ARTICLE pubs.acs.org/JPCC
Molecular Hydrogen Evolution by Organic p/n Bilayer Film of Phthalocyanine/Fullerene in the Entire Visible-Light Energy Region Toshiyuki Abe,*,† Shunsuke Tobinai,† Naohiro Taira,† Junpei Chiba,† Takashi Itoh,‡ and Keiji Nagai§ †
Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan ‡ Center for Interdisciplinary Research, Tohoku University, Aoba-ku, Sendai 980-8578, Japan § Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
bS Supporting Information ABSTRACT: This article presents a novel approach to photochemical energy conversion by employing organic compounds. We demonstrated that water can be photoelectrochemically split into H2 and O2 using an organic photodevice responsive to the entire visible-light energy range of 600 nm.3,4 Recently, a metal-free photocatalyst for H2 production was reported by Wang and Domen;5 however, the Pt (cocatalyst)-loaded compound was photocatalytically active only under irradiation of 99.5%) was purchased from Tokyo Kasei Kogyo Co., Ltd., and was used as received. H2Pc (Tokyo Kasei Kogyo) Received: October 4, 2010 Revised: March 10, 2011 Published: March 25, 2011 7701
dx.doi.org/10.1021/jp1094992 | J. Phys. Chem. C 2011, 115, 7701–7705
The Journal of Physical Chemistry C
Figure 1. Chemical structures of C60 and H2Pc, and schematic illustration of the procedure for the present photoelectrochemical measurement.
was purified by sublimation in a thermostatted vessel at 510 °C prior to use (i.e., thermal control was conducted on the exterior of the vessel). Hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6 3 6H2O) was obtained from Kanto Chemical Co., Inc. An indiumtin oxide (ITO)-coated glass plate (sheet resistance = 8 Ω cm2; transmittance > 85%; ITO thickness = 174 nm) was purchased from Asahi Glass Co., Ltd. The organic p/n bilayer film was prepared by vapor deposition (pressure = ca. 1.0 103 Pa; deposition speed = 0.03 nm s1), and it comprised H2Pc coated on ITO and C60 coated on top of the H2Pc layer (ITO/ H2Pc/C60). During vapor deposition, the temperature at the ITO plate was not controlled. Absorption spectra were recorded using a Hitachi U-2010 spectrophotometer. The resulting absorption spectra of both H2Pc15 and C6016 were identical to those previously reported, and their absorption coefficients indicated the thickness of the film employed (cf., the aggregation structure of a single layer of H2Pc was also identifiable from the absorption spectrum; it implied that the polymorph of H2Pc can be attributed to the R phase, which is also supported by earlier findings).17 It is considered that the additivity of the absorption coefficients is held in the visible-light absorption spectrum of the bilayer; thus, the two unknown parameters, the thicknesses of both layers, were estimated by solving simultaneous equations based on absorbance at two distinct wavelengths. An electrochemical glass cell of single compartment was equipped with a modified ITO working electrode (effective area = 1 cm 1 cm), a spiral Pt counter electrode, and an Ag/AgCl (in saturated KCl electrolyte) reference electrode. Such a cell has also been employed in the photoelectrochemical study where the photoinduced evolution of O2 from water was involved.11 The entire photoelectrochemical study was conducted in a phosphoric acid solution (pH = 2) in an Ar atmosphere. This study was performed using a potentiostat (Hokuto Denko, HA-301) with a
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Figure 2. Cyclic voltammograms at ITO/H2Pc/C60 (a) and ITO/ H2Pc/C60-Pt (b). Conditions: Phosphoric acid electrolyte solution (pH = 2); scan rate = 20 mV s1; film thickness of H2Pc/C60 bilayer = 75 nm (H2Pc)/125 nm (C60); amount of coated Pt = 5.0 108 mol; light intensity = 100 mW cm2.
function generator (Hokuto Denko, HB-104), a coulomb meter (Hokuto Denko, HF-201), and an XY recorder (GRAPHTEC, WX-4000) under illumination. The procedure for the photoelectrochemical measurement is illustrated in Figure 1. A halogen lamp was used as the light source under typical conditions (light intensity = ca. 100 mW cm2). Irradiation was usually conducted from the ITO side. Light intensity was measured using a power meter (Ophir Japan, Ltd., Type 2A). The lamp was also used as the light source in combination with a monochromator (Soma Optics, Ltd., S-10) when measuring the action spectrum for photocurrent. The estimation method for the incident photonto-current conversion efficiency (abbreviated as IPCE) has been described elsewhere.913 Regarding the deposition of Pt, the modified ITO electrode was photocathodically polarized from þ0.4 V (vs Ag/AgCl (sat.)) to 0.2 V in an acidic solution (pH = 2) containing 5.0 104 mol dm3 H2PtCl6 3 6H2O, wherein the amount of Pt deposited was controlled by controlling the amount of charge passed (2.0 102 C). The H2 and O2 produced were analyzed using a thermal conductivity detector (TCD) gas chromatograph (Shimadzu, GC-8A) equipped with a molecular sieve 5 Å column and Ar carrier gas. Quantification of gaseous products was performed using a chromatogram analyzer (Shimadzu, C-R8A) equipped with the chromatograph. As for the calculation procedure of the faradaic efficiency for H2 evolved, it is stated in the Supporting Information. For in situ Raman spectroelectrochemistry, a quartz cell was constructed with an optical flat window for optical measurement and three electrode holders for electrochemical measurement. Irradiation was performed using an Ar laser (Coherent, Innova 70, 514.5 nm) incident on the working electrode surface at an 7702
dx.doi.org/10.1021/jp1094992 |J. Phys. Chem. C 2011, 115, 7701–7705
The Journal of Physical Chemistry C
Figure 3. Time course of H2 produced at ITO/H2Pc/C60-Pt. Conditions: Phosphoric acid electrolyte solution (pH 2); applied potential = 0.1 V (vs Ag/AgCl (sat.)); film thickness of H2Pc/C60 bilayer = 75 nm (H2Pc)/125 nm (C60); amount of coated Pt = 5.0 108 mol; light intensity = 100 mW cm2.
angle of approximately 60°. The optical window of the electrochemical cell allowed the transmission of light scattered from the electrode surface; this light was collected by an achromatic doublet lens and focused on the entrance slit of a singlestage spectrometer (Jasco, TRS-300) after the prereduction of Rayleigh light by Raman notch filters (Kaiser Optical Systems, holographic super notch plus, 514). Raman spectra were captured using a photodiode array detector (Hamamatsu Photonics, M2493) and an intensifier. This Raman spectroscopic system captured spectra over a typical range of 10002000 cm1. The effective spectral width of the entrance slit was ca. 32 cm1. The accumulation time of the Raman data was 60 s per measurement. The wavenumber of the detection channels was decided on the basis of Raman spectra of a naphthalene pellet and the luminescence of a Ne lamp.
’ RESULTS AND DISCUSSION When recording a cyclic voltammogram (CV) at ITO/H2Pc/ C60 in the water phase, almost no response was obtained under illumination or in the dark (Figure 2); however, when a Pt catalyst was loaded onto the outer surface of C60 in the H2Pc/C60 bilayer (referred to as ITO/H2Pc/C60-Pt), a photocathodic current was found to be generated noticeably at potentials less positive than þ0.1 V (vs Ag/AgCl (sat.)) (Figure 2). Although the CV measurements were also conducted at a Pt-covered ITO electrode (ITO-Pt), a Pt-covered H2Pc/ITO electrode (ITO/ H2Pc-Pt), and a Pt-covered C60/ITO electrode (ITO/C60-Pt), each reference system was inferior to the ITO/H2Pc/C60-Pt in terms of CV characteristics (see Figure S1). Potentiostatic electrolysis was carried out at ITO/H2Pc/C60Pt under illumination wherein a potential of 0.1 V (vs Ag/AgCl (sat.)) was applied to the photocathode for 3 h. The stoichiometric photoelectrochemical splitting of water into H2 (333.5 μL/3 h) and O2 (159.7 μL/3 h) was confirmed at an applied bias potential more positive than the formal potential for Hþ/H2 couple (ca. 0.32 V vs Ag/AgCl (sat.) at pH = 2). This implies that the potential of 0.22 V is gained by the introduction of photoenergy. The faradaic efficiency for the H2 evolution was estimated to be ca. 90%. The supplements to the H2 evolution at ITO/H2Pc/C60-Pt are stated in the Supporting Information. The control experiments of potentiostatic electrolysis were also performed at ITO/H2Pc/C60 (without Pt) as well as ITO/
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Figure 4. Time course of absorption spectrum of H2Pc/C60 bilayer in ITO/H2Pc/C60-Pt by in situ spectroelectrochemistry. A white light of 100 mW cm2 was incident on ITO/H2Pc/C60-Pt prior to each spectral measurement (cf., when measuring the absorption spectrum of the bilayer, the white-light irradiation was switched off). Conditions: Phosphoric acid electrolyte solution (pH = 2); applied potential = 0.1 V (vs Ag/AgCl (sat.)); film thickness of H2Pc/C60 bilayer = 75 nm (H2Pc)/125 nm (C60); amount of coated Pt = 5.0 108 mol.
H2Pc (with Pt) by employing the same conditions as those for ITO/H2Pc/C60-Pt; no evidence for H2 evolution was found in either case. C60 has been extensively studied in terms of its properties and reactivity.1820 It functions as a good electron acceptor because it has a low-lying LUMO level; moreover, electron-photodoped C60 acts as a promising electron conductor among the few available organic n-type semiconductors, and hence it has been applied to dry-type photovoltaic cells.21,22 Therefore, the H2 production at Pt appeared to occur via the conduction of the electron carriers photogenerated at the p/n interface (vide infra), when O2 was concurrently yielded at the counter electrode. Figure 3 illustrates the time course of H2 produced; it can be seen that the photoinduced evolution of H2 started to occur noticeably after ca. 20 min. To gain insights into the appearance of an induction period prior to the steady evolution of H2, the bilayer film was characterized by in situ spectroelectrochemistry. ITO/H2Pc/C60-Pt was polarized at 0.1 V (vs Ag/AgCl) under illumination, and the time course of the absorption spectrum of the film was measured simultaneously (Figure 4). The spectrum remained almost unchanged, which may imply the stability of the film; however, a detailed observation revealed the very slight appearance of a new peak at around 960 nm (see inset). The resulting spectra indicated the formation of C602 species, which were also consistent with the spectrum of the electrochemically formed species in the water phase.23 The formation of the C602 species was also confirmed by in situ Raman spectroscopy. Figure 5 shows a Raman spectrum at ITO/H2Pc/C60-Pt after illumination for 20 min. The resulting Raman spectrum probably depended on the applied potential and irradiation (cf., pristine C60,24 1468 cm1; this can also be confirmed through a Raman spectrum measured at ITO/H2Pc/C60-Pt without applying potential in the dark (Figure S2)). Therefore, we focused on the variation in the Raman lines between 1430 and 1480 cm1. The Raman lines were fitted by Gaussian curves to elucidate the dependences on potential and irradiation, through which the peak parameters were determined precisely. Results of a line shape analysis revealed the presence of a new band at 1453 cm1 in addition to an intense band at 1465 cm1. As indicated 7703
dx.doi.org/10.1021/jp1094992 |J. Phys. Chem. C 2011, 115, 7701–7705
The Journal of Physical Chemistry C
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Figure 5. In situ Raman spectrum of ITO/H2Pc/C60-Pt and its deconvolution curves. A white light of 100 mW cm2 was incident on ITO/H2Pc/C60-Pt prior to the spectral measurement (cf., when measuring the Raman spectrum, the white-light irradiation was switched off). Conditions: Phosphoric acid electrolyte solution (pH = 2); applied potential = 0.1 V (vs Ag/AgCl (sat.)); film thickness of H2Pc/C60 bilayer = 75 nm (H2Pc)/125 nm (C60); amount of coated Pt = 5.0 108 mol.
earlier,24 a band at around 1425 cm1 in the raw spectrum can be attributed to a nontotally symmetric mode of the pristine C60; however, it has also been observed that the 1425 cm1 band disappears when the pristine C60 undergoes a complete reduction. It is also possible to conclude from Figure 5 that most of the pristine C60 remained unchanged. In addition, a frequency shift of ca. 6 cm1 per added electron has been observed previously in the Raman spectrum of C60.24,25 Therefore, the bands at 1453 and 1465 cm1 were assignable to the C602 species and the pristine species, respectively. In separate experiments, the abovedescribed in situ measurements were conducted with Pt-free ITO/H2Pc/C60. Similar characteristics of C60 were also confirmed by both VIS-near IR absorption spectroscopy (Figure S3) and Raman spectroscopy (Figure S4). These results indicate that loading of Pt is essential for rate-limiting production of H2 at the photocathode of H2Pc/C60. The action spectra for photocurrent were measured with the irradiation directions of incident light (Figure 6). These spectra show that H2 evolution was triggered by the absorption of the H2Pc/ C60 bilayer over the entire visible-light region of
