Semiconductor Electrodes . 11 . Electrochemistry at n - Type Ti 02 Electrodes in Acetonitrile Solutions

The electrode response of single crystal n-type Ti02 in acetonitrile was investigated with a large number of electroactive compounds differing widely in their standard potentials. The electrochemical behavior of various compounds can be used to investigate the band structure of the semiconductor electrode. This method is useful in detecting intermediate energy levels between the condubtion and valance bands capable of mediating electron transfer and in assigning energies to these levels. For Ti02 at least one intermediate energy level was located about 1.2 eV below the conduction band. Polycrystalline Ti02 was shown to respond in a similar fashion indicating that the intermediate energy level is characteristic of n-type Ti02. A comparison of the behavior of metal and semiconductor electrodes as a function of the redox potential of various couples and the effect of light on oxidations at the Ti02 electrode is discussed. Semiconductors show promise as electrodes for electrochemical processes.' In part this is because of the energy specificity of semiconductor (SC) reactions. A change in the potential of a SC electrode often results in large changes in the number of charge carriers (electrons or holes) a t the SC-electrolyte interface but in very little modulation of the energies of the valence and conduction bands a t the SC surface through which electron transfer must occur (in the absence of such complicating factors as surface states or energy levels between the conduction and valence bands). Only those solution redox couples with energies near the conduction or valence bands can exchange electrons with the electrode' so that SC electrodes may be used to probe the mechanism of electrode reactions.2 Moreover changes in mechanism leading to different products than those obtained a t metal electrodes may be possible (e.g., a reaction which is ECE a t a metal electrode may be EC a t a SC) with attendant synthetic implication^.^ Recently, the possible exploitation of photoeffects a t SC electrodes for solar energy conversion and storage has been discussed4 and an electrochemical cell utilizing a n-type Ti02 electrode for carrying out the photodecomposition of water to 0 2 and H2 was recently d e m ~ n s t r a t e d . ~ Similar photo effects may also have synthetic implications in organic electrosynthesis perhaps by using sunlight as an energy source to drive the desired electrode reactions. A knowledge of electrode behavior in nonaqueous solvents would be especially useful in this area since water is often not a satisfactory solvent for organic synthesis. By exciting solution species with light instead of the electrode, it may also be possible to study the electrochemistry of excited states with SC electrode^,'.^ because quenching of excited states by the electrode should be of less importance for S C compared with metal electrodes. T o date nearly all experimental studies have been concerned with semiconductor-aqueous solution interface. However, aqueous media are often not suited for studies probing the electrochemical properties of semiconductor electrodes. Adsorption of impurities, even from rigorously purified aqueous systems, can contaminate the electrode surface. Under these circumstances, reproducible background and faradaic currents a t solid electrodes may be difficult to obtain. The adsorbed impurities may also generate surface states, confusing the issue of solution and SC related phenomena.' Water also has a rather limited thermodynamic (1.23 V) and practical (ca. 1.5 V) stability range for electrochemical studies. This severely limits the potential range of stable redox couples available for study and increases the probability of undesirable electron transfer reactions between the solvent and electrode. Finally, there are a relatively small number of simple and reversible redox couples involving soluble reactants and products within the stability range of water. Many of these problems can be eliminated or a t least minimized by using a nonaqueous solvent. Adsorption of impurities is known to occur to a lesser extent in nonaqueous solvents. Dissolution of the electrode into its ionic components a t positive potentials and during illumination, often an undesirable reaction in aqueous solutions, should be less of a problem since solvation of the ions composing the SC lattice, which is a major factor in the dissolution, is usually not as strong as in water. Many aprotic solvents have a larger difference between the energies of solvent oxidation and reduction than does water (e.g., ca. 5 V for acetonitrile). Moreover, a large number of reversible redox couples with a wide range of standard potentials a re available in a number of nonaqueous solvents. With these considerations in mind, we have undertaken the investigation of the electrochemical behavior of n-type TiO2, a wide band gap SC ( E , = 3 eV) in acetonitrile (ACN). Our aim in the present investigation was to probe the relation between band structure and reactivity of solution species and, i f possible, map the band gap region to determine the presence and energies of any intermediate bands or surface states. W e have chosen to do this by studying the electrochemical behavior of a large number of compounds comprising a broad range of formal redox potentials and hence a broad energy range. In a previous investigation by Gomes and Cardon,6 a single redox couple, Fe(CN)64--Fe(CN)63-, was used to probe the band structures of a number of different S C mater iak6 The use of a single couple, however, can probe only a narrow energy range. Where more than one redox couple was used, the energy distribution of the couples was such that all could exchange electrons with the conduction band and the possible participation of surface states was not taken into account.' W e were also interested in studying the effect of light on S C electrode processes of organic species in ACN. Experimental Section Most of the compounds used in this study were obtained from commercial sources and in many cases purified by three to five recrystallizations from the appropriate solvent or solvents. Ru(TPTZ)2(C104)3 (TPTZ = 2,4,6-tripyridyl-s-triazene) and Ru(bipy)3(ClO& (bipy = bipyridine) were prepared as described in the literature.8 All of these compounds have been previously characterized in this laboratory and shown to be of satisfactory purity. However, to ensure that no decomposition or contamination Frank, Bard / Electrochemistry at n-Tj'pe Ti02 Electrodes