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Efficient Photocatalytic Hydrogen Evolution from Water without an Electron Mediator over Pt-Rose Bengal Catalysts Xiaojie Zhang,†,§ Zhiliang Jin,† Yuexiang Li,‡ Shuben Li,† and Gongxuan Lu*,† State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China, Department of Chemistry, Nanchang UniVersity, Nanjing Road 245, Nanchang 330047, P. R. China, and Graduate UniVersity, Chinese Academy of Sciences, Beijing 100101, P. R. China ReceiVed: September 27, 2008; ReVised Manuscript ReceiVed: NoVember 20, 2008
A highly efficient visible-light-induced hydrogen evolution system without an electron mediator such as methyl viologen, constructed with rose bengal as a photosensitizer, triethanolamine (TEA) as an electron donor, and Pt as a cocatalyst, has been reported. Under visible light irradiation (λ g 420 nm), the highest rate of hydrogen evolution and apparent quantum efficiency are about 56.9 µmol h-1 and 11.3%, respectively. According to the effects of urea and ionic strength on the activity of hydrogen evolution, the dimers of rose bengal play a significant role in the photosensitized hydrogen evolution process. On the basis of activity tests and UV-vis absorbance and fluorescence measurements, the probable mechanism for photosensitized hydrogen evolution has been postulated. 1. Introduction Light energy can be effectively captured, transformed, and stored during photosynthesis in green plants, which fascinates many scientists and spurs numerous attempts to construct functional and efficient artificial photosynthetic systems. Meanwhile, increasing environmental and energy concerns have motivated studies of innovative solar energy storage and conversion systems. On the basis of understanding the nature of photosynthesis, a large number of photochemical systems capable of reducing water into hydrogen have been developed in the last decades. Photoinduced hydrogen evolution systems, consisting of a photosensitizer, a quencher and electron transfer mediator, an electron donor, and a hydrogen evolution catalyst (e.g., colloidal Pt or biological enzymes such as hydrogenase), have been studied extensively.1-6 There have been many reports on metal complexes, such as metalloporphyrins and transition metal polypyridine-based complexes as photosensitizers, used to reduce water to hydrogen. However, compared with transition metal complexes, few organic dyes have been investigated for the photoreduction of water into hydrogen,7-16 although they are usually less expensive and more readily available. Xanthene appears to be one of the more potential organic dyes used for the photoreduction of water.8-11,13-16 Of particular interest is rose bengal, a well-known and extensively studied water soluble photosensitizer with a very high visible absorption band around 550 nm and with a high quantum yield of triplet state formation.17 Its triplet state exhibits a reasonably long lifetime compared with its singlet state.18 In addition, the singlet-triplet state energy gap (∆E) of rose bengal is small (∆E ≈ 0.35 eV). Therefore, the use of a xanthene dye triplet state involves a relatively small energy loss. The combination of the ground-state reduction potential, -0.51 V versus the normal hydrogen electrode (NHE in water, pH 7),19 * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: +86-931-4968178. Fax: +86-931-4968178. † Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. § Graduate University, Chinese Academy of Sciences. ‡ Nanchang University.
and the long lifetime of the one-electron reduction intermediate of rose bengal19 allows this dye to sensitize the system to photoreduce water into hydrogen. To facilitate the charge transfer, an electron relay such as methyl viologen (MV2+) or cobalt and rhodium complexes is commonly employed. Nevertheless, that in turn reduces the free energy of the system and simultaneously induces back reactions associated with electron transfer to the relay.20 In order to overcome these obstacles, extensive efforts have been devoted to avoiding an electron relay. Indeed, there are several reports on photochemical hydrogen evolution without an electron mediator.20-23 In the current study, we report herein a highly efficient photocatalytic hydrogen evolution system with readily available rose bengal as a photosensitizer, triethanolamine (TEA) as an electron donor, and Pt as a cocatalyst in the presence of an electron mediator such as MV2+. The effects of parameters such as pH value, the concentration of rose bengal, and the concentration of Pt on the activity of hydrogen evolution were investigated. In addition, the role of TEA in photosensitized hydrogen evolution and the probable mechanism of photosensitized hydrogen evolution were also discussed. 2. Experimental Section 2.1. Chemical Reagents. Commercially available rose bengal (disodium salt), namely, tetrachlorotetraiodofluorescein disodium salt, was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. Other chemicals used in our experiments were analytical reagents. For convenience, the anionic form, oxidized radicals, and reduced radicals of rose bengal are denoted hereafter as RB, RB+•, and RB-•, respectively. 2.2. Photocatalytic Activity Measurements. A 300 W tungsten halogen lamp, equipped with a cutoff filter, was used as the light source. The photocatalytic reaction was carried out in a Pyrex flask of about 108 mL with a flat window (with an efficient irradiation area of about 10 cm2). The reaction mixtures inside the cell were maintained in suspension by means of a magnetic stirrer. In a typical photocatalytic experiment, a certain
10.1021/jp8085717 CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
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J. Phys. Chem. C, Vol. 113, No. 6, 2009 2631
amount of RB was added to a 60 mL aqueous solution of TEA (10%, w/w). To enhance the photocatalytic activity of hydrogen evolution, the addition of Pt particles is essential. Pt particles as active centers for hydrogen evolution were synthesized by a photochemical method with an aqueous solution of H2PtCl6, which is similar to the method described in ref 24. Prior to irradiation, Ar gas was bubbled through the reaction mixtures for 40 min to remove oxygen. In order to reduce PtCl62- to Pt, the reaction mixtures were irradiated for 5 h and then degassed with Ar. To investigate the effect of pH value on the activity of hydrogen evolution, pH values of the solution were determined on a Markson model 6200 pH meter and adjusted by the addition of hydrochloric acid or sodium hydroxide as required. Photocatalytic activity was determined by measuring the amount of hydrogen evolved using a gas chromatograph (TCD, molecular sieve 13X column, Ar gas carrier). The apparent quantum efficiency (ΦH2) is defined as follows
ΦΗ2 ) 2 × (mole of incident photon) ⁄ (mole
Figure 1. Dependence of the average rate of hydrogen evolution upon pH. Reaction conditions: 4.0 × 10-4 mol dm-3 rose bengal, 7.5 × 10-3 g dm-3 Pt, irradiation time 5 h, and Ar saturated.
of hydrogen involved) (1)
In measuring the apparent quantum efficiency, the photon flux of the incident light was determined with a ray virtual radiation actinometer (FU 100, silicon ray detector, light spectrum of 400-700 nm, sensitivity of 10-50 µV µmol-1 m-2 s-1), and then the apparent quantum efficiency was calculated according to eq 1. Moreover, to verify the apparent quantum efficiency, the photon flux of the incident light was also determined by the method of liquid-phase chemical actinometry (Reinecke’s chemical actinometry). 2.3. Characterization. UV-vis absorption spectra of the samples were recorded with a HP8453 spectrometer. The X-ray photoelectron spectroscopy (XPS) spectra were recorded with a VG ESCALAB 210 electron spectrometer using Mg KR radiation, and binding energies were calculated with respect to C(1s) at 285.0 eV. For XPS measurements, Pt particles were adsorbed onto active carbon by an impregnation method. Photoluminescence spectra of the sample were recorded with a Hitachi M-3500 fluorescence spectrophotometer under an Ar atmosphere, with a slit of 2.5 nm. For fluorescence measurements, the concentration of RB was adjusted to be about 1.0 × 10-6 mol dm-3 and then saturated with Ar. 3. Results and Discussion 3.1. Effect of pH on the Rate of Hydrogen Evolution. As shown in Figure 1, the pH value of the solution has dramatic effects on the average rate of hydrogen evolution from an aqueous solution of TEA. When pH values vary from 2 to 13, the maximum average rate of hydrogen evolution and apparent quantum efficiency are achieved at pH 8. The corresponding hydrogen generation rate and apparent quantum efficiency are 56.9 µmol h-1 and 11.3%, respectively. The rate of hydrogen evolution decreases as the medium becomes either more acidic or more basic. In particular, when the pH value of the solution is adjusted to 2, 3, 4, or 5, no hydrogen evolution is observed. In a strongly acidic solution, TEA undergoes total protonation. As a result, the ability of donating electrons will decrease,25 which results in a shorter lifetime for the excited RB and lower efficiency of the excited dye species. The pH value of the solution will affect the reactions of hydrogen evolution and TEA oxidation. Because the reaction of hydrogen evolution involves the reduction of H+, the rate of hydrogen evolution should depend on the concentration of H+. The higher the concentration of H+, the faster the reduction of
Figure 2. Dependence of the average rate of hydrogen evolution on the concentration of rose bengal. Reaction conditions: pH 8, 7.5 × 10-3 g dm-3 Pt, irradiation time 5 h, and Ar saturated.
H+ will occur. During hydrogen evolution, TEA is oxidized by losing one of the unpaired electrons of the N atom.26 Because the protonated forms of TEA are difficult to oxidize, the photosensitized hydrogen evolution process should occur at an optimized pH. 3.2. Effect of the Concentration of Rose Bengal on the Rate of Hydrogen Evolution. Figure 2 shows the effect of the RB concentration on the average rate of hydrogen evolution at pH 8 (0-2.4 × 10-3 mol dm-3). The optimal concentration of RB is within the range of 4.0-6.0 × 10-4 mol dm-3. The corresponding rate of hydrogen evolution is about 56.9 µmol h-1, and the corresponding apparent quantum efficiency is about 11.3%. If no RB is added, no hydrogen evolution is observed. When the concentration of RB is lower than 4.0 × 10-4 mol dm-3, the rate of hydrogen evolution increases with increasing RB concentration. In contrast, if RB concentration is greater than 6.0 × 10-4 mol dm-3, the rate of hydrogen evolution decreases with further increases of RB concentration. When the concentration of RB is relatively high, the deactivation of the excited dye species could be easier via intersystem crossings as well as radiative and nonradiative transitions. At a higher concentration of RB, the adjacent dye species results in more efficient quenching and less efficient
2632 J. Phys. Chem. C, Vol. 113, No. 6, 2009
Zhang et al.
Figure 3. X-ray photoelectron spectroscopy spectrum of Pt(4f) of Pt particles synthesized by a photochemical method.
utilization of the excited dye species. On the other hand, a decrease in the rate of hydrogen evolution may be attributed to the back reaction between RB-• and TEA+ (i.e., reaction R7)
(RB · · · RB) + hV f (RB · · · RB*)
(R1)
(RB · · · RB*) f (RB · · · RB*)
(R2)
1
1
3
+·
-·
(RB · · · RB*) f (RB RB ) 3
+·
-·
+·
-·
(RB RB ) f (RB + RB ) +·
-·
RB + RB +·
-·
RB
f RB + TEA +·
+ TEA
(R4) (R5)
f RB +·
TEA + RB
(R3)
f RB + TEA
(R6) (R7)
From a comparison of the reduction potentials of the two redox couples, Eo(TEA+/TEA) ) 0.82 V versus NHE, and Eo(RB/RB-•) ) -0.51 V versus NHE; it is apparent that reaction R7 is energetically favorable by about 1.3 V. In addition, the inherent lifetime of RB-• is rather long.19 These two factors provide the driving force for the back reaction (reaction R7). Because the rate of ionic radical (e.g., RB-• and RB+•) generation increases as the concentration of RB increases, the back reaction (reaction R7) will occur. Here, the back reaction (reaction R7) more or less accounts for the decrease in the hydrogen evolution. It has been reported that xanthene dyes exhibit substantial aggregation effects at high concentrations.27,28 Rose bengal, one of the xanthene dyes, tends to form dimers or higher aggregates at higher RB concentration.28 Because photosensitized hydrogen evolution by RB could be carried out through the dimer, the rate of hydrogen evolution increases with an increase in RB concentration (e3.60 × 10-4 mol dm-3). 3.3. Effect of the Concentration of Pt on the Rate of Hydrogen Evolution. Sudeep et al. employed a photochemical method, with thionine as a photosensitizer, to synthesize Ag nanoparticles under visible light irradiation.24 On the basis of their work combined with the following data [Eo(RB*/RB+•) ) -1.3 V versus NHE,18 Eo(RB-•/RB) ) -0.51 V versus NHE,19 and Eo(PtCl62-/Pt) ) 0.73 V versus NHE], we believe that PtCl62- can be reduced by RB-• or RB* into Pt(0). Figure 3 shows the narrow scan of a Pt(4f) core level of XPS of Pt synthesized by a photochemical method. The spectrum shows a doublet containing a lower-energy band (4f7/2) at 70.8 eV and a higher-energy band (4f5/2) centered at 74.1 eV, which is 3.3
Figure 4. (a) Dependence of the average rate of hydrogen evolution on the concentration of Pt. Reaction conditions: 4.0 × 10-4 mol dm-3 rose bengal, pH 8, irradiation time 5 h, Ar saturated. (b) Time courses of hydrogen evolution from an aqueous solution of TEA. Reaction conditions: 4.0 × 10-4 mol dm-3 rose bengal, 6.0 × 10-2 g dm-3 Pt, pH 8, Ar saturated.
eV higher than that of the lower-energy band. The lower-energy band centered at 70.8 eV (4f7/2) agrees well with the literature values.29 This result confirms further that Pt exists mainly in the form of Pt(0). It is noteworthy that the intensity of the Pt(4f7/ 2) band is smaller than that of the Pt(4f5/2) band. Figure 4a describes the effect of the concentration of Pt on the average rate of hydrogen evolution. The optimal concentration region is from 7.5 × 10-3 to 1.5 × 10-2 g dm-3. The corresponding rate of hydrogen generation is about 56.9 µmol h-1. Platinum-free catalysts show very low activity for the photoreduction of water to hydrogen, only 0.4 µmol h-1. In addition, the rate increases with increasing Pt concentration when the Pt concentration is lower than 7.5 × 10-3 g dm-3. In contrast, at higher Pt concentrations, a decrease in the rate of hydrogen generation was observed. Interestingly, as shown in Figure 4b, the amount of hydrogen evolved increases significantly at the beginning of irradiation, but subsequently decreases with prolonged irradiation time at higher Pt concentrations (e.g., g 6.0 × 10-2 g dm-3). The overpotential of hydrogen evolution on Pt is rather low; hydrogen evolution can take place easily. Because the radiationless dipole-dipole energy transfer can occur between the RB excited states and metal particles30 and metal nanoparticles (e.g., Au and Ag) can accept electrons from sensitizers in metal particle-sensitizer nanoassemblies under photoexcitation,31,32 platinum-free catalysts show very low activity for water reduction. The addition of Pt promotes the hydrogen rate very significantly at lower Pt concentrations (e.g., e7.5 × 10-3 g dm-3). However, remarkable RB photobleaching took place at higher Pt concentrations and resulted in a decrease in the rate of hydrogen generation. Mills et al. have verified that colloidal Pt particles can catalyze the hydrogenation of rose bengal.8 In our previous study, the hydrogenation of rhodamine B was observed and was validated by mass spectrometry.33 In a control experiment, it was observed that the addition of colloidal Pt to a H2saturated solution of rose bengal results in RB decoloration, which further confirms that Pt particles can accelerate the photobleaching of RB. RB bleaching leads to a diminishing of the antenna effect of RB and a decrease in the hydrogen evolution rate. Besides, Pt particles may aggregate at high concentrations of Pt and lead to a filter effect.
Pt-Rose Bengal Catalysts
Figure 5. UV-vis absorption spectrum of 2.0 × 10-5 mol dm-3 rose bengal in an aqueous solution of TEA at pH 8. The insert is partial magnification of the absorption spectrum.
Figure 6. (a) Effect of ionic strength on the ratio of band intensity, I1/I2. (b) Effect of ionic strength on the rate of hydrogen evolution. Reaction conditions: 4.0 × 10-4 mol dm-3 rose bengal, 7.5 × 10-3 g dm-3 Pt, pH 8, irradiation time 5 h, Ar saturated.
4. Inference of Photosensitized Hydrogen Evolution Mechanism 4.1. Effect of Ionic Strength on the Rate of Hydrogen Evolution. As has been inferred above, rose bengal, one of the xanthene dyes, tends to form dimers or higher aggregates. Figure 5 shows the UV-vis absorption spectrum of a 2.0 × 10-5 mol dm-3 RB in an aqueous solution of TEA. The spectrum shows a peak at around 553 nm with a shoulder at about 514 nm. These bands are attributed to those of the monomer and the dimer (H-type) of RB, respectively.28 The intensities of the bands at 553 and 514 nm are denoted as I1 and I2, respectively. The ratio I1/I2 is related to the concentration of the monomer and dimer of rose bengal in the solution. Figure 6a exhibits the effect of ionic strength on aggregation. It can be seen that the I1/I2 ratio increases with increasing ionic strength, namely, the concentration of KBr, which indicates that the concentration of the dimer decreases as the concentration of KBr is increased. The counterions tend to localize the charge and thereby reduce the charge oscillation, which in turn may lead to reduced aggregation tendency because of a decrease in dispersive interaction.34 In consideration of this property, the effect of ionic strength on the rate of hydrogen evolution is shown in Figure 6b. It can be seen that the rate of hydrogen evolution decreases with increasing KBr concentration. The effect of other salts, such as KCl, NaCl, KNO3, or NaBr, is similar to that of KBr on the
J. Phys. Chem. C, Vol. 113, No. 6, 2009 2633
Figure 7. Effect of urea on the average rate of hydrogen evolution. Reaction conditions: 4.0 × 10-4 mol dm-3 rose bengal, 7.5 × 10-3 g dm-3 Pt, pH 8, irradiation time 5 h, Ar saturated.
rate of hydrogen evolution. Therefore, photosensitized hydrogen evolution could be carried out via the dimers. The apparent quantum efficiency for hydrogen evolution at wavelengths 420-600, 460-600, and 550-600 nm is 27.2, 25.9, and only 4.2%, respectively, while no hydrogen evolution is observed at wavelengths longer than 600 nm. Because the monomer of rose bengal has a very strong visible absorption band around 553 nm, as shown in Figure 5, the above result implies that electron transfer through the dimer is more efficient than through the monomer of RB. Besides, it has been reported that the dimers of bacteriochlorophyll molecules play a vital role in bacterial photosynthesis.35 4.2. Effect of Urea on the Rate of Hydrogen Evolution. It is generally accepted that the dispersive force and the hydrophobic force as well as the hydrogen bond are the main motivations for the aggregation of xanthene dyes. In addition, it has been reported that urea is able to weaken hydrophobic forces as well as hydrogen bonds,36 which could have a detrimental effect on the intermolecular electron transfer within the dimers of RB, as will be discussed in detail below. Consequently, the presence of urea in the solution helps to break apart the dimers or higher aggregates present in the aqueous solution of the dyes (e.g., methylene blue, fluorescence, Rhodamine B, etc.).37,38 Given this property of urea, the effect of urea on the rate of hydrogen evolution was investigated (Figure 7). It can be seen that the rate of hydrogen evolution decreases with increasing urea concentration. It is important to point out that urea exhibits a detrimental effect on the rate of hydrogen evolution at higher RB concentrations. These results further confirm that photosensitized hydrogen evolution could be carried out through the dimer of RB. 4.3. Inference of the Photosensitized Electron Transfer Mechanism. Excitation of an aqueous solution of rose bengal at 518 nm results in an emission spectrum, centered at about 562 nm, similar to that observed by Fleming et al.39 The results are shown in Figure 8. It is unexpected that the intensity increases rather than decreases with increasing TEA concentration, which indicates that TEA cannot efficiently quench the dye singlet state. A similar phenomenon was also found in an aqueous solution of ethanol.39 This result combined with the facts that the triplet state of RB has an inherently long lifetime18 and a high quantum yield of formation compared with that of its singlet state17 implies that the electron transfer could be accomplished via the triplet state of RB. To see whether or not hydrogen evolution could be carried out through the dimers of RB, we first investigate whether the
2634 J. Phys. Chem. C, Vol. 113, No. 6, 2009
Zhang et al. SCHEME 1: Probable Mechanism of Photosensitized Hydrogen Evolutiona
a Note that RB, D, and DOX represent the sensitizer, electron donor, and oxidized product of the electron donor, respectively.
Figure 8. Fluorescence emission spectra of a rose bengal aqueous solution (1.0 × 10-6 mol dm-3) with changes in the concentration of TEA at pH 8.
intermolecular electron transfer within the dimer of RB is feasible. It is well-known that noncovalent interactions play a critical role in mediating electron transfer (ET) processes occurring in biological systems. Work on proteins has established that specific hydrogen-bonding interactions are critical in achieving long-range donor-acceptor electronic coupling.40-42 Because the hydrogen bond is one of the main motivations for the aggregation of xanthene dyes, the intermolecular electron transfer within the dimer of RB should be feasible. On the other hand, when the geometry of the excited state does not differ from the ground state, the free energy change for the photoinduced electron transfer reactions could be estimated by the Rehm-Weller equation43,44
(
∆G (kcal mol-1) ) 23.06 × ΕD+⁄D - ΕΑ-⁄Α -
)
e2 - ∆ED * εa (2)
In consideration of the similarity of the structure between the excited state and the ground state of RB, the free energy change for the electron transfer within the dimer of RB was estimated according to eq 2, in which ED+/D - EA-/A ) 1.85 V, namely, E(RB+•/RB) - E(RB-•/RB) ) 1.85 V,45 ∆ED*(3RB*) ) 1.83 eV (42.2 kcal mol-1),8 and e2/εa is not less than 0.06 V (versus NHE), so that ∆G is not more than -0.92 kcal mol-1 (less than zero). This means that electron transfer between an excited state and a ground state molecule within the dimer of RB is spontaneous; that is, the intermolecular electron transfer within the dimer of RB is feasible. It was previously postulated that the ionic radicals of Eosin Y, one of the xanthene dyes, could be formed through intermolecular electron transfer with Eosin Y dimers yielding (EO+•EO-•) radical pairs. In addition, flash excitation of a deaerated aqueous rose bengal solution leads to the generation of the RB+• and RB-• ionic radicals.46 These two factors could further support the argument that intermolecular electron transfer within the dimer of RB is feasible. 4.4. Role of TEA in the Photosensitized Hydrogen Evolution Process. Control tests indicate that TEA is indispensable, which implies that TEA could play an essential role in the photosensitized hydrogen evolution process. TEA as an electron donor could quench the excited dye species to the ground state by means of reacting with the oxidized dye species (i.e., RB+•), as shown in reaction R6. Furthermore, as proposed earlier by Kalyanasundaram et al., the resultant TEA+• cation radical
rapidly loses H+ and decomposes to form N,N-diethanolamino aldehyde or glycolaldehyde and diethanolamine.26 As a result, the back reactions (i.e., reactions R5 and R7) should be inhibited. On the other hand, it has been reported that the fluorescence lifetime of rose bengal and the lifetime for singlet-triplet state intersystem crossings are very sensitive to solvent environments, such as 118 and 120 ps in H2O and 739 and 827 ps in ethanol, respectively.47 On the basis of this fact combined with the similarity of the structure, we infer that TEA could also prolong the lifetime of both the singlet excited sensitizers and the singlet-triplet state intersystem crossings through van der Waals interactions (e.g., hydrogen bond) with the dye species. Additionally, the polarity of the solution could increase with the addition of TEA because the dipole moment of TEA (dioxane, 11.9 D) is larger than that of water (1.8 D); therefore, the stability of the ionic radicals of RB (i.e., RB+•RB-• and RB-•) should be improved in an aqueous solution of TEA. As a result, the efficiency of both utilization of the excited dye species and photosensitized hydrogen evolution could be enhanced. 4.5. Inference of the Photosensitized Hydrogen Evolution Mechanism. On the basis of the facts above combined with the data Eo(RB/RB-•) ) -0.51 V and the long inherent lifetime of RB-•, we can infer that photosensitized hydrogen evolution could be carried out through RB-• produced through the intermolecular electron transfer within RB dimers yielding (RB+•RB-•) radical pairs. Certainly, RB-• could be produced through a reductive quenching mechanism, namely, the reductive quenching of the xanthene triplet state by electron donors, which usually occurs in biological systems. Nevertheless, as has been discussed above, electron transfer through the dimer is more efficient than that through the monomer of RB. Control tests indicate that visible light irradiation is essential to the photosensitized hydrogen process. It was also found that the total amount of hydrogen evolved under visible light irradiation is about 2000 µmol for 40 h under the optimized conductions. The turnover number (TON), which is defined as the ratio of the total amount of hydrogen evolved to the amount of sensitizer, exceeds 100, indicating that the photosensitized hydrogen evolution reaction proceeds photocatalytically and rose bengal is a sensitizer rather than a sacrificial agent. On the basis of the measurements and observations described above, we propose a probable mechanism for photosensitized hydrogen evolution using a rose bengal/TEA/Pt system (Scheme 1). The various steps involved in the mechanism are (i) excitation of photosensitizer, (ii) production of ionic radicals (i.e., RB+•RB-•, RB-•, and RB+•), and (iii) evolution of hydrogen on Pt particles and regeneration of photosensitizer. 5. Conclusion In conclusion, we have demonstrated a highly efficient visible-light-induced hydrogen evolution system without an electron mediator by using readily available rose bengal as
Pt-Rose Bengal Catalysts a photosensitizer, triethanolamine (TEA) as an electron donor, and Pt as a cocatalyst. The optimal pH, concentration of RB, and concentration of Pt are 8, 4.0-6.0 × 10-4 mol dm-3, and from 7.5 × 10-3 to 1.5 × 10-2 g dm-3, respectively. The corresponding rate of hydrogen evolution and apparent quantum efficiency are about 56.9 µmol h-1 and 11.3%, respectively, under visible light irradiation (λ g 420 nm). On the basis of the effects of urea and ionic strength on the activity of hydrogen evolution combined with the facts that hydrogen-bonding interactions play an important role in mediating electron transfer processes occurring in biological systems as well as that irradiation of deaerated RB aqueous solution leads to the generation of RB+• and RB-• ionic radicals, we postulate that photosensitized hydrogen evolution could be carried out through RB-• produced through intermolecular electron transfer with RB dimers yielding RB+•RB-• radical pairs. Further studies are in progress to elaborate why there is significant difference among fluorescein and its derivatives (e.g., Eosin Y and rose bengal) as photosensitizers. For example, the rate of hydrogen evolution is rather low when Eosin Y or fluorescein acts as a photosensitizer without any action taken to assist charge separation in spite of the similarity in their structures. Acknowledgment. Financial support from National Natural Science Foundation of China (90210027) and National Basic Research Program of China (2003CB214500, 2007CB613305, and 2009CB220003) is gratefully acknowledged. References and Notes (1) Lehn, J.-M.; Sauvage, J.-P. NouV. J. Chim. 1977, 1, 449. (2) Kiwi, J.; Gra¨tzel, M. Nature 1978, 274, 657. (3) Harriman, A.; Porter, G.; Richoux, M. C. J. Chem. Soc., Faraday Trans. 1981, 2, 833. (4) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639. (5) Astuti, Y.; Palomares, E.; Haque, S. A.; Durrant, J. R. J. Am. Chem. Soc. 2005, 127, 15120. (6) Du, P. W.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2006, 128, 7726. (7) Kalyanasundaram, K.; Dung, D. J. Phys. Chem. 1980, 84, 2551. (8) Mills, A.; Lawrence, C.; Douglas, P. J. Chem. Soc., Faraday Trans. 1986, 82, 2291. (9) Shimidzu, T.; Iyoda, T.; Koide, Y. J. Am. Chem. Soc. 1985, 107, 35. (10) Bi, Z. C.; Xie, O. S.; Yu, J. Y. J. Photochem. Photobiol., A 1995, 85, 269. (11) Abe, R.; Hara, K.; Sayama, K.; Domen, K.; Arakawa, H. J. Photochem. Photobiol., A 2000, 137, 63. (12) Abe, R.; Sayama, K.; Arakawa, H. Chem. Phys. Lett. 2003, 379, 230.
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