Guanidinium-Enhanced Production of Hydrogen ... - ACS Publications

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Guanidinium-Enhanced Production of Hydrogen on Nafion-Coated Dye/TiO2 under Visible Light

Jihee Park,† Jaeseon Yi,† Takashi Tachikawa,‡ Tetsuro Majima,‡ and Wonyong Choi*,† †

School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea, and ‡The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan

ABSTRACT Guanidinium cations (abbreviated as G) enhanced the visible lightinduced hydrogen production by three times in the dye(Ru(bpy)32þ)-sensitized system using TiO2 nanoparticles coated with nafion polymer. The enhanced photoactivity in the presence of G adsorbed on the nafion coating was related to the retardation of the charge recombination between the electrons injected into the TiO2 conduction band (CB) and the oxidized dye molecules, which was confirmed by time-resolved diffuse reflectance spectroscopic measurements. It is proposed that G cations located near the TiO2 surface repel the oxidized dyes to retard the recombination with CB electrons. In addition, the dye luminescence in nafion was enhanced in the presence of G, which was ascribed to the retardation of selfquenching of the excited dyes. The dual roles of G in the nafion layer increase the photoelectron density in TiO2 CB and subsequently enhance the production of hydrogen. SECTION Surfaces, Interfaces, Catalysis

T

and porous and cannot be a barrier for the access of dyes onto the TiO2 surface and the escape of H2 produced on Pt sites. The nafion-coated TiO2 particles were characterized in our previous studies.6,7 The Ru-bipyridyls are usually derivatized with chemical functional groups (e.g., carboxylate, phosphonate) for their anchoring on the surface of TiO2.8,9 However, the RuL complex (underivatized) can be bound onto Nf/TiO2 through the electrostatic attraction and the aqueous suspension of Nf/RuL/TiO2 successfully produced hydrogen under visible light.7 Although the effect of G in DSSC has been studied, the present Nf/RuL/TiO2 system employing G is different in many aspects. First of all, the surface of TiO2 is coated with nafion resin, which makes the surface conditions drastically different from those of DSSC. The concentration of G in the electrolyte solution in DSSC was maintained very high (typically 0.1 M) to allow the access of G onto the TiO2 surface. In the present study, the nafion coating on TiO2 preferentially attracts G cations into the surface region through the ion exchange process, and therefore, a much lower concentration of G (1-10 mM) was employed. The solvents, electrolytes, and electron donors are quite different as well. Therefore, the primary role of G in DSSC might be quite different from that of the present system. In this study, we observed that the photosensitized production of hydrogen on Nf/RuL/TiO2 was markedly enhanced in the presence of G. The effect of G on the photoconversion process was investigated both

he dye-sensitized nanocrystalline TiO2 system has attracted considerable attention as a promising method for solar energy conversion. The dye-sensitized solar cell (DSSC) and dye-sensitized hydrogen production that are based on the use of TiO2 nanoparticles are the most popular and successful applications. The most important parameter that determines the overall solar conversion efficiency is the photoinduced electron transfer between the adsorbed dye and the TiO2 nanoparticles. After the electron injection from the excited dye into the TiO2 CB, the subsequent recombination of the electron with the oxidized dye or the redox electrolyte may reduce the overall conversion efficiency. Several strategies have been employed to inhibit the interfacial charge recombination. For example, coating TiO2 with a thin overlayer of insulating oxide (e.g., Al2O3) enhanced the photoconversion efficiency by retarding the recombination.1,2 Introducing specific adsorbents may serve the purpose as well. Guanidinium cation, CH6N3þ, (abbreviated as G throughout the text, the molecular structure shown in Scheme 1) added as a molecular adsorbent to the electrolyte in the DSSC system has been reported to enhance the overall conversion efficiency by slowing down the surface charge recombination.3-5 In this work, the use of G was applied to the photosensitized production of hydrogen on nafion-coated TiO2 (Nf/TiO2). Tris(2,20 -bipyridine)ruthenium(II), Ru(bpy)32þ (abbreviated as RuL), was employed as a dye sensitizer. The nafion (perfluorosulfonate polymer, cation exchange resin) provides the anchoring sites for the cationic dye sensitizer and G (see Scheme 1). Nf/TiO2 has shown successful performances in photocatalysis when cationic substrates or cationic dyes are adsorbed.6,7 The nafion coating on TiO2 should be thin, loose,

r 2010 American Chemical Society

Received Date: March 5, 2010 Accepted Date: April 6, 2010 Published on Web Date: April 09, 2010

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DOI: 10.1021/jz100292v |J. Phys. Chem. Lett. 2010, 1, 1351–1355

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Scheme 1. (a) Molecular Structure of Guanidinium, G, and (b) Illustration of the Hydrogen Evolution in the Nf/RuL/TiO2 in the Presence of G

photocatalytically and photoelectrochemically, and how G enhances the conversion efficiency is discussed along with transient absorption and luminescence data. Figure 1a shows that the visible-light-sensitized production of hydrogen was much enhanced when G was coadsorbed on Nf/RuL/TiO2. When G was present in the absence of either RuL or EDTA, the H2 generation was negligible, which indicates that the primary role of G should be of neither a sensitizer nor an electron donor. In Figure 1b, the hydrogen production rate is shown as a function of the dye sensitizer concentration in the presence and absence of G. The G-enhanced hydrogen production is clearly seen over a range of dye concentrations. The sensitized activities of the Nf/RuL/TiO2 system were also tested in a PEC cell employing the Nf/RuL/TiO2 electrode. The hydrogen production and the photocurrent generation were simultaneously monitored in the PEC cell and compared in Figure 2. Both hydrogen generation and photocurrent production under visible light were enhanced in the presence of G. The fact that the G-enhanced photoactivity for H2 production is similarly observed in both the aqueous suspension and the PEC cell indicates that this enhancement effect should be related to the role of G in the visible-light-induced electron-transfer process. In the previous studies on the effect of G in DSSC,4,5 it was proposed that G can retard the charge recombination between TiO2 CB electrons and the oxidized dyes/triiodides. Although the presence of G in the electrolyte solution shifted the band edges toward positive potentials by 100 mV, the retardation of surface recombination offset the unfavorable band edge shift to produce a net positive effect.4 In this study, the effect of G on the charge recombination dynamics in the aqueous slurry of Nf/RuL/TiO2 without EDTA as an electron donor for dye regeneration was investigated by directly monitoring the trapped electrons in TiO2 using time-resolved diffuse reflectance (TDR) spectroscopy. Figure 3 compares the transient absorption decays of the electrons on Nf/RuL/TiO2 in the absence and presence of G. We monitored the transient absorbance of trapped electron (550-800 nm) during the 532 nm laser photolysis. The trapped electrons appeared within the laser pulse and slowly decayed as they recombined with the oxidized dye, RuLþ (Ru(bpy)33þ). The transient absorption decays show that the decay rate is retarded in the presence of G. When [G] increased from 1 to 10 mM, the decay rate was further reduced. This provides direct evidence that the presence of G retards the charge recombination


Figure 1. (a) Time profiles of hydrogen production in the visiblelight-irradiated suspension of Nf/RuL/TiO2 particles with and without G. (b) Hydrogen production rate as a function of the sensitizer (RuL) amount adsorbed on Nf/TiO2 in the presence and absence of G. The experimental conditions were [Nf/TiO2] = 0.5 g/L, [G] = 1 mM, [EDTA] = 10 mM, pH 4.6, λ > 420 nm, and initially N2saturated. RuL of 0.2 μmol was adsorbed on 10 mg of Nf/TiO2 in (a).

Figure 2. Time profiles of the simultaneous generation of the photocurrent and hydrogen in the dye-sensitized PEC cell that employed the Nf/RuL/TiO2 electrode under visible light. The photoanode was biased with þ0.4 V (versus Ag/AgCl).

between the TiO2 CB electron and RuLþ even in the presence of the Nf layer as it does in DSSC. We propose the following. The observed retardation in the recombination can be related to the role of cationic adsorbents interfering the interaction between TiO2 and the dye cations.10 The presence of cationic G in the nafion layer may repel cationic RuL and RuLþ from the surface of TiO2

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Figure 3. Normalized time traces of % absorption (%abs) at 700 nm during the 532 nm laser photolysis (5 mJ pulse-1) in the air-saturated slurry (pH 4.5) of Nf/RuL/TiO2 with [G] = 0, 1, or 10 mM. % abs = (R0 - R)/R0 100, where R and R0 represent the intensities of the diffuse reflected monitor light with and without excitation, respectively. Scheme 2. Energy Levels and the Photoinduced Electron Transfer Pathways in the Nf/RuL/TiO2 System with Ga

a 1: dye excitation; 2: electron injection from RuL* to TiO2; 3: electron recombination with the oxidized dye (RuLþ); 4: luminescent decay; 5: self-quenching; 6: regeneration of the oxidized dye by electron donor.

(see Scheme 2). As a result, the electron transfers from excited RuL (RuL*) to TiO2 and from TiO2 to RuLþ should be hindered by G. Since G cations should repel the trivalent RuLþ (Ru(bpy)33þ) more strongly than the divalent RuL (Ru(bpy)32þ), the electron recombination (path 3 in Scheme 2) should be retarded more than the electron injection (path 2 in Scheme 2) by G cations. In addition, the electron injection from RuL* to TiO2 is so fast (completed within the laser pulse in Figure 3) that it is little influenced by the presence of G. On the other hand, the electron transfer (recombination) from TiO2 to RuLþ is far slower and sensitively affected by the presence of G cations. This can be similarly compared with the case of dye-sensitized TiO2 coated with a thin layer of insulating oxide where the forward electron injection was little affected but the backward recombination was significantly retarded by the barrier layer.1,2 The G cations bound in the nafion layer seem to form a barrier for the backward electron transfer. Another possible role of G is to influence the lifetime of the excited dye (RuL*) in the Nf layer by interfering in the RuL*-RuL interaction. To investigate whether G can interact


Figure 4. Time traces of the transient emission intensity at 650 nm during the 532 nm laser photolysis (0.5 mJ pulse-1) (a) in aqueous solutions (pH 4.5, HNO3) containing RuL (abs at 532 nm = 0.13) and (b) in the air-saturated slurry (pH 4.5) of Nf/RuL/TiO2 with [G] = 0, 1, or 10 mM. (c) Static photoluminescence spectra (λex = 451 nm) of aqueous solutions containing RuL, G, and Nf resin (without TiO2); A-C without Nf and D-F with [Nf] = 0.5 wt %. Other conditions are [RuL] = 0.5 mM for all spectra, [G] = 0 mM (A, D), [G] = 1 mM (B, E), and [G] = 10 mM (C, F).

with the RuL*, the transient emission decays of RuL* were measured in an aqueous solution (Figure 4a) and aqueous suspension of Nf/TiO2 (Figure 4b). The average lifetimes Æτæ of the RuL* emission (0.37 ( 0.01 μs) in aqueous solution are independent of the concentration of G (1-10 mM), whereas Æτæ increases with increasing the concentration of G in the Nf/RuL/TiO2 system (see Supporting Information, Table S1). This indicates that the effect of G on the photoinduced electrontransfer process seems to depend on the local medium. As Figure 4a shows, there is no sign of direct electron/energy transfer from RuL* to G in aqueous solution. RuL and G (both

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cations) should repel each other electrostatically while inhibiting the direct electron transfer between them. The UV-visible absorption spectrum of RuL in aqueous solution is not influenced at all by the presence of G, which also indicates no interaction between G and RuL in the ground state (see Supporting Information, Figure S1). However, when both of them are confined within the Nf layer (Figure 4b), their mutual interaction cannot be negligible. The effect of G on the emission of RuL* in the absence of TiO2 was also observed in the static photoluminescence spectra. Figure 4c shows the emission spectra of RuL* in aqueous solution without TiO2. The spectra A, B, and C were obtained in the absence of Nf with [G] = 0, 1, and 10 mM, respectively. Similarly, the spectra D, E, and F were obtained in the presence of Nf with [G] = 0, 1, and 10 mM, respectively. Without Nf, the static emission spectra were not influenced at all by G, which confirmed the result of the transient emission (Figure 4a) that there is no direct interaction between RuL* and G. On the other hand, in spectra D-F, for which RuL cations should be bound with Nf polymer, the emission intensity increased with [G], which is also consistent with the transient emission result (Figure 4b). The G-enhanced luminescence of RuL in Nf appears to be related to the role of G in self-quenching of the excited dye sensitizers. The luminescence of RuL is efficiently quenched when the sensitizer cations are aggregated or locally concentrated.11,12 When RuL cations are bound with Nf polymer, their self-quenching occurs within the neighbor. When G cations that do not directly interact with RuL are present in the Nf layer, the self-quenching rate can be retarded presumably because G cations serve as diluents that prevent the localization of the sensitizers.13 Because the self-quenching process should compete with the electron injection into TiO2 CB (see Scheme 2), retarding the self-quenching of dyes by the addition of G should increase the photoelectron density in TiO2 CB and subsequently enhance the hydrogen production. The self-quenching process should be sensitively dependent on the ion mobility in the Nf layer. When the decay rate of the transient emission of RuL in a wet Nf film was compared with that in a dry Nf film, the former was faster than the latter (i.e., Æτæ: 0.31 μs in wet Nf versus 0.61 μs in dry Nf; see Supporting Information, Figure S2). The faster emission decay in a wet Nf film indicates that the self-quenching rate is faster in the wet state because the RuL cations should be more mobile in the wet film. In the dry film, the ion mobility should be restricted, and the self-quenching is also hindered. As a result, the emission lifetime of RuL in Nf/RuL/TiO2 was not affected at all by the presence of G in the dry power state (see Supporting Information, Figure S3). We have investigated the effect of G coadsorption on the dye-sensitized Nf/TiO2 for the visible-light-induced production of hydrogen. TDR spectra showed evidence for the retarded recombination in the presence of G, and the photoluminescence spectra (both transient and static) showed that the luminescence decay is hindered by G in the Nf layer. On the basis of these observations, the enhanced production of hydrogen in the presence of G is ascribed to both the retarded charge recombination and the retarded self-quenching of the excited dyes. The


proposed roles of G are dual, to retard the recombination between the injected electron and the oxidized dye and to serve as a diluent of dyes in the Nf layer while retarding the self-quenching of excited dyes. The primary roles of G in Nf/RuL/TiO2 are mainly related to the electrostatic effect of the cations, and other inert organic/inorganic cations might have a similar effect. Further studies for utilizing alternative cations are needed to optimize the cation-modified Nf/RuL/ TiO2 system.

EXPERIMENTAL SECTION Nf/TiO2 was prepared according to the previous method.7 Pt nanoparticles were loaded onto the surface of TiO2 (Degussa P25) as a cocatalyst for hydrogen production, and a typical photodeposition method was employed for platinization. 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. Then, Nf/TiO2 was suspended in an aqueous solution (10 mg/1 mL) that contained 1 mM (or 10 mM) guanidinium thiocyanate (Aldrich) and 0.5 mM RuII(bpy)3Cl2(Aldrich) as a dye sensitizer. The solution was equilibrated overnight for the adsorption of the dyes (RuL) and G, and then, the mixture was centrifuged, and the supernatant was removed. The G- and/or RuLadsorbed Nf/TiO2 was resuspended in an aqueous solution of EDTA (as an electron donor) in a pyrex reactor. The suspension was purged with N2 for 15 min prior to illumination. A 300 W Xe arc lamp (Oriel) was used as a light source with a 10 cm IR water filter and a cutoff filter (λ > 420 nm). During irradiation, the head space gas (∼17 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. The photoelectrochemical (PEC) experiments were carried out by using a potentiostat (Gamry, Reference 600). The PEC cell (vol 45 mL) consisted of a sensitized Nf/TiO2 photoanode, a reference electrode (Ag/AgCl), and a platinum counter electrode, which were immersed in an electrolyte solution (10 mM EDTA). To fabricate the Nf/TiO2 photoanode, calcined TiO2 electrodes were dipped into 0.5 wt % Nf solution overnight and then, after drying, into G and/or RuL solution for adsorption. The TDR measurements were performed according to the procedure described elsewhere.2,10 The static luminescence spectra of the sensitizer were measured using a spectrofluorometer (JASCO FP-6500).

SUPPORTING INFORMATION AVAILABLE The average emission lifetimes Æτæ of RuL in the Nf/TiO2 system, the UV-visible absorption spectra of the aqueous solution of RuL in the presence of G, and the transient emission spectra of RuL in the wet or dry Nf film and dry Nf/TiO2 powder. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Fax: þ82-54-2798299. E-mail: [email protected].

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ACKNOWLEDGMENT This work was supported by KOSEF NRL

program funded by the Korean government (MEST) (No. R0A-2008000-20068-0), the KOSEF EPB center (Grant No. R11-2008-05202002), the Hydrogen Energy R&D Center (21st Century Frontier R&D Program), and the Korea Center for Artificial Photosynthesis (KCAP: Sogang Univ.) funded by MEST through NRF (2009C1AAA001-2009-0093879).

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