Visible-Light-Sensitized Production of Hydrogen Using

Feb 15, 2006 - Visible-Light-Sensitized Production of Hydrogen Using Perfluorosulfonate Polymer-Coated TiO2 Nanoparticles: An Alternative Approach to ...
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Langmuir 2006, 22, 2906-2911

Visible-Light-Sensitized Production of Hydrogen Using Perfluorosulfonate Polymer-Coated TiO2 Nanoparticles: An Alternative Approach to Sensitizer Anchoring Hyunwoong Park and Wonyong Choi* School of EnVironmental Science and Engineering and Department of Chemistry, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea ReceiVed September 26, 2005. In Final Form: January 9, 2006 TiO2 sensitized by derivatized ruthenium bipyridyl complexes has been intensively investigated as a tool to utilize visible light. This article describes an alternative approach to attaching ruthenium complex sensitizers at the TiO2/H2O interface, which is a much simpler and more efficient way to produce hydrogen. The surface of TiO2 particles are simply coated with perfluorosulfonate polymer (cation-exchange resin: Nafion), and then Ru(bpy)32+ (as a cationic form), whose bipyridyl ligands are not functionalized with carboxylic acid groups, are bound within the Nafion layer through electrostatic attraction. The visible-light-induced production of H2 on Nf/TiO2 using simple Ru(bpy)32+ as a sensitizer is far more efficient than that on Ru(dcbpy)3-TiO2, upon which many sensitized photoelectrochemical conversion systems are based. Effects of various experimental parameters such as pH, concentration of Ru(bpy)32+, Nafion loading, and the kind of TiO2 were investigated. Under optimized conditions, the H2 production rate was about 80 µmol/h, which corresponds to an apparent photonic efficiency of 2.6%. The roles of the Nafion layer on TiO2 in the sensitized H2 production are proposed to be twofold: to provide binding sites for cationic sensitizers and to enhance the local activity of protons in the surface region.

Introduction The production of hydrogen and solar energy conversion through visible light photocatalysis are of paramount interest. Semiconducting materials are being intensively investigated for these purposes. TiO2 has been the most successful UV-active photocatalyst1 but lacks visible light activity. As a mean of making TiO2 active under visible light, various sensitizers including organic dyes,2,3 organometallic complexes,4-6 and inorganic quantum dots7 have been attached to the surface of TiO2, and this sensitized TiO2 has been applied not only to photocatalytic hydrogen production8-10 but also to pollutant degradation4,5 and dye-sensitized solar cells.6 Among many kinds of visible light sensitizers, Ru(bpy)32+ and its derivatives have been the most successful and widely used.11 To anchor Ru(bpy)32+ complex sensitizers on the surface of TiO2 through chemical linkages, the bipyridyl (bpy) ligands have been typically functionalized with carboxylic acid groups (i.e., RuIILx(dcbpy)y) (dc ) dicarboxyl).4-6,10 Such chemical anchoring, however, is not * Corresponding author. E-mail: [email protected]. Phone: +8254-279-2283. Fax: +82-54-279-8299. (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Cao, J.; Sun, J.-Z.; Hong, J.; Yang, X.-G.; Chen, H.-Z.; Wang, M. Appl. Phys. Lett. 2003, 83, 1896. (3) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.; Arakawa, H. Chem. Commun. 2001, 569. (4) (a) Cho, Y.; Choi, W.; Lee, C.-H.; Hyeon, T.; Lee, H.-I. EnViron. Sci. Technol. 2001, 35, 966. (b) Bae, E.; Choi, W. EnViron. Sci. Technol. 2003, 37, 147. (5) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J. Phys. Chem. B 2004, 108, 14093. (6) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (7) Peter, L. M.; Riley, D. J.; Tull, E. J.; Wijayantha, K. G. U. Chem. Commun. 2002, 1030. (8) Mau, A. W.-H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (9) (a) Hirano, K.; Suzuki, E.; Ishikawa, A.; Moroi, T.; Shiroishi, H.; Kaneko, M. J. Photochem. Photobiol., A 2000, 136, 157. (b) Sabate´, J.; Cervera-March, S.; Simarro, R.; Gime´nez, J. Int. J. Hydrogen Energy 1990, 15, 115. (10) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J. Phys. Chem. 1986, 90, 1107. (11) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159.

sufficiently stable in an aquatic environment and can be prepared only in the acidic pH region.4,5 Therefore, H2 production using sensitized TiO2 has not been very successful. Nafion, an anionic perfluorinated polymer with sulfonate groups, is a chemically and photochemically stable cation exchanger.12 It has been often used either as a film matrix to incorporate CdS13 and TiO214 nanoparticles or as a surfacemodifier resin that is coated onto TiO2 to function as a spectroscopic sensor15 or a photocatalyst.16,17 In a resent study, we prepared Nafion-coated TiO2 (Nf/TiO2) particles to modify the surface charge and thereby to enhance the photocatalytic reactivities under UV or visible light.17 Nafion-coated TiO2 exhibited significantly enhanced adsorption capacity for cationic compounds such as tetramethylammonium ((CH3)4N+), methylene blue, and rhodamine B. In addition, the photosensitized degradation of the cationic dyes on TiO2 under visible light irradiation was markedly enhanced by the Nafion coating. This indicates that electron transfer takes place freely from the excited dye adsorbed in the Nafion adlayer to the TiO2 conduction band (CB) and to dissolved oxygen as a final electron acceptor.15,18 This study tries to utilize the cation-binding ability of Nf/TiO2 in dye sensitization and photoenergy conversion processes. We (12) (a) Perfluorinated Ionomer Membranes; Eisenberg, A., Yager, H. L., Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. (b) Mauritz, K. A.; Hora, C. J.; Hopfinger, A. J. Theoretical Model for the Structures of Ionomers: Application to Nafion Materials. In Ions in Polymers; Eisenberg, A., Eds.; Advances in Chemistry Series 187; American Chemical Society: Washington, DC, 1980; pp 123-144. (13) (a) Mau, A. W.-H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (b) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 732. (14) (a) Liu, P.; Bandara, J.; Lin, Y.; Elgin, D.; Allard, L. F.; Sun, Y.-P. Langmuir 2002, 18, 10398. (c) Albu-Yaron, A.; Arcan, L.; Heitner-Wirguin, C. Thin Solid Films 1990, 185, 181. (d) Vichi, F. M.; Tejedor-Tejedor, M. I.; Anderson, M. A. Chem. Mater. 2000, 12, 1762. (15) Choi, H. N.; Cho, S.-H.; Lee, W.-Y. Anal. Chem. 2003, 75, 4250. (16) Vohra, M. S.; Tanaka, K. EnViron. Sci. Technol. 2001, 35, 411. (17) Park, H.; Choi, W. J. Phys. Chem. B 2005, 109, 11667. (18) Rabani, J.; Ushida, K.; Yamashita, K.; Stark, J.; Gershuni, S.; Kira, A. J. Phys. Chem. B 1997, 101, 3136.

10.1021/la0526176 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/15/2006

H2 Production Via Polymer-Coated TiO2 Nanoparticles Scheme 1. Comparison of Visible-Light-Induced H2 Production in (a) Ru(dcbpy)3-TiO2 and (b) Ru(bpy)32+-Nf/TiO2 Systems

attached underivatized Ru(bpy)32+ as a photosensitizer on Nf/ TiO2 particles through electrostatic attraction and investigated visible-light-sensitized H2 production on them. The basic strategy of the Ru(bpy)32+-Nf/TiO2 system is illustrated in Scheme 1b and compared with conventional chemical anchoring using the carboxylate linkage in ligand-functionalized Ru(dcbpy)3 (Scheme 1a). This alternative method of sensitizer anchoring is simpler and more efficient in producing hydrogen than the conventional approach. The roles of Nafion layers on TiO2 and the effects of various factors in sensitized hydrogen production are investigated in detail. Experimental Section Materials and Reagents. TiO2 powder (Degussa P25), a mixture of anatase and rutile (8:2) with a BET surface area of 50 m2/g and primary particle sizes of 20-30 nm, was used as a base material. Hombikat (HBK) UV-100 (anatase), Ishihara ST-01 (anatase), and Aldrich rutile were also used and compared as the support of the sensitizer. Their BET surface areas were measured to be 348, 340, and 1.9 m2/g, respectively. Pt nanoparticles were loaded onto the surface of TiO2 as a cocatalyst for hydrogen production, and a typical photodeposition method was employed for platinization.19 An aqueous TiO2 suspension (0.5 g/L) containing 1 M methanol and 0.1 mM chloroplatinic acid (H2PtCl6, Aldrich) was irradiated with a 200-W mercury lamp for 30 min. After irradiation, the suspension was filtered with a 0.45-µm filter, washed with distilled water, and collected as a powder after drying at 70 °C. The concentration of unused chloroplatinic acid remaining in the filtrate solution after the photodeposition was determined by inductively coupled plasma/ atomic emission spectroscopy (IRIS/AP, Thermo Jarrell Ash) to quantify the amount of deposited Pt. A typical Pt loading on TiO2 was estimated to be ca. 3 wt %. Transmission electron micrographic image of platinized TiO2 showed that Pt particles were in the size range of 2-5 nm and well dispersed on TiO2 particles. The notation “TiO2” represents platinized TiO2 for simplicity throughout the text. Tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)3Cl2) (Aldrich) was used as a cationic photosensitizer in the Nf/TiO2 system. Tris(4,4′-dicarboxy-2,2′-bipyridyl)ruthenium(II) chloride (Ru(dcbpy)3Cl2) was prepared according to a previous method4 and adsorbed onto the surface of TiO2. The two sensitizing systems of Ru(bpy)32+Nf/TiO2 and Ru(dcbpy)3-TiO2 are compared throughout the study. The adsorption of the sensitizer on TiO2 (or Nf/TiO2) was measured (19) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4318.

Langmuir, Vol. 22, No. 6, 2006 2907 as a function of pH: the amount of sensitizer adsorbed on TiO2 (or Nf/TiO2) was calculated from the absorbance difference between the initial sensitizer solution and the filtered solution of the sensitizeradsorbed suspension. The UV-visible absorption spectra of the sensitizer solutions were recorded with a UV-vis spectrophotometer (Shimadzu UV-2401PC). Surface Characterization. For the surface analysis of Nf/TiO2, 0.1 g of TiO2 (P25) powder was coated with 0.1 mL of Nafion solution, dried at 80 °C overnight, and formed into a thin disk with a high-pressure pelletizer (Carver). The pellet was analyzed with X-ray photoelectron spectroscopy (XPS, Kratos XSAM 800 pci) using Mg KR lines (1253.6 eV) as an excitation source. The spectrum of the sample was taken after Ar+ (3 keV) sputter cleaning. Surface charging was minimized by spraying low-energy electrons over the sample using a neutralizer gun. Binding-energy spectra were recorded in the regions of C 1s, Ti 2p, O 1s, Pt 4f, F 1s, and S 2p. The binding energies of all peaks were referenced to the Ti 2p line (459.0 eV) originating from TiO2. The electrophoretic mobilities of platinized TiO2 particles in an aqueous suspension without or with the addition of Nafion were measured to determine their ζ potentials as a function of pH. An electrophoretic light-scattering spectrophotometer (ELS 8000, Otsuka) equipped with a He-Ne laser and a thermostated flat board cell was used. Photosensitized H2 Production. Nafion-coated TiO2 (Nf/TiO2) was prepared as follows. An aliquot (typically 0.1 mL) of Nafion solution (5 wt % solution in a mixture of alcohol and water, Aldrich) was added to the platinized TiO2 powder (0.1 g), mixed well, and dried at room temperature overnight or at 90 °C for 10 min. The Nf/TiO2 powder obtained (0.0125 g) was suspended in an aqueous solution of Ru(bpy)32+ and EDTA (Aldrich) in a glass reactor (25 mL) with a quartz window. Although Nf/TiO2 powder was not mixed well initially because of the hydrophobic nature of Nafion, vigorous stirring for 30 min caused it to be well suspended in the solution. The suspension was purged with N2 for 30 min prior to illumination. A 450-W Xe arc lamp (Oriel) was used as a light source. Light passed through a 10-cm IR water filter and a cutoff filter (λ > 420 nm), and then the filtered light was focused onto the quartz window of the reactor. During irradiation, the headspace gas (ca. 10 mL) of the reactor was intermittently sampled and analyzed for H2 using a gas chromatograph (Agilent 6890A) equipped with a thermal conductivity detector and a 5-Å molecular sieve column. Light intensity was measured by chemical actinometry using (E)-R-(2,5dimethyl-3-furylethylidene) (isopropylidene)succinic anhydride (Aberchrome 540) as described elsewhere.20 A typical incident light intensity was measured to be about 4 × 10-3 einstein L-1 min-1 in the wavelength range of 420-550 nm. The apparent photonic efficiency (APE) of H2 production was calculated by APE ) 2 × [(number of H2 molecules generated)/(number of incident photons)].

Results and Discussion Effect of Nafion Coating on Surface Charge and Sensitizer Adsorption. Figure 1a shows the change in ζ potential of suspended TiO2 (platinized P25) particles in the absence or presence of Nafion (0.9 g/g of TiO2) as a function of pH. The point of zero zeta potential (PZZP) of TiO2 is determined to be around 6 without Nafion. However, the addition of Nafion drastically reduces ζ potentials of TiO2 to be negative over the entire pH range. This should be attributable to the fact that the anionic sulfonate groups (-SO3-) in the Nafion layer outnumber the positively charged surface functional groups (tTiOH2+). In addition, it indicates that a Nf/TiO2 particle has a high capacity to bind positively charged molecules such as Ru(bpy)32+. The XPS spectra of Nf/TiO2 powder shown in Figure 1b indicate the presence of a Nafion layer on TiO2 that contains the fluorine and the sulfur. (20) Heller, H. G.; Langan, J. R. J. Chem. Soc., Perkin Trans. 1981, 2, 341.

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Figure 1. (a) Zeta potentials of suspended platinized TiO2 (Degussa P25) particles in the presence or absence of Nafion as a function of pH. [TiO2] ) 2 mg/L; [Nafion] ) 0.9 g/g of TiO2. (b) XPS survey spectrum of Nafion-coated platinized TiO2 (Nf/TiO2: 45 mg of Nf/g of TiO2) powder. The insets show the S 2p and Pt 4f bands of the Nf/TiO2 powder sample.

The pH-dependent adsorption behaviors of Ru(dcbpy)3 and Ru(bpy)32+ on TiO2 and Nf/TiO2 are compared in Figure 2. The UV-visible absorption spectra of each sensitizer remaining after the equilibrated adsorption on TiO2 (or Nf/TiO2) were measured at several pH values (Figure 2a and b), and the pH-dependent adsorption of the sensitizers is plotted in Figure 2c. The dye adsorption in the Ru(dcbpy)3-TiO2 system was favored only in acidic pH and rapidly decreased with increasing pH, as reported in other studies.4,5,10 The inhibited adsorption at basic pH is ascribed to the electrostatic repulsion between the deprotonated carboxylates (-CO2-) and the negative surface charge of TiO2 (tTiO-). However, the adsorption of Ru(bpy)32+ on TiO2 gradually increased with pH, and the Nafion coating on TiO2 markedly enhanced the uptake of Ru(bpy)32+. This should be ascribed to the cation-exchange ability of Nafion adlayers that bind more Ru(bpy)32+ molecules near the surface of TiO2. Photosensitized Hydrogen Production. Figure 3 compares the time profiles of Ru-complex-sensitized H2 production between the TiO2 (Scheme 1a) and Nf/TiO2 (Scheme 1b) systems. H2 was not evolved at all in the TiO2 suspension with Ru(bpy)32+, whereas the presence of Nafion layers on TiO2 surface enabled the production of H2 at a rate of 23 µmol/h, which is faster than that for the conventional Ru(dcbpy)3-TiO2 system. Although Ru(bpy)32+ is not chemically attached to the surface of TiO2, its sensitizing action is not inhibited as long as it is bound electrostatically within the Nafion layer. In addition, the pH dependence of the sensitized H2 production is very different between the two sensitizing systems, as Figure 4 shows. pHdependent H2 production in the Ru(dcbpy)3-TiO2 system is closely related to the pH-dependent adsorption of Ru(dcbpy)3 on TiO2 (Figure 2). It rapidly decreased with increasing pH and was negligible above pH 6. H2 production in the Ru(bpy)32+-TiO2

Park and Choi

Figure 2. UV-visible absorption spectra of (a) Ru(bpy)32+ remaining after the equilibrated adsorption on Nf/TiO2 and (b) Ru(dcbpy)3 remaining after the equilibrated adsorption on bare TiO2 at several pH values. The spectra were taken after 2-fold dilution. (c) pH-dependent adsorption of Ru sensitizers in TiO2 (platinized) and Nf/TiO2 (i.e., Nafion-coated platinized TiO2) suspensions. [TiO2 (P25)] ) 0.5 g/L; [Ru(dcbpy)3]i ) [Ru(bpy)32+]i ) 20 µM; [Nafion] ) 45 mg/g of TiO2.

Figure 3. Comparison of H2 production in Ru(dcbpy)3-TiO2, Ru(bpy)32+-TiO2, and Ru(bpy)32+-Nf/TiO2 suspensions. [TiO2 (P25)] ) 0.5 g/L; [Ru(dcbpy)3] ) [Ru(bpy)32+] ) 10 µM; [Nafion] ) 45 mg/g of TiO2; [EDTA] ) 4 mM; purged with N2 for 30 min prior to visible light (λ > 420 nm) irradiation. The pH values of Ru(dcbpy)3-TiO2, Ru(bpy)32+-TiO2, and Ru(bpy)32+-Nf/TiO2 systems were 3.0, 6.0, and 6.0, respectively.

system was completely absent over the whole pH range, although a fraction of Ru(bpy)32+ was adsorbed on TiO2 through electrostatic attraction (Figure 2), which is consistent with a previous report.10 It has been reported that H2 production in the Ru(bpy)32+-TiO2 system was possible only with extremely high concentrations (e.g., 0.1 mM Ru(bpy)32+ + 17 g/L platinized TiO2 or 0.5 mM Ru(bpy)32+ + 0.5 g/L platinized TiO2) to achieve a production rate as low as 10 µmol H2/h.9 However, the system of Ru(bpy)32+-Nf/TiO2 exhibited higher activity and a much wider pH range for H2 production compared with the conventional sensitizing system of Ru(dcbpy)3-TiO2. The H2 production rate

H2 Production Via Polymer-Coated TiO2 Nanoparticles

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Figure 4. pH-dependent H2 production rate in Ru(dcbpy)3-TiO2, Ru(bpy)32+-TiO2, and Ru(bpy)32+-Nf/TiO2 suspensions. Experimental conditions are identical to those of Figure 3.

was maximal around pH 6, where the Ru(dcbpy)3-TiO2 system exhibited negligible activity. Control experiments with Nf/TiO2 alone or with Ru(bpy)32+ and colloidal platinum in the absence of TiO2 generated no hydrogen under visible light. Although excited Ru(bpy)32+ has a sufficient reduction potential for H2 production (Eo[Ru(bpy)33+/ Ru(bpy)32+*] ) -0.84 VNHE), it alone cannot generate H2 on a Pt catalyst unless suitable electron-transfer mediators such as methyl viologen and TiO2 are present.11 The present results indicate that Ru(bpy)32+ sensitizers are able to diffuse through the Nafion layer and inject electrons into TiO2 within the lifetime of the excited state.18 Incidentally, the lifetime of Ru(bpy)32+* in the Nafion environment is prolonged: the lifetime of the triplet excited state of Ru(bpy)32+ in wet Nafion was measured to be around 900-1200 ns, which is much longer than that in water or in dried Nafion (600 ns).21 The role of the Nafion layer in extending the lifetime of excited sensitizers should contribute to the overall process as well. Effects of Component Concentration and Optimization. Figure 5 shows the effect of Ru(bpy)32+ and EDTA (electron donor) concentrations on H2 production in a Nf/TiO2 suspension. As the concentration of the sensitizer (or electron donor) increases, the H2 production rate (inset) increases rapidly in the initial stage and then reaches a saturation level at around [Ru(bpy)32+] ) 20 µM and [EDTA] ) 2 mM. No hydrogen was produced in the absence of either the sensitizer or EDTA. The kind of TiO2 substrate also influences H2 production. Two anatase (HBK and ST01), a rutile (Aldrich), and an anataserutile mixture (P25) are compared in Figure 6. The order of the H2 production rate follows the order of surface area: HBK > ST01 > P25 > rutile. Rutile was completely inactive for H2 production because the CB edge potential in rutile is slightly more positive than in anatase (i.e., ca. +0.1 VNHE vs -0.1 VNHE at pH 0),22 which is not sufficient to drive proton reduction (H+ + e- f 1/2H2: 0.0 VNHE at pH 0). The sensitized H2 production was greatly enhanced with Nf/HBK (ca. 80 µmol h-1) compared to that obtained with Nf/P25. This corresponds to an apparent photonic efficiency of 2.6% according to the actinometric measurement described in the experimental. However, the higher efficiency of TiO2 with a larger surface area appears not to be related to the sensitizer adsorption because the amount of Ru(bpy)32+ adsorbed on the TiO2 surface depends on the Nafion loading, not on the surface area of TiO2. Two TiO2 samples having different specific surface areas (P25 with 50 m2/g vs (21) (a) Lin, R.-J.; Onikubo, T.; Nagai, K.; Kaneko, M. J. Electroanal. Chem. 1993, 348, 189. (b) Yi, X.-Y.; Wu, L.-Z.; Tung, C.-H. J. Phys. Chem. B 2000, 104, 9468. (22) Gra¨tzel, M. Heterogeneous Photochemical Electron-Transfer Reactions; CRC Press: Boca Raton, FL, 1987.

Figure 5. Effects of (a) Ru(bpy)32+ and (b) EDTA concentrations on H2 production in Ru(bpy)32+-Nf/TiO2 suspensions. [TiO2 (P25)] ) 0.5 g/L; [Nafion] ) 45 mg/g of TiO2; [EDTA] ) 4 mM (in part a); [Ru(bpy)32+] ) 10 µM (in part b); pH ≈ 6; λ > 420 nm irradiation.

Figure 6. Comparison among different kinds of TiO2 substrates for sensitized H2 production as a function of Ru(bpy)32+ concentration. The inset compares the adsorption isotherms of Ru(bpy)32+ between Nf/P25 and Nf/HBK systems. [TiO2] ) 0.5 g/L; others are identical to those of Figure 5a.

HBK with 350 m2/g) but the same Nafion loading were nearly identical in their adsorption capacity for Ru(bpy)32+ (Figure 6 inset). In addition, Figure 7 shows that the optimal Nafion loading was the same, about 45 mg/g of TiO2 for both P25 and HBK despite the marked difference in the specific surface area. On the basis of the average TiO2 particle (P25) diameter of 30 nm and the densities of Nafion and TiO2 of 0.9 and 3.9 g/cm3, respectively, we calculate that 45 mg of Nf/g of TiO2 corresponds to the thickness of the Nafion adlayer of 0.98 nm. As for Nf/HBK TiO2 having a much higher specific surface area, the corresponding Nafion layer should be even thinner. The average distance between Ru(bpy)32+ in the Nafion layer and the TiO2 surface should be shorter in Nf/HBK than in Nf/P25. This may explain why Nf/ HBK shows a higher sensitization activity than Nf/P25 despite the fact that the two systems have the same loadings of Nafion and Ru(bpy)32+. However, excessive Nafion loading should result in a thicker layer, which retards both the electron transfer from

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Figure 7. Effect of Nafion concentration on H2 production in Ru(bpy)32+-Nf/TiO2 suspensions. [TiO2 P25] ) [TiO2 HBK] ) 0.5 g/L; [Ru(bpy)32+] ) 20 µM; [EDTA] ) 4 mM; pH ≈ 6; λ > 420 nm irradiation.

the excited sensitizer to TiO2 and the diffusion of electron donors within the layer. The higher H2 production with the larger surface area also seems to be related to the larger number of Pt catalyst sites on TiO2. Role of the Nafion Layer. Ru(bpy)32+-Nf/TiO2 is better than the Ru(dcbpy)3-TiO2 system in sensitized H2 production despite the lack of direct chemical linkage of the sensitizer to the TiO2 surface. First of all, the role of the Nafion layer on TiO2 should be related to the electrostatic binding between the permanent cationic dyes (Ru(bpy)32+) and the sulfonate groups (-SO3-) attached to the poly(tetrafluoroethylene) backbone. With 12.5 mg of Nf/TiO2 (45 mg of Nf/g of TiO2) suspended in aqueous solution and 0.2 µmol of Ru(bpy)32+ adsorbed on Nf/TiO2 (Figure 2), it is estimated that about 80% of the total number of ionexchange sites of the Nafion (ca. 0.9 mequiv/g of Nf) coating on TiO2 are associated with Ru(bpy)32+. This ion pairing in Ru(bpy)32+-Nf/TiO2 is certainly less influenced by pH than the chemical linkage through the carboxylate group in the Ru(dcbpy)3-TiO2 system (Figure 4). However, the role of the Nafion layer seems to be more than simply providing binding sites for cationic sensitizers because H2 production was not proportional to the sensitizer adsorption. For example, the amount of Ru(bpy)32+ adsorbed onto the TiO2 surface in the neutral pH region was about 30% of that on Nf/TiO2, but H2 production in Ru(bpy)32+-TiO2 was completely absent. In addition, H2 production in Ru(bpy)32+-Nf/TiO2 at pH 3 was higher than that in Ru(dcbpy)3-TiO2 despite the fact that the sensitizer adsorption at pH 3 in the former was only one-third of that in the latter (cf. Figures 2 and 4). Results from a related study that measured the photocurrent action spectra of Nf/TiO2 and bare TiO2 electrodes, which were sensitized by Ru(bpy)32+ and Ru(dcbpy)3, respectively, showed that the photocurrents generated with the Ru(bpy)32+-Nf/TiO2 electrode were much smaller than those obtained with the Ru(dcbpy)3-TiO2 electrode in the visible illumination region.23 That is, the intrinsic efficiency of visible light sensitization in Ru(bpy)32+-Nf/TiO2 should be lower than that in the Ru(dcbpy)3-TiO2 system. Despite this fact, we obtained higher hydrogen production with Ru(bpy)32+-Nf/TiO2. To investigate the role of the Nafion layer in the sensitization process, another reductive conversion reaction was tested. We compared the photosensitized reductive dechlorination of CCl4 in Ru(dcbpy)3-TiO2, Ru(bpy)32+-TiO2, and Ru(bpy)32+-Nf/ TiO2 as function of pH (Figure 8). In this case, 2-propanol was used as an electron donor (D) instead of EDTA. The photoreductive dechlorination of CCl4 can be driven by a dye-sensitized (23) Kavan, L.; Gra¨tzel, M. Electrochim. Acta 1989, 34, 1327.

Park and Choi

Figure 8. Ru-complex-sensitized dechlorination of CCl4 in TiO2 and Nf/TiO2 suspensions as a function of pH under visible light. [TiO2 (P25)] ) 0.5 g/L; [Nafion] ) 45 mg/g of TiO2; [Ru complex] ) 10 µM; [CCl4]0 ) ca. 1 mM; [2-propanol]0 ) 0.4 M; purged with N2 for 30 min before irradiation.

process on TiO2 (reactions 1-4).4

RuIIL3-TiO2 + hν (visible light) f RuIIL3*-TiO2 (1) RuIIL3*-TiO2 f RuIIIL3-TiO2 + ecb-

(2)

ecb- + CCl4 f •CCl3 + Cl-

(3)

RuIIIL3-TiO2 + D f RuIIL3-TiO2 + D•+

(4)

If the role of Nafion layers is mainly to provide binding sites for cationic sensitizers, then we should expect the sensitized dechlorination of CCl4 on Nf/TiO2 to take place with an efficiency similar to that of the sensitized H2 production on Nf/TiO2. However, the two sensitized reactions taking place on Nf/TiO2 are drastically different. The CCl4 dechlorination under visible light could be achieved only with Ru(dcbpy)3-TiO2 and was negligible with Ru(bpy)32+-Nf/TiO2 whereas the H2 production was the most active with the latter. It appears that the electron transfer from TiO2 CB to CCl4 is strongly inhibited on Nf/TiO2. This implies either that the diffusion of CCl4 onto the surface site is hindered by the Nafion layer or that most CB electrons are transferred to protons, not to CCl4 on Nf/TiO2. The hidden role of the Nafion layer in promoting H2 production seems to be related to the enhanced proton activity in the surface region of Nf/TiO2. Owing to the presence of sulfonate groups within the Nafion layer, the proton concentration in the surface region of TiO2 could be highly enhanced from that of the bulk solution ([H+]Nf > [H+]aq), and hence the photoreductive H2 production (H+ (Nf) + ecb- f 1/2H2) is enhanced (Scheme 1b). It is well known that the pH in the Nafion internal channel is much lower than the pH in the aqueous bulk phase and that the aqueous suspension of Nafion behaves as a source of H+ (pKa ≈ 0.3).24 Because of this locally enhanced proton activity within the Nafion layer, most CB electrons seem to be scavenged preferentially by protons even if alternative electron acceptors such as CCl4 are presented in the Nafion layer. It also explains why EDTA efficiently serves as an electron donor on Nf/TiO2. Note that EDTA is a hexaprotic molecule (H6Y2+) having four acidic carboxyl protons and two basic ammonium protons. Nearly all EDTA molecules are present as an anionic form of H2Y2or HY3- from pH 3-10 and should be repelled from the negatively charged surface. However, EDTA in the Nafion layer might be fully protonated and present as a cationic form. Otherwise, the (24) Sondheimer, S. J.; Bunce, N.; Fyfe, C. A. J. Macromol. Sci., Polym. ReV. 1986, C26, 353.

H2 Production Via Polymer-Coated TiO2 Nanoparticles

diffusion of anionic EDTA to the oxidized sensitizer (Ru(bpy)33+) should have been blocked by the Nafion layer. In a similar context, the Ru(dcbpy)3-Nf/TiO2 system exhibited hydrogen production activity comparable to that of Ru(bpy)32+-Nf/TiO2 (data not shown), which implies that Ru(dcbpy)3 is incorporated within the Nafion layer. Ru(dcbpy)3 is normally an anion in aqueous solution, but its fully protonated form is a cation such as Ru(bpy)32+. Ru(dcbpy)3 seems to be present as a cationic species in the Nafion layer. The overall hydrogen production efficiency should be determined by the balance among the sensitizers, the electron donors, and the protons that are all incorporated within the Nafion layer. Excessive incorporation of the cationic sensitizers and electron donors may deplete protons in the Nafion layer because they are exchanged for protons. Therefore, the hydrogen production rate is saturated beyond the critical concentration level of Ru(bpy)32+ and EDTA (Figure 5) or is maximized at [Ru(bpy)32+] ) 20 µM (Figure 6). The fact that the H2 production rate was maximal around pH 6 in the Ru(bpy)32+-Nf/TiO2 system (Figure 4) also implies that the optimal balance among three components (sensitizer, electron donor, proton) is met under this condition.

Langmuir, Vol. 22, No. 6, 2006 2911

Conclusions This study demonstrates that the visible-light-induced production of H2 on Nf/TiO2 using simple Ru(bpy)32+ as a sensitizer is more efficient than that of Ru(dcbpy)3-TiO2, upon which many sensitized photoelectrochemical conversion systems are based. The present Ru(bpy)32+-Nf/TiO2 system can be prepared in a very simple way and is effective, whereas the conventional Ru(dcbpy)3-TiO2 system requires a complicated synthetic procedure for preparing derivatized bpy ligands and the sensitizer linkage to TiO2 surface is not stable enough in aquatic environments. The Nafion layer on TiO2 seems to play dual roles: (1) binding Ru(bpy)32+ sensitizers through electrostatic attraction and (2) enhancing the local activity of protons near the surface region. The present strategy provides an attractive way to develop efficient sensitized photochemical systems for hydrogen production. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Nano R&D program and by POSTECH through the POSRIP program. LA0526176