Langmuir 2000, 16, 6077-6080
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H2 Generation by Cycling Dark Adsorption and Successive Photoinduced Desorption of 2-Mercaptopyridine on/from Ag-Core/Pt-Shell Nanoparticles Loaded on TiO2 Hiroaki Tada,*,† Kazuaki Teranishi,‡ Seishiro Ito,‡ Hisayoshi Kobayashi,§ and Susumu Kitagawa| Environmental Science Research Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka 577-8502, Japan, Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, 577-8502, Japan, Department of Chemical Technology, Kurashiki University of Science and Arts, 2640 Nishinoura, Tsurajima, Kurashiki 712, Japan, and Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Received February 2, 2000. In Final Form: May 6, 2000 Generation of H2 with liquid-phase adsorption of a thiol (2-mercaptopyridine, RSH) on transition metal surfaces has been confirmed for the first time. This reaction consists of photoinduced reduction of RS adsorbed on the metal to RSH and its re-adsorption during the subsequent dark process. Nearly perfect overcoating of nanometer-sized Ag particles with Pt by photodeposition (Pt-shell/Ag-core/TiO2) remarkably increases the rate of H2 production, and its reproducibility with cycle time is also much improved. X-ray photoelectron spectroscopy has provided clear evidence for the S-H bond cleavage with adsorption of RSH on Ag and Pt-shell/Ag-core particles loaded on TiO2.
Self-assembled monolayers (SAMs) have attracted much interest as a model system for many fundamental and technological phenomena including supramolecular assembly, wetting, tribology, and corrosion inhibition.1 Particularly, SAMs consisting of a variety of thiols adsorbed on Ag(111) and Au(111) have been extensively studied. It has been established that well-ordered SAMs form on the surfaces through strong S-Ag or S-Au bonds accompanied by cleavage of the S-H bonds.1 On the other hand, the outcome of the counterpart, the H• atom (for homolytic S-H bond scission) or H+ (for heterolytic S-H bond scission), remains unclear despite it being essential for the understanding of the mechanism of the formation of SAMs.2 We have recently found that bis(2-pyridyl) disulfide (RSSR) is selectively reduced to 2-mercaptopyridine (RSH) by H2O when TiO2 is used as a photocatalyst, and the reduction is remarkably enhanced with the loading of Ag nanoparticles on TiO2 (Ag/TiO2).3 This Letter describes the cycle of the dark adsorption and the photoinduced desorption of RSH on/from Ag-core/Pt-shell particles loaded on TiO2. To the best of our knowledge, * To whom correspondence should be addressed: TEL: +81-66721-2332. FAX: +81-6-6721-3384. E-mail: h-tada@ apsrv.apch.kindai.ac.jp. † Environmental Science Research Institute, Kinki University. ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University. § Department of Chemical Technology, Kurashiki University of Science and Arts. | Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University. (1) Ulman, A. Chem. Rev. 1996, 96, 1533 and references therein. (2) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (b) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (3) (a) Tada, H.; Teranishi, K.; Inubushi, Y.-i.; Ito, S. Chem. Commun. 1998, 2345. (b) Tada, H.; Teranishi, K. Ito, S. Langmuir 1999, 20, 7084.
this is the first study detecting H2 as a final product during liquid-phase adsorption of a thiol on transition metal surfaces. Figure 1A (a) shows the variation of the RSH concentration induced by a light on-off cycle in the Ag/TiO2 system.4 In the first photoprocess (0-2.5 h), irradiation leads to rapid reduction of RSSR to RSH (conversion φ2.5h ) 55.7%). Evolution of a small amount of O2 as well as a decrease in pH has been confirmed in a closed system.5 After the light is turned off, the concentration of RSH slowly decreases because of the selective adsorption of RSH on the surface of Ag.6 X-ray photoelectron spectra (XPS) indicated that the S2p binding energy of RSH adsorbed on Ag/TiO2 (161.8 eV) shifts to lower energy as compared to that for authentic RSH (163.2 eV). This is evidence for the adsorption to occur via formation of a Sδ--Agδ+ bond with a large ionic character.7 In the second photoprocess (26.5-27.5 h), illumination quickly reproduces RSH (φ27.5h ) 54.8%), which is removed again from (4) After 0.25 g of Ag(0.24 wt %)/TiO2 (or Pt(0.16 wt %)/Ag/TiO2) particles had been dispersed in 50 mL of a 5.41 × 10-5 M RSSR solution (solvent ) H2O/acetonitrile ) 99/1 v/v), the suspension was irradiated with UV light (λ > 300 nm, light intensity integrated from 320 to 400 nm ) 0.69 mW cm-2) under an N2 flow (6.1 mL min-1). The initial pH of the suspension was 5.8. The concentration of RSH was determined from the absorbance of the peak at 342 nm in the electronic absorption spectra (max ) 7.18 × 103 M-1 cm-1) as a function of time. The temperature was kept at 30 ( 1 °C throughout the reaction. (5) Stoichiometric O2 generation would be obstructed by the successive reaction of O2 with RSH or peroxide formation on the surface of TiO2 (Gu, B.; Kiwi, J.; Graetzel, M. Nouv. J. Chim. 1985, 9, 539). However, it must be stressed that there is no reductant for RSSR (or RSad) but H2O in this system because the reduction does not occur at all in the presence of acetonitrile above 1.5 vol % (ref 3b). (6) Electronic absorption spectroscopy confirmed that the concentration of RSSR is invariant during the dark process. Separate adsorption experiments on RSH provided the areas occupied by one RS group of ca. 0.1 nm2 group-1 for Ag loaded on TiO2 and 6.4 nm2 group-1 for TiO2 in each saturated adsorption state at 25 °C, respectively. The former value is in good agreement with that in a close-packing state of the RS group adsorbed with a vertical orientation (ref 3a).
10.1021/la0001530 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/23/2000
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Figure 1. (A) Variations of the RSH concentration during the light on-off cycle: (a) Ag/TiO2; (b) Pt/Ag/TiO2. (B) Time courses of H2 evolution: (a) Ag/TiO2; (b) Pt/Ag/TiO2.
the solution during the successive dark process. The adsorption rate of RSH is much smaller than that reported for octadecanethiol in the 10-5 M range.8 The solvent is known to have a large effect on the adsorption rate, e.g., octadecanethiol hardly adsorbs on Au surfaces from ethanol.8 Thus acetonitrile involved in the solution to dissolve RSSR (1% v/v)4 may cause a reduction in the rate of RSH adsorption. Figure 1B(a) shows the time course of H2 evolution in the Ag/TiO2 system.9 H2 is generated during the dark adsorption process. A small amount of H2, i.e., 0.78% of the value expected from a plausible adsorption process, RSH + Ag/TiO2 f RSad-Ag/TiO2 + (7) XPS measurements were performed using a PHI 5600Ci spectrophotometer with a monochromated Al KR X-ray source operated at 14 kV and 150 W. The takeoff angle was 45°, and typical operating pressures were 1 × 10-8 Pa. All the binding energies were referenced with respect to the carbon peak of C-H at 285.0 eV. Detailed XPS data on S2p for SAMs formed on Ag and Au surfaces have been reported in the following literature; Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (8) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
is detected during the first dark process (2.5-26.5 h). In the second dark process (27.5-51.5 h), the rate of H2 evolution decreases. No H2 was produced during illumination in the closed system. Without RSSR in the system, H2 was not formed during either the illumination or dark processes. Figure 2A shows a high-resolution transmission electron micrograph (HRTEM) of Pt-photodeposited Ag/TiO2 particles (Pt/Ag/TiO2).10 Metal particles spanning from 5 to 10 nm are observed on the surface of TiO2. Irradiation by an electron beam focused on the arrowed single metal particle yielded the energy-dispersive X-ray spectrum (EDS) (Figure 2B).11 This particle consists of Ag and Pt. The metal-loaded TiO2 particles (1 g) were treated with two kinds of etchants (50 mL), aqua regia (a) and concentrated HNO3 (b), and the concentrations of Ag and Pt were determined by inductively coupled plasma emission spectrophotometry (ICPS). Etchant (a) dissolves both Ag and Pt, whereas etchant (b) dissolves only Ag. The concentrations of Ag in the etchants (a) and (b) treating the Ag/TiO2 particles were 50.3 and 48.6 ppm, respectively. On the other hand, the concentrations of Ag and Pt in the etchant (b) treating the Pt/Ag/TiO2 particles were, respectively, little more than 0.4 and 1.0 ppm, while 60.8 ppm of Ag and 30.4 ppm of Pt were detected from the etchant (a) treating the same particles. These results indicate that most Ag particles are covered with Pt; i.e., the metal particles have an Ag-core/Pt-shell type form (Agc/Pts). The action of the Ag nanoparticles as reduction sites in the photocatalytic reaction, where both oxidation (H2O + 4h+ f 4H+ + O2) and reduction (Pt4+ + 4e- f Pt0) occur simultaneously on the TiO2 surface, explains the coverage of Ag with Pt.12 Figure 1A(b) shows the time dependence of the RSH concentration in the Pt/Ag/TiO2 system. The rates of the first photoreaction (0-1 h, φ1h ) 66.2%) and the second photoreaction (25-26 h, φ26h ) 67.6%) are greater than those of the Ag/TiO2 system. The rate depends on both the kind and the amount of metals deposited on TiO2.13 The overcoating with Pt drastically enhances H2 generation (Figure 1B(b)). For the first dark process (1-25 h) and for the second dark process (26-51.5 h), the quantities of H2 rise to 3.44% and 3.22%, respectively, of the values expected from the adsorption amounts of RSH. Even at the sixth cycle, ca. 5.0 × 10-8 mol of H2 was detected. Just as in the Ag/TiO2 system, both RSSR and preservation of the illuminated suspensions in the dark are necessary for H2 formation. A chemical shift of the S2p binding energy to lower energy by 1.4 eV with adsorption of RSH on Pt/ Ag/TiO2 was also observed in the XPS spectra, suggesting that the adsorption on Agc/Pts takes place through S-H (9) After irradiation had been stopped, the H2 evolved in the closed system was quantified by gas chromatography (tcd column Shimadzu SHINCARBON T (6 m × 2 mm) as a function of time. The carrier gas was Ar (350 kPa) at an injection temperature of 50 °C and a column temperature of 50 °C. (10) Ag/TiO2 particles (10 g) were dispersed in a 5.52 × 10-4 M H2PtCl6‚6H2O aqueous solution (245 mL), and then 5 mL of CH3OH was added as a reducing agent. After the suspension had been allowed to stand overnight with stirring under dark conditions, it was irradiated with UV light (λ > 300 nm) under an N2 atmosphere for 1 h. (11) HRTEM measurements were carried out using a JEOL JEM3000F (applied voltage ) 300 kV). The EDS was collected at a beam radius below 3 nm during 100 s with a takeoff angle of 20° and a solid angle of 0.09 (steradian). (12) Sclafani, A.; Mozzanega, M.-N.; Herrmann, J.-M. J. Catal. 1997, 168, 117. (13) Tada, H.; Teranishi, K.; Inubushi, Y.-i.; Ito, S. Langmuir 2000, 16, 3304.
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Figure 2. (A) A typical HRTEM of Pt/Ag/TiO2 particles. (B) The ED spectrum of the single metal particle shown by an arrow in the left micrograph. Scheme 1. A Proposed Mechanism of the Cycle Reaction
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bond cleavage.14 Also, almost an equal amount of H2 was evolved by adding directly authentic RSH into the suspension of Pt/Ag/TiO2 in an N2 atmosphere. The amounts of RSH adsorbed on Pt/Ag/TiO2 (Γ ) (1.10 ( 0.15) × 10-5 mol g-1) and Ag/TiO2 (Γ ) (0.92 ( 0.22) × 10-5 mol g-1), determined from the decreases in the concentrations of RSH during the dark processes, are comparable. Accordingly, the decrease in the activation energy for H2 generation from the metal surface seems to be partly responsible for the effect of the Pt overcoating. Scheme 1 illustrates the fundamental mechanism proposed for the cycle reaction. In the initial stage of the photoreaction, RSSR adsorbs selectively on the surface of Ag (or Pt/Ag) via the S-S bond cleavage.1 Electron-hole pairs (e-‚‚‚h+) are generated by the band gap excitation of TiO2. A portion of the conduction band electrons (e-cb) escaping from the recombination flow into Ag (or Agc/ Pts),15 while the holes are left in the valence band (h+vb). Reduction of the adsorbed RS group to RSH by e-cb3a,16 and oxidation of H2O by h+vb proceed simultaneously on (14) Hubbard et al. and Bryant et al. drew the same conclusion for the SAMs of thiols on Pt surfaces by means of high-resolution electron energy loss spectroscopy and Raman scattering spectroscopy, respectively. (a) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davison, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N., Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (b) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (15) Pichat, P.; Herrmann, J.-M. Photocatalysis fundamentals and Applications; Serpone, N.; Pelizzetti, E. Eds.; John Wiley & Sons: New York, 1989. (16) A general description of photodesorption has recently been given in the following literature: Bickley, R. I. Heterogeneous Photocatalysis; Schiavello, M., Ed.; John Wiley & Sons: Chichester, 1997.
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the metal surface and the TiO2 surface, respectively. Under dark conditions, RSH spontaneously adsorbs on Ag (or Pt/Ag) by virtue of strong Sδ--Agδ+ or Sδ--Ptδ+ bonds. Coupling of a part of H adatoms on the metal surfaces results in H2 formation. This is consistent with the accelerating effect due to the coverage of Ag with Pt, a good catalyst for H2 generation.17 However, it is wellknown that Pt (not Agc/Pts) loaded on TiO2 also promotes the back reaction in the usual photocatalytic H2O splitting, where H2 and O2 are concurrently generated during illumination. In such cases, most H2 produced photocatalytically is consumed when the light is turned off.18 This is in marked contrast to the behavior in the present system. It has been confirmed that H2 is generated upon adsorption of a thiol on Ag or Pt/Ag surfaces in a low yield. This study will provide significant information about SAM formation on transition metal surfaces. A subsequent study is currently being conducted to elucidate the origin of the unique action of Agc/Pts metal deposits. Acknowledgment. The authors express sincere gratitude to Dr. Tomoki Akita (Osaka National Research Institute) for the HRTEM measurements. Supporting Information Available: Description of the reactor. This material is available free of charge via the Internet at http://pubs.acs.org. LA0001530 (17) Sakata, T.; Kawai, T.; Hashimoto, K. Chem. Phys. Lett. 1982, 88, 50. (18) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72, 83.
