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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 optically transparent 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
  • Optically Transparent Electrode - Part 1

Chapter 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.

Chapter II - Optically Transparent electrode (Part 1)

1. Requirements for optically transparent electrodes

The optically transparent electrode used in spectroelectrochemistry should meet the requirements of both spectral and electrical properties. The ideal optically transparen electrode should have good light transmission and low resistance value. In fact there are not many materials that meet these two requirements, and a compromise approach is generally used 2-1).
Several types of optically transparent electrodes are discussed below.

2. SnO2 and In2O3 optically transparent electrode

2.1 SnO2 optically transparent electrode

SnO2 glass coated with a thin layer of Sb doped with its trade name is Nesa glass. The coating is usually about 0.8 to 1.0 µm. It is made by spraying an acidic solution of SnCl4 onto a glass or quartz substrate and thermally decomposing it. The pure tin oxide film has polycrystalline characteristics, and its conductivity is due to the oxygen defect and the defect structure of interstitial tin atoms, and the doping of oxygen impurities in the cassiterite crystal. Doped tin oxide is more commonly made by adding antimony atoms. As the core of Sn(IV) in the tin oxide lattice is replaced by antimony Sb(III), the density of n-type carriers increases by more than 1020 carrier⁄cm2 can reach a surface resistance of less than 15 Ω⁄sq. These SnO2 films can serve as a completely inert and chemically stable surface in electrochemical research.

The typical spectrum of n-type tin oxide SnO2 coating on the transparent substrate is shown in Fig. 2-1. At 360 nm on the glass substrate, the sharp attenuation of the light transmittance is due to the absorption of the glass itself. Coated on a quartz plate, the outside direction of the optical window can be extended to the wavelength of 300nm. Therefore, the SnO2 optically transparent electrode is only suitable for research in the visible light range.

Fig. 2-1. Transmission spectra of tin oxide coating on different substrates.
Fig. 2-1. Transmission spectra of Sn02 coating on different substrates.
(a) Glass, 3 Ω⁄sq; (b)Vicor, 6 Ω⁄sq; (c) Quartz,12 Ω⁄sq2-1)

Fig. 2-2.  Comparison of current-potential curves in 1 M sulfuric acid solution.
Fig. 2-2. Comparison of current-potential curves of Pt, Au, SnO2 and In2O3 in 1M H2SO4 solution.

The range of potentials applied to the SnO2 coated electrode in aqueous solution is much wider than that applied to the Pt and Au electrodes 2-2). Figure 2-2 is a comparison of the current-potential curves for Pt, Au, SnO2 and In2O3 in a 1 M H2SO4 medium. The current-voltage (i-E) curves can be divided into three main regions, which have been studied extensively for both platinum and gold electrodes. For both electrodes, if the potential is maintained in the hydrogen emitting region for a period of time, the metal film will be irreversibly damaged.

For tin oxide and indium oxide, the charge on the electrode changes monotonously in the entire area. Because the electrode is already an oxide, it is usually the highest oxidation state of the metal. When the potential increases in the positive direction, there is no oxide formation. Obviously, the potential of Faraday current region 1 is as wide as 1.5 V. The boundary of these oxide films in the direction of positive potential zone III′ is for the precipitation of oxygen, and the boundary zone II′ in the direction of negative potential is the oxide reduction to metal The lower valence state of the oxide, hydrogen evolution or surface reduction to the metal state zone II″, so similar to the metal electrode. Therefore, delay electrolysis at a very negative potential will produce irreversible and harmful changes.

It can be seen from Fig. 2-2. that compared with platinum or gold electrodes, SnO2 and In2O3 tin oxide and indium oxide electrodes have a wider redox potential window, however, carriers will be generated at a potential that is more positive than the potential window. Therefore, the rate of any Faraday transfer (such as oxidation of trivalent cerium) under a very positive potential is limited by the charge transfer process of the semiconductor film. In the case of low doping content, the tunneling effect may be the main electron transfer mechanism. In the case of high doping content, other factors including the composition of the surface oxide may be the main factor affecting electron transfer.

The hydrogen evolution zone II on the metal film electrode is replaced by II′, II″ on the semiconductor. The difference between the zone I and II′ is that the small Faraday current may be related to the surface electrode reaction, and its current size is the same as that of the zinc oxide electrode surface Part of the reduction is related to the current, which is only observed in deoxygenated solutions (less than 10 - 5 mol⁄L O2 and increases in alkaline solutions. The potential passes through the region II&pirme; to produce irreversible reduction of semiconductors. Due to some surface degradation such as the dissolution of the layer, zinc oxide coated electrodes are not suitable for research in high alkaline pH solutions. Indium oxide motors behave similarly to tin oxide.

In addition, the polyester sheet coated with metal oxide can also be used as a optically transparent electrode, showing a lower impedance (10 -200 Ω), and it is much cheaper than quartz and glass as a substrate. The flexibility of the sheet makes it suitable for non-planar geometries, and the electrode is relatively stable in water and non-aqueous solvents. Its main disadvantage is still limited due to the absorption of the polyester plastic substrate in the infrared and ultraviolet regions. For applications in the visible light range.

2. 2 In2O3 optically transparent electrode

In2O3 crystal has two crystal types, cubic phase (wurstite type) and triangular phase (corundum type). Theoretically, pure indium oxide film has high resistance and almost no conductivity, but its performance can be effectively adjusted by doping. Doping Sn, Mo, Sb and other elements can obtain N-type indium oxide film materials with ideal electrical properties , Significantly improve the conductivity of the indium oxide film and improve the conductivity of the film. At present, the most studied and most excellent comprehensive performance is mainly tin-doped indium oxide film, namely ITO film (Indium Tin Oxide). Tin-doped indium oxide (ITO) films are widely used as transparent electrodes in optoelectronic devices. Including liquid crystal displays, plasma display panels and solar cells. Although the ITO film shows transparency and conductivity at high room temperature, the latter is severely damaged at high temperature. In practice, when the ITO film is exposed to high temperatures above 300°C, its electrical properties will be affected. The resistance has increased by more than three times.

Indium tin oxide (ITO) films are widely used as transparent electrodes in optoelectronic devices. Including liquid crystal displays, plasma display panels and solar cells. Although the ITO film shows transparency and conductivity at high room temperature, the latter is severely damaged at high temperature. In practice, when the ITO film is exposed to high temperatures above 300 °C, its electrical properties will be affected. The resistance has increased by more than three times.

Fig. 2-3. The light transmittance of the ITO electrode on the quartz substrate is compared with the 0.5 mm and 1 mm quartz substrate.
Fig. 2-3. The light transmittance of the ITO electrode on the quartz substrate is compared with the 0.5 mm and 1 mm quartz substrate.
Fig. 2-4. Comparison of the absorption spectra of the ITO electrode on the glass substrate and the glass substrate.
Fig. 2-4. Comparison of the absorption spectra of the ITO electrode on the glass substrate and the glass substrate.

Fig. 2-3. shows the light transmittance comparison between the ITO electrode on the quartz substrate and the 0.5 mm, 1 mm quartz substrate. The light transmittance of the quartz substrate decreases slightly as its thickness increases. It can be seen that the ITO on the quartz substrate transmits light in the visible light range, and light in the ultraviolet range cannot pass. It is difficult to measure the absorption spectrum in the ultraviolet region.

Figure 2-4 shows the comparison of the absorption spectra of the ITO electrode on the glass substrate and the glass substrate. It can be seen that for the ITO electrode, there is no obvious difference between the absorption spectrum shape of the quartz substrate and the glass substrate.

3. Pt and Au Thin film transparent electrode

The main disadvantages of the above-mentioned metal oxide electrodes are the poor reproducibility and high resistance of the electrodes, and they are only suitable for research in the visible light range. They are later replaced by vacuum sprayed Pt and Au thin films (<5000 angstroms), which can be properly prepared. These films have good mechanical stability, low resistance and reasonable light transmittance.
The film manufacturing steps are as follows.

  1. Strictly clean the transparent substrate surface (glass or quartz), and then ion bombard the substrate surface under vacuum to ensure good adhesion of the metal film on the substrate.
  2. On the substrate coated with Bi and Pb in advance, gold is deposited in a high vacuum, and the thickness of the film is controlled by controlling the deposition time.
  3. Control temperature annealing to change the conductivity and light transmittance of the film.

The deposition of Pt film is similar to this, the difference is that Pt can be sprayed directly on the substrate. The Pt film is mechanically stable. When the film is oxidized or polarized for a few seconds at the potential of hydrogen evolution, or when mercury is electrochemically reduced to the surface of the Pt film, the Pt metal is easily detached from the substrate, but through normal cleaning, heated in concentrated nitric acid for a few seconds, immersed in hydrochloric acid for 12 hours or in contact with metallic mercury, but does not fall off from the electrode surface.

The gold film with similar impedance has higher light transmittance than Pt film in the visible light range. Since gold has lower adhesion on the substrate than Pt, a layer of oxide metal oxide is deposited on the substrate in advance. For example, the bismuth oxide substrate has better mechanical stability and lower impedance while changing the light transmittance. Attempts have also been made to reduce the impedance of the Pt film by using a substrate method, but the film prepared in this way is similar to non-transparent, especially with higher impedance. This impermeability may be due to the difference in refractive index between Pt and metal oxide. The Au film is also not easy to fall off unless it is brushed, immersed in hydrochloric acid, nitric acid, or polarized as early as at particularly positive and negative potentials.

      Fig. 2-5. The effect of annealing time on Pt film (glass substrate).
Fig. 2-5. The effect of annealing time on Pt film (glass substrate)
(a) Absorbance-time, (b) Impedance-time; annealing temperature 560 °C2-3)
      Fig. 2-6. The effect of annealing time on Au films.
Fig. 2-6. The effect of annealing time on Au film (PbO2 on glass substrate).
(a) Absorbance-time, (b) Impedance-time; annealing temperature 200 °C2-4)

The resistance of these films can be reduced by about 10% to 30% by annealing in a muffle furnace. This artificial aging process may cause the discontinuous metal islands on the surface of the substrate to condense into a more continuous metal film, which reduces the resistance and improves mechanical adhesion and light transmittance, especially in the case of gold films. Fig. 2-5. shows the effect of annealing time on Pt film (glass substrate). It can be seen that the impedance decreases sharply at the beginning and reaches a normal value after 1 hour. In most cases, the impedance decreases by about 55% to 66%, and the permeability of the film lightness, gradually decreases with the annealing time. For the Au films on bismuth or lead oxide and ground in Fig. 2-6., the impedance and absorbance both decrease with annealing time.

Fig. 2-7. Absorption spectra of various films on quartz substrate.
      Fig. 2-7. Absorption spectra of various films on quartz substrate.)

Fig. 2-7. shows the absorption spectra of various films. The absorbance of Pt film is approximately linear in the wavelength range of 250 to 800 nm, while the absorption spectrum of gold film has a minimum at a wavelength of about 550 to 650 nm. When these metal films are deposited on a quartz substrate, the optically transparent electrode can be used for research in the ultraviolet region. Except that the overpotential of hydrogen ion discharge on the Pt thin film electrode is slightly lower than that of the bulk Pt electrode, the electrochemical properties of the gold thin film electrode are very similar to the bulk electrode.

2-1) T. Kuwana and N. Winograd in Electroanalytical Chemistry, A. J.Bard. Ed., vol. 7, Marcel Dekker, New York, 1974.
2-2) N.R. Armstrong, A.W.C. Lin, M. Fujihira and T. Kuwana, Anal. Chem., 48, 741 (1976).
2-3) R. Cielinski and N. R. Armstrong, Anal, Chem., 51, 565 (1979)
2-4) W. von Benken and T. Kuwana, Anal. Chem., 42, 1114 (1970).

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