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Enhanced Photosensitized Hydrogen Production by Encapsulation of Ferrocenyl Dyes into Single-Walled Carbon Nanotubes Noritake Murakami, Hideaki Miyake, Tomoyuki Tajima, Kakeru Nishikawa, Ryutaro Hirayama, and Yutaka Takaguchi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018
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Enhanced Photosensitized Hydrogen Production by Encapsulation of Ferrocenyl Dyes into Single-Walled Carbon Nanotubes Noritake Murakami,† Hideaki Miyake,‡ Tomoyuki Tajima,† Kakeru Nishikawa,† Ryutaro Hirayama,† and Yutaka Takaguchi＊,† †
Graduate School of Environmental and Life Science, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 7008530, Japan ‡
Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan Supporting Information Placeholder ABSTRACT: Dye-encapsulated single-walled carbon
nanotubes (SWCNTs) were employed for the construction of a coaxial three-component dye/SWCNT/C60 heterojunction. Despite the larger diameter (~1.4 nm) of the SWCNTs relative to that set by Flavel’s rule (0.95 nm), the photo-induced electron transfer from dyeencapsulated SWCNTs to C60 proceeded smoothly, resulting in the photosensitized evolution of H2 from H2O using a ferrocenyl-based photosensitizer, which was confirmed by the action spectra.
Absorbers of solar light based on semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted great attention, because their absorption properties can be tuned via their chiral indices (n,m), which allows absorption in the visible to NIR range.1-3 Tune and Shapter have estimated that the potential power conversion efficiency (PCE) of a single-species photovoltaic (PV) device with (7,5)SWCNT should be ~7%.4 Moreover, a hypothetical tandem device, consisting of (6,4), (9,1), (7,3), and (7,5)SWCNTs, has been reported to potentially provide PCEs as high as 28%.4 To develop such PV devices, increasing attention has been focused on sSWCNT/C60 heterojunctions, in which the exciton dissociation at the donor–acceptor interface drives the solar energy conversion, despite the large binding energy (>100 meV) of the excitons produced from the optical absorption of the s-SWCNTs.5-9 Recently, we have reported water-dispersible coaxial nanowires with SWCNT/C60 heterojunctions, which can be used as photosensitizers to produce H2 from water in a dispersion/solution system.10-12 Photo-induced electron transfer (PET) triggered by the photo-excitation of the SWCNTs smoothly proceeds to generate H2 in the presence of a cocatalyst.12 However, there is a limitation in terms of
the s-SWCNTs’ ability to generate mobile charge carriers upon photo-irradiation of the s-SWCNT/C60 heterojunction: Flavel and colleagues have reported that the diameter limit for SWCNTs in SWCNT/C60 solar cells is 0.95 nm ((8,6)SWCNT).13 Blackburn and colleagues have described an optimum LUMO offset between the SWCNTs and C60 of approximately 130 meV, which is satisfied only by nanotubes with small diameter, e.g., (8,3), (9,1), or (6,5)SWNT.14 Hence, another approach to tune the absorption wavelength is required, especially with respect to an effective use of solar energy. A promising alternative would be the fabrication of PV systems that contain organic dye molecules. These dyes absorb a wide range of wavelengths, and the hybrids that result from encapsulation of these dyes within s-SWCNTs should find potential applications in solar cells, sensors, and photocatalysts. However, a clear-cut example for the generation of mobile charge carriers upon photoirradiation of dye-encapsulated CNTs has not yet been reported. Imahori and colleagues have reported the photophysical properties of three-component heterojunctions based on C60-encapsulated SWCNTs (C60@SWCNTs). For example, the C60/s-SWCNT/poly(3-hexylthiophene) (P3HT) heterojunction, obtained from the physical modification of C60@SWCNTs with P3HT, did not exhibit a charge-separated state, but an exciplex between C60@SWCNTs and P3HT upon photo-irradiation.15 On the other hand, a charge-separated state between C60@SWCNTs and the zinc porphyrin (ZnP) of a C60/SWCNT/ZnP heterojunction, obtained from the covalent modification of C60@SWCNTs with ZnP, was observed by transient absorption spectroscopy.16 However, the contribution from the encapsulated C60 molecules as light absorbers remains unclear. Although PET between s-SWCNTs and encapsulated molecules has not yet been reported, the photo-induced energy transfer from organic dyes to s-SWCNTs has often been ob-
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served in dye-encapsulated s-SWCNTs.17-20 Kataura and colleagues have used photoluminescence spectroscopy to describe the efficient energy transfer from an encapsulated squarylium dye to the s-SWCNTs.18 This context prompted us to investigate a new type of coaxial threecomponent heterojunction based on dye-encapsulated sSWCNTs and C60, wherein the s-SWCNT acts as an intermediate layer, despite the fact that its diameter exceeds Flavel’s limit. Herein, we describe the encapsulation of organic dye 1 (Figure 1), which is expected to act as an absorber of visible light and as a donor for C60,21 within s-SWCNTs to produce the novel dyeencapsulated s-SWCNT (1@SWCNT), whose diameter (~1.4 nm) exceeds the limit set by Flavel’s rule. We also present the fabrication of a coaxial heterojunction by physical modification of (1/SWCNT/C60) 1@SWCNT with a fullerodendron.22,23 Furthermore, we report the photo-induced evolution of H2 from H2O via the PET from the encapsulated 1 to the C60 molecules located on the lateral surface of the s-SWCNTs. To explore the photosensitizing properties of a dye molecule encapsulated within SWCNTs, we employed donor–acceptor–donor (D–A–D) dye 1 for the construction of a coaxial photosensitizing system consisting of dye-encapsulated SWCNTs and fullerodendron (Figure 1). In a typical experiment, SWCNTs (20 mg) and ferrocenyl dye 1 (20 mg) were refluxed in 1,2dimethoxyethane for 3 h. After filtration, the resulting solid was washed repeatedly with chloroform to remove any non-encapsulated molecules of 1, which afforded
Figure 1. Molecular structure of (a) thiocarbonyl dye 1 and (b) fullerodendron. Schematic illustration of (c) the stepwise fabrication of 1@SWCNT/fullerodendron and (d) its coaxial nanostructure.
1@SWCNTs. Subsequently, 1@SWCNTs (1.0 mg) was added to a water solution (10 mL) of a fullerodendron (25.5 mg, 0.01 µmol), which was then sonicated in a bath-type ultrasonicator at 17–25 °C for 4 h. After the suspension was centrifuged (3000g, 30 min), the black supernatant dispersion containing 1@SWCNT/fullerodendron was collected and purified by dialysis for 3 days to remove any excess fullerodendron molecules. The TEM image of 1@SWCNTs illustrates the encapsulation of 1 in a SWCNT (Figure 2). The image shows how the inner space of the SWCNT is filled with molecules of 1. An energy-dispersive X-ray (EDX) analysis of 1@SWCNT detected signals corresponding to sulfur and iron atoms, which are both contained in 1.
Figure 2. TEM image of 1 encapsulated in an SWCNT, as indicated by the arrows.
To confirm the formation of 1@SWCNT/fullerodendron nanocomposites, the absorption spectra of 1@SWCNT/fullerodendron and SWCNT/fullerodendron were recorded in D2O (Figure 3), and the two spectra were normalized at 900 nm to facilitate a comparison. The pink dotted line corresponds to the absorption spectrum of 1 in 1,1,2,2-tetrachloroethane (TCE), and the difference between 1@SWCNT/fullerodendron (red) and SWCNT/fullerodendron (blue) is shown (green line). The absorption band of 1 is clearly observed in the spectrum of 1@SWCNT/fullerodendron. The broadening of the absorption band for the encapsulated dye may be
Figure 3. UV-NIR spectra of 1@SWCNT/fullerodendron (red) and SWCNT/fullerodendron in D2O (blue), as well as that of 1 in TCE (pink dotted line) and the difference spectrum between 1@SWCNT/fullerodendron and SWCNT/fullerodendron (green).
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attributed to dispersion interactions with the SWCNTs, which have been reported by Yanagi et al.17,18 Furthermore, the Raman spectrum of the 1@SWCNT/fullerodendron nanocomposite was a rough superposition of the spectra of SWCNT/fullerodendron and 1, wherein the position of the G-band (1580 cm-1) of the SWCNT hosts was not shifted (Figure S2). This result indicates that charge transfer between the encapsulated dye molecules and SWCNTs does not occur in 1@SWCNT/fullerodendron.24 Meanwhile, Raman peaks associated with 1 shifted to lower wavenumbers after the encapsulation due to the interactions between 1 and the SWCNT (Figure S2). In terms of the formation of photo-induced charge carriers in heterojunctions consisting of s-SWCNTs, the 1@SWCNT/fullerodendron nanocomposite is quite interesting as it represents a new class of heterostructure with an SWCNT intermediate layer. Moreover, the photo-excitation of SWCNTs cannot lead to the formation of mobile carriers, as the diameter of the SWCNTs (~1.4 nm) is much thicker than the limit (0.95 nm) reported for SWCNT/C60 heterojunction solar cells. In order to explore the formation of photo-induced charge carriers in the 1/SWCNT/C60 heterostructure, we examined the photosensitized evolution of H2 from water using 1@SWCNT/fullerodendron in the presence of methyl viologen dication (MV2+), 1-benzyl-1,4-dihydronicotinamide (BNAH), and Pt nanoparticles. Figure 4a shows the energy level diagram for the conduction (C1 and C2) and valence (V1 and V2) bands of a typical
SWCNT ((13,8)SWCNT), the LUMOs and HOMOs of 1 and C60, and the energy levels of BNAH, MV2+, and PVP-Pt. After photo-excitation of 1 encapsulated in a SWCNT, the photogenerated electron in the LUMO of 1 is most likely transferred to the LUMO of C60 through the C2 band of the SWCNT. Simultaneously, hole transfer occurs from the HOMO of 1 to the V1 band of SWCNT, which generates the charge-separated state 1@SWCNT+・/C60−・ (Figure 4b). Subsequently, the hole of 1@SWCNT+・/C60−・oxidizes BNAH, while the electron is consumed by the production of H2 on a Pt nanoparticle via electron transfer assisted by the electronrelay molecule MV2+. It should be noted that the formation of MV•+ via PET from 1 to MV2+was confirmed by absorption spectra under illumination (lex = 650 nm) (Figure S7). Figure 5 shows the action spectra for the evolution of H2 using 1@SWCNT/fullerodendron and SWCNT/fullerodendron under monochromatic light (450, 510, 550, and 650 nm), as well as the UV-vis spectrum of 1. Under monochromatic illumination at 450 nm, the H2 production rates of 1@SWCNT/fullerodendron (3.1 µmol/h) and SWCNT/fullerodendron (3.4 µmol/h) are almost identical, as the C60 moiety of the fullerodendron acts as the light absorber in both cases. However, marked differences in the photocatalytic activity are observed for the photosensitized systems in the presence and absence of 1 upon irradiation at wavelengths >500 nm, wherein the C60 moiety is not the dominant light-absorber. Although the SWCNT/fullerodendron displays a relatively low reactivity (510 nm: 1.7 µmol/h; 550 nm: 0.15 µmol/h; 650 nm: 0.011 µmol/h), the H2 production rates using 1@SWCNT/fullerodendron are relatively high (510 nm: 3.2 µmol/h; 550 nm: 1.2 µmol/h; 650 nm: 1.3 µmol/h). Furthermore, the obtained action spectra of 1@SWCNT/fullerodendron closely match the absorb1.2
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Figure 4. (a) Energy-level diagram and (b) mechanism of the photocatalytic evolution of H2 using 1@SWCNT/fullerodendron.
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Figure 5. UV-vis spectrum of 1 in TCE, as well as action spectra for the evolution of H2 from H2O using 1@SWCNT/fullerodendron and SWCNT/fullerodendron photocatalysts.
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ance profile of 1. This result indicates that the light absorber in this photosensitized system is the encapsulated dye 1 rather than C60 or the SWCNTs. Furthermore, the importance of the dye/SWCNT/C60 heterojunction was confirmed by control experiments (Figure S12). It is noteworthy that the electron transfer from 1 to C60 via the C2 band of the intermediate SWCNT layer seems to be faster than the relaxation of the electron from the C2 to the C1 band of the SWCNT, which stands in marked contrast to the ultrafast relaxation from the E22 to the E11 exciton of SWCNTs.25 However, the detailed mechanism of the PET processes remains unclear. Further theoretical and experimental studies on this complicated multicomponent system are in progress. In summary, we have fabricated a new class of heterojunction (dye/SWCNT/C60) that consists of dyeencapsulated SWCNTs and an external fullerodendron. Although SWCNTs with a larger diameter (~1.4 nm) than that stated by Flavel’s rule (0.95 nm) were employed, the production of H2 from water using 1@SWCNT/fullerodendron as the photosensitizer proceeded efficiently. The action spectra showed good H2evolution rates for irradiation wavelengths between 450 and 650 nm, which is consistent with the absorption profile of the encapsulated dye (1), confirming that 1 acts as the photosensitizer in this H2-evolution system. To the best of our knowledge, this is the first example of PET from an encapsulated dye molecule to adsorbed molecules on the lateral surface of an SWCNT, even though the photo-induced energy transfer from encapsulated dyes to SWCNTs has been widely reported. Further studies on photosensitized systems using dye-encapsulated SWCNTs are currently in progress in our laboratories. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: #####. Preparation of dye-encapsulated SWCNTs and dyeencapsulated SWCNT/fullerodendron, UV-NIR spectra, TEM images and EDX mappings, AFM images, and results of the photocatalytic H2 evolution experiments.
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AUTHOR INFORMATION Corresponding Author
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[email protected] (22)
Funding Sources
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
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This work was partially supported by the JSPS KAKENHI grants 15H03519 (Y.T.) and 16K05895 (T.T.).
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