Stability of Semiconducting TiO2 Photoanodes

As shown in the extensive study by Harris and Wilson ( i) , TiO2 photoanodes tend to undergo aging effects characterized by changes in the action spectra, quantum efficiency, and surface capacitance. Specifics of the aging effect, as well as photoelectrochemical behavior in general, depend on the method of anode Preparation (2-4) 9 Migration of interstitial Ti +a (5) and hydrogen ions (2, 4) has been proposed as contrib-uting to the effect although the detailed mechanisms remain obscure. The situation with hydrogen-loaded specimens is complicated by the observations that hy-drogen in both TiO2 single crystals (6) and sputtered films (7) appears to occupy different sites not all of which are electrochemically active. Reported here are some striking aging effects ob-served with relatively highly doped TiO ~ anodes which have been exposed in the dark to alkaline solutions for somewhat over 100 hr. No detailed explanation for the effects noted is available at this time. Single crystals of TiO2 (National Lead Corporation) oriented in the (110) plane were employed. These were polished with 0.3/ ~ A120 ~ abrasive and then converted to n-type material by heating to 700 ~ in hydrogen for 20 rain. While a number of specimens were so pre-pared, we report on a sample with a carrier concentra-tion of 4.8 • 1019/cm 3 as determined by the Mott-Schottky method. The reliability of the carrier concen-tration was checked with resistivity measurements using the Van derPauw technique (8). These gave a calculated mobility of 1.87 cm2/V-sec at room tem-perature, in agreement ws the value reported in the literature (9). Indium solder and silver-plated copper wires provided ohmic contacts to the samples. The contacts were protected by a coating of silicone rub-ber and a glass tube. The cell was fabricated of Pyrex and provided with a quartz window to admit illumina-tion. A platinu m counterelectrode and a saturated calomel reference electrode (SCE) were employed9 The electrolyte used was 09 NaOH