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Electroluminescence of TiO2 film and TiO2:Cu2+ film prepared by the sol-gel method
~
4
° !,
• i,~
17May 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
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.
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
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
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).
3. Results and discussion
The absorption spectra o f a TiO 2 film coated on an SiO 2 substrate by the s o l - g e l method were investigated. An intense absorption assigned to TiO 2 was observed near 400 nm. The X-ray diffraction pattern of these films indicated that anatase-TiO 2 and rutileTiO 2 coexisted in the TiO 2 and TiO2:Cu 2+ films. The crystalline phases o f the TiO 2 and TiO2:Cu 2+ films changed from anatase-TiO 2 to rutile-TiO 2 upon heating till 700°C. Table 1 shows the intensity ratios of the main peaks in the X-ray diffraction pattern of the TiO 2 and TiO2:Cu 2+ films heated at various temperatures. The crystalline phases of the TiO 2 and TiO2:Cu 2+ films were rutile-TiO 2 when heated above 700°C, anatase-TiO 2 when heated at 500°C, and a mixture of anatase-TiO 2 and rutile-TiO 2 when heated at 600°C, respectively, as shown in Table 1. The EL spectra of the TiO 2 and TiO2:Cu 2+ films formed on titanium electrodes were measured. Although t h e s e e l e c t r o d e s did not show any EL in 0.1 M aqueous solution of Na2SO 4 with imposed potential, they did show EL by applying a cathodic potential in 0.1 M aqueous solution of N a 2 S 2 0 8, which worked as a hole injecter [12]. This fact suggests that
Table 1 X-ray diffraction pattern for the TiO2 and TiO2:Cu 2+ film beat-treated at various temperatures Sample
Heat treatment temperature (°C)
Main peak ratio of XRD pattern (rutile: anatase)
TiO2
500 600 700 800
0 : 100 47 : 53 95 : 5 100:0
TiO2 : Cuz+ (Cu 2+ :5 mol %)
500 600 700 800
0:100 13:87 100:0 100:0
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 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
,d t~
300
400 500 600 700 Wavelength / nm
Fig. 1. Electroluminescence spectrum of (a) anatase-TiO~ film and (b) rutile-TiO2 film in 0.1 M aqueous solution of Na 2SzOs at - 3 V vs. Ag/AgCI.
111
T. Houzouji et a l . / Chemical Physics Letters 254 (1996) 109-113
Anatase .0
TiO 2
Rutile
TiO2
00
1d-(
6~ t~[(~ • S2082"
Ti 3+ level .................J ....................t K~SO4 ""
52082"
•.................................... " ~ 8 0 4 ""
I 3.~Ue
~,
3.11 eV
d(:so
[
" ...................
"~ 8 0 4 2 "
~so4-~ t.....~...~ ,:4.SO42.
~0.2 eV
Fig. 2. Electroluminescence mechanism of TiO 2 films in aqueous solution of Na2S208.
assumed luminescence mechanism is illustrated in Fig. 2. The band gap o f anatase-TiO 2 is = 3.2 eV and that of rutile-TiO 2 = 3.0 eV [13-15]. It is considered that the luminescence center was Ti 3÷ and that the luminescence is due to recombination of an electron trapped by Ti 3+ with a hole in the valence band. If it is assumed that the energy level of Ti 3+ in anatase-TiO 2 is identical to that in rutileTiO 2, it is estimated that the valence band of anatase-TiO 2 is located 0.2 eV lower than that of rutile-TiO 2. In fact, the separation between the peak positions o f the EL is --- 0.2 eV as shown in Fig. 1. This result supports the present assumption. The E L intensity of the TiO 2 film increased slowly with time under cathodic polarization. This suggests that the hole injection from SO 4" formed at the TiO 2 surface into the valence band of TiO 2 is the rate-determining step and the concentration o f SO~- at the interface increases gradually with time.
is five times stronger than that of the TiO 2 film. The EL intensity of the TiO2:Cu 2+ film shows a maxim u m and decreases gradually with time under cathodic polarization. This behavior is different from that of the TiO 2 films and is explained by a decrease in Cu + concentration due to the reduction to copper metal under cathodic potential. Figs. 3 - 5 show the E L behavior for the TiOE:CU 2+ films of different crystal structure. Fig. 3 shows the EL spectra of anatase-TiO2:Cu 2+ films at various electrode potentials. The EL band near 500 nm o f anataseTiO2:Cu 2+ films at - 2 V vs. A g / A g C 1 is assigned to the Cu + monomer. Its intensity is increased under cathodic polarization from - 1 V to - 2 V vs. A g / A g C 1 . By further increasing the applied cathodic polarization, the intensity decreases. Along with the intensity decrease, the peak position shifts to longer wavelength near 600 nm assigned to Cu + dimer
-2V
3.2. Electroluminescence o f T i O 2 :Cu 2 + films d
The EL spectra of TiO2:Cu 2+ films are different from those of TiO 2 films. The EL spectra o f TiO2:Cu 2+ films show two bands near 500 and near 600 nm. Barrie et al. reported that the PL spectrum of the Cu + monomer in A1203:Cu + showed a peak at 450 nm and that of the Cu + dimer in A1203:Cu + a peak at 550 nm [8]. Shortening the C u + - C u + distance would produce a red shift in the dimer emission spectrum [8,9]. F r o m these observations it may be assumed that the band near 500 nm is due to the Cu ÷ monomer and the band near 600 nm to the Cu + dimer. The E L intensity of the TiO2:Cu 2÷ film
.9
-1 V 300
400 500 600 700 Wavelength / nm
Fig. 3. Potential dependence of the electroluminescence spectrum of anatase-TiO2:Cu2+ (1 tool% (Cu2+/TiO2)) films in 0.1 M aqueous solution of Na2S20 s. Potential vs. Ag/AgCI.
112
T. Houzouji et al. / Chemical Physics Letters 254 (1996) 109-113
3.3. Mechanism o f electroluminescence o f T i O 2 : Cu 2 ÷ films
1 300
From the above results, we assume the following mechanism for the EL of TiO2:Cu 2+ films. First, Cu 2+ in the films is reduced to Cu + under cathodic polarization: Cu2÷+e--->Cu*; 400
500 600 700 Wavelength / nm
Fig. 4. Potential dependence of the eleclroluminescence spectrum of (anatase, rutile)-TiO2:Cu2+ (1 mol% (Cu2+/TiO2)) films in 0.1 M aqueous solution of Na2S208. Potential vs. Ag/AgCI.
(3d) l°.
This Cu ÷ reacts with electrons injected into the conduction band of TiO 2 and holes injected into the valence band of TiO 2 from SO 4' in the solution. Thus, (Cu+) * ions in excited states of Cu + are produced: Cu++h++e--o(Cu+)*;
emission, as shown in Fig. 3. The EL spectrum of (anatase, mtile)-TiO2:Cu 2+ films, in which two crystal structures of anatase-TiO 2 and rutile-TiO 2 coexist, shows a band near 500 nm assigned to the Cu + monomer at - 3 V vs. A g / A g C 1 , as shown in Fig. 4. The EL peaks of (anatase, mtile)-TiO 2 :Cu 2 + films assigned to Cu + dimer emission shift gradually to longer wavelength near 600 nm with increasing cathodic potential. On the other hand, the intensity of the EL spectra of rutile-TiO2:Cu 2+ films increases with increasing cathodic polarization, while the peak position near 600 nm assigned to the Cu + dimer hardly change at 0 to - 6 V vs. A g / A g C 1 , as shown in Fig. 5. Thus, the EL spectra of various TiO2:Cu 2+ films change depending on the electrode potential and the crystal structure of the TiO 2 films.
-6V
¢i
I _a
300
4O0
50O
60O
(1)
(3d)a(4s) I.
(2)
( C u + ) " emits a photon to generate luminescence: (Cu+) * ~ Cu++hv.
(3)
The shift to longer wavelength near 600 nm of the EL peak for anatase-TiO2:Cu 2+ films and (anatase, rutile)-TiO2:Cu 2+ films with increasing cathodic potential ( < - 2 V vs. A g / A g C 1 ) is considered to have been caused by the increase in Cu + dimer concentration by increasing cathodic potential. In the case of rutile-TiO2:Cu 2+, Cu + dimer emission is observed already at - 2 V vs. A g / A g C 1 . This suggests that Cu + dimers are formed in rutileTiO2:Cu 2÷ films more easily than in anataseTiO2:Cu 2+ and (anatase, rutile)-TiO2:Cu 2+ films. This seems to have been caused by the different surroundings of Cu 2+ in the TiO2:Cu 2+ films. EL of (anatase, rutile)-TiO2:Cu 2+ films under ac (alternating current, frequency from 5 to 500 Hz) cathodic potential was investigated. The EL spectrum under ac cathodic potential showed a band near 500 nm which is assigned to Cu + monomer and Cu + dimer emission was not observed. This behavior is explained by a milder reduction of Cu 2+ to Cu + under ac potential than dc (direct current) cathodic potential.
70O
Wavelength / nm
Fig. 5. Potential dependence of the electroluminescence spectrum of rutile-TiO2:Cu2+ (1 tool% (Cu2÷/TiO2)) films in 0.1 M aqueous solution of Na2S20 s. Potential vs. Ag/AgCI.
4. C o n c l u s i o n
We prepared the TiO2:Cu 2+ films by the sol-gel method and observed two EL peaks near 500 and
T. Houzouji et al. / Chemical Physics Letters 254 (1996) 109-113
600 nm by controlled heat-treatment (crystal structure) and electrode potential.
References [1] H. Tang, H. Berger, P.E. Schmid, F. Levy and G. Burri, Solid State Commun. 87 (1993) 847. [2] L.G.J. De Haart and G. Blasse, J. Solid State Chem. 61 (1986) 135. [3] L.G.J. De Haart, A.J. de Vries and G. Blasse, J. Solid State Chem. 59 (1985) 291. [4] S.K. Poznyak, V.V. Sviridov, A.I. Kulak and M.P. Samtsov, J. Electroanal. Chem. 340 (1992) 73. [5] Y. Nakato, A. Tsumura and H. Tsubomura, J. Phys. Chem. 87 (1983) 2402.
113
[6] G. Nogami, R. Shiratsuchi and S. Ohkubo, J. Electrochem. Soc. 138 (1991) 751. [7] R.N. Noufi, P.A. Kohl, S.N. Frank and A.J. Bard, J. Electrochem. Soc. 125 (1978) 246. [8] J.D. Barrie, B. Dunn, G. Hollingsworth and J.l. Zink, J. Phys. Chem. 93 (1989) 3958. [9] N. Yamashita, Japan J. Appl. Phys. 30 (1991) 3335. [10] M. Anpo, T. Nomura, T. Kitao, E. Giamello, M. Che and M.A. Fox, Chem. Letters (1991) 889. [11] T. Hashimoto, T. Yoko and S. Sakka, Bull. Chem. Soc. Japan 67 (1994) 653. [12] R.N. Noufi, P.A. Kohl, S.N. Frank and A.J. Bard, J. Electrochem. Soc. 125 (1978) 246. [13] K. Vos, J. Phys. C. 10 (1977) 3917. [14] M.V. Rao, K. Rajeshwar, V.R. Pai Verneker and J. Dubow, J. Phys. Chem. 84 (1980) 1987. [15] M. Gratzel and F.P. Rotzinger, Chem. Phys. Letters 118 (1985) 474.
4
° !,
• i,~
17May 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
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.
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
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
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).
3. Results and discussion
The absorption spectra o f a TiO 2 film coated on an SiO 2 substrate by the s o l - g e l method were investigated. An intense absorption assigned to TiO 2 was observed near 400 nm. The X-ray diffraction pattern of these films indicated that anatase-TiO 2 and rutileTiO 2 coexisted in the TiO 2 and TiO2:Cu 2+ films. The crystalline phases o f the TiO 2 and TiO2:Cu 2+ films changed from anatase-TiO 2 to rutile-TiO 2 upon heating till 700°C. Table 1 shows the intensity ratios of the main peaks in the X-ray diffraction pattern of the TiO 2 and TiO2:Cu 2+ films heated at various temperatures. The crystalline phases of the TiO 2 and TiO2:Cu 2+ films were rutile-TiO 2 when heated above 700°C, anatase-TiO 2 when heated at 500°C, and a mixture of anatase-TiO 2 and rutile-TiO 2 when heated at 600°C, respectively, as shown in Table 1. The EL spectra of the TiO 2 and TiO2:Cu 2+ films formed on titanium electrodes were measured. Although t h e s e e l e c t r o d e s did not show any EL in 0.1 M aqueous solution of Na2SO 4 with imposed potential, they did show EL by applying a cathodic potential in 0.1 M aqueous solution of N a 2 S 2 0 8, which worked as a hole injecter [12]. This fact suggests that
Table 1 X-ray diffraction pattern for the TiO2 and TiO2:Cu 2+ film beat-treated at various temperatures Sample
Heat treatment temperature (°C)
Main peak ratio of XRD pattern (rutile: anatase)
TiO2
500 600 700 800
0 : 100 47 : 53 95 : 5 100:0
TiO2 : Cuz+ (Cu 2+ :5 mol %)
500 600 700 800
0:100 13:87 100:0 100:0
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 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
,d t~
300
400 500 600 700 Wavelength / nm
Fig. 1. Electroluminescence spectrum of (a) anatase-TiO~ film and (b) rutile-TiO2 film in 0.1 M aqueous solution of Na 2SzOs at - 3 V vs. Ag/AgCI.
111
T. Houzouji et a l . / Chemical Physics Letters 254 (1996) 109-113
Anatase .0
TiO 2
Rutile
TiO2
00
1d-(
6~ t~[(~ • S2082"
Ti 3+ level .................J ....................t K~SO4 ""
52082"
•.................................... " ~ 8 0 4 ""
I 3.~Ue
~,
3.11 eV
d(:so
[
" ...................
"~ 8 0 4 2 "
~so4-~ t.....~...~ ,:4.SO42.
~0.2 eV
Fig. 2. Electroluminescence mechanism of TiO 2 films in aqueous solution of Na2S208.
assumed luminescence mechanism is illustrated in Fig. 2. The band gap o f anatase-TiO 2 is = 3.2 eV and that of rutile-TiO 2 = 3.0 eV [13-15]. It is considered that the luminescence center was Ti 3÷ and that the luminescence is due to recombination of an electron trapped by Ti 3+ with a hole in the valence band. If it is assumed that the energy level of Ti 3+ in anatase-TiO 2 is identical to that in rutileTiO 2, it is estimated that the valence band of anatase-TiO 2 is located 0.2 eV lower than that of rutile-TiO 2. In fact, the separation between the peak positions o f the EL is --- 0.2 eV as shown in Fig. 1. This result supports the present assumption. The E L intensity of the TiO 2 film increased slowly with time under cathodic polarization. This suggests that the hole injection from SO 4" formed at the TiO 2 surface into the valence band of TiO 2 is the rate-determining step and the concentration o f SO~- at the interface increases gradually with time.
is five times stronger than that of the TiO 2 film. The EL intensity of the TiO2:Cu 2+ film shows a maxim u m and decreases gradually with time under cathodic polarization. This behavior is different from that of the TiO 2 films and is explained by a decrease in Cu + concentration due to the reduction to copper metal under cathodic potential. Figs. 3 - 5 show the E L behavior for the TiOE:CU 2+ films of different crystal structure. Fig. 3 shows the EL spectra of anatase-TiO2:Cu 2+ films at various electrode potentials. The EL band near 500 nm o f anataseTiO2:Cu 2+ films at - 2 V vs. A g / A g C 1 is assigned to the Cu + monomer. Its intensity is increased under cathodic polarization from - 1 V to - 2 V vs. A g / A g C 1 . By further increasing the applied cathodic polarization, the intensity decreases. Along with the intensity decrease, the peak position shifts to longer wavelength near 600 nm assigned to Cu + dimer
-2V
3.2. Electroluminescence o f T i O 2 :Cu 2 + films d
The EL spectra of TiO2:Cu 2+ films are different from those of TiO 2 films. The EL spectra o f TiO2:Cu 2+ films show two bands near 500 and near 600 nm. Barrie et al. reported that the PL spectrum of the Cu + monomer in A1203:Cu + showed a peak at 450 nm and that of the Cu + dimer in A1203:Cu + a peak at 550 nm [8]. Shortening the C u + - C u + distance would produce a red shift in the dimer emission spectrum [8,9]. F r o m these observations it may be assumed that the band near 500 nm is due to the Cu ÷ monomer and the band near 600 nm to the Cu + dimer. The E L intensity of the TiO2:Cu 2÷ film
.9
-1 V 300
400 500 600 700 Wavelength / nm
Fig. 3. Potential dependence of the electroluminescence spectrum of anatase-TiO2:Cu2+ (1 tool% (Cu2+/TiO2)) films in 0.1 M aqueous solution of Na2S20 s. Potential vs. Ag/AgCI.
112
T. Houzouji et al. / Chemical Physics Letters 254 (1996) 109-113
3.3. Mechanism o f electroluminescence o f T i O 2 : Cu 2 ÷ films
1 300
From the above results, we assume the following mechanism for the EL of TiO2:Cu 2+ films. First, Cu 2+ in the films is reduced to Cu + under cathodic polarization: Cu2÷+e--->Cu*; 400
500 600 700 Wavelength / nm
Fig. 4. Potential dependence of the eleclroluminescence spectrum of (anatase, rutile)-TiO2:Cu2+ (1 mol% (Cu2+/TiO2)) films in 0.1 M aqueous solution of Na2S208. Potential vs. Ag/AgCI.
(3d) l°.
This Cu ÷ reacts with electrons injected into the conduction band of TiO 2 and holes injected into the valence band of TiO 2 from SO 4' in the solution. Thus, (Cu+) * ions in excited states of Cu + are produced: Cu++h++e--o(Cu+)*;
emission, as shown in Fig. 3. The EL spectrum of (anatase, mtile)-TiO2:Cu 2+ films, in which two crystal structures of anatase-TiO 2 and rutile-TiO 2 coexist, shows a band near 500 nm assigned to the Cu + monomer at - 3 V vs. A g / A g C 1 , as shown in Fig. 4. The EL peaks of (anatase, mtile)-TiO 2 :Cu 2 + films assigned to Cu + dimer emission shift gradually to longer wavelength near 600 nm with increasing cathodic potential. On the other hand, the intensity of the EL spectra of rutile-TiO2:Cu 2+ films increases with increasing cathodic polarization, while the peak position near 600 nm assigned to the Cu + dimer hardly change at 0 to - 6 V vs. A g / A g C 1 , as shown in Fig. 5. Thus, the EL spectra of various TiO2:Cu 2+ films change depending on the electrode potential and the crystal structure of the TiO 2 films.
-6V
¢i
I _a
300
4O0
50O
60O
(1)
(3d)a(4s) I.
(2)
( C u + ) " emits a photon to generate luminescence: (Cu+) * ~ Cu++hv.
(3)
The shift to longer wavelength near 600 nm of the EL peak for anatase-TiO2:Cu 2+ films and (anatase, rutile)-TiO2:Cu 2+ films with increasing cathodic potential ( < - 2 V vs. A g / A g C 1 ) is considered to have been caused by the increase in Cu + dimer concentration by increasing cathodic potential. In the case of rutile-TiO2:Cu 2+, Cu + dimer emission is observed already at - 2 V vs. A g / A g C 1 . This suggests that Cu + dimers are formed in rutileTiO2:Cu 2÷ films more easily than in anataseTiO2:Cu 2+ and (anatase, rutile)-TiO2:Cu 2+ films. This seems to have been caused by the different surroundings of Cu 2+ in the TiO2:Cu 2+ films. EL of (anatase, rutile)-TiO2:Cu 2+ films under ac (alternating current, frequency from 5 to 500 Hz) cathodic potential was investigated. The EL spectrum under ac cathodic potential showed a band near 500 nm which is assigned to Cu + monomer and Cu + dimer emission was not observed. This behavior is explained by a milder reduction of Cu 2+ to Cu + under ac potential than dc (direct current) cathodic potential.
70O
Wavelength / nm
Fig. 5. Potential dependence of the electroluminescence spectrum of rutile-TiO2:Cu2+ (1 tool% (Cu2÷/TiO2)) films in 0.1 M aqueous solution of Na2S20 s. Potential vs. Ag/AgCI.
4. C o n c l u s i o n
We prepared the TiO2:Cu 2+ films by the sol-gel method and observed two EL peaks near 500 and
T. Houzouji et al. / Chemical Physics Letters 254 (1996) 109-113
600 nm by controlled heat-treatment (crystal structure) and electrode potential.
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