ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL INVESTIGATION OF SINGLE-CRYSTAL ANATASE

Single crystals of TiO2 anatase containing 0.22% of Al and traces of V , Zr, Nb, and La were grown by chemical transport reactions employing TeCl4 as the transporting agent. Electrodes having the (101) face expo sed doped by reduction with hydrogen were employed, The electrochemica l and photoelectrochemical behavior of a single crystal of anatase wer e scrutinized for the first time. Properties were compared to those of single-crystal rutile having the (001) face exposed. Impedance analys is established that the flatband potential of anatase (101) is shifted negatively by 0.2 V with regards to that of rutile (001). Interfacial capacitance measurements under forward bias indicate smaller density bf surface states on anatase. Photoelectrochemical oxidation of water occurs on both rutile and anatase with incident photon-to-current conv ersion efficiencies close to unity at lambda = 300 nm. From the compar ison of U-fb and E(g), it follows that anatase (101) and rutile (001) electrodes differ mainly in the position of the conduction band edge. The complete photoelectrolysis of water to H-2 and O-2 is thermodynami cally possible on anatase only. Photosensitized electron injection fro m adsorbed cis-Ru[L(2)(SCN)(2)] (L=2,2'-bipyridyl-4,4'-dicarboxylic ac id) proceeds with similar efficiency on both types of electrodes. Howe ver, light-induced charge separation on the single-crystal electrodes is about three times less efficient compared with nanoscopic anatase f ilms. Anatase (101) is strikingly more active for electrochemical inse rtion of Li+ than rutile (001). The diffusion coefficients for Li+ ins ertion and extraction were estimated to be 2 x 10(-13) and 6 x 10(-13) cm(2)/s, respectively.