06 TiO2 film and TiO2

1. Introduction
Electroluminescence (EL) is the reverse process of photoelectro-conversion. It is interesting in view of the application to optical devices and optical communication. Many studies of TiO 2 as photocatalyst have been carried out from the point of view of electron transfer at the interface. The photoluminescence (PL) and the EL of TiO 2 were studied to investigate the surface states involved in charge transfer at the T~O2-electrolyte interface [1-7]. It has been reported that the PL spectrum of TiO 2 showed a band at 5 0 0 - 6 0 0 nm, which was assigned to the oxide lattice defects (Ti 3÷) [1,3,4]. Nakato et al. reported that the EL of TiO 2 near 850 nm was due to an oxidative surface species which acted as an intermediate in the photo-oxidation reaction of water at the TiO2-solution interface [5]. Poznyak et al. studied the EL and the PL of TiO 2 in detail and reported the visible EL of TiO 2 near 650 nm [4]. They proposed that the EL of TiO 2 near 650 nm was due to
~
4
° !,
• i,~
17May 1996
Hale Waihona Puke ELSEVIERCHEMICAL PHYSICS LETTERS
Chemical Physics Letters 254 (1996) 109-113
Electroluminescence of TiO 2 film and TiOa:Cu 2÷ film prepared by the sol-gel method
Tomoaki Houzouji, Nobuhiro Saito, Akihiko Kudo, Tadayoshi Sakata
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226, Japan
Received 27 December 1995; in final form 2 February 1996
Abstract
The electroluminescence (EL) of TiO 2 films and TiO 2 films doped with Cu 2+ (ZiO2:Cu 2+) prepared by the sol-gel method was studied. The EL spectra of anatase-TiO 2 film showed a band at about 520 nm and that of rutile-TiO 2 film a band at about 570 rim. The EL spectra of TiO2:Cu2+ film showed two bands at about 500 and 600 nm assigned to Cu + monomer and Cu ÷ dimer, respectively. The EL spectra of TiO2:Cu2+ films changed remarkably with the crystal structure and electrode potential.
luminescence centers produced by the penetration of an alkali metal cation into the near-surface region of the TiO 2 electrode [4]. Barrie et al. studied the PL of Na+-/3"-A1203 doped with Cu+(A1203:Cu +) [8]. Yamashita studied the PL of MgS, CaS and BrS doped with Cu+(MgS:Cu ÷, CaS:Cu ÷, BrS:Cu ÷) [9]. Anpo et al. studied the PL of SiO 2 doped with Cu÷(SiO2:Cu ÷) [10]. They reported two emission bands due to the Cu ÷ monomer and to the Cu ÷ dimer [8-10]. In the present Letter, we would like to report the visible EL of TiO 2 film prepared by the sol-gel method and the EL of TiO 2 film doped with Cu 2÷ as luminescence center (hereafter abbreviated as TiO2:Cu 2+ ).
2. Experimental
The TiO 2 films were prepared by the sol-gel method as reported by Hashimoto et al. [11]. As a
000%2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PH S0009-2614(96)00284-9
Table 1 X-ray diffraction pattern for the TiO2 and TiO2:Cu 2+ film beat-treated at various temperatures Sample Heat treatment temperature (°C) 500 600 700 800 500 600 700 800 Main peak ratio of XRD pattern (rutile: anatase) 0 : 100 47 : 53 95 : 5 100:0 0:100 13:87 100:0 100:0
The EL spectra of TiO 2 films in 0.1 M aqueous solution of Na2S208 are shown in Fig. 1. The EL spectrum of anatase-TiO 2 is different from that of rutile-TiO 2 as shown in Fig. 1. The EL spectra of anatase-TiO 2 film show a band near 520 nm (--- 2.4 eV) and that o f rutile-TiO 2 film a band near 570 nm (--- 2.2 eV). These spectra are different from the EL spectrum observed near 650 nm as reported by Poznyak et al. [4], but agree with the reported PL spectrum o f TiO 2 [1-4]. This suggests that the EL of TiO 2 films occurs in the bulk, probably due to the oxide lattice defects (Ti 3÷) of the oxide [1,3,4]. An
3. Results and discussion
110
T. Houzouji et al. / Chemical Physics Letters 254 (1996) 109-113
coating solution, T i ( O C 3 H ~ ) 4 - i - C 3 H 7 O H - H 2 0 HN(CH2CH2OH) 2 in a 1 : 2 0 : 1 : 1 molar ratio was used. The coating solution was allowed to stand at 25°C for 24 h prior to use. After a supporting substrate was dipped into the coating solution, it was heated at 500, 600, 700 or 800°C in air for 10 min. This procedure was repeated five times. For the preparation of the TiO2:Cu z+ films, a given amount of CuC12(Cu2+/TiO2 = 0 . 1 - 1 0 mol%) was added to the above coating solution and the further operation was carried out in a similar manner as above. The crystalline phases of the TiO 2 and TiO2:Cu 2+ films were identified by X-ray diffraction. The absorption spectra of the TiO 2 and TiO2:Cu 2÷ films were measured in the wavelength region from 200 to 800 nm by a U V - v i s i b l e spectrophotometer (UV-2100 PC, Shimazu). The EL spectra of the TiO 2 and TiO2:Cu 2+ films were measured in the wavelength region from 250 to 800 nm by a fluorescence analyzer (Fluoro Max TM, Spex Industries Inc. DM3000).
TiO2
TiO2 : Cuz+ (Cu 2+ :5 mol %)
the EL o f TiO 2 and TiO2:Cu 2+ films needs supply of electrons and injection of holes.
3.1. Electroluminescence of TiO 2 films