Current-doubling effect

Fujishima, A., T. Kato, E. Maekawa and K. Honda (1981). Mechanism of the current doubling effect. I. The ZnO photoanode in aqueous solution of sodium formate. Bulletin of the Chemical Society of Japan, 54,1671-1674. 

This leads to the so-called current-doubling -effect, as shown for p-GaP under illumination in Fig. 33. The origin of this effect is based on the result that only for the first step an excitation by light is required. At n-type electrodes, the complete reaction occurs already in the dark. The hole injection was proved by luminescence measurements in n-GaP [151, 15]. The same result has been obtained with S20i" [150] and for the reduction of quinones [152]. 

In this case, a hole created by light excitation is transferred in the first reaction step. Similar observations have been made for the oxidation of alcohols at CdS-[158], ZnO [161] and Ti02-electrodes [159], [160]. Interestingly, it has been observed with the latter electrode material that the current-doubling effect increased with increasing doping, with respect to the oxidation of CH3OH in... 

In solutions without Cu ", the photocurrent is enhanced upon addition of CH3OH, due to the current-doubling effect (see Sect. 4.5, Fig. 34), i.e. 

Upon addition of Cu -ions, the current-doubling effect disappears. Accordingly, reaction (95b) did not take place. It has been concluded that the radical must be capable of reducing Cu ", i.e. 

Two-step reduction processes and the corresponding current-doubling effects have also been found with Br2 [73], C10 [74] and Br03 [73] at p-GaAs. 

An analogous description can be given for the oxidation of various organic compounds. It is clear that an anodic photocurrent and its doubling can only be observed with n-type electrodes. For Instance, Morrison and co-workers [75,15] have studied the oxidation of formic acid at n-ZnO electrodes and found a current doubling upon addition of HCOOH. The basic photocurrent without any HCOOH was due here to the dissolution of ZnO. Morrison was actually the first to realise the importance of the current-doubling effect. He interpreted this result in terms of the reaction... 

According to this reaction scheme, only one hole created by light excitation is required for the first reaction step. In the second step, an electron is injected from the radical into the conduction band, which proceeds without any light excitation. Upon addition of Cu ions, the current-doubling effect disappears. Accordingly, the second reaction (Eq. 11.33) does not take place anymore. It has been concluded from this result that... 

CH2OH - HCHO + H+ + 2ecB (Current Doubling Effect)... 

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