Semiconductor Electrodes: XIV . Electrochemistry and Electroluminescence at n‐Type in Aqueous Solutions

Abstract

The electrochemical behavior of single crystal n-TiO2 was investigated in aqueous solutions. Charge transfer at the semiconductor/electrolyte interface was probed with several redox couples with standard redox potentials spanning a wide range. Most reductions occurred at potentials close to Vfb, the flatband potential and the current-voltage behavior suggested involvement of surface states in the charge transfer process. Evidence for the adsorption of phosphate ion and its effect on the reduction of Fe (CN)68-is also presented. Electroluminescence was observed during reduction of S2Os 2-at wavelengths longer than 700 nm, suggesting the existence of intermediate levels.The electrochemical behavior of n-TiO2 has been the subject of several recent investigations including a previous report in this series (1), which utilized an aprotic solvent, acetonitrile (ACN), and which postulated that electron transfer can occur via intermediate levels or surface states within the bandgap region. Studies in aqueous solutions with TiO2 (2) and other semiconductors (3, 4) have also investigated the mechanism of charge transfer of added redox couples at the semiconductor/solution interface. Although not as many Nernstian, one-electron redox couples are available in aqueous, as compared to nonaqueous solutions, the relative location of the energy levels of TiO2 at the semiconductor/solution interface (i.e., the fiatband potential) can be changed in aqueous solutions by relatively large amounts by changing the solution pH. This provides another variable for investigating the band structure. This approach has previously been used to study SiC (4) and SnO2 (5).In the present study the semiconductor/solution interface was investigated by several techniques. Electron transfer reactions of couples, including some with standard redox potential well positive of those used in a previous study (2), were investigated at different pH's. To probe the existence and role of intermediate levels within the bandg~ap, electroluminescence of the n-TiO2 semiconductor caused by electron-hole recombination from minority carrier injection from the electrolyte was studied. Hole injection into n-type semiconductors leading to radiative recombination has previously been demonstrated for GaP (6), SnO2, ZnO, CdS, and GaAs (7).In addition a study of the space charge capacitance, Csc, as a function of potential, E, was undertaken in an * Electrochemical Society Student Member.

Luminescent Properties of Semiconductor Photoelectrodes

“…Electrode decomposition and electrolyte redox reactions are also non radiative but result from separation of electron-hole pairs, as described above. at extreme potentials or in strongly oxidizing or reducing media (38)(39)(40)(41)(42)(43)(44). These are frequently transient effects and not appropriate for sustained optical energy conversion.…”

A novel synthesis of Zn2SiO4 thin film on n-type ZnO semiconductor electrode and its electrochemical luminescence under the anodic polarization

“…If the Zn 2 SiO 4 thin film can be modified on the semiconductor surface readily and directly, the ECL study will contribute to the information by using it as the semiconductor electrode [10]. In this paper, we have developed a novel synthesis method of the Zn 2 SiO 4 thin film and studied the ECL properties of the Zn 2 SiO 4 thin film under the anodic polarization.…”

A Photoelectrochemical Solar Cell Consisting of a Cadmium Sulfide Photoanode and a Ruthenium–2,2′‐Bipyridine Redox Shuttle in a Non‐aqueous Electrolyte

“…[8,11] Thus,approaches to developing aPEC PVs with ahigh output voltage for direct connection to aw ater electrolyzer have rarely been reported. [17][18][19][20] While CdS photoanodes are well known to degrade by photocorrosion in aqueous media, [21,22] aremarkable durability can be expected in anonaqueous medium. [15,16] Disadvantages of direct water splitting by the PEC and PC methods are:1)the semiconductors have to be able to function in aqueous solutions without any degradation;2 )the energy levels of the conduction band minimum (CBM) and valence band maximum (VBM) have to exceed the hydrogen and oxygen evolution potentials,respectively;and 3) photoexcited minority carriers have to perform the multi-electron processes of hydrogen (two-electron) and oxygen (four-electron) evolution at the interface.Because of these requirements,t he efficiencyo fP EC or PC water splitting has remained limited, and only afew examples have been reported for PEC and PC water splitting with one-step excitation by visible light irradiation.…”