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 exposed doped by reduction with hydrogen were employed, The electrochemical and photoelectrochemical behavior of a single crystal of anatase were scrutinized for the first time. Properties were compared to those of single-crystal rutile having the (001) face exposed. Impedance analysis 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 conversion efficiencies close to unity at lambda = 300 nm. From the comparison 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 thermodynamically possible on anatase only. Photosensitized electron injection from adsorbed cis-Ru[L(2)(SCN)(2)] (L=2,2'-bipyridyl-4,4'-dicarboxylic acid) proceeds with similar efficiency on both types of electrodes. However, light-induced charge separation on the single-crystal electrodes is about three times less efficient compared with nanoscopic anatase films. Anatase (101) is strikingly more active for electrochemical insertion of Li+ than rutile (001). The diffusion coefficients for Li+ insertion and extraction were estimated to be 2 x 10(-13) and 6 x 10(-13) cm(2)/s, respectively.