1236 Nuclear Instruments and Methods in Physics Research BS9/60 (1991) 1236-1239 North-Holland Effects of ion implantation on the photoelectrochemical properties of TiO, Nobuyoshi Koshida and Hirotoshi Yabumoto Deporiment of Electronic Engineering. Faculty of Technologv, Tokyo Unruersi<v of Agriculture and Technology. Koganei, Tokyo 184. Japan The possible use of Ion implantation for the control of photoelectrochemical properties has been studied for single-crystalline TiO,. Thermionic ions (Nat. Li+ ) are implanted into TiO, (rutile) at energies of about 10 keV. Effects of ion implantation are evaluated in terms of the optical absorption spectra, the interfacial capacitance-voltage characteristics, and the photoelectrochemical current-voltage characteristics. It is shown that ion implantation is useful both for an enhancement in the absorption of visible light and for a shift of the flatband potential toward the negative side with respect to the reference electrode. Further studies are necessary in order to obtain the photoelectrochemical activation without the influence of irradiation damage. zyxwvutsrqponmlkjihgfedcbaZYXW 1. Introduction To improve the energy conversion efficiency of pho- toelectrochemical solar cells, it is necessary to arrange the optical properties and the band edge energy of semiconductors such that the photoelectronic charge transfer proceeds efficiently at the semiconductor-elec- trolyte interfaces \I]. In the present article. the possible use of ion implan- tation for this purpose has been studied for a single crystal of TiO,. A semiconducting (n-type) oxide TiOz with an optical band gap of 3 eV is known as one of the most promising materials for efficient photoelectro- chemical energy conversion [2]. There is a possibility that implantation of some alkali metal ions into TiO, modifies its optical, electri- cal, and photoelectrochemical properties through the formation of titanium bronze phases [3]. Here we show that low energy implantation of Na+ and Li+ into TiO, is useful both for an enhancement in the optical absorp- tion of visible light and for control of the flatband potential. 2. Experimental The samples used in this experiment were optical- grade, single-crystalline TiO, (rutile). The size was 10 X 10 mm2 in area and 1 mm in thickness. Prior to ion implantation, the samples were reduced by heat treat- ment in vacuum at about 500” C. The resistivity after reduction was about 100 Q cm. Thermionic ions of Naf or Li* were implanted into the reduced specimens at an accelerating voltage of IO kV, in the same way as described previously [4-61. Effects of ion implantation were evaluated in terms of the optical absorption spectra, the interfacial capaci- tance-voltage (Cd-V,,) characteristics in the dark and the current-voltage (j-V,) characteristics in a photo- electrochemical cell with a standard three-electrode con- figuration. The potentiostat-controlled experimental cell is composed of a TiO, working electrode, a Pt counter- electrode and a reference electrode (saturated calomel electrode: SCE). An aqueous solution of Na,SO, buffered with sodium phosphate (pH = 7) was used as an electrolyte. For the measurements of the j-V,, curves, a 500 W Xe lamp was used as the light source. 3. Results and discussion 3.1. Optical properties Fig. 1 shows the optical absorption spectra of TiO, before and after a Na+ implantation. The absorption spectrum before reduction and that of the thermally annealed sample after implantation are also shown in this figure. Before reduction, no appreciable absorption can be seen in the visible-light region of our interest below the main edge (3 eV). In reduced TiO,, the characteristic absorption (peaked at 0.8-0.9 eV) due to oxygen vacancies or interstitial titanium atoms [7] be- comes apparent. and correspondingly, the sample has a pale-blue appearance. This color band is enhanced by ion implantation. An additional absorption m the region of 1-3 eV. observed for the as-implanted specimen, decreased after 0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)