Photoelectrochemical Electrode Kinetics

Photocurrent transient methods i o-iss intensity modulated photocurrent spectroscopy (IMPS) ggjj established as good techniques 

1 day ( ) and 6 days (--) after preparation. (Reprinted from ref.[44] with permission 

The back reactions corresponding to the detrapping from surface defects (A i) and dissociation of electron-hole pairs to a trapped minority carrier and a majority carrier (k-2) were energetically not favored at room temperature (kT = 0.048 eV as compared to a frontier orbital gap of about 1.65 eV) and were therefore assumed to be negligible in first approximation. Electron injection 

As mentioned above, the anodic charging currents and cathodic discharging currents at FiePcZn in aqueous Ce / solution were found to be independent of the electrolyte concentration and could therefore not be analyzed by Equation (10.10), although /m did show a saturation behavior comparable to PcZn. 

Measured Constant PeZn PeZn FigPcZn FlsPcZn 

---

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

Prior to the 1970 s, electrochemical kinetic studies were largely directed towards faradaic reactions occurring at metal electrodes. While certain questions remain unanswered, a combination of theoretical and experimental studies has produced a relatively mature picture of electron transfer at the metal-solution interface f1-41. Recent interest in photoelectrochemical processes has extended the interest in electrochemical kinetics to semiconductor electrodes f5-151. Despite the pioneering work of Gerischer (11-141 and Memming (15), many aspects of electron transfer kinetics at the semiconductor-solution interface remain controversial or unexplained. 

The flat band potential in electrochemistry of semiconductors is equivalent to the zero-charge potential in electrochemistry of metals (more exactly, to the potential of the zero free charge—see Frumkin, 1979). As with the zero-charge potential in the electrochemistry of metals, the flat band potential is rather important in the kinetics of both dark and photoelectrochemical reactions at semiconductor electrodes. Several methods, including photoelectrochemical ones, have been developed to determine q>tb (see Section 7). 

At present there is a sufficiently complete picture of photoelectrochemical behavior of the most important semiconductor materials. This is not, however, the only merit of photoelectrochemistry of semiconductors. First, photoelectrochemistry of semiconductors has stimulated the study of photoprocesses on materials, which are not conventional for electrochemistry, namely on insulators (Mehl and Hale, 1967 Gerischer and Willig, 1976). The basic concepts and mathematical formalism of electrochemistry and photoelectrochemistry of semiconductors have successfully been used in this study. Second, photoelectrochemistry of semiconductors has provided possibilities, unique in certain cases, of studying thermodynamic and kinetic characteristics of photoexcited particles in the solution and electrode, and also processes of electron transfer with these particles involved. (Note that the processes of quenching of photoexcited reactants often prevent from the performing of such investigations on metal electrodes.) The study of photo-electrochemical processes under the excitation of the electron-hole ensemble of a semiconductor permits the direct experimental verification of the applicability of the Fermi quasilevel concept to the description of electron transitions at an interface. 

Most detailed studies of water photodissociation on SrTi03 and Ti02 have concentrated on photoelectrochemical cells (PEC cells) operating under conditions of optimum efficiency, that is with an external potential applied between the photoanode and counterelectrode. We have become interested in understanding and improving reaction kinetics under conditions of zero applied potential. Operation at zero applied potential permits simpler electrode configurations (11) and is essential to the development of photochemistry at the gas-semiconductor interface. Reactions at the gas-sold, rather than liquid-solid, interface might permit the use of materials which photocorrode in aqueous electrolyte. 

A significant drawback of metals for photoelectrochemical applications lies in their ability to efficiently quench excited states via energy transfer processes, as discussed below. Direct detection of photosensitized electron transfer to or from a metal electrode surface has been observed [30]. However, unlike dye-sensitized semiconductor systems, little examination of the kinetics of such systems has yet been undertaken. 

Consider the photoelectrochemical kinetics as a function of potential at photocurrent densities that are 1% of the limiting photocurrent and less. What will be the nature of the photocurrent electrode potential in these regions Explain your reasoning. (Bockris)... 

Owing to its extraordinary chemical stability, diamond is a prospective electrode material for use in theoretical and applied electrochemistry. In this work studies performed during the last decade on boron-doped diamond electrochemistry are reviewed. Depending on the doping level, diamond exhibits properties either of a superwide-gap semiconductor or a semimetal. In the first case, electrochemical, photoelectrochemical and impedance-spectroscopy studies make the determination of properties of the semiconductor diamond possible. Among them are the resistivity, the acceptor concentration, the minority carrier diffusion length, the flat-band potential, electron phototransition energies, etc. In the second case, the metal-like diamond appears to be a corrosion-stable electrode that is efficient in the electrosyntheses (e.g., in the electroreduction of hard to reduce compounds) and electroanalysis. Kinetic characteristics of many outer-sphere... 

We first discuss current-potential curves in supporting electrolyte solutions [38, 39] as a base characteristic of diamond electrodes. It is these curves that are the background against which the kinetic, impedance, photoelectrochemical, electro-analytical properties of diamond electrodes manifest themselves. 

Before considering the kinetics of photoelectrochemical reactions, it is useful to compare electron transfer at metal electrodes with electron trans-... 

In an IMPS experiment the light intensity is modulated to produce an ac photocurrent that is analysed to obtain kinetic information. An alternative approach is to modulate the electrode potential while keeping the illumination intensity constant. This method is called PhotoElectrochemical Impedance Spectroscopy (PEIS). 

---