Semiconductor electrodes thermodynamic stability

Park SM, Barber ME (1979) Thermodynamic stabilities of semiconductor electrodes. Electroanal Chem 99 67-75... 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

The charge carriers may reduce or oxidize the semiconductor itself leading to decomposition. This poses a serious problem for practical photoelectrochemical devices. Absolute thermodynamic stability can be achieved if the redox potential of oxidative decomposition reaction lies below the valence band and the redox potential of the reductive decomposition reaction lies above the conduction band. In most cases, usually one or both redox potentials lie within the bandgap. Then the stability depends on the competition between thermodynamically possible reactions. When the redox potentials of electrode decomposition reactions are thermodynamically more favored than electrolyte redox reactions, the result is electrode instability, for example, ZnO, Cu20, and CdS in an aqueous electrolyte. 

The reactions that are more favored thermodynamically tend to be also favored kineti-cally. Semiconductor electrodes can be stabilized by using this effect. For this purpose, redox couples in the electrolyte are established with the redox potential more negative than the oxidative decomposition potential, or more positive than reductive decomposition potential in such a manner that the electrolyte redox reaction occurs preferentially compared to the electrode decomposition reaction. 

Fig. 9-17. Thermodynamic stability of electrodes of compound semiconductors relative to oxidative and reductive dissolution in the state of band edge level pinning (a) oxidative dissolution is thermodynamically impossible (eFXp.< Cb) oxidative dissolution may occur (er(p.dK)> Ev)> (c) reductive dissolution is thermodynamically impossible (cnn.M> E ), (d) reductive dissolution may occur < Cc) pip. sk) (cpbi. d i) = equivalent Fermi...

Tin (IV) oxide, Sn02, (rutile-type structure), a well-established n-type semiconductor with a wide band gap ( gap = 3.6 eV at 300 K) also has potential applications as a catalyst support,as transparent conducting electrodes,and as a gas sensor.i This material possesses many advantages, such as (i) high thermodynamic stability in air (at least up to 500 °C), (ii) low cost, and (iii) the possibility of the introduction of catalysts or dopants to enhance the sensitivity or selectivity. ... 

The lack of photostability of M-type semiconductor electrodes is a severe problem when using aqueous solutions. Holes excited by light excitation and then driven toward the surface can be either used for the decomposition or are transferred to a redox system. In the case of large band-gap oxides, the stability is thermodynamically controlled i.e. the anodic decomposition occurs only at potentials that are considerably more positive than typical redox reactions. In all other cases, the competition between these two processes is kinetically controlled (Memming, 1990, 1994). In several cases, a decrease of the stability of n-type semiconductors with increasing... 

Figure 14. Diagram illustrating the thermodynamic stability of a semiconductor electrode against decomposition in the solution (a) semiconductor is absolutely stable (b) stable against cathodic decomposition (c) stable against anodic decomposition and (d) unstable.

The reactive semiconductor-electrolyte interface makes stability a major issue in photoelectrochemical solar energy conversion devices, and aspects of thermodynamic and kinetic stability are briefly reviewed here. Thermodynamic stability considerations are based on so-called decomposition levels [56, 57] that are determined by combining the decomposition reaction with the redox reaction of the reversible hydrogen reference electrode. The anodic and cathodic decomposition reactions of a compound semiconductor MX can be written for aqueous solutions as... 

When photoelectrochemical solar cells became popular in the 1970s, many reports appeared concerning the stability, dissolution, and flat-band potential of semiconductors in solutions. These papers investigated parameters such as the energy level of the band edges, which is critical for the thermodynamic stability of the semiconductor and how to determine the potential for the onset of the (photo) electrochemical etching [38-40]. The criterion for thermodynamic stability of a semiconductor electrode in an electrolyte solution is determined by the position of the Fermi level with respect to the decomposition potential of the electrode with either the conduction band electrons or valence band holes E. Under illumination, the quasi-Fermi level replaces the Fermi level. The Fermi level is usually found within the band gap of the semiconductor and its position is not easily evaluated (especially the quasi-Fermi level of minority carriers). Therefore it was found more practical to use the conduction band minimum (Eq) and valence band maximum (Ey) as criteria for electrode corrosion. Thus, a semiconductor will be corroded in a certain electrolyte by the conduction band electrons if its... 

Compound Semiconductors, Electrochemical Decomposition, Fig. 1 Possible thermodynamic electrochemical stability windows for semiconductor electrodes relative to the potentials of the band edges at the electrolyte interface... 

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