Improved Superiority by Covalently Binding Dye to Graphene for

Article pubs.acs.org/JPCC

Improved Superiority by Covalently Binding Dye to Graphene for Hydrogen Evolution from Water under Visible-Light Irradiation Bin Xiao,† Xiaomei Wang,‡ Hui Huang,† Mingshan Zhu,§ Ping Yang,*,† Yong Wang,*,† and Yukou Du† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China School of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215011, China § CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

ABSTRACT: A novel nanohybrid composed of bipyridine ruthenium complex ((2,2′-bipyridyl)-4-pyridyl-chlororuthenium(II), Ru(bpy)2(py)Cl), covalently functionalized graphene (Ru(bpy)2(py)Cl/G), has been synthesized successfully via coordination of Ru(bpy)2Cl2 with pyridine covalently functionalized graphene (py/G), which was synthesized through 1,3-dipolar cycloaddition of azomethine ylides. Ru(bpy)2(py)Cl/G was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra, Fourier transform infrared (FTIR), and ultraviolet−visible absorption (UV−vis). The fluorescence quenching efficiency of the nanohybrid excited at 290 and 535 nm was calculated to be 83% and 71%, respectively. The photocurrent density of the nanohybrid also significantly improved. The results revealed that fast photoinduced electron transfer from Ru(bpy)2(py)Cl moiety to graphene sheet occurred. Ru(bpy)2(py)Cl/G modified with platinum nanoparticles demonstrates remarkable enhanced photocatalytic activity for hydrogen evolution from water. Under 10 h UV−vis or visible light irradiation (>400 nm), the total amount of H2 evolution was 39.3 and 7.6 μmol mg−1, respectively. In addition, Ru(bpy)2(py)Cl/G/Pt nanohybrid shows sufficient stability for photocatalytic H2 evolution, and the hydrogen yield remains virtually unchanged in 50 h of irradiation. This study suggests that the carbon based nanohybrid composed of organic dye molecules covalently functionalized graphene is a promising candidate as a novel photocatalyst for photocatalytic hydrogen evolution.

1. INTRODUCTION Graphene is a very promising candidate in the design of new nanomaterials because of its superior physical and chemical properties,1−6 including excellent mobility of charge carriers (200 000 cm2 V−1 s−1),7,8 half-integer quantum Hall effect at room temperature,9−14 large theoretical surface areas (2600 m2 g−1),15 and nice ferromagnetism.16 Modification of graphene sheets with semiconductor nanoparticles17−20 or photosensitizers21−23 is a significant and available technique for the design of graphene-based hybrid material because it can take full advantages of the both components and also offers the potential to create new properties to extend the range of application.24−28 In the recent review of Gong et al.,29 the development about graphene-based materials for photocatalytic hydrogen generation from water is well summarized. It is reasonable to believe that not too far in the future graphenebased hybrid materials will demonstrate their peculiar properties in many scientific realms.30−37 In this paper, we report here the synthesis, characterization, and photocatalysis of a novel nanohybrid composed of bipyridine ruthenium complex ((2,2′-bipyridyl)-4-pyridylchloro-ruthenium(II), Ru(bpy)2(py)Cl) covalently function© 2013 American Chemical Society

alized graphene (Ru(bpy)2(py)Cl/G). Scheme 1 illustrates the synthesis process of Ru(bpy)2(py)Cl/G nanohybrid. Ru(bpy)2(py)Cl moiety as one of typical ruthenium bipyridyl complexes with polynuclear arrays in the nanocomposite acts as a light-harvesting sensitizer covalently attached on the graphene surface and broadens the light-absorption of the nanohybrid into the visible-light range. The graphene moiety in the composite serves not only as a superior supporting matrix for bonding the sensitizer molecules and Pt cocatalyst but also as an excellent electron mediator to adjust electron transfer, thus, restrain the recombination of photoexcited charges. Compared with that of Ru(bpy)2(py)Cl noncovalently functionalized graphene, the new covalently functionalized graphene nanohybrid demonstrated much better photocatalytic activities for hydrogen evolution from water under UV−vis and visible light irradiation without an electron mediator. The research revealed the potential application of organic dye covalently functionReceived: June 4, 2013 Revised: July 28, 2013 Published: September 20, 2013 21303

dx.doi.org/10.1021/jp405497j | J. Phys. Chem. C 2013, 117, 21303−21311

The Journal of Physical Chemistry C

Article

Scheme 1. Synthetic Route Toward Ru(bpy)2(py)Cl/G

alized graphene hybrid as a novel photocatalyst in the field of solar energy conversion.

resulting in Ru(bpy)2(py)Cl functionalized graphene nanocomposite (Ru(bpy)2(py)Cl/G). Preparation of Ru(bpy)2(py)Cl/G/Pt Nanocomposite. In order to get a photocatalyst for hydrogen evolution, we deposited Pt nanoparticles as a cocatalyst on Ru(bpy)2(py)Cl/ G nanosheets via photodeposition.45,46 In a 50 mL flask, 15 mL (ca. 0.2 mg mL−1) of Ru(bpy)2(py)Cl/G ethanol suspension and 0.18 mL (7.723 mmol L−1) of H2PtCl6 were mixed with 30 mL of distilled water under magnetic stirring. The mixture was irradiated by a GY-10 xenon lamp (150 W) at room temperature for 2 h under argon, resulting in a platinum nanoparticles modified Ru(bpy)2(py)Cl/G nanocomposite. 2.2. Characterization. Fourier transform infrared (FTIR) spectra of the samples were obtained with a Thermo Scientific Nicolet 6700 instrument. Raman spectra were measured with a Jobin Yvon HR-800 spectrometer using a He−Ne laser (λ = 633 nm, spot size ∼ 1 μm). Atomic force microscopy (AFM) images were recorded on a Digital Instrument Nanoscope IIIa Multimode system (Santa Barbara, CA) with a silicon cantilever using tapping mode. The sample for AFM measurement was prepared by spraying the diluted suspension of graphene (or Ru(bpy)2(py)Cl/G) (ca. 0.01 mg mL−1) onto a freshly cleaved mica surface and dried under vacuum at room temperature. Scanning electron microscopy (SEM) measurement equipped with energy dispersive X-ray (EDX) was carried out with an S4700 microscope (Hitachi, Limited). Transmission electron microscopy (TEM) studies were conducted on a TECNAI-G20 electron microscope operating at an accelerating voltage of 200 kV. UV−vis absorption spectra of the samples were recorded on a TU1810 SPC spectrophotometer. Fluorescence spectra were taken on an FL-2500 fluorospectrophotometer, the concentration of as-prepared Ru(bpy)2Cl2 or Ru(bpy)2(py)Cl/G in the suspension basing on the content of ruthenium was ca. 0.0015 mg mL−1. 2.3. Photoelectrochemical Measurement. The measurements of photoelectrochemical experiments were performed with a CHI 660B potentiostat/galvanostat electrochemical analyzer in a three-electrode system consisting of a reference electrode, a working electrode, and a counter electrode. The working electrode for photocurrent measurement was prepared by dipping ca. 1.35 mL of the sample solution (0.2 mg mL−1) on the clean indium tin oxide (ITO) glass and drying under ambient conditions. The area of the sample on the electrode is ca. 1.0 cm2. The platinum wire and the saturated calomel electrode (SCE) acted as the counter electrode and reference electrode, respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution. The working electrode was irradiated by using a GY-10 xenon lamp (150 W) equipped with a UV cutoff filter (>400 nm) during the measurement.

2. EXPERIMENTAL SECTION 2.1. Preparation. Materials. Graphite, 4-Formylpyridin, Nmethyl-glycine, 1-Methyl-2-Pyrrolidinone (NMP), Cis-bis(2,2′bipyridine)dichlororuthenium(II) hydrate (Ru(bpy)2Cl2) and organic solvents were purchased from J&K Company and were used without further purification. Synthesis. Preparation of Exfoliated Graphene. Single- or few-layered graphene was prepared by exfoliation of graphite in an organic solvent.38−40 In a typical experiment, 75 mg of graphite was dispersed in 150 mL of NMP in a 250 mL roundbottom flask. The mixture was sonicated for 30 h at room temperature, resulting in a dark brown suspension. The suspension was centrifuged for 30 min (3000 rpm), and the residue was removed by decantation. The concentration of asprepared single- or few-layered graphene in the suspension was ca. 0.2 mg mL−1. Synthesis of Ru(bpy)2(py)Cl/G. Scheme 1 illustrates the synthesis process of Ru(bpy)2(py)Cl/G nanosheets. First, pyridine covalently functionalized graphene (py/G) was synthesized via 1,3-dipolar cycloaddition of azomethine ylides.41−43 To a 250 mL three-necked, round-bottomed flask equipped with a pressure-equalizing dropping funnel, 100 mL of exfoliated graphene suspension was added and stirred in a nitrogen atmosphere at 160 °C. After 0.5 h of vigorous stirring, 100 mg of N-methyl-glycine (1.1 mmol) and 100 μL of 4formylpyridin (1.08 mmol) were added to the flask through the funnel. The temperature was kept at 160 °C with magnetic stirring. The addition of N-methyl-glycine (1.1 mmol) and 4formylpyridin (1.08 mmol) was repeated each 24 h for four times. After the addition was completed, the reaction was continued for 24 h at 160 °C. Then the reaction mixture was cooled to room temperature and filtered with a Millipore system (JH 0.45 μm filter). The obtained filter cake was washed thoroughly with deionized water and ethanol. The solid was dried under vacuum at room temperature for 5 h, resulting in pyridine covalently functionalized graphene (py/G). (2,2′-Bipyridyl)-4-pyridyl-chloro-ruthenium(II) (Ru(bpy)2(py)Cl) functionalized graphene nanocomposite (Ru(bpy)2(py)Cl/G) was synthesized using py/G and Ru(bpy)2Cl2 as starting materials via a coordination reaction.44 To a 100 mL three-necked, round-bottomed flask, 60 mL of the mixed solvent of ethanol and acetone (Vethanol/Vacetone = 2:1), 8 mg of Ru(bpy)2Cl2, and 20 mg of py/G were added. The mixture was heated at reflux temperature for 10 h under nitrogen atmosphere. The resulting reddish-brown suspension was filtered, and the solid was washed several times with water and ethanol, respectively, and dried at room temperature, 21304



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Figure 1. FTIR (A) and Raman (B) spectra of graphene (a), Ru(bpy)2Cl2 (b), and Ru(bpy)2(py)Cl/G (c).

G-band position in graphene, which is centered at 1583 cm−1, the G-band of Ru(bpy)2(py)Cl/G shift to 1580 cm−1. The redshift of the band demonstrates that Ru(bpy)2(py)Cl moiety as an electron-donor component has been linked on graphene surface.54−57 The intensity ratio of ID/IG for graphene and Ru(bpy)2(py)Cl/G is 0.44 and 0.48, respectively, implying that Ru(bpy)2(py)Cl moiety introduced into graphene network left more topological defects and vacancies.49,58 Most of all, compared with the band of 1477 cm−1 in the Raman spectrum of Ru(bpy)2Cl2, which is associated with C−N stretching vibrations of bipyridyl groups,59 the band at 1423 cm−1 in the spectrum of Ru(bpy)2(py)Cl/G suggests that a new Ru−N band has formed. Another notable evidence of forming Ru(bpy)2(py)Cl/G is that the Raman spectrum of the nanohybrid also includes peaks centered at 1028, 1121, 1159, 1551, and 1618 cm−1, dovetailing the Raman peaks of Ru(bpy)2Cl2 relating to the vibrations of bpy ring stretching modes. These results also provide an evidence that graphene grafts Ru(bpy)2(py)Cl moiety successfully. The AFM image of Ru(bpy)2(py)Cl/G, as illuminated in Figure 2, reveals that the apparent thickness of the nanohybrid is about 1.67 nm. Compared with the thickness of the exfoliated graphene flake, which is ca. 0.5−1.0 nm,53 the existence of the

2.4. Photocatalytic Reaction. The photocatalytic reactions were run in a 50 mL quartz flask quipped with a flat optical entry window. In a typical photocatalytic experiment, 10 mL of CH3OH, 40 mL of distilled water, and 1 mL of Ru(bpy)2(py)Cl/G/Pt (or Ru(bpy)2(py)Cl/G) suspension (ca. 0.2 mg mL−1) were added to the quartz flask with vigorous magnetic stirring. The pH value of the system was adjusted by addition of hydrochloric acid or sodium hydroxide solution. The system was deaerated by bubbling argon into the solution for 30 min before light irradiated. The solution was stirred continuously and irradiated by a GY-10 xenon lamp (150 W) at 298 K and atmospheric pressure. The visible-light irradiation was obtained from the xenon lamp equipped with a UV cutoff filter (>400 nm). The gases produced were analyzed with an online gas chromatograph (GC1650) equipped with a thermal conductivity detector and 5 Å molecular sieve columns using argon as carrier gas. The standard H2/Ar gas mixtures of known concentrations were used for GC signal calibration.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology Characterization. The FTIR spectra of the as-prepared samples are shown in Figure 1A. The absorption band around 1670−1560 cm−1 in the FTIR spectrum of Ru(bpy)2(py)Cl/G may be attributed to skeletal vibrations of aromatic domains of graphene like in most observations.47,48 The absorption bands around 2964 and 1384 cm−1 show the existence of a stretching vibration of C−H of the methyl group, and the peaks at 1261, 1097, and 1020 cm−1 are assigned to the stretching vibration of aliphatic C−N, indicating the existence of aziridino in the hybrid.49,50 The absorptions of 1680−1520 cm−1 can be appointed to the stretching vibration of the pyridine CC bonds and CN bonds, and the absorption band around 800 cm−1 in fingerprint region and weak absorption band around 3030 cm−1 can be attributed to the stretching vibration of C−H of the pyridine ring. The characteristic vibrations of bipyridyl groups are found around 1605, 1588, 1468, 1448, 1425, 1315, 1275, 1244, 763, and 731 cm−1.51 All of these indicate that Ru(bpy)2(py)Cl moiety has been successfully grafted on the surface of graphene. The Raman spectra of Ru(bpy)2(py)Cl/G as well as graphene and Ru(bpy)2Cl2 are shown in Figure 1B. Compared with that of graphene, the Raman spectrum of Ru(bpy)2(py)Cl/G demonstrates stronger and broader D-band and G-band, indicating a higher level of disorder and more defects existing in functionalized graphene.52,53 In addition, compared with the

Figure 2. Tapping-mode AFM images of graphene nanosheets (a) and Ru(bpy)2(py)Cl/G (b) height profile along the line shown in the AFM image. Image dimension for (a) is 20 nm × 10 μm and (b) 20 nm × 5 μm. 21305



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Figure 3. SEM images of graphene (a, b), Ru(bpy)2(py)Cl/G (c, d), and EDX analysis result of Ru(bpy)2(py)Cl/G (e).

Figure 4. TEM image of Ru(bpy)2(py)Cl/G/Pt (left) and the histogram of the size distribution of the Pt nanoparticles (right).

should be in the range of the half of the distance of Pt nanoparticles (∼0.7 nm, estimated from the TEM image). 3.2. Optical and Photoelectrochemical Properties. UV−vis absorption spectra of the exfoliated graphene, Ru(bpy)2Cl2, and Ru(bpy)2(py)Cl/G are shown in Figure 5. The exfoliated graphene suspension in ethanol shows an absorption centered at 238 nm arising from the π−π* transition of electrons.61 Ru(bpy)2Cl2 shows four major absorption bands centered at 522, 382, 295, and 245 nm, which are in agreement with other observations.59,62 The absorption spectrum of Ru(bpy)2(py)Cl/G also exhibits four absorption peaks centered at 547, 397, 296, and 254 nm. The broad band centered at 254 nm may be attributed to the overlap of the absorptions of graphene and Ru(bpy)2(py)Cl moiety. Compared with the band position in the graphene spectrum, the bathochromic shift of the band in the visible region of Ru(bpy)2(py)Cl/G may be interpreted as the strong electronic coupling between individual components in Ru(bpy)2(py)Cl/G nanocomposite. The similar phenomenon was also observed by Guldi’s group when they investigated phthalocyanine−PPV conjugated with graphene.21 The fluorescence spectra of Ru(bpy)2Cl2 and Ru(bpy)2(py)Cl/G samples excited at 290 and 535 nm, respectively, are demonstrated in Figure 6. Compared with Ru(bpy)2Cl2,

functional moiety on the graphene surface may be confirmed. SEM images of exfoliated graphene and Ru(bpy)2(py)Cl/G are shown in Figure 3. Both samples demonstrate that the flakes with irregular shapes have limited lateral dimensions ranging from several hundred nanometers to several micrometers. More importantly, the sheets of the exfoliated graphene and Ru(bpy)2(py)Cl/G appear transparent to the electron beams, giving an evidence that monolayer or few-layer graphene is obtained. EDX data demonstrate the elementary composition of Ru(bpy)2(py)Cl/G (Figure 3e). The atomic percent of Ru and C in the nanocomposite is 0.55 and 68.24, respectively, which means that approximately one dye molecule covalently bonds on a graphene sheet with 96 carbon atoms (ca. 2.5 nm2).15,60 The TEM image of Pt nanoparticles modified Ru(bpy)2(py)Cl/G clearly shows that the Pt nanoparticles as dark dots well deposite on the graphene sheet (Figure 4). The size of Pt nanoparticles is in the range of 2−5 nm. The number of the Pt nanoparticles calculated within a square of graphene sheet with an area of 2809 nm2 is 118. Therefore, we may conclude that one Pt nanoparticle is surrounded by ca. 10 dye molecules and the average distance between the Ru(bpy)2(py) Cl moieties and Pt nanoparticles on the graphene surface 21306



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Figure 7. Photocurrent responses of ITO-electrode (a), graphene/ ITO-electrode (b), graphene/Pt/ITO-electrode (c), Ru(bpy)2Cl2/ ITO-electrode (d), and Ru(bpy)2(py)Cl/G/ITO-electrode (e) to visible light irradiation in an aqueous solution containing 0.5 M Na2SO4 as supporting electrolyte recorded at −0.005 V. The illumination from a 150 W xenon lamp equipped with a UV cutoff filter (>400 nm) was interrupted every 30 s.

Figure 5. UV−vis spectra of graphene (a), Ru(bpy)2Cl2 (b), and Ru(bpy)2(py)Cl/G (c) in ethanol (ca. 0.01 mg mL−1).

Ru(bpy)2(py)Cl/G nanocomposite only demonstrates a weak fluorescence emission at the same wavelength excitation. The calculated quenching efficiencies are 83% and 71% excited at 290 and 535 nm, respectively. The high quenching efficiencies of Ru(bpy)2(py)Cl/G suggest that covalently bonding between the dye moiety and graphene enhances the photoinduced electron transfer from Ru(bpy)2(py)Cl to graphene.63 Figure 7 shows photocurrent density-time curve (J−t) of the ITO electrode covered with graphene, graphene/Pt, Ru(bpy)2Cl2, or Ru(bpy)2(py)Cl/G under visible light irradiation (>400 nm). Compared with weak photocurrent responses detected from the other samples (0.05, 0.14, and 0.37 μA cm−2 for graphene/ITO, graphene/Pt/ITO, and Ru(bpy)2Cl2/ITO electrode, respectively), steady and recyclable photocurrent responses were observed from Ru(bpy)2(py)Cl/G/ITO electrode. The photocurrent density reached ca. 2.23 μA cm−2. The enhancement of the photocurrent may be owing to the nice absorption of Ru(bpy)2(py)Cl/G to visible light and efficient charge transfer from the excited dye moiety to graphene.64 3.3. Photocatalytic Hydrogen Evolution. The photocatalytic results of the catalysts under 10 h UV−vis light irradiation are shown in Figure 8A. The total amount of H2 produced over Ru(bpy)2(py)Cl/G is 22.5 μmol mg−1, which is much higher than that over graphene and graphene/Pt under the same reaction conditions (4.7 and 5.6 μmol mg−1,

respectively). The result can be attributed to nice absorption to the visible light of Ru(bpy)2(py)Cl moiety and its covalent connection with graphene in the nanohybrid. After loading Pt nanoparticles on Ru(bpy)2(py)Cl/G, 39.3 μmol mg−1 of H2 was produced from Ru(bpy)2(py)Cl/G/Pt. Enhancement of the photocatalysis may be attributed to that Pt nanoparticles acting as cocatalyst can reduce the overpotential of H2 evolution.65 Under UV−vis light irradiation, the photoexcited electrons are transferred from the dye moiety to graphene and then subsequently are shuttled to Pt nanoparticles deposited on graphene nanosheets because of the excellent conductivity of graphene and lower work function of Pt nanoparticles. The photocatalytic results over as-prepared catalysts under visible light irradiation (>400 nm) are shown in Figure 8B. In 10 h visible light irradiation, the amount of hydrogen evolved from Ru(bpy)2(py)Cl/G/Pt was 7.6 μmol mg−1, while graphene and graphene/Pt did not produce hydrogen basically under the same conditions because graphene does not absorb visible light. In addition, the fact that the photocatalytic activity of Ru(bpy)2(py)Cl/G under visible light irradiation is relatively lower than that under UV−vis light irradiation may be interpreted to lower photoexcited energy of the visible light. The nice photocatalytic performance of the nanohybrid under both UV−vis and visible light irradiation may be attributed to

Figure 6. Fluorescence emission spectra of Ru(bpy)2Cl2 (a) and Ru(bpy)2(py)Cl/G (b) in ethanol excited at 290 nm (A) and 535 nm (B), respectively. 21307



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Figure 8. Hydrogen production from graphene (a), graphene/Pt (b), noncovalently functionalized graphene (c), Ru(bpy)2(py)Cl/G (d), and Ru(bpy)2(py)Cl/G/Pt (e) under UV−vis (A) and visible (B) light irradiation. Reaction conditions: 0.2 mg of the catalyst dispersed in 50 mL of CH3OH (20%) solution, pH = 3, T = 298 K.

Figure 9. (A) Influence of pH on hydrogen production from Ru(bpy)2(py)Cl/G/Pt under UV−vis light irradiation. Reaction conditions: 0.2 mg of Ru(bpy)2(py)Cl/G/Pt dispersed 50 mL CH3OH (20%) solution, T = 298 K, time = 10 h. (B) Cycling measurements of hydrogen production using Ru(bpy)2(py)Cl/G/Pt as photocatalyst under UV−vis light irradiation. Reaction conditions: 0.2 mg of Ru(bpy)2(py)Cl/G/Pt dispersed in 20% CH3OH solution, pH = 3, T = 298 K.

the hydrogen evolution rate. However, the ability of the donating electron of protonated methanol decreases at low pH, reducing the rate of hydrogen evolution. Figure 9B shows the stability of Ru(bpy)2(py)Cl/G/Pt nanocomposite under UV− vis light irradiation. As shown in the figure, the hydrogen yield remains virtually unchanged after 50 h of irradiation, indicating sufficient stability of Ru(bpy)2(py)Cl/G/Pt nanocomposite for photocatalytic H2 evolution. These results suggest that the carbon based nanohybrid composed of organic dye molecules covalently functionalized graphene is a promise candidate as a novel photocatalyst for photocatalytic hydrogen evolution. The mechanism of H2 production over Ru(bpy)2(py)Cl/G/ Pt can be illustrated in Scheme 2. Ru(bpy)2(py)Cl moiety of the nanocomposite acting as a light-harvesting sensitizer absorbs irradiated light. The photoelectrons are transferred from the excited sensitizers to graphene and then to Pt nanoparticles for the hydrogen evolution from water. The covalent linkage between the sensitizer and graphene accelerates the electron transfer from the excited sensitizer to graphene. Graphene herein serves as an excellent supporting matrix for anchoring the light-harvesting molecules and Pt cocatalyst as well as a superior electron mediator to adjust electron transfer, enhancing greatly the separation efficiency of photoexcited charges and photocatalytic activity.

the ruthenium bipyridine complex covalently bonded with graphene, which enhances greatly the photoinduced electron transfer from Ru(bpy)2(py)Cl to graphene. Graphene here serves not only as an excellent supporting matrix for anchoring the sensitizer molecules but also as a superior electron mediator to adjust electron transfer, which can separate efficiently the electron−hole pairs and lengthen greatly their lifetimes in the process of photocatalytic reaction. In addition, the photocatalytic activity of Ru(bpy)2(py)Cl noncovalently functionalized graphene has also been investigated. The results are shown in Figure 8. Under 10 h UV− visible or visible light irradiation, the amount of hydrogen evolved from the nanohybrid is 7.1 and 1.4 μmol mg−1, respectively, which were much inferior to the results of Ru(bpy)2(py)Cl/G under the same conditions. These results demonstrate that the efficiency of the intramolecular electron transfer in Ru(bpy)2(py)Cl/G is much higher than that of intermolecular one in Ru(bpy)2(py)Cl noncovalently functionalized graphene. The dependence of hydrogen production amount on pH of the system using Ru(bpy)2(py)Cl/G/Pt as catalyst is shown in Figure 9A. The maximum amount of photocatalytic H2 evolution was observed at pH = 3 when pH value of the system changes from 1 to 5. Both the reduction potential of water and the ability of the donating electron of methanol influence the hydrogen evolution rate.66,67 As the pH value decreases, the reduction potential of proton augments, raising 21308



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Scheme 2. Diagram of the Electron Transfer and Hydrogen Evolution in Ru(bpy)2(py)Cl/G/Pt Photocatalyst

4. CONCLUSIONS In summary, a novel bipyridine ruthenium complex covalently functionalized graphene nanohybrid has been successfully synthesized and used as catalyst for photocatalytic hydrogen evolution. The dye moiety in the nanohybrid acts as a sensitizer to harvest irradiated light. The covalent linkage between the dye moiety and graphene enhances the efficiency of the intramolecular photoinduced electron transfer from the sensitizer to the Pt cocatalyst. Graphene moiety acts as an excellent supporting matrix for anchoring the sensitizer molecules and also as a superior electron mediator to adjust electron transfer. The recombination of photoexcited charges is depressed and their lifetimes are lengthened greatly. The nanohybrid may be used as a photocatalyst for water reduction to produce hydrogen under UV−vis or visible-light irradiation, which demonstrates nice and stable photocatalytic performance. This study provides a new strategy for developing highly efficient carbon-based photocatalysts for photoinduced hydrogen evolution.

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*Tel: +86-512-6588 0361. Fax: +86-512-6588 0089. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of this research by the National Natural Science Foundation of China (21373143, 21243009, and 51273141), the Natural Science Foundation of Jiangsu Province (BK2010209), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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