studies on the electrode/electrolyte solution interface structure rely on capacitance measurement, and molecular level information cannot be obtained.

Spectroelectrochemistry has the following advantages over the usual electrochemical methods.

  • It provides molecular information on electrode reaction products and intermediates. The absorption spectrum of a substance on the surface of a solution or electrode can be recorded while changing the form of the active substance present on the electrode by applying an excitation potential signal. Using fast scanning spectrophotometry, it can also monitor the useful information of the intermediate reaction intermediate molecular spectrum1-2).
  • It is highly selective. Spectroelectrochemistry makes use of both electrochemical substances with different redox potentials to control them, as well as various substances. With different molecular spectral properties, many electrical processes that are difficult to distinguish electrochemically can be distinguished by spectroelectrochemical methods1-3).
  • Not affected by charging current and residual current. For example, when spectroelectrochemical methods are used to monitor protein characteristics, changes in absorption spectrum can be easily studied without being affected by the electrocatalyst medium.
  • Very slow heterogeneous electron transfer and homogeneous chemical reactions can be studied. For example, the first electronic step in the reduction of vitamin B12 is very slow, and it can be easily studied by thin-layer spectroelectrochemistry1-4).
  • The adsorption orientation of electroactive substances on the electrode surface can be studied. As long as the substance has spectral absorption in the ultraviolet-visible range, the adsorption amount of the adsorbed substance on the electrode surface and its adsorption orientation can be obtained.

1-1) T. Kuwana, R.K.Darlington and D.W. Leedy, Anal. Chem., 36,2036(1964).
1-2) Lin Zhonghua, Ye Siyu, Huang Mingdong and Shen Cultivation. Optical methods in electrochemistry. Beijing Science Press, (1990).
1-3) Bcwick, Jhon M, Meller and B.S. Pons, Electrochim Acta. 23,77 (1978).
1-4) Xie Yuanwu, Dong Shaojun, Spectroelectrochemical Method-Theory and Application, Jilin Science and Technology Press.

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