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Cite this: Chem. Commun., 2014, 50, 1950 Received 2nd September 2013, Accepted 17th December 2013 DOI: 10.1039/c3cc46701e
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Decomposition of hydrazine by an organic fullerene–phthalocyanine p–n bilayer photocatalysis system over the entire visible-light region† Toshiyuki Abe,*a Naohiro Taira,a Yoshinori Tanno,a Yuko Kikuchia and Keiji Nagaib
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An organic p–n bilayer photocatalysis system successfully induced the oxidative decomposition of hydrazine (N2H4) into N2 and simultaneously yielded H2 from H+. This demonstrates the first instance of the stoichiometric decomposition of N2H4 under visible-light irradiation.
Solar fuel has attracted attention as an alternative energy source to fossil fuels and as a means of restricting global warming. Molecular hydrogen (H2) is a typical fuel and a source of potential clean energy, which can contribute to a sustainable society. Since the report of a photoelectrochemical water-splitting system using a UV-responsive TiO2 by Honda and Fujishima,1 several types of visible-light-responsive photocatalysts have been designed and fabricated by band engineering,2–6 aiming for the efficient utilization of solar energy with a high activity. However, there are few examples of inorganic-semiconductor-based photocatalysts that are capable of H2 evolution, even in the nearinfrared region;7–9 moreover, band-engineered photocatalysts usually become inactive, because a visible-light response of the photocatalyst may decrease its activity because of band-gap reduction. Distinct from the above-mentioned conventional approach to inorganic semiconductors, photocatalysis systems featuring an organic p–n bilayer have recently been revealed.10–13 In the organophotocatalysis system, a series of photophysical events (i.e. formation of an exciton by light absorption, charge separation at the p–n interface and carrier conduction through each layer) within the bilayer can occur in a manner similar to the corresponding photovoltaic system.14–16 However, the collection of photogenerated carriers (i.e. electrons and holes) can take place through reduction and oxidation on the surface
of the n- and p-type semiconductors, respectively,17–20 thus resulting in photocatalysis over the full visible-light region.10–13 In this study, a photocatalysis system of a fullerene (C60, an n-type semiconductor)/zinc phthalocyanine (ZnPc, a p-type semiconductor) bilayer (Scheme 1) was utilized for the decomposition of hydrazine (N2H4). N2H4 is a carbon-free and a H-storage material, because one molecule of N2H4 can produce double the amount of H2 during the multi-electron transfer oxidation of N2H4 to N2. However, only a few examples of TiO2 photocatalysts have been reported for the photocatalytic decomposition of N2H4.21,22 This system presents a novel instance of N2H4 decomposition in the entire range of visible-light irradiation. The organic p–n bilayer of C60 and ZnPc was prepared by vapour deposition (pressure, o1.0 10 3 Pa; deposition speed, ca. 0.03 nm s 1), and an indium-tin oxide (ITO)-coated glass plate was used as the base material. The organic bilayer was composed of C60 coated on ITO, and ZnPc coated on top of the C60 layer (denoted as ITO/C60/ZnPc). Furthermore, a Nafion membrane (denoted as Nf) was used in combination with the bilayer, and a 1 mm-thick Nf was loaded on the ZnPc surface. The resulting photocatalytic device is
a
Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan. E-mail: [email protected] b Chemical Resources Laboratory, Tokyo Institute of Technology, Suzukake-dai, Midori-ku, Yokohama 226-8503, Japan † Electronic supplementary information (ESI) available: Experimental details, structural information, illustration of a twin-compartment cell employed for photoelectrolysis, cyclic voltammograms, photoelectrolysis data for N2H4 decomposition with respect to applied potentials, photocatalysis data for N2H4 decomposition with respect to light intensity and film thickness of the photocatalyst device C60/ZnPc/Nf, and the energy diagram for N2H4 decomposition in the present photocatalysis system. See DOI: 10.1039/c3cc46701e
1950 | Chem. Commun., 2014, 50, 1950--1952
Scheme 1 Schematic illustration of a twin-compartment cell employed for photocatalysis experiments and the structures of chemicals employed in this study.
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abbreviated as ITO/C60/ZnPc/Nf. A cell made up of twin compartments separated by a salt bridge was utilized for photocatalytic experiments (see Scheme 1). Other experimental details and structural information (Fig. S1) are provided in the ESI.† ITO/C60/ZnPc and ITO/C60/ZnPc/Nf exhibited photoanodic characteristics due to the oxidation of N2H4 (Fig. S2, ESI†), and were supported by our previous studies (vide supra).17–19 Furthermore, Fig. S2 (ESI†) also reveals that ITO/C60/ZnPc/Nf is superior to ITO/ C60/ZnPc; in other words, Nf is considered to function as an absorbent for N2H4. The details are interpreted in the ESI.† By applying ITO/C60/ZnPc/Nf to a photoanode, potentiostatic electrolysis was conducted at a potential of +0.3 V (vs. Ag/AgCl (sat.)) for 1 h (the cell employed in this study is illustrated in Scheme S1, ESI†). The typical results are shown in Table 1, and the electrolysis data of the controlled systems are also included. The oxidative production of N2 from N2H4 was confirmed for both ITO/C60/ZnPc/Nf (entry 1) and ITO/C60/ZnPc (entry 2), simultaneously yielding H2 from H+ at the Pt counter. In both cases, the Faradaic efficiency for N2 formation was estimated to be >90% when assuming the four-electron-transfer oxidation of N2H4 (eqn (1)). N2H4 - N2 (oxidation product) + 2H2 (reduction product) (1) As for the other possible pathway, the two-electron-transfer oxidation of N2H4 to diazene (N2H2) can be considered (eqn (2)), which can be further followed by the spontaneous decomposition of N2H2 into N2 and H2 (eqn (3)).22,23 N2H4 - N2H2 + H2
(2)
N2H2 - N2 + H2
(3)
However, when N2H4 decomposes through eqn (2) and (3), the Faradaic efficiency for N2 formation is estimated to be ca. 50%; furthermore, in compartment A, generation of H2 was not confirmed. Therefore, based on the above-mentioned consideration and evidence, the stoichiometric decomposition of N2H4 into N2 and H2 occurred photoelectrochemically (entries 1 and 2). A comparison of entry 1 with entry 2 evidently supported the voltammetric results (see Fig. S2, ESI†). In the negative controls, i.e. entries 3–5, the
Table 1
Results of photoelectrolysis in the presence of N2H4a
N2 evolved/mL H2 evolved/mL System (compartment A) (compartment B) Note Entry 1b Entry 2c Entry 3c Entry 4c Entry 5c
69.7 31.2 1.3 0 0
147.6 65.8 0 0 0
Full conditions Without Nf Without applied potentiald Without irradiation In the absence of N2H4
a ITO/C60/ZnPc/Nf was used as the photoanode except for entry 2. b Film thickness: C60 – 200 nm, ZnPc – 150 nm, Nf – 1 mm; effective area (i.e. geometrical area) of the photoelectrode: 1 cm2; electrolyte solution in compartment A, 5 mM N2H4 (pH = 11); electrolyte solution in compartment B, H3PO4 (pH = 2); applied potential, +0.3 V vs. Ag/AgCl (sat.); light intensity, ca. 70 mW cm 2; irradiation direction, back side of the ITO-coated face; electrolysis time, 1 h. c Each potentiostatic electrolysis experiment was performed by employing conditions similar to ITO/C60/ ZnPc/Nf (i.e. entry 1). d Irradiation of ITO/C60/ZnPc/Nf in compartment A was conducted in an open circuit.
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Table 2 Results of photocatalytic N2H4 decomposition using the photocatalysis system of ITO/C60/ZnPc/Nf and the Pt wire (see Scheme 1)a
System
N2 evolved/mL H2 evolved/mL (compartment A) (compartment B) Note
Entry 1b 10.7 Entry 2c 14.6
20.4 —
Entry 3c
1.3
0
Entry 4c Entry 5c
0 0
0 0
Full conditions Aerobic atmosphere in compartment B No connection between the ITO/C60/ZnPc and Pt wire Without irradiation In the absence of N2H4
a These experiments were conducted by employing greater chemical bias between the compartments, in comparison with those in Table 1. b Film thickness: C60 – 200 nm, ZnPc – 150 nm, Nf – 1 mm; effective area (i.e. geometrical area) of the photocatalytic device, 1 cm2; electrolyte solution in compartment A, 5 mM N2H4 (pH = 11); electrolyte solution in compartment B, H3PO4 (pH = 0); light intensity, ca. 70 mW cm 2; irradiation direction, back side of the ITO-coated face; irradiation time, 1 h. c Controlled experiments were performed by employing conditions similar to entry 1.
decomposition of N2H4 into N2 and H2 was not confirmed (cf. data for photoelectrolysis are shown in Fig. S3, ESI†). A photocatalysis system composed of ITO/C60/ZnPc/Nf and a Pt wire was constructed for investigating the decomposition of N2H4 (Scheme 1), where there was no application of bias potentials to the system. Typical photocatalysis data for that system are shown in Table 2. Similar to the photoelectrolysis result (entry 1 in Table 1), the stoichiometric decomposition of N2H4 to N2 and H2 was confirmed (entry 1). A control study was carried out (entry 2), in which only compartment B was operated under an aerobic atmosphere. Almost an equal amount of N2 compared to entry 1 was produced in compartment A, although there was no evolution of H2 in compartment B. This implies that the rate-limiting oxidation of N2H4 occurs in ITO/C60/ZnPc/Nf. In addition, when other controlled experiments were also performed (see entries 3–5 in Table 2), almost none of the gaseous product was found in either case. Factors affecting the photocatalytic decomposition of N2H4 were investigated in terms of light intensity (Fig. S4, ESI†) and film thickness of C60, ZnPc and Nf (Fig. S5, ESI†). From the results, it was revealed that the experimental conditions for entry 1 in Table 2 were appropriate for operating the present photocatalysis system. A prolonged study, which repeatedly used the photocatalytic device ITO/C60/ZnPc/Nf, was conducted to test the durability of the photocatalysis system (Fig. 1). This photocatalysis study was carried out under the conditions for entry 1 as seen in Table 2, with the exception of the irradiation time. Fig. 1 shows the linear relationships between the total amount of N2 and H2 generated and the number of cycles, explicitly demonstrating stable photocatalysis even after several cycles. The photocatalytic decomposition of N2H4 was also investigated under monochromatic-light irradiation. The external quantum efficiency (EQE) was estimated for each of the incident wavelengths tested (see the ESI† for the calculation procedure), and the results are shown in Fig. 2. The dependence of the EQE on the irradiated wavelength was consistent with the absorption spectrum of the C60/ZnPc bilayer employed. Furthermore, photocatalytic N2H4 decomposition was induced at all the visible-light regions, i.e. l o 750 nm.
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Fig. 1 Prolonged study of N2H4 decomposition using the ITO/C60/ZnPc/ Nf and Pt wire photocatalysis system. The conditions employed in entry 1 of Table 2 were applied to this study, except that the irradiation time (6 h in one cycle) was varied. For each cycle, the electrolyte solutions in both compartments were replaced with fresh solutions.
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conditions by means of photoelectrochemistry20 and photocatalysis,11 through which the mechanistic details of H2 evolution were clarified. Of course, it appears that in the present system, a Pt wire also functions as a co-catalyst for H2 evolution. In summary, this study exhibited a novel photocatalysis system using an organic p–n bilayer and is also the first instance of overall N2H4 decomposition under visible-light irradiation. Accomplishing the multi-charge transfer oxidation of N2H4 to N2 was also confirmed photoelectrochemically to involve the simultaneous formation of H2 at the counter electrode. Different from conventional photocatalysis systems, the present organophotocatalysis system demonstrated that N2H4 decomposition occurs over the entire visible-light energy range (l o 750 nm), along with H2 formation. In addition to our previous studies,10–13 various types of organophotocatalysis systems could be designed and fabricated on the basis of the abundant varieties of organic semiconductors and their easy processing. Organophotocatalysts can be expected to result in innovative technology for the large-scale photocatalytic production of solar fuel, for which the structural optimization for each objective will be a vital issue for drawing out high photocatalytic activity. This work was partly supported by a grant for Hirosaki University Institutional Research and a Grant-in-Aid for both Scientific Research C and Scientific Research on innovative areas (T.A.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References
Fig. 2 Dependence of the EQE value (closed circle) on the incident wavelength in the ITO/C60/ZnPc/Nf and Pt wire photocatalysis system, and the absorption spectrum (solid line) of the employed C60/ZnPc bilayer. The conditions for entry 1 in Table 2 were applied to the EQE measurements, except that monochromatic light was used. The irradiated intensity for the photocatalytic device was 0.83 mW cm 2.
The mechanism of photocatalytic N2H4 decomposition in the present system is thought to proceed as follows (also see Fig. S6, ESI†). The photoanode of the organic p–n bilayer such as ITO/C60/ZnPc can generate oxidizing power at the p-type semiconductor/water interface,24 originating from holes photogenerated through photophysical events similar to the corresponding photovoltaic cell in a dry state (vide supra).16 That is, when the C60/ZnPc bilayer is applied to a photocatalytic device, the ZnPc/water interface can induce oxidation, which is an alternative to charge collection at an electrode in the solar cell. After comparing the formal potential of ZnPc+/ZnPc (cf. it was estimated to be +0.64 V vs. Ag/AgCl from ref. 15, 16 and 20) with that of N2/N2H4 ( 1.18 V (at pH = 11) vs. Ag/AgCl),24 it is reasonable to assume that in compartment A, the photocatalytic oxidation of N2H4 to N2 can occur at the ZnPc/water interface. On the other hand, in compartment B, the reduction of H+ to H2 can take place at the Pt wire to which the reducing power photogenerated at C60 is transferred from compartment A [the formal potential of H+/H2 (pH = 0) = 0.20 V vs. Ag/AgCl]. The authors have previously shown that Pt-loaded C60 in the phthalocyanine/C60 bilayer can induce the evolution of H2 under acidic
1952 | Chem. Commun., 2014, 50, 1950--1952
1 A. Fujishima and K. Honda, Nature, 1972, 238, 37. 2 X. Chen, S. Shen, L. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503. 3 S. Hara, M. Yoshimizu, S. Tanigawa, L. Ni, B. Ohtani and H. Irie, J. Phys. Chem. C, 2012, 116, 17458. 4 Y. Sasaki, H. Nemoto, K. Saito and A. Kudo, J. Phys. Chem. C, 2009, 113, 17536. 5 F. Zhang, K. Maeda, T. Takata and K. Domen, Chem. Commun., 2010, 46, 7313. 6 M. Xu, J. Zai, Y. Yuan and X. Qian, J. Mater. Chem., 2012, 22, 23939. 7 R. Abe, K. Shinmei, K. Hara and B. Ohtani, Chem. Commun., 2009, 3577. 8 H. Kaga, K. Saito and A. Kudo, Chem. Commun., 2010, 46, 3779. 9 O. Game, U. Singh, A. A. Gupta, A. Suryawanshi, A. Banpurkar and S. Ogale, J. Mater. Chem., 2012, 22, 17302. 10 K. Nagai, T. Abe, Y. Kaneyasu, Y. Yasuda, I. Kimishima, T. Iyoda and H. Imaya, ChemSusChem, 2011, 4, 727. 11 T. Abe, J. Chiba, M. Ishidoya and K. Nagai, RSC Adv., 2012, 2, 7992. 12 S. Zhang, A. Prabhakarn, T. Abe, T. Iyoda and K. Nagai, J. Photochem. Photobiol., A, 2012, 244, 18. 13 K. Nagai, Y. Yasuda, T. Iyoda and T. Abe, ACS Sustainable Chem. Eng., 2013, 1, 1033. 14 C. W. Tang, Appl. Phys. Lett., 1986, 48, 183. 15 K. Suemori, T. Miyata, M. Yokoyama and M. Hiramoto, Appl. Phys. Lett., 2005, 86, 063509. 16 Z. R. Hong, R. Lessmann, B. Maennig, Q. Huang, K. Harada, M. Riede and K. Leo, J. Appl. Phys., 2009, 106, 064511. 17 T. Abe, K. Nagai, M. Kaneko, T. Okubo, K. Sekimoto, A. Tajiri and T. Norimatsu, ChemPhysChem, 2004, 5, 716. 18 T. Abe, K. Nagai, S. Kabutomori, M. Kaneko, A. Tajiri and T. Norimatsu, Angew. Chem., Int. Ed., 2006, 45, 2778. 19 T. Abe, Y. Tanno, T. Ebina, S. Miyakushi and K. Nagai, ACS Appl. Mater. Interfaces, 2013, 5, 1248. 20 T. Abe, S. Tobinai, N. Taira, J. Chiba, T. Itoh and K. Nagai, J. Phys. Chem. C, 2011, 115, 7701. 21 Y. Oosawa, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 1507. 22 H. Yuzawa, T. Mori, H. Itoh and H. Yoshida, J. Phys. Chem. C, 2012, 116, 4126. 23 D. M. Stanbury, Inorg. Chem., 1991, 30, 1293. 24 J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka and P. Strasser, J. Am. Chem. Soc., 2011, 133, 5425.
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Cite this: Chem. Commun., 2014, 50, 1950 Received 2nd September 2013, Accepted 17th December 2013 DOI: 10.1039/c3cc46701e
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Decomposition of hydrazine by an organic fullerene–phthalocyanine p–n bilayer photocatalysis system over the entire visible-light region† Toshiyuki Abe,*a Naohiro Taira,a Yoshinori Tanno,a Yuko Kikuchia and Keiji Nagaib
www.rsc.org/chemcomm
An organic p–n bilayer photocatalysis system successfully induced the oxidative decomposition of hydrazine (N2H4) into N2 and simultaneously yielded H2 from H+. This demonstrates the first instance of the stoichiometric decomposition of N2H4 under visible-light irradiation.
Solar fuel has attracted attention as an alternative energy source to fossil fuels and as a means of restricting global warming. Molecular hydrogen (H2) is a typical fuel and a source of potential clean energy, which can contribute to a sustainable society. Since the report of a photoelectrochemical water-splitting system using a UV-responsive TiO2 by Honda and Fujishima,1 several types of visible-light-responsive photocatalysts have been designed and fabricated by band engineering,2–6 aiming for the efficient utilization of solar energy with a high activity. However, there are few examples of inorganic-semiconductor-based photocatalysts that are capable of H2 evolution, even in the nearinfrared region;7–9 moreover, band-engineered photocatalysts usually become inactive, because a visible-light response of the photocatalyst may decrease its activity because of band-gap reduction. Distinct from the above-mentioned conventional approach to inorganic semiconductors, photocatalysis systems featuring an organic p–n bilayer have recently been revealed.10–13 In the organophotocatalysis system, a series of photophysical events (i.e. formation of an exciton by light absorption, charge separation at the p–n interface and carrier conduction through each layer) within the bilayer can occur in a manner similar to the corresponding photovoltaic system.14–16 However, the collection of photogenerated carriers (i.e. electrons and holes) can take place through reduction and oxidation on the surface
of the n- and p-type semiconductors, respectively,17–20 thus resulting in photocatalysis over the full visible-light region.10–13 In this study, a photocatalysis system of a fullerene (C60, an n-type semiconductor)/zinc phthalocyanine (ZnPc, a p-type semiconductor) bilayer (Scheme 1) was utilized for the decomposition of hydrazine (N2H4). N2H4 is a carbon-free and a H-storage material, because one molecule of N2H4 can produce double the amount of H2 during the multi-electron transfer oxidation of N2H4 to N2. However, only a few examples of TiO2 photocatalysts have been reported for the photocatalytic decomposition of N2H4.21,22 This system presents a novel instance of N2H4 decomposition in the entire range of visible-light irradiation. The organic p–n bilayer of C60 and ZnPc was prepared by vapour deposition (pressure, o1.0 10 3 Pa; deposition speed, ca. 0.03 nm s 1), and an indium-tin oxide (ITO)-coated glass plate was used as the base material. The organic bilayer was composed of C60 coated on ITO, and ZnPc coated on top of the C60 layer (denoted as ITO/C60/ZnPc). Furthermore, a Nafion membrane (denoted as Nf) was used in combination with the bilayer, and a 1 mm-thick Nf was loaded on the ZnPc surface. The resulting photocatalytic device is
a
Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan. E-mail: [email protected] b Chemical Resources Laboratory, Tokyo Institute of Technology, Suzukake-dai, Midori-ku, Yokohama 226-8503, Japan † Electronic supplementary information (ESI) available: Experimental details, structural information, illustration of a twin-compartment cell employed for photoelectrolysis, cyclic voltammograms, photoelectrolysis data for N2H4 decomposition with respect to applied potentials, photocatalysis data for N2H4 decomposition with respect to light intensity and film thickness of the photocatalyst device C60/ZnPc/Nf, and the energy diagram for N2H4 decomposition in the present photocatalysis system. See DOI: 10.1039/c3cc46701e
1950 | Chem. Commun., 2014, 50, 1950--1952
Scheme 1 Schematic illustration of a twin-compartment cell employed for photocatalysis experiments and the structures of chemicals employed in this study.
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abbreviated as ITO/C60/ZnPc/Nf. A cell made up of twin compartments separated by a salt bridge was utilized for photocatalytic experiments (see Scheme 1). Other experimental details and structural information (Fig. S1) are provided in the ESI.† ITO/C60/ZnPc and ITO/C60/ZnPc/Nf exhibited photoanodic characteristics due to the oxidation of N2H4 (Fig. S2, ESI†), and were supported by our previous studies (vide supra).17–19 Furthermore, Fig. S2 (ESI†) also reveals that ITO/C60/ZnPc/Nf is superior to ITO/ C60/ZnPc; in other words, Nf is considered to function as an absorbent for N2H4. The details are interpreted in the ESI.† By applying ITO/C60/ZnPc/Nf to a photoanode, potentiostatic electrolysis was conducted at a potential of +0.3 V (vs. Ag/AgCl (sat.)) for 1 h (the cell employed in this study is illustrated in Scheme S1, ESI†). The typical results are shown in Table 1, and the electrolysis data of the controlled systems are also included. The oxidative production of N2 from N2H4 was confirmed for both ITO/C60/ZnPc/Nf (entry 1) and ITO/C60/ZnPc (entry 2), simultaneously yielding H2 from H+ at the Pt counter. In both cases, the Faradaic efficiency for N2 formation was estimated to be >90% when assuming the four-electron-transfer oxidation of N2H4 (eqn (1)). N2H4 - N2 (oxidation product) + 2H2 (reduction product) (1) As for the other possible pathway, the two-electron-transfer oxidation of N2H4 to diazene (N2H2) can be considered (eqn (2)), which can be further followed by the spontaneous decomposition of N2H2 into N2 and H2 (eqn (3)).22,23 N2H4 - N2H2 + H2
(2)
N2H2 - N2 + H2
(3)
However, when N2H4 decomposes through eqn (2) and (3), the Faradaic efficiency for N2 formation is estimated to be ca. 50%; furthermore, in compartment A, generation of H2 was not confirmed. Therefore, based on the above-mentioned consideration and evidence, the stoichiometric decomposition of N2H4 into N2 and H2 occurred photoelectrochemically (entries 1 and 2). A comparison of entry 1 with entry 2 evidently supported the voltammetric results (see Fig. S2, ESI†). In the negative controls, i.e. entries 3–5, the
Table 1
Results of photoelectrolysis in the presence of N2H4a
N2 evolved/mL H2 evolved/mL System (compartment A) (compartment B) Note Entry 1b Entry 2c Entry 3c Entry 4c Entry 5c
69.7 31.2 1.3 0 0
147.6 65.8 0 0 0
Full conditions Without Nf Without applied potentiald Without irradiation In the absence of N2H4
a ITO/C60/ZnPc/Nf was used as the photoanode except for entry 2. b Film thickness: C60 – 200 nm, ZnPc – 150 nm, Nf – 1 mm; effective area (i.e. geometrical area) of the photoelectrode: 1 cm2; electrolyte solution in compartment A, 5 mM N2H4 (pH = 11); electrolyte solution in compartment B, H3PO4 (pH = 2); applied potential, +0.3 V vs. Ag/AgCl (sat.); light intensity, ca. 70 mW cm 2; irradiation direction, back side of the ITO-coated face; electrolysis time, 1 h. c Each potentiostatic electrolysis experiment was performed by employing conditions similar to ITO/C60/ ZnPc/Nf (i.e. entry 1). d Irradiation of ITO/C60/ZnPc/Nf in compartment A was conducted in an open circuit.
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Table 2 Results of photocatalytic N2H4 decomposition using the photocatalysis system of ITO/C60/ZnPc/Nf and the Pt wire (see Scheme 1)a
System
N2 evolved/mL H2 evolved/mL (compartment A) (compartment B) Note
Entry 1b 10.7 Entry 2c 14.6
20.4 —
Entry 3c
1.3
0
Entry 4c Entry 5c
0 0
0 0
Full conditions Aerobic atmosphere in compartment B No connection between the ITO/C60/ZnPc and Pt wire Without irradiation In the absence of N2H4
a These experiments were conducted by employing greater chemical bias between the compartments, in comparison with those in Table 1. b Film thickness: C60 – 200 nm, ZnPc – 150 nm, Nf – 1 mm; effective area (i.e. geometrical area) of the photocatalytic device, 1 cm2; electrolyte solution in compartment A, 5 mM N2H4 (pH = 11); electrolyte solution in compartment B, H3PO4 (pH = 0); light intensity, ca. 70 mW cm 2; irradiation direction, back side of the ITO-coated face; irradiation time, 1 h. c Controlled experiments were performed by employing conditions similar to entry 1.
decomposition of N2H4 into N2 and H2 was not confirmed (cf. data for photoelectrolysis are shown in Fig. S3, ESI†). A photocatalysis system composed of ITO/C60/ZnPc/Nf and a Pt wire was constructed for investigating the decomposition of N2H4 (Scheme 1), where there was no application of bias potentials to the system. Typical photocatalysis data for that system are shown in Table 2. Similar to the photoelectrolysis result (entry 1 in Table 1), the stoichiometric decomposition of N2H4 to N2 and H2 was confirmed (entry 1). A control study was carried out (entry 2), in which only compartment B was operated under an aerobic atmosphere. Almost an equal amount of N2 compared to entry 1 was produced in compartment A, although there was no evolution of H2 in compartment B. This implies that the rate-limiting oxidation of N2H4 occurs in ITO/C60/ZnPc/Nf. In addition, when other controlled experiments were also performed (see entries 3–5 in Table 2), almost none of the gaseous product was found in either case. Factors affecting the photocatalytic decomposition of N2H4 were investigated in terms of light intensity (Fig. S4, ESI†) and film thickness of C60, ZnPc and Nf (Fig. S5, ESI†). From the results, it was revealed that the experimental conditions for entry 1 in Table 2 were appropriate for operating the present photocatalysis system. A prolonged study, which repeatedly used the photocatalytic device ITO/C60/ZnPc/Nf, was conducted to test the durability of the photocatalysis system (Fig. 1). This photocatalysis study was carried out under the conditions for entry 1 as seen in Table 2, with the exception of the irradiation time. Fig. 1 shows the linear relationships between the total amount of N2 and H2 generated and the number of cycles, explicitly demonstrating stable photocatalysis even after several cycles. The photocatalytic decomposition of N2H4 was also investigated under monochromatic-light irradiation. The external quantum efficiency (EQE) was estimated for each of the incident wavelengths tested (see the ESI† for the calculation procedure), and the results are shown in Fig. 2. The dependence of the EQE on the irradiated wavelength was consistent with the absorption spectrum of the C60/ZnPc bilayer employed. Furthermore, photocatalytic N2H4 decomposition was induced at all the visible-light regions, i.e. l o 750 nm.
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Fig. 1 Prolonged study of N2H4 decomposition using the ITO/C60/ZnPc/ Nf and Pt wire photocatalysis system. The conditions employed in entry 1 of Table 2 were applied to this study, except that the irradiation time (6 h in one cycle) was varied. For each cycle, the electrolyte solutions in both compartments were replaced with fresh solutions.
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conditions by means of photoelectrochemistry20 and photocatalysis,11 through which the mechanistic details of H2 evolution were clarified. Of course, it appears that in the present system, a Pt wire also functions as a co-catalyst for H2 evolution. In summary, this study exhibited a novel photocatalysis system using an organic p–n bilayer and is also the first instance of overall N2H4 decomposition under visible-light irradiation. Accomplishing the multi-charge transfer oxidation of N2H4 to N2 was also confirmed photoelectrochemically to involve the simultaneous formation of H2 at the counter electrode. Different from conventional photocatalysis systems, the present organophotocatalysis system demonstrated that N2H4 decomposition occurs over the entire visible-light energy range (l o 750 nm), along with H2 formation. In addition to our previous studies,10–13 various types of organophotocatalysis systems could be designed and fabricated on the basis of the abundant varieties of organic semiconductors and their easy processing. Organophotocatalysts can be expected to result in innovative technology for the large-scale photocatalytic production of solar fuel, for which the structural optimization for each objective will be a vital issue for drawing out high photocatalytic activity. This work was partly supported by a grant for Hirosaki University Institutional Research and a Grant-in-Aid for both Scientific Research C and Scientific Research on innovative areas (T.A.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References
Fig. 2 Dependence of the EQE value (closed circle) on the incident wavelength in the ITO/C60/ZnPc/Nf and Pt wire photocatalysis system, and the absorption spectrum (solid line) of the employed C60/ZnPc bilayer. The conditions for entry 1 in Table 2 were applied to the EQE measurements, except that monochromatic light was used. The irradiated intensity for the photocatalytic device was 0.83 mW cm 2.
The mechanism of photocatalytic N2H4 decomposition in the present system is thought to proceed as follows (also see Fig. S6, ESI†). The photoanode of the organic p–n bilayer such as ITO/C60/ZnPc can generate oxidizing power at the p-type semiconductor/water interface,24 originating from holes photogenerated through photophysical events similar to the corresponding photovoltaic cell in a dry state (vide supra).16 That is, when the C60/ZnPc bilayer is applied to a photocatalytic device, the ZnPc/water interface can induce oxidation, which is an alternative to charge collection at an electrode in the solar cell. After comparing the formal potential of ZnPc+/ZnPc (cf. it was estimated to be +0.64 V vs. Ag/AgCl from ref. 15, 16 and 20) with that of N2/N2H4 ( 1.18 V (at pH = 11) vs. Ag/AgCl),24 it is reasonable to assume that in compartment A, the photocatalytic oxidation of N2H4 to N2 can occur at the ZnPc/water interface. On the other hand, in compartment B, the reduction of H+ to H2 can take place at the Pt wire to which the reducing power photogenerated at C60 is transferred from compartment A [the formal potential of H+/H2 (pH = 0) = 0.20 V vs. Ag/AgCl]. The authors have previously shown that Pt-loaded C60 in the phthalocyanine/C60 bilayer can induce the evolution of H2 under acidic
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