Introduction
Over the last several decades, photoinduced hydrogen generation has been studied for the conversion of solar energy into chemical energy (Sprick et al. Reference Sprick, Bonillo, Clowes, Guiglion, Brownbill, Slater, Blanc, Zwijnenburg, Adams and Cooper2016; Stevenson et al. Reference Stevenson, Marguet, Schneider and Shafaat2017; Wang et al. Reference Wang, Hisatomi, Suzuki, Pan, Seo, Katayama, Minegishi, Nishiyama, Takata, Seki, Kudo, Yamada and Domen2017). Methylviologen (MV2+) has been used commonly to evolve hydrogen, because its one-electron reduced species (methylviologen cation radical, MV+•) generates hydrogen in the presence of a Pt catalyst (Toshima et al. Reference Toshima, Kuriyama, Yamada and Hirai1981; Matheson et al. Reference Matheson, Lee, Meisel and Pelizzetti1983; Bard and Fox Reference Bard and Fox2002). Many researchers have studied the generation of MV+• and/or hydrogen from MV2+ with visible light irradiation in the presence of photosensitizers (Wasielewski Reference Wasielewski1992; Yonemoto et al. Reference Yonemoto, Riley, Kim, Atherton, Schmehl and Mallouk1992; Inoue et al. Reference Inoue, Ichiroku, Torimoto, Sakata, Mori and Yoneyama1994). In addition, some researchers have found that, under UV light irradiation, MV+• is generated in solution in the presence of solid surfaces such as silica gels, layered transition metal oxides, and zeolites (Miyata et al. Reference Miyata, Sugahara, Kuroda and Kato1988; Mao et al. Reference Mao, Breen and Thomas1995; Alvaro et al. Reference Alvaro, García, García, Márquez and Scaiano1997; Peon et al. Reference Peon, Tan, Hoerner, Xia, Luk and Kohler2001). Among inorganic layered materials, clay minerals are an attractive group because of their following properties: (1) flat surfaces; (2) regularly arranged anionic charges on their surfaces; (3) exfoliation and stackability of individual nanolayers depending on the medium; (4) optical transparency in the visible region in the exfoliated state when the particle size is small (<100 nm); and (5) easy formation of hybrid materials with cationic molecules by electrostatic interaction (Shichi & Takagi Reference Shichi and Takagi2000; Egawa et al. Reference Egawa, Watanabe, Fujimura, Ishida, Yamato, Masui, Shimada, Tachibana, Yoshida, Inoue and Takagi2011). Various researchers have reported hybrid materials of clay minerals and MV2+ (Raupach et al. Reference Raupach, Emerson and Slade1979; Villemure et al. Reference Villemure, Detellier and Szabo1986, Reference Villemure, Detellier and Szabo1991; Miyata et al. Reference Miyata, Sugahara, Kuroda and Kato1987; Rytwo et al. Reference Rytwo, Nir and Margulies1996), and Ogawa and co-workers (Kakegawa et al. Reference Kakegawa, Kondo and Ogawa2003) reported that MV+• was generated when intercalated MV2+ in clay minerals was irradiated by UV light. The mechanism of MV+• generation in clay minerals under UV light was mentioned as being due to the generation of MV+• via an electron-transfer reaction from the bridging Si-O-Al oxygen lone pair to the excited MV2+. However, in the solid hybrid systems above, estimating the efficiency of photogeneration of MV+• was difficult, and the reaction details were not revealed because the amount of MV+• generated and decreased amount of MV2+ could not be estimated.
Previous work revealed the interesting structure of porphyrin−clay mineral hybrids, in which the porphyrin molecules adsorb densely on the exfoliated clay nanolayers without aggregation (Takagi et al. Reference Takagi, Tryk and Inoue2002, Reference Takagi, Shimada, Ishida, Fujimura, Masui, Tachibana, Eguchi and Inoue2013). The crucial factor for the very dense adsorption of porphyrins was the matching of the distances between cationic sites in porphyrin and the average distance between anionic sites on the clay nanolayer surfaces (called the intercharge distance matching effect or size-matching effect). In these systems, the fluorescence intensity of some dyes adsorbed on saponite surfaces was quenched at higher adsorption densities (self-fluorescence quenching reaction) (Ishida et al. Reference Ishida, Shimada, Tachibana, Inoue and Takagi2012b; Ohtani et al. Reference Ohtani, Ishida, Ando, Tachibana, Shimada and Takagi2014). This observation suggested that collision and/or electron transfer between excited and adjacent ground-state dye molecules might be one of the reasons for the self-fluorescence quenching reaction. The detail of the self-fluorescence quenching reaction, however, was not revealed.
MV2+ adsorbed on clay minerals has been reported to show self-fluorescence quenching behavior (Villemure et al. Reference Villemure, Detellier and Szabo1986, Reference Villemure, Detellier and Szabo1991). Judging from those studies and previous results, the principal mechanism for the generation of MV+• either on the surface or in the interlayer space of saponite could be an electron transfer reaction between the excited methylviologen (abbreviated as MV2+*) and the adjacent ground-state MV2+. The objective of the current study was to measure the photogeneration efficiency of MV+• in a semi-transparent hybrid film composed of MV2+ and saponite, in which the amount of MV2+ and MV+• could be estimated by UV-Vis absorption spectra, and to determine whether the principal mechanism of the self-fluorescence quenching reaction is indeed electron transfer between MV2+* and the adjacent ground-state MV2+ molecules, all with an eye to understanding whether this electron transfer reaction in the interlayer space of the clay mineral could be utilized further to construct other photochemical reaction systems.
Methods
Preparation of MV2+/Saponite Hybrid Film
A synthetic saponite (Sumecton SA ((Na0.49Mg0.14)[(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4]), abbreviated as SSA, from Kunimine Industries Co., Ltd., Chiyoda-ku, Tokyo, Japan) was used, and its theoretical surface area and CEC were 750 m2 g −1 and 1.00 meq g −1, respectively (Egawa et al. Reference Egawa, Watanabe, Fujimura, Ishida, Yamato, Masui, Shimada, Tachibana, Yoshida, Inoue and Takagi2011). MV2+ dichloride was purchased from Nacalai Tesque, Inc (>98%, Chukyo-ku, Kyoto, Japan). The purity of the MV2+ was checked by 1H-NMR and thin layer chromatography. All of the chemicals were used as received.
Quartz substrates were sonicated with water for 30 min and then sonicated with ethanol for another 30 min. These substrates were further treated in sulfuric acid (Kanto Chemical, Chuo-ku, Tokyo, Japan) overnight at room temperature and washed with > 150 mL of water to remove the sulfuric acid. MV2+/SSA hybrid films were prepared according to the literature (Kawamata et al. Reference Kawamata, Suzuki and Tenma2010; Takagi et al. Reference Takagi, Eguchi, Yui and Inoue2004, Reference Takagi, Shimada, Masui, Tachibana, Ishida, Tryk and Inoue2010; Suzuki et al. Reference Suzuki, Tenma, Nishioka, Kamada, Ohta and Kawamata2011). MV2+ aqueous solution (2.0×10–4 mol/L, 600 μL) was added to the saponite dispersion (2400 μL) within 2 s to prepare a MV2+/SSA complex dispersion in the exfoliated state (Suquet et al. Reference Suquet, Iiyama, Kodama and Pezerat1977; Auerbach et al. Reference Auerbach, Carrado and Dutta2004). The MV2+ loading level as a fraction of the % CEC of the clay mineral was adjusted from 10 to 80% CEC by changing the concentration of the saponite dispersion (from 1.00 to 1.25×10–1 g/L). The complex dispersions were filtered through PTFE membrane filters (pore size = 0.1 μm, Advantec, Bunkyo-ku, Tokyo, Japan). The residue was transferred onto a quartz substrate (Agri, Oume-city, Tokyo, Japan) and dried under vacuum overnight at room temperature to form a semi-transparent MV2+/SSA hybrid film.
Photogeneration of MV+• in Hybrid Film
MV2+/hybrid films were placed in a cuvette and vacuumed for >1 h to remove the oxygen that quenches MV+•. These films were irradiated with UV light (270 nm, FWHM of wavelength was 12 nm) from a Xe lamp through a diffraction grating to examine the photoreduction of MV2+. The generation of MV+• was monitored with a UV-Vis absorption spectrometer. The cuvette was kept under vacuum during photo-irradiation and measurement of UV-Vis absorption spectra.
Analysis
The UV-Vis absorption spectra were obtained on a UV-3150 system (Shimadzu, Chukyo-ku, Kyoto, Japan). The fluorescence spectra were measured using an FP-6600 spectrofluorometer (Jasco, Hachioji-city, Tokyo, Japan), with an excitation wavelength of 272 nm. The hybrid film was set at 45° relative to the axis of the excitation beam and the emission detector. X-ray diffraction (XRD) patterns were measured using a RINT TTR III system (Rigaku, Akishima-city, Tokyo, Japan), using CuKα radiation. These measurements were carried out at room temperature.
Results and Discussion
XRD Patterns and Absorption and Fluorescence Spectra of the Hybrid Film
The hybrid film was sufficiently transparent (Fig. 1a) for transmission-mode measurements. The photograph of the hybrid film under 254 nm UV light irradiation (Fig. 1b) revealed blue or purple emission from MV2+*, while MV2+ in the aqueous medium exhibited no emission. Fluorescence was observed, however, when MV2+ was intercalated in the clay mineral or adsorbed on the clay surfaces, as reported by Detellier and Szabo (Villemure et al. Reference Villemure, Detellier and Szabo1986, Reference Villemure, Detellier and Szabo1991). The rotation of pyridyl groups of MV2+ was suppressed by adsorption on the SSA surface, and then the rate constant of non-radiative deactivation decreased. This phenomenon has been commonly recognized as surface-fixation induced emission (S-FIE) (Ishida et al. Reference Ishida, Shimada and Takagi2014; Tokieda et al. Reference Tokieda, Tsukamoto, Ishida, Ichihara, Shimada and Takagi2017).
Fig. 1. Photographs of MV2+/SSA hybrid film: (a) light transmission and (b) under UV irradiation
From the XRD patterns of MV2+/SSA hybrid films at various MV2+ loading levels (Fig. 2), the interlayer spaces of hybrid films were estimated to be 0.30–0.32 nm, which is almost the same as that of montmorillonite and other clay minerals where MV2+ was intercalated (Raupach et al. Reference Raupach, Emerson and Slade1979; Rytwo et al. Reference Rytwo, Nir and Margulies1996; Kakegawa et al. Reference Kakegawa, Kondo and Ogawa2003). Judging from the thickness of pyridyl groups and the basal space of SSA in the hybrid film, the orientation of MV2+ aromatic rings in the interlayer of SSA was parallel to the SSA layer. The XRD pattern of MV2+/SSA shifted slightly to a higher diffraction angle, indicating the decrease in interlayer distance, when the loading level of MV2+ was increased. This may indicate that the interlayer space of MV2+/SSA hybrid film became hydrophobic with the increased MV2+, and then the interlayer water was excluded.
Fig. 2. XRD patterns of MV2+/SSA hybrid films, with MV2+ loading levels of 20%, 40%, 60%, and 80% CEC
The UV-Vis absorption spectra of hybrid films varied with different loading levels of MV2+ (Fig. 3). The film with no MV2+ loading displayed no absorption band in the 200–600 nm region (not shown). The background of each film was different because of the scattering by SSA; however, the absorption band ascribed to MV2+ was clearly observed in the UV region in all cases. Note that when the loading level of MV2+ was low, a correspondingly greater amount of SSA was in the film, thus causing greater scattering. The maximum absorption wavelength (λmax) of MV2+ was shifted to longer wavelength (272 nm) compared to that in the aqueous solution (257 nm, Fig. S1). A similar spectral shift was observed when MV2+ was adsorbed on the surfaces of SSA or other clay minerals (Villemure et al. Reference Villemure, Detellier and Szabo1991). This result indicated that the π-conjugated system of MV2+ was extended by the co-planarization of its two pyridinium groups (Fig. S2) because of the steric effect of SSA (Kuykendall & Thomas Reference Kuykendall and Thomas1990; Chernia & Gill Reference Chernia and Gill1999; Ishida et al. Reference Ishida, Masui, Shimada, Tachibana, Inoue and Takagi2012a). The λmax of MV2+ was independent of the MV2+ loading level, and the spectral shape was also almost the same. Hence, the MV2+ intercalated in the interlayer of SSA did not aggregate in spite of its dense intercalation. When the MV2+ loading level in the hybrid film was set as high as 80% CEC, the absorbance at 272 nm was relatively small compared to other films, and MV2+ was observed in the filtrate, according to the UV-Vis absorption spectra. This indicated that a small amount of MV2+ was not adsorbed on SSA, because its saturation adsorption amount on the SSA surface was <80% CEC. Judging from the optical absorbance of excess MV2+ in the filtrate solution and the extinction coefficient of MV2+ at 257 nm (ε = 20700 mol·L–1·cm–1), the maximum adsorption of MV2+ in the above hybrid film was calculated more accurately to be 75% CEC (Watanabe & Honda Reference Watanabe and Honda1982). In the fluorescence spectra of MV2+/SSA hybrid films (Fig. 4), fluorescence of MV2+ was observed at the maximum emission wavelength of 344 nm.
Fig. 3. UV-Vis absorption spectra of MV2+/SSA hybrid films, with MV2+ loading levels of 10% (black solid line), 20% (black broken line), 40% (black dotted line), 60% (gray solid line), and 80% (gray broken line) CEC
Fig. 4. Fluorescence spectra of MV2+/SSA hybrid films at various MV2+ loadings, with MV2+ loading levels of 10% (black solid line), 20% (black broken line), 40% (black dotted line), 60% (gray solid line), and 80% (gray broken line) CEC. The excitation wavelength was set at 272 nm
The fluorescence of MV2+ is known to occur when MV2+ is adsorbed on/intercalated in clay minerals, although aqueous solutions of MV2+ do not show the fluorescence (Villemure et al. Reference Villemure, Detellier and Szabo1986, Reference Villemure, Detellier and Szabo1991). This phenomenon is explained as being due to the suppression of rotation of the pyridinium groups of MV2+ by the steric effect of the clay minerals, and the rate constant of internal conversion is decreased, as described above.
The shape of the emission spectrum was independent of the loading level of MV2+. This suggests the absence of new luminescent species, such as J-type aggregates (head-to-tail type aggregate), and of excimers that could be formed in the interlayer space of SSA nanolayers, in spite of the large MV2+ loadings. The emission intensities of MV2+/SSA hybrid films decreased when the loading level of MV2+ was increased, in spite of the same amount of MV2+. The formation of H-type aggregates, which is one cause of fluorescence quenching, was not observed judging from the above-mentioned XRD and absorption spectra analyses. These results indicate that the fluorescence of MV2+ was quenched via a self-fluorescence quenching reaction. The occupied surface area of an anion on SSA was calculated as 1.25 nm2, based on the theoretical surface area and the cation exchange capacity. On the other hand, assuming that MV2+ is rectangular (dihedral angle of pyridinium substrate is 0°), the cross-sectional footprint of MV2+ would be 0.844 nm2/molecule (viewed from the normal direction of the pyridinium group (Raupach et al. Reference Raupach, Emerson and Slade1979). Calculating from these values, the occupancy ratio of the SSA surface by MV2+ was 50% (at 75% CEC). Judging from the result of this calculation, MV2+ should be intercalated into SSA without aggregation.
Generation of MV+• with UV Irradiation in the Interlayer of Saponite
The UV-Vis absorption spectra of MV2+/SSA hybrid films (75% CEC) changed under UV irradiation at 270 nm (Fig. 5); the absorbance of MV2+ decreased quickly under UV irradiation, and new absorption bands appeared at ~400 and ~600 nm. The newly appeared absorption bands at ~350–420 nm, the broadened band at ~450–750 nm, and their characteristic vibrational structures were almost the same as in the reported absorption spectrum of MV+•, which is the one-electron reduced species of MV2+ (Watanabe & Honda Reference Watanabe and Honda1982; Bockman & Kochi Reference Bockman and Kochi1990). In the photograph of the MV2+/SSA hybrid film after UV irradiation (Fig. 6), the color of the film changed from colorless to blue, the latter being the typical color of MV+• (Palenzuela et al. Reference Palenzuela, Vinuales, Odriozola, Cabanero, Grande and Ruiz2014). In addition, this absorption peak disappeared upon exposure to air, and then the absorption band of MV2+ increased. This result also supports the conclusion that the newly generated species was MV+• because it reacted with oxygen in air to produce MV2+. The peak absorption wavelengths of MV+• in the interlayer space of SSA were 398 and 608 nm. Interestingly, λmax of MV+• was shifted slightly to a longer wavelength (by 2 nm) compared to that in water, although the λmax of MV2+ was shifted to an even longer wavelength (by >10 nm) by intercalation into the interlayer space of SSA. These observations could be ascribed to the different stable structures of MV2+ and MV+• in water. In the case of MV2+, the pyridinium groups are not co-planar in water because of the steric effect, while the stable structure of MV+• is approximately co-planar (Bockman & Kochi Reference Bockman and Kochi1990; Kubota Reference Kodaka and Kubota1999; Porter & Vaid Reference Porter and Vaid2005). Thus, the λmax of intercalated MV+• in SSA was almost the same as in water, because the effect of co-planarization by intercalation is relatively weak compared to that for MV2+. MV+• was not generated without UV light irradiation, indicating that the MV2+ intercalated in SSA was reduced by UV irradiation.
Fig. 5. UV-Vis absorption spectra of MV2+/SSA hybrid film during UV-irradiation (concentration of MV2+: 75% CEC) at 0 min (gray solid line), 1 min (black dotted line), 3 min (black broken line), and 5 min (black solid line)
Fig. 6. Photograph of MV2+/SSA hybrid film after UV light irradiation
Several reports have discussed the photoreduction of MV2+ with UV irradiation in several media, such as water, organic solvents, and solid surfaces, as mentioned above. The electron donor to reduce the MV2+ via a photoinduced electron transfer reaction was discussed in those reports. Although some reports indicated that the chloride ion is the electron donor to reduce MV2+* (Ebbesen et al. Reference Ebbesen, Manring and Peters1984; Peon et al. Reference Peon, Tan, Hoerner, Xia, Luk and Kohler2001), MV2+ adsorbs on SSA by electrostatic interaction and, thus, few chloride ions should be present in the hybrid film. Kohler and co-workers (Peon et al. Reference Peon, Tan, Hoerner, Xia, Luk and Kohler2001) reported that gas-phase water has a high ionization potential (12.62 eV), and they did not observe the quenching of MV2+* via electron transfer. Considering the previous reports, the residual interlayer water should not be the electron donor. In aluminosilicate media (clays and zeolites), some reports (Alvaro et al. Reference Alvaro, García, García, Márquez and Scaiano1997; Kakegawa et al. Reference Kakegawa, Kondo and Ogawa2003) suggested that the bridging Si-O-Al oxygen lone pair can donate an electron to MV2+*. To confirm this electron transfer pathway, MV2+/SSA hybrid films with very low adsorption density of MV2+ (0.13% and 0.06% CEC) were prepared to avoid the self-fluorescence quenching reaction. The fluorescence intensities of these films were approximately the same in spite of their different loading levels of MV2+. Thus, the self-quenching reaction was absent in these films (Fig. S3). The UV-Vis absorption spectra of MV2+/SSA hybrid film (MV2+ loading level of 0.13% CEC) varied during UV irradiation (Fig. S4). The absorbance ascribed to MV2+ decreased with UV irradiation because of decomposition after prolonged UV exposure, and a small absorption band was observed at ~400 nm. The absorption peak at ~600 nm attributed to MV+•, however, was not observed, and the existing peak did not disappear in the presence of oxygen. These observations suggested that MV+• was not generated because it would be quenched by oxygen molecules. This absorption band was similar to that of the decomposition product of MV2+ (Solar et al. Reference Solar, Solar, Getoff, Holcman and Sehested1982; Bahnemann et al. Reference Bahnemann, Fischer, Janata and Henglein1987). Hence, the new species was not MV+• but a decomposition product. All these results, taken together, indicate that the electron transfer from the bridging Si-O-Al oxygen lone pair to MV2+* did not take place. Considering these results and the dependency of MV+• photogeneration on the adsorption density of MV2+ (as described below), the photoinduced electron transfer reaction between excited MV2+ and adjacent ground-state MV2+ would be one of the principal pathways of the self-quenching reaction. An electron transfer reaction between electron donor and acceptor depends on their redox potentials and re-organization energy, requiring a difference between the reduction potential of the acceptor and the oxidation potential of the donor. In this case, the redox potential of MV2+ would be affected by the negative charges of SSA and co-planarization of the dye molecules, both of which will depend on the distance between the anionic site on SSA and the cationic site of MV2+. Thus, a difference in the adsorption site might change the redox potentials slightly, thereby allowing the photoinduced electron transfer reaction to happen.
Although a back electron transfer reaction between MV+• and oxidized MV2+ was expected, MV+• could be observed by the typical steady-state UV-Vis spectrophotometer. The reason is that the one-electron oxidized MV2+ will be unstable and, thus, undergo a decomposition reaction, and then the back electron transfer may be suppressed.
The generation of MV+• with UV irradiation in MV2+/SSA hybrid films at various adsorption densities of MV2+ is summarized in Fig. 7. The amount of MV+• generated was calculated from Δabs. = abst – abs0 and the extinction coefficient of MV+• in water (Watanabe & Honda Reference Watanabe and Honda1982). The amount of MV+• generated depended on the adsorption density of MV2+, and reached a plateau in all hybrid films. The maximum conversion ratio from MV2+ to MV+• (generated MV+• / initial MV2+) was 0.144 (75% CEC), although the maximum value of the conversion ratio will be 0.5. This is because MV2+* may be quenched by the MV+• generated and decomposition products. According to the quantum yield (φ) at 1 min of the photogeneration of MV+• (Table 1), the photogeneration efficiency of MV+• increased with increasing adsorption density of MV2+, indicating that the self-fluorescence quenching reaction is important for generating MV+•. φ drastically increased at large dye loadings (>60% CEC), suggesting that the roaming range of MV2+* within the excited lifetime will be limited, because MV2+ molecules have to approach each other to some extent.
Fig. 7. Generation of MV+• with UV irradiation at different MV2+ loading levels: 75% (open circle), 60% (solid circle), 40% (open square), 20% (solid square), and 10% (solid triangle) CEC
Table 1. Quantum yields (φ) of the MV+• photogeneration in hybrid films
Conclusions
The generation of MV+• in semi-transparent MV2+/SSA hybrid films under UV irradiation was observed clearly by transmission spectra. The efficiency of MV+• generation depended on the adsorption density of MV2+. When the adsorption density of MV2+ was low, where a self-quenching reaction was not observed, MV+• was not generated. This indicates that MV+• was produced via electron transfer between MV2+* and MV2+ on the saponite surfaces.
Electronic supplementary material
The online version of this article (https://doi.org/10.1007/s42860-019-00047-8) contains supplementary material, which is available to authorized users.
Acknowledgments
This work was partly supported by the PRESTO/JST Program, Innovative Use of Light and Materials/Life; a Grant-in-Aid for Scientific Research on Innovative Areas; a Grant-in-Aid for Scientific Research (B) (No. 24350100); and a Grant-in-Aid for Scientific Research on Innovative Areas “All Nippon Artificial Photosynthesis Project for Living Earth” (AnApple, No. 25107521).
