Ruthenium Dye N749 Covalently Functionalized Reduced Graphene

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Ruthenium Dye N749 Covalently Functionalized Reduced Graphene Oxide: A Novel Photocatalyst for Visible Light H Evolution 2

Jie Huang, Dandan Wang, Zongkuan Yue, Xia Li, Dongmei Chu, and Ping Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09483 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

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The Journal of Physical Chemistry

Ruthenium Dye N749 Covalently Functionalized Reduced Graphene Oxide: A Novel Photocatalyst for Visible Light H2 Evolution

Jie Huang,a, b Dandan Wang,a Zongkuan Yue,a Xia Li,a Dongmei Chu,a Ping Yang*a

a

College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou, 215123, China.

b

Key Laboratory of Nano-Bio Interface, Division of Nanobiomedicine, Suzhou

Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China.

1

ACS Paragon Plus Environment


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ABSTRACT To improve the photocatalytic activity of graphene-based catalysts, an efficient photocatalytic hydrogen evolution system based on black dye N749 covalently functionalized reduced graphene oxide (rGO-N749) was synthesized. The obtained product was characterized with transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), Raman spectra, X-ray photoelectron spectroscopy (XPS), Fluorescence and UV-vis spectroscopy. The results demonstrate that N749 has been successfully grafted on the surface of rGO. The rGO-N749 nanocomposite exhibits high light-harvesting efficiency covered a range of wavelengths from the ultraviolet to visible light. The efficient fluorescence quenching and the enhanced photocurrent response confirm that the photoinduced electron transfers from N749 moiety to rGO sheet. Moreover, we chose Pt nanoparticles (NPs) as cocatalyst loading on rGO-N749 sheets to obtain the optimal H2 production effect. The platinized rGO-N749 (rGO-N749-Pt) demonstrates much high photocatalytic activity for hydrogen evolution from water under both UV-vis and visible light (λ>400 nm) irradiation. The apparent quantum yields are 0.54% and 0.21% at 365 and 420 nm, respectively. These results reveal that rGO-N749-Pt nanocomposite consolidated the advantages of N749, rGO and Pt NPs can be a potential candidate for hydrogen evolution from water under UV-vis or visible-light irradiation.

KEYWORDS: photocatalysis; hydrogen evolution; reduced graphene oxide; N749 2


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1. Introduction Because of the increasing global energy demand and deteriorating environmental pollution, hydrogen has been considered as one of the most promising candidates to substitute fossil fuels due to that it may be produced from water using freely available sunlight as input energy.1-4 During the past few decades, photocatalytic hydrogen production over semiconductors, metal organic complexes, biomolecules, organic donor-acceptor systems, and recently graphene-based materials has been extensively investigated.5-11 Graphene, due to its unique structure and attractive properties, has attracted immense attention in many applications such as nanoelectronics, sensors, catalysts, and energy conversion.12-17 Particularly, reduced graphene oxide (rGO) as a building block for fabricating organic photocatalysts for water splitting have attracted great attention. Many organic dyes such as Ru-tris(dicarboxybipyridine),18 Eosin Y19 and phthalocyanine20 have been used as photosensitizers to enhance the H2 production performance of graphene-based photocatalysts. However, the absorbance peak of these organic dyes is only under UV-vis and visible light,21 which result in the low utilization ratio of sunlight. Therefore, synthesizing novel functionalized graphene nanomaterials

to

fulfill

the

requirement

of

the

high-absorption

and

high-charge-separation efficiency of light-sensitive photocatalysts are still required. Ru-based

organic

complexes

such

as

cis-dithiocyanate-N,N’-bis(4,4’-

dicarboxylate-2,2’-bipyridine)ruthenium(II) (N3),22 cis-bis(isothiocyanato)bis(2,2’bipyridyl-4,4’-dicarboxylic acid)ruthenium(II) (N719)23 and tri(isothiocyanato) (2,2’; 3



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6’2’’,-terpyridyl-4,4’; 4’’-tricarboxylic acid)ruthenium(II) (N749)24 used as sensitizers have attracted more attention because of their low lying metal-ligand charge transfer (MLCT) transition that extends into the red and near-infrared region of the solar spectrum. The application of the black dye N749 as a sensitizer to modify semiconductor has been successfully explored,25 which demonstrated that the absorption of the modified semiconductor extended to the near infrared region. In this work, we report design, synthesis and photocatalysis of a novel N749 covalently functionalized reduced graphene oxide (rGO-N749) nanocomposite. Here, N749 grafted on the rGO acts as a sensitizer to harvest incident light, and the rGO acts as an excellent electron accepter and mediator to adjust electron transfer. The rGO-N749 nanocomposite exhibits much improved photocatalytic activity compared to rGO, which is attributed to nice absorption to the visible light of N749 moiety and its covalent bonding to the rGO sheet. The covalent bonding of N749 and rGO is beneficial to the photoexcited electrons transfer from the sensitizer to rGO as well as the effective restraint of the recombination of the photoexcited electrons and holes. Moreover, platinized rGO-N749 (rGO-N749-Pt) shows satisfactory photocatalytic activity under both UV-vis and visible light (λ>400 nm) irradiation. These results reveal that rGO-N749-Pt nanocomposite with the full advantages of N749, rGO and Pt NPs can act as a novel candidate to produce hydrogen from water under solar light irradiation.

2. EXPERIMENTAL SECTION 4


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2.1. Materials. Native graphite flake was purchased from Alfa Aesar; black dye N749

N-(3-dimethylaminopropyl-N’-ethylcarbodiimide)

hydrochloride

(EDC),

amine- terminated six-armed polyethylene glycol (PEG) were purchased from Sigma; HCl, NaOH and other reagents were purchased from China National Medicine Corporation and used as received. 2.2. Synthesis of rGO-N749. The preparation of rGO-N749 is shown in Scheme 1. GO was prepared by a modified Hummers method, using flake expandable graphite as the original material according to the previous protocol.26,27 To prepare GO-PEG, the aqueous suspension of GO (5.0 mL) at a concentration of 4.5 mg mL-1 was washed twice by centrifugation at 8,000 rpm for 5 min to give a clear solution (15.0 mL). NaOH (1.8 g) was added to the GO suspension, and then the mixed solution was further stirred at 55 °C for 4 h. The resulting solution was neutralized by HCl, and purified by repeated washing and centrifugation. Subsequently, aqueous solution of PEG (2.0 mg mL-1) was added to the GO solution (0.3 mg mL-1) and the mixture was sonicated for 5 minutes. EDC aqueous solution was then added to the mixture twice to give a final concentration of 1.0 mg mL-1. The reaction was stirred overnight, and finally the solution of GO-PEG was obtained after purification by ultra-filtration through a 100-kDa filter (Millipore) and stored at 4 °C. To prepare rGO-PEG, 5 µL of hydrazine hydrate was added into GO-PEG colloidal solution (0.1 mg mL-1, 10 mL) under vigorous stirring. The reaction mixture was kept Teflon autoclave at 90 oC for 1 h. After being cooled to room temperature, the products were dialysis against deionized water for 24 h and concentrated to 1 mg 5



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mL-1. To prepare rGO-N749, 2.0 mL ethanol solution of N749 (0.25 mg mL-1) and 0.25 mg EDC were added to 2.0 mL 75% ethanol solution of rGO-PEG (0.25 mg mL-1), and the mixed solution was stirred for 48 h in the darkness. Free N749 was then removed by ultra-filtration through a 100-kDa filter (Millipore) until the filtrate was colorless. The remaining rGO-N749 solution was re-dispersed in ultrapure water. The final concentration of N749 conjugated on GO was evaluated by the standard curve which was measured by the UV-vis absorbance at 615 nm of N749 in ethanol solution. The calculated loading ration of N749 on rGO-PEG nanosheet is ca.5% (the weight percentage of N749 in total rGO-N749), which was in agreement with the results measure by coupled plasma optical emission spectrometer (ICP-OES). 2.3. Characterization of materials. Morphological features of the GO-PEG, rGO-PEG and their nanocomposites were characterized by transmission electron microscopy (TEM, Tecnai G2 F20 S-Twin) and atomic force microscopy (AFM, Veeco Dimension 3100). The absorbance and fluorescent spectra of the rGO-N749 were measured using UV-vis spectrophotometer (UV2600, Shimadzu) and fluorescence spectrophotometer (LS 55, PerkinElmer), respectively. Fourier transform infrared (FTIR) spectra of the samples (KBr pellet) were collected using a Thermo Nicolet 6700 FTIR spectrometer. Raman spectra were obtained using a confocal microprobe Raman system (HR 800) equipped with a holographic notch filter and a CCD detector. A long working distance 50× objective was used to collect the Raman scattering signal. The size of the laser spot is 1.7 mm. The excitation wavelength was 6


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532 nm from a He-Ne laser. XPS of the materials were collected by Thermao Scientific ESCALA 250Xi XPS spectrometer. The content of Pt NPs loaded on rGO-N749 sheet was measured by ICP-OES (Thermo scientific iCAP 6200). ICP results of the platinized samples demonstrated that the values of Pt loading on rGO-N749 are roughly in agreement with the corresponding values estimated from the starting materials (Table S1). 2.4.

Photoelectrochemical

measurement.

The

photoelectrochemical

experiments were carried out on a CHI 660D potentiostat/galvanostat electrochemical analyzer in a three-electrode system consisted of a saturated calomel electrode (SCE) as a reference electrode, a working electrode and a platinum wire as counter electrode. The working electrode for photocurrent measurement was prepared by drop-casting ca. 0.5 mg of the sample on the clean indium tin oxide (ITO) glass with the surface areas of ca. 0.8 cm2. The electrolyte was 0.2 M Na2SO4 aqueous solution. The working electrode was irradiated by a GY-10 xenon lamp (150 W) during the measurement. The oxidation potentials and reduction potential of N749 were measured by cyclic voltammetry using ferrocene as the standard. All the measurements were carried out in anhydrous acetonitrile containing 0.3 M lithium perchlorate (LiClO4) as the supporting electrolyte. Scan rate = 0.1 V s-1. 2.5. Photocatalytic reaction. The photocatalytic reaction was run in a 50 mL quartz flask equipped with a flat optical entry window. In a typical photocatalytic experiment, 1 mg of the catalyst dispersed in TEA (50 mL, 10%) solution. The solution was stirred continuously and irradiated by a GY-10 xenon lamp (150 W) at 7



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298 K under atmospheric pressure. The produced gases were analyzed with an online gas chromatograph (GC1650 equipped with a thermal conductivity detector and 5 Å molecular sieve columns) using argon as the carrier gas. The standard H2/Ar gas mixtures of the known concentrations were used for GC signal calibration. The apparent quantum efficiency ( Φ H 2 ) was measured under the same photocatalytic reaction condition except that the photocatalytic reaction was triggered at wavelength of 365 nm and 420 nm light sources. The apparent quantum efficiency ΦH2 is defined by the equation: ΦH2 =

2nH 2 (mol ) I 0 (mol ⋅ s −1 ) × t (s)

× 100%

Where I0 is the number of photons per unit time. I0 was found to be 1.09×10-8 mol s-1 and 2.46×10-8 mol s-1 for wavelength of 365 nm and 420 nm light sources, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization of rGO-N749. TEM and AFM measurements indicate that the size of GO-PEG prepared is in the range of 15 to 20 nm, which is much smaller than that of GO synthesized using modified Hummer’s method.28,29 After reduction, the size of rGO-PEG did not change noticeably, but the thickness decreased from 1.3 nm (GO-PEG) to 1.1 nm (rGO-PEG), (Figure 1 and Figure S1) which may be attributed to the significant removal of hydroxyl and epoxyl groups. Furthermore, the average size and thickness of rGO-N749 are about 30 nm (Figure 1C) and 1.8 nm (Figure 1D), respectively, which may ascribe to that one N749 molecules can 8


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conjugate with two rGO-PEG sheets. The energy-dispersive X-ray spectrometry (EDX) pattern (Figure 1E) demonstrates the elementary composition of rGO-N749. The existence of Ru confirms that N749 has been successfully grafted on the rGO-PEG sheets. Figure 2A presents the FTIR spectrum of rGO-N749. For comparison, the spectra of GO-PEG, rGO-PEG, and N749 are also presented in the same figure. For the GO-PEG sample, the IR band at ~3400 cm-1 assignable to the stretching mode of the hydroxyl groups of GO is obvious, while it becomes significantly weaker for the rGO-PEG sample, suggesting the partial removal of the hydroxyl groups of the GO sheets. A broad peak at ~2880 cm-1 due to the symmetric and asymmetric stretching modes of the methylene groups of the PEG moiety30 and a strong peak at ~1100 cm-1, which is attributed to the vibration of the -C-O- groups of PEG, from both spectra of GO-PEG and rGO-PEG,31 indicating that PEG molecules have been successfully conjugated to the graphene sheet, and that such conjugation is robust against chemical reduction.31 From the spectrum of rGO-N749, strong peaks at around 1400 cm-1 in the ﬁngerprint region, which can be appointed to the various aromatic vibration of terpyridine of N749 moiety,32 and the characteristic C=N stretching vibration of N749 moiety at 2085 cm-1 can be observed.32,33 The facts demonstrate that the N749 molecules are covalently bonded with rGO-PEG in rGO-N749. As shown by Raman spectra of GO-PEG and rGO-PEG (Figure 2B), the peaks at 1325 cm-1 and 1606 cm-1 are assigned to the D and G bands of graphene, respectively.28,29 The D/G intensity ratio is 2.4 and 1.8 for rGO-PEG and GO-PEG, 9



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respectively, which also confirms the reduction of GO-PEG to rGO-PEG after treating it with hydrazine. Furthermore, the G band position of rGO-N749 appears at 1598 cm-1, which downshifts by 8 cm-1 compared to that of rGO-PEG at 1606 cm-1. The shift of the G band can be attributed to charge transfer from donor (N749) to acceptor (rGO-PEG).34 Another notable evidence of forming rGO-N749 is that the Raman spectrum of the nanocomposite also includes the peak centered at 1469 cm-1 and 1538 cm-1, dovetailing the Raman peaks of N749 relating to the vibrations of terpyridine ligand.35 These Raman spectra further prove that rGO-PEG nanosheet has grafted with N749 moiety successfully. The XPS results are shown in Figure 3A. The XPS spectrum of GO shows the presence of carbon (285.1 eV) and oxygen (532.5 eV) elements. The high-resolution XPS spectrum of C 1s peaks can be fitted to three components, corresponding to the C-O, C=C and O=C bonds in GO (Figure S2), which is the results of oxidation and destruction of the sp2 atomic structure of graphite.36 The content of O element in GO is ca. 54 at.%, which also suggests high oxidation of graphene. Compared to that of GO, N 1s peak appeared in the XPS spectrum of GO-PEG, because of amine-terminated PEG covalently conjugation on GO sheets. After reduction of GO-PEG, the content of the oxygen element in rGO-PEG is ca. 23 at.%, which is obviously lower than that of GO-PEG, indicating the efficient reduction of GO-PEG to rGO-PEG. For rGO-N749, the high-resolution XPS spectrum (Figure 3B) shows that the peaks at 280.9 eV and 284.8 eV can be identiﬁed as Ru 3d5/2 and 3d3/2, respectively.37,38 Although the C 1s peak overlaps with Ru 3d3/2 at the binding energy 10


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~284 eV, the Ru 3d5/2 peak at ~280 eV does not overlap with any carbon signals. Another two features at 286.2 eV and 287.3 eV are also observed for rGO-N749, which is due to the carbon contamination.37,38 These results further demonstrate that N749 has been covalently bonded on rGO-PEG nanosheet successfully. 3.2. Investigation of optical and photoelectrochemical properties. The UV-vis absorption spectrum of rGO-PEG aqueous solution, along with the spectra of N749, rGO-N749 in ethanol, is shown in Figure 4A. N749 shows the band in the range 250-450 nm, which can be broadly attributed to the ligand centered π-π* electronic transition and/or metal to ligand charge transition (MLCT).39 The band corresponding to the longer wavelength (λmax) centered at 615 nm can be broadly attributed to the MLCT from the occupied 4d orbitals of ruthenium to the lowest unoccupied π* orbitals of the terpyridine ligand.24,39 The absorption spectrum of rGO-N749 also exhibits the same two typical electronic absorptions, indicating the presence of the N749 moiety in the nanocomposites. According to the standard curve which was measured by the UV-vis absorbance at 615 nm of N749 in ethanol solution, the calculated loading ration of N749 on rGO-PEG nanosheet is ca.5%. Figure 4B shows the fluorescence emission spectra of N749 and rGO-N749 in ethanol solution excited at 610 nm. Compared with N749, rGO-N749 demonstrates a weak fluorescence emission at the same concentration of N749. The calculated quenching efficiency is 90%, suggesting efficient photoinduced electron transfer from N749 to rGO-PEG due to covalently bonding between the sensitizer moiety and rGO-PEG nanosheet.40 Photocurrent measurements were also performed for investigating the electron 11



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transfer between N749 moiety and rGO-PEG (Figure 5). The photocurrent response of the bare ITO electrode was negligible under the performed conditions. The GO-PEG and rGO-PEG electrode demonstrated a weak photocurrent density (ca. 0.3 µA cm-2 and 0.5 µA cm-2) due to their weak absorption to UV-visible light irradiation. However, the photocurrent density increased to 0.9 µA cm-2 for the N749/ITO electrode owing to the optical absorption of N749 and efficient photoexcited electron transfer from the sensitizer to ITO. For the rGO-N749/ITO electrode, the steady and reproducible photocurrent response reached to ca. 2.8 µA cm-2, which is ca. 3 times as high as that of the N749/ITO electrode. The significant improvement of photocurrent response of the rGO-N749/ITO electrode may be attributed to the nice absorption of rGO-N749 in visible-light range and efficient electron transfer from the photoexcited N749 moiety to covalently bonded rGO-PEG nanosheet.41,42 The energy levels of N749 are determined by cyclic voltammograms which were recorded on film sample and the potentials were obtained relative to an internal ferrocene reference (Figure 6), then the energy levels of LUMO and HOMO of N749 are defined by the equations:43

[

(

)

]

[

(

)

]

EHOMO = − E ox − E Fc / Fc + + 4.8 eV ELUMO = − E red − E Fc / Fc + + 4.8 eV where E ( Fc / Fc + ) is the equilibrium potential of ferrocene. According to the cyclic voltammogram of ferrocene (Figure 6 (A)), E ( Fc / Fc + ) is 0.355 V (vs. SCE). From Figure 6 (B) and (C), the oxidation potentials ( E ox ) and reduction potential ( E red ) of N749 are 1.55 V (vs. SCE) and -0.49 V (vs. SCE), respectively. Thus, the measured 12


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energy levels of LUMO and HOMO of N749 are -3.955 eV (vs. Vacuum) and -5.995 eV (vs. Vacuum), respectively. Compared with the energy level of the conduction band of rGO (-4.42 eV vs. Vacuum),18 we know that photoexcited electrons generated from the N749 moiety of rGO-PEG could transfer to the conduction band of the rGO moiety of the composite (Scheme 2A).

3.3. Photocatalytic hydrogen evolution. The photocatalytic performances of H2 production from water over the as-prepared samples under 6 h UV-vis light irradiation are shown in Figure 7A. The total amount of H2 production over rGO-PEG is only 2.65 µmol mg-1, due to the energy level of the conduction band edge of rGO is high enough to supply an overpotential for H2 generation.44 For the rGO-N749 sample, the total amount of H2 production increased to 3.94 µmol mg-1 under the same reaction conditions. This result can be attributed to nice absorption to the visible light of N749 moiety and its covalent connection with rGO-PEG in the nanocomposite. To further improve the photocatalytic activity, Pt NPs were chose as cocatalyst,45 and loaded on rGO-N749 via photodeposition.46,47 The results show that H2 evolution over the 1 wt% Pt-deposited rGO-N749 (rGO-N749-Pt) can reach to 7.36 µmol mg-1, nearly 2 times compared to that of rGO-N749. Enhancement of the photocatalytic activity can be attributed to that Pt NPs acting as cocatalyst reduce the overpotential in the production of H2 from water and suppress the fast backward reaction as well.45,48,49 With the increasing of the loading amount of Pt, the evolved H2 also increased. When the Pt content reached to 5 wt%, the amount of H2 production reached its maximum 8.2 µmol mg-1. (Figure 7B) However, higher loading of Pt over rGO-N749 does not 13



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improve photocatalytic activity any more, since the Pt loading on rGO-N749 sheet has an optimum content for hydrogen production. Therefore the intimate interaction between rGO-N749 and Pt NPs would be beneficial to the photocatalysis. Under UV-vis light irradiation, the photoexcited electrons transfer from the dye moiety to rGO-PEG surface and then subsequently are shuttled to Pt NPs deposited on rGO-PEG nanosheets because of the excellent conductivity of rGO-PEG and lower work function of Pt NPs. Such an ordered photoinduced electron flow would certainly promote the separation of electron-hole pairs and enhance the photoconversion efficiency. The photocatalytic results over the as-prepared catalysts under visible light irradiation (> 400 nm) are shown in Figure 7C. In 6 h visible light irradiation, the amount of hydrogen evolved from rGO-N749-Pt was 1.5 µmol mg-1, however, rGO-PEG-Pt does not produce detectable hydrogen under the same conditions because rGO-PEG nanosheet does not absorb visible light. In addition, the calculated apparent quantum yield was 0.54% and 0.21% at 365 nm and 420 nm, respectively. The fact that the photocatalytic activity of rGO-N749-Pt under visible light irradiation is relatively lower than that under UV-vis light irradiation can be interpreted to lower photoexcited energy of the visible light. Moreover, the catalytic performance of rGO-N749-Pt under visible light irradiation mainly own to the red wavelength absorption of the catalyst, demonstrating agreeable ability of the material in the using red light of the solar spectrum. The nice photocatalytic performance of the nanocomposite under both UV-vis and visible light irradiation can be attributed to the 14


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N749 moiety covalently bonded with rGO-PEG nanosheet. Here, rGO-PEG nanosheet serves not only as an excellent supporting matrix for anchoring the sensitizer molecules and Pt cocatalyst but also as a superior electron mediator to adjust electron transfer. Figure 7D shows the photocatalytic stability of rGO-N749-Pt nanocomposite under UV-vis light irradiation. As shown by the figure, the amount of hydrogen in the first 6 h UV-vis irradiation is 8.2 µmol mg-1, and the activity of the photocatalyst remains virtually unchanged in the next four runs, indicating sufficient catalytic stability of rGO-N749-Pt. These results suggest that this black dye N749 molecules covalently functionalized rGO-PEG nanosheet is a promise candidate as a novel photocatalyst for hydrogen evolution. The mechanism of H2 production over rGO-N749-Pt can be illustrated in Scheme 2B. N749 moiety covalently bonded on the rGO-PEG nanosheets acting as a light harvesting sensitizer absorbs light irradiation. The photoelectrons transfer from the excited sensitizers to rGO-PEG nanosheet through PEG, which covalently bonds rGO and N749 in the nanocomposite, then to Pt NPs loaded on the rGO-PEG nanosheets, where the water molecules accept the electrons to form H2. The photoexcited N749 moiety returns back to the ground state by accepting electrons from TEA.

4. CONCLUSIONS In summary, a novel black dye N749 covalently functionalized rGO-PEG 15



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nanocomposite has been successfully synthesized and used for photocatalytic hydrogen evolution under UV-vis or visible-light irradiation. In this rGO-N749 hybrid, efficient photoinduced electron transfer from N749 to rGO nanosheets was shown. After Pt nanoparticles were deposited on rGO-N749 sheets, the photocurrent and photocatalytic activity of the nanocomposites were improved greatly compared with that of rGO-N749 under visible-light irradiation. The enhancement of the photocurrent and photocatalytic performance was attributed to the photosensitizer molecules covalently contacting with rGO. Here, rGO nanosheet served not only as an excellent supporting matrix for anchoring the sensitizer molecules but also as a superior electron mediator to adjust electron transfer. This study provides a new strategy for developing highly efficient carbon-based nanomaterials for photoinduced hydrogen evolution.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. AFM images of GO-PEG and rGO-PEG; High-resolution C 1s, O 1s XPS spectra of GO and N 1s XPS spectra of GO-PEG; The content of Pt loaded on rGO-N749 by photodeposition.

AUTHOR INFORMATION Corresponding Author 16


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*E-mail: [email protected] (P. Yang).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for the financial support of this research by the National Natural Science Foundation of China (21373143), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207), and the Outstanding Talent Training Plan of Soochow University (5832000213).

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Figure captions

A

B

C

D

E

Figure 1. TEM images of GO-PEG (A), rGO-PEG (B) and rGO-N749 (C). AFM image of rGO-N749 (D). (E) EDX pattern of the particles indicated in (C).

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PEG

N749

rGO-N749

1000

2000

3000

4000

1000

Wavelength (cm-1)

1606 1606

1540

GO

1607

2880 3400

rGO-PEG

1598

1630

GO-PEG

1470

1100

rGO-PEG

1538

1640

GO-PEG

1469

2085

Intensity (a.u.)

N749

1325

B

rGO-N749

1325

A Transmittance %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1325

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1200 1400 1600 Raman shift (cm-1)

1800

Figure 2. (A) FTIR spectra of PEG, GO, GO-PEG, rGO-PEG, N749 and rGO-N749. (B) Raman spectra of GO-PEG, rGO-PEG, N749 and rGO-N749.

27



GO rGO-PEG

A 600000

GO-PEG rGO-N749

35000 O 1s

C 1s

450000

N 1s 300000

150000

Intensity (arb. units)

750000

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

286.2

B

30000 Ru 3d 25000 284.8

20000 15000 10000 280.9

5000

287.3

0

0

200

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300

400

500

279

600

Binding Energy (eV)

282 285 288 Binding Energy (eV)

291

Figure 3. (A) XPS spectra of GO, GO-PEG, rGO-PEG and rGO-N749, respectively. (B) High-resolution Ru 3d XPS spectra of rGO-N749.

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400

4 3

B

rGO-PEG N749 rGO-N749

Intensity (a.u.)

A Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1 0 200

200 100 0

400 600 Wavelength (nm)

800

N749 rGO-N749 Ex: 610 nm

300

700 800 Wavelength (nm)

900

Figure 4. (A) UV-vis spectra of rGO-PEG, N749 and rGO-N749 in ethanol solution, (B) Fluorescence spectra N749 and rGO-N749 in ethanol solution.

29



3.6 GO-PEG rGO-N749

J (µA cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

rGO-PEG blank

N749

2.4

1.2

0.0 100

200 300 Time (s)

400

Figure 5. Photocurrent responses of ITO-electrode, GO-PEG/ITO-electrode, rGO-PEG/ITO-electrode, N749/ITO-electrode, and rGO-N749/ITO-electrode, to UV-visible light irradiation in an aqueous solution containing 0.2 M Na2SO4 as supporting electrolyte recorded at -0.4 V. The illumination from a 150 W xenon lamp was interrupted every 20 s.

30


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A

0.00006

Current (A)

Ferrocene 0.00003

0.00000

-0.00003

0.28V -0.6

-0.3

0.0

0.3

0.43 V 0.6

0.9

1.2

E (V vs. SCE)

0.0004

B

N749-Oxidation potentials

Current (A)

0.0003

0.0002

0.0001

0.0000

1.55 V 0.0

0.5

1.0

1.5

2.0

2.5

3.0

E (V vs. SCE) 0.00005

C

N749-reduction potential

0.00000

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.00005

-0.00010

-0.49 V

-0.00015 -4

-3

-2

-1

0

E (V vs. SCE)

Figure 6. Oxidation potentials and reduction potential of N749 by cyclic voltammetry with ferrocene as the standard.

31


A

6

rGO rGO-N749 rGO-N749-1% Pt

4 2 0 0

1

2

3 4 Time (h)

5

6

Hydrogen Production (µ mol mg-1)

8

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



9

1.5 C rGO-PEG-5% Pt rGO-N749-5% Pt

1.0

0.5

0.0 0

1

2

3 4 Time (h)

5

6

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B

6

3

0

1% Pt 3% Pt 5% Pt 10% Pt Concentration of Pt

D

6

3

0 0

6 12 18 24 Reaction time (h)

30

Figure 7. (A) Hydrogen production by rGO-N749 with and without Pt NPs under UV-vis irradiation. (B) Hydrogen production by rGO-N749 with different loading amount of Pt NPs (w/w). (C) Hydrogen production by rGO-PEG-Pt and rGO-N749-Pt under visible irradiation (λ > 400 nm). (D) Stability of rGO-N749-Pt (5 wt% Pt) nanocomposite under UV-vis light irradiation. Reaction conditions: 1 mg of the catalyst dispersed 50 mL TEA (10%) solution, pH=10, T = 298 K.

32


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic illustration of synthesis of rGO-N749 composites.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

B

Scheme 2. (A) Energy band structure diagram of heterostructure between rGO-PEG and N749. (B) Diagram of the electron transfer and hydrogen evolution in rGO-N749-Pt photocatalyst.

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The table of contents entry: We have developed ruthenium dye N749 covalently functionalized reduced graphene oxide with much high photocatalytic activity for hydrogen evolution from water under both UV-vis and visible light (λ>400 nm) irradiation. Keywords: photocatalysis; hydrogen evolution; reduced graphene oxide; N749 Authors: Jie Huang, Dandan Wang, Zongkuan Yue, Xia Li, Dongmei Chu, Ping Yang* Title: Ruthenium Dye N749 Covalently Functionalized Reduced Graphene Oxide: A Novel Photocatalyst for Visible Light H2 Evolution ToC figure:

35


Ruthenium Dye N749 Covalently Functionalized Reduced Graphene

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