Visible-Light-Induced Photocatalytic Hydrogen Generation on Dye-Sensitized Multiwalled Carbon Nanotube/Pt Catalyst Qiuye Li,��,���,�� Liang Chen,��,��� and Gongxuan Lu*,�� State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China, The Graduate School of the Chinese Academy of Sciences, Beijing, 10080, China, and Key Laboratory of Special Functional Materials, Henan UnVersity, KaiFeng, 475001, China ReceiVed: March 30, 2007 In Final Form: May 27, 2007 Under visible light irradiation (�� g 420 nm), photocatalytic water reduction for hydrogen generation was achieved over Eosin Y sensitized multiwalled carbon nanotube (MWCNT)/Pt catalyst with triethanolamine (TEOA) as the electron donor. The highest apparent quantum yield reached 12.14%. Dilute HNO3 treated MWCNT catalyst exhibited a higher hydrogen generation rate than a concentrated HNO3 treated one. Characterization results indicated that HNO3 treatment led to formation of -COOH and -OH on MWCNT, which provided anchoring sites for Eosin Y. The results of photocurrent, photoluminescence, and photocatalytic experiments indicated that MWCNT trapped photogenerated electrons and inhibited the recombination of electron-hole pairs. MWCNT in catalyst might work as a charge-transfer carrier. The catalysts presented rather stable properties for hydrogen generation in a long-term run. 1. Introduction Photocatalytic hydrogen generation has been a challenging topic for decades for realization of the storage and conversion of solar energy.1-3 Since the discovery of photocatalytic water splitting on TiO2 single-crystal electrodes by Fujishima and Honda,4 many semiconductor catalysts have shown activities for splitting water into hydrogen under UV light irradiation, such as TiO2, ZrO2, SrTiO3, Ta2O5, Sr2M2O7 (M ) Nb, Ta), and ATaO3 (A ) Li, Na, and K).5-10 However, solar light contains only 5% ultraviolet light. In order to utilize more energy of solar light, it is necessary to develop visible-light-active photocatalysts or to modify wide-band-gap semiconductors with visible-light-sensitive materials. Recently, a series of noble- metal-loaded perovskite photocatalysts11,12 and a new type of solid solution catalysts3,13 were found to be photocatalytically active for hydrogen generation under visible light irradiation. Modification of wide-band-gap semiconductors via metal dop- ing, nonmetal doping, and narrow-band-gap semiconductor combining was identified as effective. For example, dye sensitization of TiO2 was used successfully in Gra ��tzel cell- dye-sensitized solar cells (DSC). In 2004, the solar-to-electric power conversion efficiency of dye-sensitized porous TiO2 film reached 11%.14-16 The method of dye sensitization is also used in photocatalytic hydrogen generation from water. Bae et al. reported that ruthenium complex sensitized TiO2 produced hydrogen under visible light irradiation the corresponding rate of hydrogen generation was 50 ��mol h-1 g-1.17 Merocyanine and coumarin dye sensitized Pt/TiO2 photocatalysts were also reported to be active for hydrogen evolution from a water- acetonitrile mixed solution, and the quantum efficiencies were about 2-2.5%.18,19 A higher quantum efficiency, for example, 10% at 520 nm, was obtained over Eosin Y fixed Pt/TiO2 catalyst.20 Recently, Jin et al. reported that the apparent quantum yield of dye-sensitized M/TiO2 (M ) Pt, Ru, and Rh) photo- catalysts could be increased to 10.27% in triethanolamine (TEOA) aqueous solution under irradiation with a wavelength longer than 420 nm.21 Although many extensive studies have been explored for hydrogen evolution, it is necessary to develop new photocatalytic catalysts with satisfying properties, high solar conversion efficiency, and good stability under visible light irradiation. Carbon nanotubes (CNTs) have attracted intensive interest because of their peculiar one-dimensional structure with the rolled-up graphene sheet in nanoscale diameter.22 However, the lack of solubility and the difficult manipulation in solvent have imposed great limitations on the usage of CNTs. Indeed, freshly synthesized CNTs are insoluble in all organic solvents and aqueous solutions.23 In order to improve their solubility, many approaches have been attempted, among which acid oxidation is an efficient way. It has been known that oxidation treatment of CNT with acid can cut it into short lengths, decrease the diameter of multiwalled carbon nanotubes (MWCNTs), and remove amorphous carbon and contaminating metals. Perhaps more importantly, this treatment introduces oxygen-containing groups, such as hydroxyl, carbonyl, and carboxylic function- alities. These groups provide electrostatic stabilization when the CNT is dispersed in water for further functionalization.24 In recent years, composites of carbon materials such as C60 and MWCNT with conjugated polymers were used in photovoltaic devices and photochemical solar cells, for the reason that these carbon materials can act as electron traps due to their high electron affinities. Charge separation occurs: the electron transfers to carbon material and the hole transfers to polymer.25-29 This function of charge separation of MWCNT can be employed in photocatalytic reaction. Lee et al. reported that the TiO2- MWCNT composite catalyst could be used as a spore-deactivat- ing agent under UV or solar light irradiation.30 Ou et al. found * Corresponding author. Fax: +86-931-4968178. E-mail: gxlu@lzb.ac.cn. �� Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. ��� The Graduate School of the Chinese Academy of Sciences. �� Henan Unversity. 11494 J. Phys. Chem. C 2007, 111, 11494-11499 10.1021/jp072520n CCC: $37.00 �� 2007 American Chemical Society Published on Web 07/12/2007

that MWCNT-TiO2/Ni composite photocatalyst could be used to generate hydrogen from water, although the hydrogen evolution rate was only 38.1 ��mol h-1 g-1.31 To the best of our knowledge, a report about applying functionalized-MWCNT in photcatalytic hydrogen evolution under visible light irradiation is lacking. In this paper, the photocatalytic water reduction for hydrogen generation over Eosin Y sensitized MWCNT/Pt catalyst is reported. By the investigation of the transient photocurrent- time curves and photoluminescence (PL) spectra, the function of MWCNT was elucidated. At the optimized experimental conditions, the rate of hydrogen generation could reach 3.06 mmol h-1 g-1. The highest apparent quantum yield achieved 12.14%. This new catalyst showed a satisfying long-term stability for photocatalytic hydrogen generation, and the activity still maintained 70% of that of the fresh sample after reaction for 100 h. 2. Experimental Section 2.1. Materials and Apparatus. All reagents were of analyti- cal grade and used without further purification. Eosin Y dye (bisodium salt), named 2���,4���,5���,7���-tetrabromofluorescein, was used as the photosensitizer of the catalyst. The chemical structure of Eosin Y dye molecule is shown in Figure 1. FTIR spectra were obtained on a Bruker IFS 120HRFT-IR spectrometer. UV-vis spectra were obtained with a Hewlett- Packard 8453 spectrophotometer. Photoluminescence spectra were determined by a SPEX F 212 spectrometer. 2.2. Treatment of MWCNT. The hydrophobic property of MWCNT led it to disperse hardly in water. In this work, two kinds of carboxylated MWCNTs were prepared by refluxing MWCNT in nitric acid solutions with different concentrations. One was refluxing the MWCNT in dilute, 2.6 M HNO3 for 24 h. This purified MWCNT was denoted as DMWCNT. Another was refluxing the MWCNT in concentrated nitric acid for 48 h at about 123 ��C. This MWCNT sample was denoted as CMWCNT. These two functionalized MWCNTs were washed with water to neutral and then dried under vacuum. The untreated MWCNT was denoted as UMWCNT. 2.3. Photocatalytic Hydrogen Generation. Photocatalytic reactions were carried out in a Pyrex flask of 140 mL with a flat window (an efficient irradiation area of 10 cm2). Eosin Y sensitized MWCNT/Pt photocatalysts were prepared using the photoreduction method in situ. Typically, 20 mg of MWCNT, a calculated amount of Eosin Y dye, and aqueous H2PtCl6 were suspended in TEOA-H2O mixture (80 mL, 15 (vol) % aqueous solution) by magnetic stirring. A 300 W tungsten halogen lamp, equipped with a 420 nm cutoff filter (Toshiba, SY44.2), was used as the light source. Prior to irradiation, the suspension of the catalysts was dispersed in an ultrasonic bath for 1 min, and then Ar gas was bubbled through the reaction mixture for 40 min to remove oxygen. The illumination time of in situ preparation was optimized. Once the illumination was started, Pt nanoparticles were formed. Increase of the illumination time resulted in more Pt nanoparticles formation. However, further illumination longer than 10 min led to no variation of dispersion of Pt nanoparticles. The photocatalytic activity was estimated by measuring the amount of hydrogen evolution using gas chromatography (thermal conductivity detector (TCD), molec- ular sieve 5A column, Ar as gas carrier). The pH values of the reaction solution were adjusted by addition of hydrochloric acid or sodium hydroxide using a Markson 6200 model pH meter. 2.4. Measurement of the Adsorption Amount of Eosin Y. The adsorption curves of Eosin Y on different MWCNTs were conducted in the same experimental conditions. A 20 mg sample of UMWCNT (or CMWCNT or DMWCNT) and Eosin Y with a calculated amount were added to 80 mL of TEOA solution first, ultrasonically dispersed for 10 min, and stirred for 24 h to saturate. The adsorption amount of Eosin Y was calculated from the absorbance difference between the initial sensitizer solution and the filtered solution of MWCNT-added suspension. The effect of Pt on the adsorption amount was determined in a similar way. The difference was that the adsorption system was photoreduced for some time for Pt deposition before Eosin Y was added. 2.5. Photoelectrochemical Experiments. Photoelectrochemi- cal behavior of the photocatalysts was measured by transient photocurrent-time curves using a standard three-electrode cell at the computer-controlled CHI660A electrochemical worksta- tion. The photocatalysts were dip-coated on the indium tin oxide (ITO) surface, which acted as the working electrode. The supporting electrolyte was TEOA mixed with 0.5 M Na2SO4 aqueous solution. A Pt wire and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. 3. Results and Discussion 3.1. Effect of Different MWCNTs on the Photocatalytic Activity for Hydrogen Generation. The effect of different MWCNT samples on the photocatalytic activity for hydrogen generation is listed in Figure 2. Eosin Y sensitized Pt catalyst could produce hydrogen from water reduction under visible light irradiation, but the hydrogen evolution rate was only 3.04 ��mol h-1. When MWCNT samples were added to the reaction system, the rate improved remarkably. Especially, the rate reached 54.20 ��mol h-1 over DMWCNT catalyst. Fourier transform infrared (FTIR) spectroscopy was used to identify the surface groups on the catalysts. The FTIR spectra of two typical catalysts are shown in Figure 3. The peaks at 1720 and 1582 cm-1 correspond to the carboxylic acid group and carboxylate group at the MWCNT surface, respectively. These results are in good accordance with that of the literature.32 Figure 1. Structure of Eosin Y dye molecule. Figure 2. Effect of different MWCNT samples on the photocatalytic activity of Eosin Y sensitized Pt (1%) photocatalyst. Reaction condi- tions: pH 7, E/MWCNT ) 1, Ar atmosphere. Hydrogen Generation on MWCNT/Pt Catalyst J. Phys. Chem. C, Vol. 111, No. 30, 2007 11495