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Modification of semiconductor surface with ultrafine metal particles for efficient photoelectrochemical reduction of carbon dioxide
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surface science ELSEVIER
Applied Surface Science 121/ 122 (1997) 301-304
Modification of semiconductor surface with ultrafine metal particles for efficient photoelectrochemical reduction of carbon dioxide R. Hinogami, Y. Nakamura, S. Yae, Y. Nakato * Department q]"Chemistry, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials. Osaka Unit,ersity, 1-3 Machikaneyama, Tovonaka Osaka 560, Japan
Received 3i October 1996; accepted 12 February 1997
Abstract A p-type silicon (p-Si) electrode modified with small metal (Cu, Au and Ag) particles works as an ideal-~pe electrode for photoelectrochemical reduction of carbon dioxide, producing carbon monoxide, methane, ethylene, etc., with a large photovoltage of 0.5 V. It is discussed that two types of surface-structure control on atomic and nanometer-sized levels are important for getting a high efficiency. © 1997 Elsevier Science B.V. Keywords: Silicon; Solar energy conversion; Ideal semiconductor electrode; Nanometer control
1. Introduction Photoelectrochemical (PEC) reduction of carbon dioxide with a semiconductor electrode can be regarded as one of the solar energy conversion technologies and is important from the viewpoint of global environmental problems. The reaction proceeds by essentially the same mechanism as photosynthesis and is of much interest as an artificial model for it. A number of studies have been made on the PEC reduction of CO 2 on semiconductor electrodes [1-3], but the energy conversion efficiency still remains relatively low. The main reason lies in that it is difficult to meet all the requirements for high effi-
ciency, such as high surface catalytic activity, little surface carrier recombination, formation of a high energy barrier, and good matching of energy levels between the semiconductor and solution reactants. The difficulty is serious because the requirements are in general incompatible with each other. We have found, through studies of n-Si based solar cells, that a semiconductor electrode sparsely modified with ultrafine metal particles can meet all the requirements and becomes an ideal-type electrode [4,5]. In the present paper, we report results of application of this principle to the PEC reduction of CO 2 on p-Si [6].
2. Electrode structure and operation mechanism Corresponding author. Tel.: +81-6-8506235; fax: +81-68506236; e-mail: [email protected].
A schematic cross-sectional view of the electrode is shown in Fig. 1. Metal is deposited sparsely in the
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00310-3
302
R. Hinogami et al./ Applied Surface Science 121 / 122 (1997) 301-304
Si02 ,-~ h ~
metal particle Cu etc. CH 4, C2H4, etc. C02 + H + ~,~. e l e c t r o l y t e Fig. 1. Schematic cross section of the metal-particle modified p-Si electrode.
form of small particles. The ideal size of the areas of metal-Si direct contact and their separation are estimated theoretically to be about 5 and 20 nm, respectively [4,5]. Photogenerated holes in p-Si enter into its interior
whereas photogenerated electrons come out to the surface due to the presence of band bending near the p-Si surface. The electrons then transfer into the metal particles and reduce CO 2 at the surface with aid of their catalytic activity. The naked p-Si surface is covered with a thin Si-oxide layer and passivated. Because the major part of the p-Si surface is passivated, it is kept nearly free from recombination centers. Thus, the electrode has both high catalytic activity and a very low surface recombination rate. Another important point is that a high energy barrier is formed. Fig. 2 shows a two-dimensional energy band diagram. The surface band energies of p-Si are modulated by the deposited metal particles, but the modulation is limited spatially only to a region of the same size as the area of the metal-Si direct contact [4,5]. Thus, the effect of the modulation is negligible if the metal particles are much smaller than the width of the space charge layer of p-Si. Consequently, the effective barrier height for metal-particle deposited p-Si ( e 0 ~ ) in a redox solution is nearly equal to that for naked p-Si (e0~),
u~
-
j
Fig. 2. Two-dimensionalenergy band diagram for the metal-particle modified p-Si electrode.
303
R. Hinogami et al. /Applied Surface Science 121 / 122 (1997) 301-304
irrespective of the barrier height at the p-Si/metal contact (echO'), and can increase up to the band-gap (the maximum barrier height) in case where the redox potential of the solution is negative enough.
Potential vs. SCE / V -1 -0.5
- 1.5 I
.
naked p-Si
4. Results The photocurrent ( j ) vs. potential (U) curve for the Cu-particle modified p-Si electrode in a CO 2saturated 0.1 M KHCO 3 electrolyte was reported in our Letter [6]. Similar results were obtained for Auand Ag-particle modified p-Si electrodes. Fig. 3 shows the j - U curve for an Au-particle modified p-Si electrode as compared with those for other related electrodes. For the reduction reaction, the
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.
.
.
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3. Experimental Single crystal p-Si wafers (Shin-Etsu Handotai, CZ, (100), 1.30-1.75 11 cm) were cut into pieces of 1 × 1 c m 2, washed with boiling acetone and water, etched with CP4A (a mixture of HF, HNO 3, CH3COOH and H20, 3 : 5 : 3 : 2 2 in volume) and 12% HF. Ohmic contact was made on the rear side with the I n - G a - Z n alloy. The p-Si piece thus obtained was placed in a Teflon holder (effective area: 0.5 cm2). Metal (Cu, Au and Ag) particles were deposited photoelectrochemically, using acidic solutions (pH 0.5-1.0) of 0.01 M CuSO 4, 0.01 M H2AuCI 4 and 0,01 M AgNO 3. Scanning electron micrographs showed that the metals were deposited in the form of small particles of 0.1-0.5 /.tin in diameter. The photoelectrolysis was performed using an H-type Pyrex-glass cell, the cathode and anode compartments being separated with a cation exchange membrane (Nafion 117). Highly pure CO 2 (99.99%) was bubbled into the stirred catholyte (aqueous 0.1 M KHCO 3 solution) until the air in the cathode compartment was completely substituted for CO 2. Then, the cathode compartment was sealed and the photoelectrolysis was carried out potentiostatically under illumination with a 300 W tungsten-halogen lamp. The reduction products were analyzed with gas chromatography and high performance liquid chromatography.
.
continuous-Au/p-Si (ill. dark) ~. ~..-r....~,,,-~--7~.~ Au "~.5" / (dark) "..,a ~' /
~"
~e
~
_lO~>
particulate-Au/p-Si
(ill.)
,.~ -15
Fig. 3. Currents vs. potential for particulate Au/p-Si and other related electrodes.
more efficient the electrode, the more positive the onset potential of the photocurrent lies. In Fig. 3, the j - U curve for a continuous-Au coated p-Si electrode, prepared by vacuum deposition, is nearly the same as that for an Au metal electrode and hardly affected by illumination, indicating that the Au/p-Si contact is nearly ohmic. On the other hand, the photocurrent onset for the Au-particle modified p-Si lies ca. 0.50 V more positive than that for the Au metal, clearly indicating the beneficial effect of particulate coating. The photocurrent onset for naked p-Si lies at nearly the same spot as that for the Au metal. This is attributed to low catalytic activity of the Si surface for the CO~ reduction. Product analyses for the photoelectrolysis in CO~-saturated 0.1 M KHCO 3 have shown that formic acid (HCOOH) and carbon monoxide (CO) are formed for naked p-Si, apart from hydrogen (H2), whereas CO is mainly produced for the Au metal electrode. For the Au-particle modified p-Si electrode, CO is the main product with no HCOOH, indicating that particulate Au on p-Si acts as an effective catalyst for the PEC reduction of CO 2. For the Cu-particle modified p-Si, hydrocarbons like methane and ethylene were produced [6].
5. Discussion The present results clearly indicate that a p-Si electrode modified with small metal particles shows a high photovoltage and high catalytic activity. The
304
R. Hinogami et al./Applied Surface Science 121 / 122 (1997) 301-304
essential mechanism was explained in the foregoing section. Another notable point is that the reduction levels of CO 2 ( - 1.1 ~ - 1.4 V vs. SCE) are above the bottom of the conduction band at the p-Si surface ( - 1.0 V). Thus, the CO 2 photoreduction is achieved only after upward shift of the p-Si band energies as well as the metal Fermi level by accumulation of photogenerated electrons. This argument implies that the present-type electrode has an ability to shift its own band energies so as to get good energy-level matching between the semiconductor and solution reactants. In order to get an actually high efficiency, two types of control of the surface structure are important: (1) The control on an atomic level for decrease in surface carrier recombination and improvement in the catalytic activity of metal particles, and (2) the control on a nanometer-sized level for obtaining a high barrier height and good energy-level matching
between the semiconductor and solution. Because the present-type electrode has a merit in that the kinds of semiconductors and metals are varied independently, further studies will lead to highly efficient electrodes.
References [1] I. Taniguchi, in: J.O'M. Bockris, P.E. Conway (Eds.), Modern Aspects of Electrochemistry, No. 20, Plenum, 1989, p. 349. [2] J.O'M. Bockris, J.C. Wass, J. Electrochem. Soc. 136 (1989) 2521. [3] S. Ikeda. Y. Saito, M. Yoshida, H. Noda, M. Maeda, K. Ito. J. Electroanal. Chem. 260 (1989) 335. [4] Y. Nakato, K. Ueda, H. Yano, H. Tsubomura, J. Phys. Chem. 92 (1988) 2316. [5] J. Jia, M. Fijitani, S. Yae, Y. Nakato, Electrochim. Acta 42 (1996) 431. [6] R. Hinogami, T. Mori, S. Yae, Y. Nakato, Chem. Lett. (1994) 1725.
surface science ELSEVIER
Applied Surface Science 121/ 122 (1997) 301-304
Modification of semiconductor surface with ultrafine metal particles for efficient photoelectrochemical reduction of carbon dioxide R. Hinogami, Y. Nakamura, S. Yae, Y. Nakato * Department q]"Chemistry, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials. Osaka Unit,ersity, 1-3 Machikaneyama, Tovonaka Osaka 560, Japan
Received 3i October 1996; accepted 12 February 1997
Abstract A p-type silicon (p-Si) electrode modified with small metal (Cu, Au and Ag) particles works as an ideal-~pe electrode for photoelectrochemical reduction of carbon dioxide, producing carbon monoxide, methane, ethylene, etc., with a large photovoltage of 0.5 V. It is discussed that two types of surface-structure control on atomic and nanometer-sized levels are important for getting a high efficiency. © 1997 Elsevier Science B.V. Keywords: Silicon; Solar energy conversion; Ideal semiconductor electrode; Nanometer control
1. Introduction Photoelectrochemical (PEC) reduction of carbon dioxide with a semiconductor electrode can be regarded as one of the solar energy conversion technologies and is important from the viewpoint of global environmental problems. The reaction proceeds by essentially the same mechanism as photosynthesis and is of much interest as an artificial model for it. A number of studies have been made on the PEC reduction of CO 2 on semiconductor electrodes [1-3], but the energy conversion efficiency still remains relatively low. The main reason lies in that it is difficult to meet all the requirements for high effi-
ciency, such as high surface catalytic activity, little surface carrier recombination, formation of a high energy barrier, and good matching of energy levels between the semiconductor and solution reactants. The difficulty is serious because the requirements are in general incompatible with each other. We have found, through studies of n-Si based solar cells, that a semiconductor electrode sparsely modified with ultrafine metal particles can meet all the requirements and becomes an ideal-type electrode [4,5]. In the present paper, we report results of application of this principle to the PEC reduction of CO 2 on p-Si [6].
2. Electrode structure and operation mechanism Corresponding author. Tel.: +81-6-8506235; fax: +81-68506236; e-mail: [email protected].
A schematic cross-sectional view of the electrode is shown in Fig. 1. Metal is deposited sparsely in the
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00310-3
302
R. Hinogami et al./ Applied Surface Science 121 / 122 (1997) 301-304
Si02 ,-~ h ~
metal particle Cu etc. CH 4, C2H4, etc. C02 + H + ~,~. e l e c t r o l y t e Fig. 1. Schematic cross section of the metal-particle modified p-Si electrode.
form of small particles. The ideal size of the areas of metal-Si direct contact and their separation are estimated theoretically to be about 5 and 20 nm, respectively [4,5]. Photogenerated holes in p-Si enter into its interior
whereas photogenerated electrons come out to the surface due to the presence of band bending near the p-Si surface. The electrons then transfer into the metal particles and reduce CO 2 at the surface with aid of their catalytic activity. The naked p-Si surface is covered with a thin Si-oxide layer and passivated. Because the major part of the p-Si surface is passivated, it is kept nearly free from recombination centers. Thus, the electrode has both high catalytic activity and a very low surface recombination rate. Another important point is that a high energy barrier is formed. Fig. 2 shows a two-dimensional energy band diagram. The surface band energies of p-Si are modulated by the deposited metal particles, but the modulation is limited spatially only to a region of the same size as the area of the metal-Si direct contact [4,5]. Thus, the effect of the modulation is negligible if the metal particles are much smaller than the width of the space charge layer of p-Si. Consequently, the effective barrier height for metal-particle deposited p-Si ( e 0 ~ ) in a redox solution is nearly equal to that for naked p-Si (e0~),
u~
-
j
Fig. 2. Two-dimensionalenergy band diagram for the metal-particle modified p-Si electrode.
303
R. Hinogami et al. /Applied Surface Science 121 / 122 (1997) 301-304
irrespective of the barrier height at the p-Si/metal contact (echO'), and can increase up to the band-gap (the maximum barrier height) in case where the redox potential of the solution is negative enough.
Potential vs. SCE / V -1 -0.5
- 1.5 I
.
naked p-Si
4. Results The photocurrent ( j ) vs. potential (U) curve for the Cu-particle modified p-Si electrode in a CO 2saturated 0.1 M KHCO 3 electrolyte was reported in our Letter [6]. Similar results were obtained for Auand Ag-particle modified p-Si electrodes. Fig. 3 shows the j - U curve for an Au-particle modified p-Si electrode as compared with those for other related electrodes. For the reduction reaction, the
.
.
I
.
.
.
.
I
(ill.) " ~
0 =
/
,:it"
3. Experimental Single crystal p-Si wafers (Shin-Etsu Handotai, CZ, (100), 1.30-1.75 11 cm) were cut into pieces of 1 × 1 c m 2, washed with boiling acetone and water, etched with CP4A (a mixture of HF, HNO 3, CH3COOH and H20, 3 : 5 : 3 : 2 2 in volume) and 12% HF. Ohmic contact was made on the rear side with the I n - G a - Z n alloy. The p-Si piece thus obtained was placed in a Teflon holder (effective area: 0.5 cm2). Metal (Cu, Au and Ag) particles were deposited photoelectrochemically, using acidic solutions (pH 0.5-1.0) of 0.01 M CuSO 4, 0.01 M H2AuCI 4 and 0,01 M AgNO 3. Scanning electron micrographs showed that the metals were deposited in the form of small particles of 0.1-0.5 /.tin in diameter. The photoelectrolysis was performed using an H-type Pyrex-glass cell, the cathode and anode compartments being separated with a cation exchange membrane (Nafion 117). Highly pure CO 2 (99.99%) was bubbled into the stirred catholyte (aqueous 0.1 M KHCO 3 solution) until the air in the cathode compartment was completely substituted for CO 2. Then, the cathode compartment was sealed and the photoelectrolysis was carried out potentiostatically under illumination with a 300 W tungsten-halogen lamp. The reduction products were analyzed with gas chromatography and high performance liquid chromatography.
.
continuous-Au/p-Si (ill. dark) ~. ~..-r....~,,,-~--7~.~ Au "~.5" / (dark) "..,a ~' /
~"
~e
~
_lO~>
particulate-Au/p-Si
(ill.)
,.~ -15
Fig. 3. Currents vs. potential for particulate Au/p-Si and other related electrodes.
more efficient the electrode, the more positive the onset potential of the photocurrent lies. In Fig. 3, the j - U curve for a continuous-Au coated p-Si electrode, prepared by vacuum deposition, is nearly the same as that for an Au metal electrode and hardly affected by illumination, indicating that the Au/p-Si contact is nearly ohmic. On the other hand, the photocurrent onset for the Au-particle modified p-Si lies ca. 0.50 V more positive than that for the Au metal, clearly indicating the beneficial effect of particulate coating. The photocurrent onset for naked p-Si lies at nearly the same spot as that for the Au metal. This is attributed to low catalytic activity of the Si surface for the CO~ reduction. Product analyses for the photoelectrolysis in CO~-saturated 0.1 M KHCO 3 have shown that formic acid (HCOOH) and carbon monoxide (CO) are formed for naked p-Si, apart from hydrogen (H2), whereas CO is mainly produced for the Au metal electrode. For the Au-particle modified p-Si electrode, CO is the main product with no HCOOH, indicating that particulate Au on p-Si acts as an effective catalyst for the PEC reduction of CO 2. For the Cu-particle modified p-Si, hydrocarbons like methane and ethylene were produced [6].
5. Discussion The present results clearly indicate that a p-Si electrode modified with small metal particles shows a high photovoltage and high catalytic activity. The
304
R. Hinogami et al./Applied Surface Science 121 / 122 (1997) 301-304
essential mechanism was explained in the foregoing section. Another notable point is that the reduction levels of CO 2 ( - 1.1 ~ - 1.4 V vs. SCE) are above the bottom of the conduction band at the p-Si surface ( - 1.0 V). Thus, the CO 2 photoreduction is achieved only after upward shift of the p-Si band energies as well as the metal Fermi level by accumulation of photogenerated electrons. This argument implies that the present-type electrode has an ability to shift its own band energies so as to get good energy-level matching between the semiconductor and solution reactants. In order to get an actually high efficiency, two types of control of the surface structure are important: (1) The control on an atomic level for decrease in surface carrier recombination and improvement in the catalytic activity of metal particles, and (2) the control on a nanometer-sized level for obtaining a high barrier height and good energy-level matching
between the semiconductor and solution. Because the present-type electrode has a merit in that the kinds of semiconductors and metals are varied independently, further studies will lead to highly efficient electrodes.
References [1] I. Taniguchi, in: J.O'M. Bockris, P.E. Conway (Eds.), Modern Aspects of Electrochemistry, No. 20, Plenum, 1989, p. 349. [2] J.O'M. Bockris, J.C. Wass, J. Electrochem. Soc. 136 (1989) 2521. [3] S. Ikeda. Y. Saito, M. Yoshida, H. Noda, M. Maeda, K. Ito. J. Electroanal. Chem. 260 (1989) 335. [4] Y. Nakato, K. Ueda, H. Yano, H. Tsubomura, J. Phys. Chem. 92 (1988) 2316. [5] J. Jia, M. Fijitani, S. Yae, Y. Nakato, Electrochim. Acta 42 (1996) 431. [6] R. Hinogami, T. Mori, S. Yae, Y. Nakato, Chem. Lett. (1994) 1725.