Photoelectrochemical Behavior of Nb-Doped TiO 2 Electrodes Alexei V. Emeline, Yutaka Furubayashi, Xintong Zhang, Ming Jin, Taketoshi Murakami, and Akira Fujishima* Kanagawa Academy of Science and Technology, Kawasaki 213-0012, Japan ReceiVed: September 9, 2005; In Final Form: NoVember 2, 2005 The photoelectrochemical behavior of degenerate Nb-doped TiO 2 (Ti 1-x Nb x O 2 : x ) 0, 0.01, 0.03, 0.06, 0.1) electrodes prepared by pulsed laser deposition on LaAlO 3 (LAO) and SrTiO 3 (STO) was examined, revealing that an increase in Nb concentration causes a significant decay of titania photoactivity. One reason for such behavior may be a Burstein-Moss effect, which leads to a blue shift of the spectral limit of photoactivity. Another reason typical for metal-doped photocatalysts is the increase of the efficiency of charge carrier recombination. Introduction Since the discovery of the photoelectrochemical splitting of water on a titania electrode by Honda and Fujishima some two decades ago, 1 major research efforts have focused on the problem of increasing the efficiency of photoelectrochemical processes. Metal and nonmetal ion doping of TiO 2 have been studied extensively in photoelectrochemistry and in photo- catalysis to achieve higher activities and spectral sensitivities of titania. 2-6 Despite the relatively long history of studies of metal-doping modification of titania, only a few reports might be considered as having had some success thus far. The major reason for this lack of success is probably the higher efficiency of electron-hole recombination in metal-doped titania as a result of the formation of new types of defects. 2-4,7 New methods of synthesis and major properties of new heavily Nb-doped TiO 2 materials have been reported recently. 8 It was demonstrated that extensive doping of titania with Nb turned an otherwise n-type semiconductor into a degenerate semiconductor with high metallic-like conductivity. This was explained in terms of formation of an impurity band overlapping with the conduction band of titania. The present article report our recent results of an experimental photoelectrochemical study of the same set of heavily Nb-doped Ti 1-x Nb x O 2 films (where x ) 0.01, 0.03, 0.06, and 0.1; film thickness, 1.2 μm). Experimental Section The Nb-doped titania samples were prepared by pulsed laser deposition on LaAlO 3 (LAO) and SrTiO 3 (STO) by a procedure described elsewhere. 8 All Nb-doped samples belong to the class of degenerate semiconductors with a metallic type of conductiv- ity and concentration of free electrons in the conduction band 10 21 -10 22 cm -3 , directly proportional to the content of Nb ions in samples. 8 Sample plates (5 × 5 mm 2 ) were used to make electrodes with a working area of about 15 mm 2 . Measurements were carried out with a standard three-electrode system that included an SCE reference electrode and a Pt counter electrode connected to a Hokuto Denko HZ-5000 potentiostat. The electrolyte solution (0.1 M KCl) was prepared with reagent grade chemical and Milli-Q water; it was mechanically stirred and purged with nitrogen prior to and during the experiments. Irradiation of the sample electrode was carried out through the cell quartz-window with either a 150-W Xe lamp (Hayashi LA- 251Xe) or in the case of spectral measurements with a 300-W Xe lamp (Oriel MLH-300) combined with a monochromator (Oriel Corner Stone). In the latter case, the spectral half-widths of the band path of the actinic light were in the range of ca. 6-8 nm. Light intensity was measured with a Nova power meter. Results and Discussion Figure 1 illustrates the stationary photocurrent-voltage dependencies for the set of samples examined. As evident from the experimental curves, Nb doping causes a drastic decay of the photocurrent despite the increase in the conductivity of the samples. Miyagi and co-workers 7 showed that titania doped with Nb leads to the formation of new defects and deep electron traps, which they assumed could serve as efficient recombination centers that caused complete loss of photocatalytic activity of the modified titania samples. However, the recombination rate is expected to be faster even if the concentration of recombina- tion centers were to remain the same because of the high concentration of electrons in the conduction band, which scales linearly with the Nb concentration. In fact, at such high values of electron concentration, band-to-band recombination with * Address correspondence to this author. E-mail: fujishima@ newkast.or.jp. Figure 1. Experimental dependencies of stationary photocurrent on applied voltage for Ti1-xNbxO2 grown on LAO electrodes: (1) x ) 0; (2) x ) 0.01; (3) x ) 0.03; (4) x ) 0.06; and (5) x ) 0.1. 24441 J. Phys. Chem. B 2005, 109, 24441-24444 10.1021/jp055090e CCC: $30.25 © 2005 American Chemical Society Published on Web 12/06/2005