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Plasma oxidation of Cu, Ti and Ni and the photoelectrochemical properties of the oxide layers formed
ELSEVIER
Materials Chemistry and Physics 43 (1996) 283-286
Materials
Science Communication
Plasma oxidation of Cu, Ti and Ni and the photoelectrochemical properties of the oxide layers formed Masayoshi
Masui, Tetsuo Muranoi, Ryoichi Urao, Yoshihiro Mohammed Rafiqul Islam, Manabu Takeuchi* School
of Engineering,
Ibaraki
University,
4-12-I
Nakanarusawa,
Hitachi
Momose,
316, Japan
Received 16 December 1994; revised 12 June 1995; accepted 15 June 1995
Abstract Plasma oxidation of Ti, Cu and Ni was carried out using a conventional photoelectrochemical properties of the oxide layers formed were examined. TiOz, Cu,O/CuO, and NiO layer on the Ti, Cu, and Ni surfaces, respectively, solar cells were fabricated using the oxide layers formed as a semiconductor photoresponse under anodic potentials in a methanol aqueous solution, while photoresponse under cathodic potentials. In conclusion, the TiO, layers
semiconductor Keywords:
electrode for a photoelectrochemical
diode-type glow discharge system. Structural and Electron diffraction confirmed the formation of a after the plasma oxidation. Photoelectrochemical electrode. The TiO, electrodes showed moderate the Cu,O/CuO and NiO electrodes showed a weak prepared by plasma oxidation work well as a
cell and are stable in methanol solution.
Anodic potentials; Plasmas; Semiconductor electrodes
1. Introduction The photoelectrolysis of water with single crystalline TiO, was proposed by Fujishima and Honda in 1972, in which water was resolved into hydrogen and oxygen by solar energy using a photoelectrochemical effect with semiconductors [ 11. Studies on direct conversion of solar energy into electricity, instead of resolving water, were also reported [2]. Among the semiconductor materials, such as TiO,, Cu,O [3], CuInSe, [4], etc., which have been examined for semiconductor electrodes, TiO, has become the subject of great attention, mainly for its stability in many liquid electrolytes. Fundamental studies have been made with rutile single crystals to clarify the basic phenomena [ 5,6]. For practical application, however, low cost polycrystalline materials are required for large area electrodes. Many extensive studies with polycrystalline TiO, films and layers have also been carried out,
* Corresponding author. 0254-0584/96/$15.00 G 1996 Elsevier Science S.A. All rights reserved .SSIIIO254-0584(95)01631-4
to determine its usage in the domain of practical application [7-91. Plasma oxidation is a convenient technique to produce oxide layers on the surface of materials at low temperatures [lo]. The oxidation performance depends on the apparatus for plasma treatment and on the operating conditions [ 111.We have studied surface oxidation of Ti, Cu and Ni using a simple plasma reactor and examined the structural and photoelectrochemical properties of the oxide layers formed.
2. Experimental 2.1. Plasma oxidation A conventional diode-type glow discharge system was used in this study. Air was employed as the discharge gas. The discharge voltage and the air pressure were kept constant at 4 kV and 0.5 torr, respectively. Electrode spacing was 12 mm. Metal (Ti, Cu, Ni) pieces (5 mm
284
M. Masui et al. 1 Materials Chemistry and Physics 43 (1996) 283-286 Table 1 Analysis of electron diffraction patterns in Fig. 1 d (ASTM) (A)
Oxide
(W
3.07 2.11 2.78 2.56 1.91 1.67 1.58 1.49
3.02 2.13 2.75 2.53 1.95 1.71 1.59 1.50
cu,o cu,o cue cue cue cue cue cue
(110) (200) (110) (OW, (111) (112) (020) (202)
TiO, rutile 3.27 2.54 2.17 1.73
3.24 2.48 2.18 1.68
TiO, TiO, TiO TiO:
(110) (101) (111) (211)
2.08
NiO NiO NiO NiO NiO
(200) (220) (311) (400) (331)
d (obs.) (A)
cu,o/cuo
NiO 2.13 1.50 1.24
1.08 0.95
1.47 1.25 1.04 0.95
(113)
2.2. Photoelectrochemical measurements Photoelectrodes were fabricated from the plasma oxidized TiO,, Cu,O/CuO or NiO layer by a conventional procedure [ 31. The photoelectrochemical measurements were made using a simple cell with three electrodes. The liquid electrolyte used in this study was 50% methanol aqueous solution. A 500 W Xe lamp was used as the light source, and was projected through a quartz window on the working electrode.
3. Results and discussion 3.1. Characterization of oxide layers formed
Fig. 1. Electron diffraction patterns of oxide layers formed by plasma oxidation for 10 h: (a) Cu,O/CuO, (b) TiO,, (c) NiO.
in diameter, 2 mm thick), with a polished surface, were placed on both the anode and the cathode during the glow discharge. In this paper, only the results for the specimens placed on the anode will be described. Plasma oxidation was carried out for 10 or 20 h. After the plasma oxidation, the surfaces of the specimens were characterized by X-ray diffraction (XRD), electron diffraction and ESCA analyses.
According to the electron diffraction results, formation of TiO,, Cu,O/CuO (mixture of Cu,O and CuO) and NiO was confirmed on the surface of the Ti, Cu and Ni pieces, respectively, after the plasma oxidation, as shown in Fig. 1 and Table 1. However, no peaks corresponding to the oxides were found in the XRD patterns for all the specimens, as shown in Fig. 2. This indicates that the surface oxide layers formed by the glow discharge in this study are very thin. The ESCA spectra for plasma oxidized Ti and Ni layers are shown in Figs. 3 and 4, respectively. The Oi, spectrum for the original Ti layer (Fig. 3(a)) has two peaks. The high energy component corresponds to adsorbed oxygen like OH, while the low energy component corresponds to oxygen bonded with Ti
M. Masui el al. 1 Marerials Chemistry and Physics 43 (1996) 283-286
1
I
535
530
Binding
I
I
40
50
I
I
60
285
Energy
Fig. 4. ESCA O,, spectra for plasma (original), (b) 10 h, (c) 20 h.
I
(eV)
oxidized
Ni layers:
(a) 0 h
70
28
(“1
Fig. 2. X-ray diffraction patterns of metal pieces exposed plasma for 10 h: (a) Cu. (b) Ti, (c) Ni.
to the air
[ 121. As the plasma oxidation period increased (Fig. 3(b), (c)), the former decreased and the latter increased. This result is reasonable, since oxygen plasma attacks the surface of the Ti layer, sputters adsorbed gases and oxidizes the surface. Kim and Davis reported binding energies for the Ni-0 system as follows: for O,, 529.1 (NiO), 530.9 (N&O,), and 532.7 (atomic oxygen adsorbed on Ni) eV [ 131. According to their report, the original Ni piece that we used has atomic oxygen adsorbed on the Ni surface (Fig. 4(a)), and with the plasma oxidation for 10 h, further oxygen was adsorbed to form N&O, and a small amount of NiO (Fig. 4(b)), while the surface of
I
I
Fig. 5. I-V illumination (10 h).
characteristics in dark (solid line) and under white light (dashed line) for the cell with a plasma oxidized Ti layer
Fig. 6. I- V characteristics in dark (solid line) and under white light illumination (dashed line) for the cell with a plasma oxidized Cu layer (10 h).
the specimen plasma oxidized for 20 h consists of N&O, and NiO (Fig. 4(c)). I
I
535
530
Binding
Energy
Fig. 3. ESCA O,, spectra for plasma (original), (b) 10 h, (c) 20 h.
3.2. Photoelectrochemical
(eV)
oxidized
Ti layers:
(a) 0 h
behavior
The I- V characteristics in the dark and under Xe lamp light illumination were measured in the methanol
M. Masui et al. / Materials Chemistry and Physics 43 (1996) 283-286
286
Fig. 7. I- V characteristics in dark (solid line) and under white light illumination (dashed line) for the cell with a plasma oxidized Ni layer (20 h).
The rise and decay of the photocurrent associated with irradiation and cessation of the Xe lamp light was measured at 0.7 V (versus R.E.). The results are shown in Fig. 8 for the cells with the TiO, electrode. The photo-current in the cell with the TiO, plasma oxidized for a longer period is larger than that oxidized for a shorter period. The TiO, layer electrode prepared by this method was very stable as a single crystalline TiO, electrode, while the Cu,O/CuO and NiO layers prepared in this study were unstable in the methanol solution and dissolved in a short period. The spectral dependence of the photocurrent for the cell with the TiO, electrode is shown in Fig. 9. The peak wavelength at approximately 360 nm corresponds to the band gap energy of the TiO,. Although it was confirmed that both the Cu,O/CuO and NiO electrodes show photoresponse to visible light, the spectral response could not be measured due to the weak photoresponse and to its instability in the methanol solution.
4. Conclusions
0
25 Time
50
15
Fig. 8. Current response associated with white light illumination at 0.7 V (versus R.E.) for the cell with a plasma oxidized Ti layer: (a) IO h, (b) 20 h.
Plasma oxidation of Ti, Cu and Ni was carried out using a conventional diode-type glow discharge system. Structural and photoelectrochemical properties of the oxide layers formed were examined. Formation of an oxide layer was confirmed on the surface of the plasma oxidized Ti, Cu and Ni by electron diffraction. The TiO, layers prepared by plasma oxidation work well as a semiconductor electrode for a photoelectrochemical cell and are stable in methanol solution, while both the Cu,O/CuO and NiO layers are unstable in the same solution.
References [II A. Fujishima and K. Honda, Nature (London), 238 (1972) 37. [21 A.B. Ellis, S.W. Kaiser and M.S. Wrighton, J. Am. Chem. Sot., 300
400 Wavelength
500 lnm)
Fig. 9. Spectral response of the photocurrent plasma oxidized Ti layer (20 h).
for the cell with a
98 (1976) 6418. 131 M. Takeuchi, F.L. Weichman, K. Morosawa, M. Kawakami and H. Nagasaka, Appl. Surf Sci., 33/34 (1988) 972. [41 D. Haneman and J. Szot, Appl. Phys. Lett., 46 (1985) 778. [51 A.J. Bard and S.N. Frank, J. Am. Chem. Sot., 97 ( 1975) 7427. [61 J.-L. Desplat, J. Appl. Phys., 47(1976) 5102. [71 A. Fujishima, K. Kohayakawa and K. Honda, J. Electrochem. Sot., 122 (1975) 1487.
solution on cells with plasma oxidized layers. The results are shown for the cells with TiO,, Cu,O/CuO and NiO in Figs. 5, 6 and 7, respectively. The TiO, electrode showed photoresponse under anodic potentials. This indicates that the plasma oxidized TiO, layers work as an n-type semiconductor. The Cu,O/CuO and NiO electrodes showed photoresponse under cathodic potentials, which indicates that the Cu,O/CuO and NiO work as p-type semiconductors.
PI D. Laser and A.J. Bard, J. Electrochem. Sot., 123 (1976) 1027. [91 H. Morisaki, T. Watanabe, M. Iwase and K. Yazawa, Appl. Phys. Lett., 29 (1976) 338.
[lOI Y. Yasuda, S. Zaima, T. Kaida and Y. Koide, Proc. Japan-
China-Korea Trilateral Symp. on Plasma Chemistry, Tokyo, 1990, p. 125. [Ill R.B. Geck, Appl. Surf. Sci., 30 (1987) 32. [121 Y. Momose and T. Yasukawa, Proc. 2nd Japan-Soviet Symposium on Mechanochemistry, Tokyo, 1988, p. 79. 1131 K.S. Kim and R.E. Davis, J. Electron Spectrosc. Relat. Phenom., I (1972/1973) 251.
Materials Chemistry and Physics 43 (1996) 283-286
Materials
Science Communication
Plasma oxidation of Cu, Ti and Ni and the photoelectrochemical properties of the oxide layers formed Masayoshi
Masui, Tetsuo Muranoi, Ryoichi Urao, Yoshihiro Mohammed Rafiqul Islam, Manabu Takeuchi* School
of Engineering,
Ibaraki
University,
4-12-I
Nakanarusawa,
Hitachi
Momose,
316, Japan
Received 16 December 1994; revised 12 June 1995; accepted 15 June 1995
Abstract Plasma oxidation of Ti, Cu and Ni was carried out using a conventional photoelectrochemical properties of the oxide layers formed were examined. TiOz, Cu,O/CuO, and NiO layer on the Ti, Cu, and Ni surfaces, respectively, solar cells were fabricated using the oxide layers formed as a semiconductor photoresponse under anodic potentials in a methanol aqueous solution, while photoresponse under cathodic potentials. In conclusion, the TiO, layers
semiconductor Keywords:
electrode for a photoelectrochemical
diode-type glow discharge system. Structural and Electron diffraction confirmed the formation of a after the plasma oxidation. Photoelectrochemical electrode. The TiO, electrodes showed moderate the Cu,O/CuO and NiO electrodes showed a weak prepared by plasma oxidation work well as a
cell and are stable in methanol solution.
Anodic potentials; Plasmas; Semiconductor electrodes
1. Introduction The photoelectrolysis of water with single crystalline TiO, was proposed by Fujishima and Honda in 1972, in which water was resolved into hydrogen and oxygen by solar energy using a photoelectrochemical effect with semiconductors [ 11. Studies on direct conversion of solar energy into electricity, instead of resolving water, were also reported [2]. Among the semiconductor materials, such as TiO,, Cu,O [3], CuInSe, [4], etc., which have been examined for semiconductor electrodes, TiO, has become the subject of great attention, mainly for its stability in many liquid electrolytes. Fundamental studies have been made with rutile single crystals to clarify the basic phenomena [ 5,6]. For practical application, however, low cost polycrystalline materials are required for large area electrodes. Many extensive studies with polycrystalline TiO, films and layers have also been carried out,
* Corresponding author. 0254-0584/96/$15.00 G 1996 Elsevier Science S.A. All rights reserved .SSIIIO254-0584(95)01631-4
to determine its usage in the domain of practical application [7-91. Plasma oxidation is a convenient technique to produce oxide layers on the surface of materials at low temperatures [lo]. The oxidation performance depends on the apparatus for plasma treatment and on the operating conditions [ 111.We have studied surface oxidation of Ti, Cu and Ni using a simple plasma reactor and examined the structural and photoelectrochemical properties of the oxide layers formed.
2. Experimental 2.1. Plasma oxidation A conventional diode-type glow discharge system was used in this study. Air was employed as the discharge gas. The discharge voltage and the air pressure were kept constant at 4 kV and 0.5 torr, respectively. Electrode spacing was 12 mm. Metal (Ti, Cu, Ni) pieces (5 mm
284
M. Masui et al. 1 Materials Chemistry and Physics 43 (1996) 283-286 Table 1 Analysis of electron diffraction patterns in Fig. 1 d (ASTM) (A)
Oxide
(W
3.07 2.11 2.78 2.56 1.91 1.67 1.58 1.49
3.02 2.13 2.75 2.53 1.95 1.71 1.59 1.50
cu,o cu,o cue cue cue cue cue cue
(110) (200) (110) (OW, (111) (112) (020) (202)
TiO, rutile 3.27 2.54 2.17 1.73
3.24 2.48 2.18 1.68
TiO, TiO, TiO TiO:
(110) (101) (111) (211)
2.08
NiO NiO NiO NiO NiO
(200) (220) (311) (400) (331)
d (obs.) (A)
cu,o/cuo
NiO 2.13 1.50 1.24
1.08 0.95
1.47 1.25 1.04 0.95
(113)
2.2. Photoelectrochemical measurements Photoelectrodes were fabricated from the plasma oxidized TiO,, Cu,O/CuO or NiO layer by a conventional procedure [ 31. The photoelectrochemical measurements were made using a simple cell with three electrodes. The liquid electrolyte used in this study was 50% methanol aqueous solution. A 500 W Xe lamp was used as the light source, and was projected through a quartz window on the working electrode.
3. Results and discussion 3.1. Characterization of oxide layers formed
Fig. 1. Electron diffraction patterns of oxide layers formed by plasma oxidation for 10 h: (a) Cu,O/CuO, (b) TiO,, (c) NiO.
in diameter, 2 mm thick), with a polished surface, were placed on both the anode and the cathode during the glow discharge. In this paper, only the results for the specimens placed on the anode will be described. Plasma oxidation was carried out for 10 or 20 h. After the plasma oxidation, the surfaces of the specimens were characterized by X-ray diffraction (XRD), electron diffraction and ESCA analyses.
According to the electron diffraction results, formation of TiO,, Cu,O/CuO (mixture of Cu,O and CuO) and NiO was confirmed on the surface of the Ti, Cu and Ni pieces, respectively, after the plasma oxidation, as shown in Fig. 1 and Table 1. However, no peaks corresponding to the oxides were found in the XRD patterns for all the specimens, as shown in Fig. 2. This indicates that the surface oxide layers formed by the glow discharge in this study are very thin. The ESCA spectra for plasma oxidized Ti and Ni layers are shown in Figs. 3 and 4, respectively. The Oi, spectrum for the original Ti layer (Fig. 3(a)) has two peaks. The high energy component corresponds to adsorbed oxygen like OH, while the low energy component corresponds to oxygen bonded with Ti
M. Masui el al. 1 Marerials Chemistry and Physics 43 (1996) 283-286
1
I
535
530
Binding
I
I
40
50
I
I
60
285
Energy
Fig. 4. ESCA O,, spectra for plasma (original), (b) 10 h, (c) 20 h.
I
(eV)
oxidized
Ni layers:
(a) 0 h
70
28
(“1
Fig. 2. X-ray diffraction patterns of metal pieces exposed plasma for 10 h: (a) Cu. (b) Ti, (c) Ni.
to the air
[ 121. As the plasma oxidation period increased (Fig. 3(b), (c)), the former decreased and the latter increased. This result is reasonable, since oxygen plasma attacks the surface of the Ti layer, sputters adsorbed gases and oxidizes the surface. Kim and Davis reported binding energies for the Ni-0 system as follows: for O,, 529.1 (NiO), 530.9 (N&O,), and 532.7 (atomic oxygen adsorbed on Ni) eV [ 131. According to their report, the original Ni piece that we used has atomic oxygen adsorbed on the Ni surface (Fig. 4(a)), and with the plasma oxidation for 10 h, further oxygen was adsorbed to form N&O, and a small amount of NiO (Fig. 4(b)), while the surface of
I
I
Fig. 5. I-V illumination (10 h).
characteristics in dark (solid line) and under white light (dashed line) for the cell with a plasma oxidized Ti layer
Fig. 6. I- V characteristics in dark (solid line) and under white light illumination (dashed line) for the cell with a plasma oxidized Cu layer (10 h).
the specimen plasma oxidized for 20 h consists of N&O, and NiO (Fig. 4(c)). I
I
535
530
Binding
Energy
Fig. 3. ESCA O,, spectra for plasma (original), (b) 10 h, (c) 20 h.
3.2. Photoelectrochemical
(eV)
oxidized
Ti layers:
(a) 0 h
behavior
The I- V characteristics in the dark and under Xe lamp light illumination were measured in the methanol
M. Masui et al. / Materials Chemistry and Physics 43 (1996) 283-286
286
Fig. 7. I- V characteristics in dark (solid line) and under white light illumination (dashed line) for the cell with a plasma oxidized Ni layer (20 h).
The rise and decay of the photocurrent associated with irradiation and cessation of the Xe lamp light was measured at 0.7 V (versus R.E.). The results are shown in Fig. 8 for the cells with the TiO, electrode. The photo-current in the cell with the TiO, plasma oxidized for a longer period is larger than that oxidized for a shorter period. The TiO, layer electrode prepared by this method was very stable as a single crystalline TiO, electrode, while the Cu,O/CuO and NiO layers prepared in this study were unstable in the methanol solution and dissolved in a short period. The spectral dependence of the photocurrent for the cell with the TiO, electrode is shown in Fig. 9. The peak wavelength at approximately 360 nm corresponds to the band gap energy of the TiO,. Although it was confirmed that both the Cu,O/CuO and NiO electrodes show photoresponse to visible light, the spectral response could not be measured due to the weak photoresponse and to its instability in the methanol solution.
4. Conclusions
0
25 Time
50
15
Fig. 8. Current response associated with white light illumination at 0.7 V (versus R.E.) for the cell with a plasma oxidized Ti layer: (a) IO h, (b) 20 h.
Plasma oxidation of Ti, Cu and Ni was carried out using a conventional diode-type glow discharge system. Structural and photoelectrochemical properties of the oxide layers formed were examined. Formation of an oxide layer was confirmed on the surface of the plasma oxidized Ti, Cu and Ni by electron diffraction. The TiO, layers prepared by plasma oxidation work well as a semiconductor electrode for a photoelectrochemical cell and are stable in methanol solution, while both the Cu,O/CuO and NiO layers are unstable in the same solution.
References [II A. Fujishima and K. Honda, Nature (London), 238 (1972) 37. [21 A.B. Ellis, S.W. Kaiser and M.S. Wrighton, J. Am. Chem. Sot., 300
400 Wavelength
500 lnm)
Fig. 9. Spectral response of the photocurrent plasma oxidized Ti layer (20 h).
for the cell with a
98 (1976) 6418. 131 M. Takeuchi, F.L. Weichman, K. Morosawa, M. Kawakami and H. Nagasaka, Appl. Surf Sci., 33/34 (1988) 972. [41 D. Haneman and J. Szot, Appl. Phys. Lett., 46 (1985) 778. [51 A.J. Bard and S.N. Frank, J. Am. Chem. Sot., 97 ( 1975) 7427. [61 J.-L. Desplat, J. Appl. Phys., 47(1976) 5102. [71 A. Fujishima, K. Kohayakawa and K. Honda, J. Electrochem. Sot., 122 (1975) 1487.
solution on cells with plasma oxidized layers. The results are shown for the cells with TiO,, Cu,O/CuO and NiO in Figs. 5, 6 and 7, respectively. The TiO, electrode showed photoresponse under anodic potentials. This indicates that the plasma oxidized TiO, layers work as an n-type semiconductor. The Cu,O/CuO and NiO electrodes showed photoresponse under cathodic potentials, which indicates that the Cu,O/CuO and NiO work as p-type semiconductors.
PI D. Laser and A.J. Bard, J. Electrochem. Sot., 123 (1976) 1027. [91 H. Morisaki, T. Watanabe, M. Iwase and K. Yazawa, Appl. Phys. Lett., 29 (1976) 338.
[lOI Y. Yasuda, S. Zaima, T. Kaida and Y. Koide, Proc. Japan-
China-Korea Trilateral Symp. on Plasma Chemistry, Tokyo, 1990, p. 125. [Ill R.B. Geck, Appl. Surf. Sci., 30 (1987) 32. [121 Y. Momose and T. Yasukawa, Proc. 2nd Japan-Soviet Symposium on Mechanochemistry, Tokyo, 1988, p. 79. 1131 K.S. Kim and R.E. Davis, J. Electron Spectrosc. Relat. Phenom., I (1972/1973) 251.