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Uniformly Sized (112) Facet CoP on Graphene for Highly Effective Photocatalytic Hydrogen Evolution Bin Tian, Zhen Li, Wenlong Zhen, and Gongxuan Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00680 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Uniformly Sized (112) Facet Co2P on Graphene for Highly Effective Photocatalytic Hydrogen Evolution Bin Tian,a,b Zhen Li,a,b Wenlong Zhenc and Gongxuan Lu*a a

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute

of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. b

University of Chinese Academy of Science, Beijing 100049, China

c

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and

Chemical Engineering, Lanzhou University, Lanzhou, China.

ABSTRACT: We show that photocatalytic hydrogen evolution reaction (HER) rate is highly dependent on Co surface state, indicating by binding energy data. The key process of hydrogen generation, Co2P-H species formation, follows lower hydrogen adsorption free energies (∆GH) route. Such low surface energy species can dramatically decrease the overpotential for HER (about 35 mV for HER in basic electrolyte at pH 11, and 150 and 196 mV overpotentials at current density 5 and 15 mA/cm2, respectively). This could explain why Co2P loaded on RGO reached high hydrogen generation rate, 1068 µmol·h-1, much higher than that of Pt/RGO catalyst (822 µmol·h-1) under same reaction condition, while a high apparent quantum efficiency (AQE) (33.3%) was achieved at 520 nm. Moreover, it opens a design strategy for development of co-catalyst with enhanced efficiencies through change of surface H species formation. 1

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1. INTRODUCTION The photocatalytic water cleavage into molecular hydrogen and oxygen is considered as one of the most promising routes for simultaneous solving the current global energy and environmental problems.1 However, since water splitting is a nonspontaneous process in thermodynamics, in-put extra energy and an efficient co-catalyst are necessary for hydrogen evolution reaction (HER). Platinum group metal co-catalyst shows ultrahigh activity for HER, however, low-abundance and high-cost limit their scale-up applications.2 Just recently, some of active HER cocatalysts have been reported, e.g., MoB, MoS2 and CoS2, but the stability during the reaction is still a problem.3-5 Transition-metal phosphides (TMPs) have been used as anode materials of lithium ion battery and efficient catalyst for hydrodesulfurization reaction (HDS),6-8 and show potential application as co-catalyst for photocatalytic HER.9 Particularly, Co-Pi showed good activity and stability as anode materials for oxygen generation from water.10 Other TMPs were also reported, such as MoP, WP, FeP, CoP, Co2P, Ni5P4, and Ni2P,11 to be active for photocatalytic HER.12 Unfortunately, the synthesis of TMPs meets tremendous challenge due to the high preparation temperature and high toxic phosphorus sources. It is known that the superior electrical coductivity and charge fast transfer properties are key factors for high photocatalytic water splitting performance.6-8 Zhen et al. reported that the high activity not only relay on superior electrical conductivity and fast charge transfer, but depend on the particles size of cocatalyst and the

2


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dispersion of active species.13 High specific surface area materials (i.e. RGO,4 MOFs14) was applied to enhance the co-catalyst dispersion and speed up the charge transfer. Kevin C. Leonard and Allen J. Bard found that catalytic activity was related to the binding state between H and surface state of co-catalyst, but quantitatively explanation needs futher investigation.15 Herein, we developed catalyst consisting decorated iniformly sized Co2P nanohybrid on RGO by chemical reduction method, for high efficient HER under visible light irradiation after sensitized by Eosin Y (EY). The results show that photocatalytic hydrogen evolution reaction (HER) rate is highly dependent on Co surface state. Co2P-H species formation follows lower ∆GH route and makes the hydrogen generation enhanced. Such low surface energy species can dramatically decrease the overpotential for HER, about 35 mV for HER in basic electrolyte at cpH 11, and 150 and 196 mV overpotentials at current density 5 and 15 mA/cm2, respectively. Co2P loaded on RGO reached high hydrogen generation rate, 1068 µmol•h-1, much higher than that of Pt/RGO catalyst (822 µmol•h-1), and high apparent quantum efficiency (AQE) (33.3%) was achieved at 520 nm. This paper opens a design strategy for co-catalyst synthesis with enhanced efficiencies through modification of surface H species formation.

2. EXPERIMENTAL SECTION Materials: Cobalt sulfate (CoSO4·7H2O), sodium hypophosphite (NaH2PO2·H2O), sodium borohydride (NaBH4), sodium citrate (C6H5Na3O7·2H2O), sodium nitrate (NaNO3) ammonium sulfate ((NH4)2SO4), concentrated sulfuric acid (98%, H2SO4), potassium 3



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permanganate(KMnO4), graphite, barium chloride(BaCl2), hydrogen peroxide (30%, H2O2), sodium hydroxide (NaOH), hydrochloric acid, triethanolamine (TEOA), Eosin Y (EY), chloroplatinic acid (H2PtCl6·6H2O) were analytical grade and used without further purification. De-ionized water with a specific resistance of 18.2 MΩ·cm was obtained by reverse osmosis followed by ion-exchange and filtration (RFD 250NB, Toyo Seisakusho Kaisha, Ltd., Japan).

2.1. Preparation of graphite oxide (GO) The graphite oxide (GO) was prepared by a modified Hummers method.4 Briefly, at 0℃, 10 g graphite and 5 g NaNO3 were added into 230 ml concentrated H2SO4, subsequently, KMnO4 (30 g) was slowly added under strongly stirring and cooling so that the temperature of the mixed solution was maintained below 20℃, then stirred at 35℃ for 2 h. Deionized water (460 ml) was added into the solution tardily and stirred for 15 min. A total of 1.4 L deionized water and 25 ml H2O2 (30%) were needed in the whole reaction. Finally, the product was filtered, washed with HCl solution (1:10, v/v) repeatedly until sulfate ion could not be detected with BaCl2, Then the precursor was washed with pure water till no Cl- ions were detected, and dried in a vacuum oven for 24 h at 40℃. 2.2. Preparation of catalysts The Co@Co2P/RGO nanohybrid was prepared using a mild method.16 In short, 250 mg graphite oxide (GO) was dispersed into 100 ml ultrapure water and ultrasonic treatment (25 kHz, 250 W) for 1 h, and then the mixture was transferred to a three-necked flask. Subsequently, added 0.242 mol·L-1 CoSO4·7H2O into the flask 4


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and stirred for 30 min at room temperature. After that, (NH4)2·SO4 (0.780 mol·L-1), Na3C6H5O7 (0.525 mol·L-1) and NaH2PO2 (0.121 mol·L-1) were added successively. The pH of the mixture was adjusted to 11 with NaOH (1 M) and the mixture was kept at 90℃ for 30 min in Ar atmosphere, then 0.5 mol·L-1 NaBH4 was added into the mixture. The mixture was kept at 90℃ for another 4.5h, letting reaction completed. The gray-black precursor was separated, washed with ethanol and deionized water, and then dried in a vacuum at 60℃ for 12 h, and annealed for 2 h at 280℃ in reducing atmosphere (8% H2/N2). The Co/RGO and Pt/RGO were prepared by one-step photoreduction and in-situ chemical deposition according to previously reported methods.17 2.3. Characterizations X-ray diffraction (XRD) data were obtained on a Rigaku B/Max-RB diffractometer with a nickel filtrated Cu Kα radiation operated at 40 kV and 40 mA. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded by a Tecnai-G2-F30 field emission transmission electron microscope with an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) analysis was implemented on a VG Scientific ESCALAB 210-XPS with a Mg Kα X-ray resource. The binding energy C 1s peak from surface adventitious carbon (284.8 eV) was adopted as a reference for the binding energy measurements. UV-vis absorption spectra were acquired on a Hewlette Packard 8453 spectrophotometer. Fluorescence lifetime measurements were performed on a Horiba Jobin Yvon Data Station HUB operating in time-correlated single photon counting mode (TCSPC) with 5



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a time resolution of 200 ps. Nano LED diode emitting pulses at 464 nm with 1 MHz repetition rate was used as an excitation source. 2.4. Electrochemical measurements All the electrochemical measurements were achieved in a typical three-electrode (a Pt counter electrode, a reference electrode of saturated calomel electrode (SCE) and a working electrode) electrochemical workstation (CHI660E). In this test, TEOA aqueous (10%, v/v) and 0.1 M Na2SO4 were used as electrolyte (pH 11). The transient photocurrent responses of EY-sensitized Co2P, Co@Co2P/RGO, and Co/RGO samples coated on ITO were recorded for several on-off cycles of intermittent visible light irradiation (20 s). The rate of Linear sweep voltammetry (LSV) was 1 mV·s−1. A 300-W Xe lamp equipped with an optical cutoff filter of 420 nm was used for light source. 2.5. Preparation of working electrodes Catalyst powder (2 mg) and EY (1×10-5 mol·L-1) were dispersed into 1 mL anhydrous ethanol and ultrasonic treatment for 30 min. 75 µL sample mixture was drop-coated onto the precleaned indium tin oxide (ITO) glass or glassy carbon electrode (GCE) surface (1 cm2) and was air-dried (loaded about 0.15 mg/cm2) before measurement. 2.6. The activity measurement of catalysts The catalytic activity was measured in a Pyrex flask (170 ml), and Co/RGO and Pt/RGO were used for comparison. The catalyst (8 mg include 2 mg Co2P and 6 mg RGO), TEOA (10%, 100 ml), and EY (1×10-5 mol·L-1) were added to the flask. Prior 6


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to irradiation, the air in the flask was excluded by Ar gas for 30 min. The illuminant was a 300 W xenon lamp with a 420 nm cut off filter. The amount of H2 production was detected by GC (Aglient 6820, TCD, 13 × column, Ar carrier). The apparent quantum efficiency (AQE) was measured by a band-pass filter (430, 460, 490, 520 and 550 nm respectively) under the same condition and photon flux of the incident light was tested by a ray virtual radiation actinometer (FU100, silicon ray detector, light spectrum, 400-700 nm, sensitivity, 10-50 µv·µmol-1·m-2·s-1). The mixture was irradiation for 30 min with band-pass filter for AQE experiments. The following Equation is a formula to calculate the AQE. AQE =

2 × the number of evolved hydrogen molecules × 100% the number of incident photons

2.7. The stability measurement of catalysts The stability tests were carried out for 360 min (include 4 runs), after first run of 90 min, the catalyst was separated by magnetic or centrifugal method. After washed thoroughly by deionized water, the catalysts were mixed with TEOA solution and fresh EY. The procedure was repeated several times according to the required experiment design.

3. Results and Discussion Co@Co2P/RGO nanoparticles were prepared using an in-situ deposited method. X-ray diffraction (XRD) results (pattern (a) in Figure S1, see Supporting Information in detail) indicated that six peaks, matched well with the typical graphene phase,4 (112), (211), (020), (024) planes of orthorhombic Co2P (JCPDS# 65-2380, a=5.646Å, 7



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b=6.608 Å, c=3.513 Å) and (111) planes of hexagonal Co (JCPDS#89-4308, a=2.505 Å, b=2.505 Å, c=4.089 Å), respectively. The pattern (b) showed slight displacement of peaks position, which might be attributed to the interaction between Co2P and graphene. The strongest diffraction peak of Co2P phase at 40.7° indicated that graphene could facilitate the growth of the Co2P (112) planes because of the confined function of graphene.18 These results were consistent with the HRTEM characterization images, in which a clearly core-shell structure and lattice patterns implied cobalt phosphide species in both catalysts should exist in mixed phases of orthorhombic (112) Co2P and hexagonal Co. The morphology and microstructure of the Co@Co2P/RGO and Co2P nanocomposites were studied by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). A typical TEM image was given in Figure 1a. The catalyst had a spherical morphology with an average size of about 10 nm (Figure S2) and were uniformly distributed and well wrapped by the graphene sheets, indicated that Co2P particles had a compact contact with graphene sheets (Figure 1b) even after the reaction. A high-resolution TEM (HRTEM) image is given in Figure 1c. The catalyst shows a clear core-shell structure with shell thickness about 1.5 nm. The Figure 1d shows clearly lattice fringes, suggesting good crystallinity of the Co2P, in accordance with that revealed by the XRD experiment. The lattice fringes with d spacing 0.221 nm correspond to the (112) planes of Co2P. Energy dispersive X-ray spectroscopy (EDX) recorded from nanoparticles confirmed the presence of Co and P in the catalysts (Figure S3). To further confirm the composition and elements 8


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distribution of the photocatalysts, elemental mapping analysis was tested. The left image in Figure 1e showed the TEM image of the region where elemental mapping was carried out. The green, yellow, orange and claret colors in Figure 1e correspond to the mapping of cobalt, phosphorus, oxygen and carbon elements, respectively. It was observed that all four elements existed in the photocatalysts and four elements (Co, P, C and O) uniformly dispersed in photocatalysts, and the distributional ranges of cobalt and phosphorus elements were excellently overlapped, revealing that cobalt and phosphorus was in the form of Co2P composition in our photocatalysts. These results are in good agreement with the results of XRD. The formation of core-shell structure transition metal phosphide nanoparticles might follow the so-called “nanoscale Kirkendall mechanism”,19 related to the outward migration of the metal in the phosphating process. The driving force for this reaction is still unclear.20 We believed that the driving force formed Co@Co2P/RGO nanohybrid could be attributed to the surface defects in the cobalt particles and diffusion promoted by the temperature. Cobalt ions were firstly absorbed on the surface of graphite oxide (GO). After addition of NaBH4, cobalt ions and GO were reduced to metal cobalt particles and RGO, respectively, meanwhile NaH2PO2 conversion to PH3. Then the hydrogen phosphide were adsorbed on the Co particle surface, formed an intermediate (Co-PH3) and Co2P. In addition, as the outermost electron configuration of Co is 3d74s2, existing three one-electron d-orbitals on the basis of Hund rule and Pauli Exclusion Principle, according to the lowest energy principle, that the electron energy of d-orbital is higher than energy of s-orbital results 9



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in d-orbital of electrons attacking the sp3 hybridized orbital formed by P and H, thus P-H bonds rupture and forming of Co-P bonds gradually, which is in well agreement with the result of theoretical calculation (Figure S7). Eventually, the graphene decorated Co@Co2P with core-shell structure nanocomposites has been formed (Scheme 1). Furthermore, formation Co2P covered Co metal phase might lead to core-shell structure in the catalysts, which was supported by XRD analysis. Co2P and Co@Co2P/RGO nanohybrid were further characterized by XPS. Figure 2a and b showed three peaks appeared in the Co 2p3/2 and P 2p spectra, the peak at 778.2 and 129.2 eV corresponds to the reduced Co species and the phosphorus species in Co2P, respectively.21 The Co 2p3/2 XPS spectra of Co@Co2P/RGO in Figure 2a manifested a higher BE of Co 2p (778.9 eV) than that of Co2P (778.2 eV), and at same time, the P 2p XPS spectra in Figure 2a also slightly shifted to higher energy field to 129.4 eV (Co@Co2P/RGO) compared to the data of P in Co2P (129.2 eV),21-22 suggested that there was a strong interaction between Co2P and RGO, which corresponded to the slight low energy shift of C 1s (Figure S4a) compared with the values of literatures.4 Such a binding energy shift could be ascribed to so-called coelectron charge interaction, similarly to that of the previous report,18 in which Kong et al. reported the electron interaction between the S atom on NiSx and its adjacent carbon layer by forming a coelectron cloud. The interaction of between Co2P and RGO by the formation of coelectron cloud would help the electrons transfer from graphene to Co2P in HER. In addition, the peaks of Co 2p were observed at 781.7 and 785.6 eV could be categorized into a oxidation state of cobalt (Figure S4b and c). 10


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The photocatalytic activity of the Co@Co2P/RGO catalyst was studied by measure of amount of H2 evolution. In Figure 3a, the photocatalytic hydrogen evolution activity markedly enhanced by introduction of graphene and Co2P. In the Co2P/EY, the average rate of hydrogen evolution rate reached ~136 µmol·h-1, indicated that Co2P was a high-efficient cocatalyst for HER. The Co@Co2P/RGO nanocomposite exhibited higher catalytic activity than Co2P, Co/RGO and Pt/RGO, and the amount of hydrogen generated was 1175 µmol within 1.5 h, which was about 5 times higher than that of Co2P/EY, about 2 times higher than that of Co/RGO/EY under the same conditions. We also studied the influence of Co2P weight ratio to graphene on hydrogen evolution activity. The maximum was reached when the ratio was 1/3 (see Figure S5). Further increase of that ratio resulted in a slight decrease of the activity of the Co@Co2P/RGO nanohybrid, which might lead to the decrease of the RGO absorption site to EY in Co@Co2P/RGO. The effect of pH on photocatalytic activity of Co@Co2P/RGO for hydrogen generation was also researched, indicated that the Co@Co2P/RGO nanohybrid was insensitive to the reaction pH, and showed excellent photocatalytic H2 generation in a wide pH range from 3-13, and the hydrogen evolution activity was the highest at pH 11 (Figure 3b). The amount of hydrogen was decrease when the solution was held under the strong acid and alkaline condition (pH 3 and 13). The possible reason was that the protonation of TEOA, which was rendered an inefficient electron donor, led to a short lifetime for EY3* in more acidic solutions. Then resulted in the concentration of reductively quench EY3* become low, which was not conducive to 11



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the evolution of hydrogen from water. In addition, the carboxyl groups of EY might appeared deprotonate under the condition of strongly basic and the dye could not be adsorbed on graphene effectively because of electrostatic repulsion resulted in a relatively low hydrogen evolution.4 In Figure 3c, the highest apparent quantum efficiencies (AQE) reached 40.6% at 430 nm for Pt/EY system. However, The Co@Co2P/RGO/EY system exhibited a highest AQE (44.1%) at 430 nm, the second high AQE was appeared at 520nm, which corresponded to the strongest absorption wavelength of EY at 519 nm. Higher AQE at 430nm might ascribe to its stronger photon energy, while the high AQE at 520nm might result from the stronger absorbance of EY (Figure S6). The stability of the Co@Co2P/RGO nanohybrids was evaluated, and the results were shown in Figure 3d. Our results showed that Co@Co2P/RGO nanohybrid exhibited a higher stability than Pt/RGO, and could maintain its activity at least 4 h. To recover the reasons of the activity differnece between different catalyts, the ∆GH data were calculated using DFT (see Supporting Information in detail). According to Table S2, it was clear for decrease of HOMO-LUMO gap (∆E) with the introduction of P element in Co, and results indicated that the ∆GH of Co2P (60.9 kJ·mol-1) was much lower than that of Co (602 kJ·mol-1), indicated that dehydrogenation was easier on Co2P than metal cobalt. The low activity of the HER on the Co could be attributed to the stronger strength of the Co-H bond hinders the smooth desorption of hydrogen from the cobalt in comparison with Co2P, which was similar to the results of theoretical calculation. 12


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Figure 4a exhibits a distinct emission peak at about 535 nm in EY solution, which is consistent with previous reports.13 These emissions were dramatically quenching as the introduction of Co2P and Co@Co2P/RGO, and the quenching efficiency of Co@Co2P/RGO (82.9%) was remarkably higher than Co2P (62.9%) due to the rapid transfer of photogenerated electrons from EY to graphene and improved the photocatalytic activity. Meanwhile, slight blue shift were observed in Co2P/EY and Co@Co2P/RGO/EY system, which could be attribute to the noncovalent interaction of graphene and Co2P with EY and the interfacial electron transfer from the attached EY* to the graphene sheets and Co2P.23-24 The Figure 4b further showed the lifetime of Co2P/EY and Co@Co2P/RGO/EY were longer than that of EY, indicated that the effective electrons transfer from EY to RGO and Co2P and the data of fluorescence lifetimes see Table S3. Figure

4c

showed

dramatically

enhancement

in

photocurrent

with

Co@Co2P/RGO/EY/ITO in relative to Co2P/EY/ITO, indicated that the electrons transfer from EY−• to Co@Co2P/RGO was fast as well as that to the ITO glass, and as excellent electrons acceptor and transporter, graphene efficiently extended the lifetime of photo-generated electrons, and thus the hydrogen generation activity was improved. In addition, the photocurrent of Co/RGO/EY/ITO electrode was much less than that of Co@Co2P/RGO/EY/ITO indicated that the introduction of P element was beneficial to electrons transfer. In Figure 4d, it was surprised that the Co2P nanohybrid showed a high cathodic current density with a low onset overpotential of -0.17 V. In contrast, a lower 13



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overpotential (-0.13 V) was needed under the same cathodic current in ITO/Co@Co2P/RGO system and only required overpotentials of 150 and 196 mV to reach current densities of 5 and 15 mA/cm2, respectively, indicated that graphene could synergistically accelerate the reduction of protons into H2. In addition, the overpotential was markedly decrease because of the implantation of P element (Figure S8). The mechanism of Co@Co2P/RGO/EY used for photocatalytic hydrogen evolution was present in Scheme 2. EY was combined with the graphene with the noncovalent π-π interactions. The excited electron was input to graphene, then transfer to Co2P very efficiently with the aids of excellent conductive graphene sheets, and the proton was reduced to hydrogen on Co2P loaded on the graphene surface. The adsorbed EY transformed into singlet excited state (EY1*), and subsequently formed a lowest-lying triplet excited state species (EY3*) by an efficient intersystem crossing (ISC).18 The oxidation EY3* could be reductively quenched by sacrificial donor TEOA to EY−•. The electron transfers to the coupling orbit of Co 3d and P 3p in Co2P photocatalyst, and reduced state of Co catalyze proton reduction to H2. Here low energy Co2P-H species determined the hydrogen formation rate, as evidenced by DFT calculation results. In addition, graphene display a key role in extending the lifetime of photo-production electrons by efficient charge separation.

4. CONCLUSIONS In summary, the Co@Co2P/RGO nanohybrid was prepared by chemical reduction method. The Co@Co2P/RGO co-catalyst exhibited excellent HER activity 14


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about 1068 µmol·h-1, which is much higher than that of Co/RGO (593 µmol·h-1) and Pt/RGO (822 µmol·h-1). The XPS spectra showed that the binding energy of Co2P alloy shifted toward to positive direction compared to Co metal. Using the DFT and FMO theory calculations analysis, we disclosed that the excellent photocatalytic performance for HER dependent on the low ∆GH, (the ∆GH of Co2P-H and Co-H were 60.9 and 602 kJ·mol-1, respectively). In addition, the Co@Co2P/RGO photocatalyst exhibited a low overpotential (-0.13 V), higher apparent quantum efficiency (33.3%, at 520 nm), stability, and longer fluorescence lifetime (1.58 ns). It suggested that Co2P is a promising high efficient candidate for HER cocatalyst. This work opens up a design strategy for co-catalyst with enhanced efficiencies through fabricating the low ∆GH surface Co2P-H species.

■ ASSOCIATED CONTENT Supporting Information Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +86-931-4968 178. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT 15



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This work has been supported by the National Natural Science Foundation of China (Grant Nos. 21433007 and 21373245) and the 973 Program of Department of Sciences and Technology China (Grant Nos. 2013CB632404).

■ REFERENCES (1) Tong, H.; Ouyang, S. X; Bi, Y. P; Umezawa, N.; Oshikiri, M.; Ye, J. H. Nano-photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229-251. (2) McIntosh, E. M.; Wikfeldt, K. T.; Ellis, J.; Michaelides, A.; Allison, W. Quantum Eﬀects in the Diﬀusion of Hydrogen on Ru (0001). J. Phys. Chem. Lett. 2013, 4, 1565-1569. (3) Cheng, H.; Lv, X.; Cao, S.; Zhao, Z.; Chen, Y.; Fu, W. Robustly Photogenerating H2 in Water Using FeP/CdS Catalyst under Solar Irradiation. Sci Rep. 2016, 6, 19846-19855. (4) Min, S. X.; Lu, G. X. Sites for High Efficient Photocatalytic Hydrogen Evolution on a Limited- Layered MoS2 Cocatalyst Confined on Graphene Sheets-The Role of Graphene. J. Phys. Chem. C 2012, 116, 25415-25424 (5) Faber, M. S.; Park, K.; Cabán-Acevedo, M.; Santra, P. K.; Song, J. EarthAbundant Cobalt Pyrite (CoS2) Thin Film on Glass as a Robust, High-Performance Counter Electrode for Quantum Dot-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1843-1849. 16


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Figure Captions Figure 1. Before (a) or after (b) the reaction Co@Co2P/RGO TEM; (c), (d) before reaction Co@Co2P/RGO HRTEM; (e) regional elements mapping. Figure 2. XPS spectra of the (a) Co (2p3/2) in Co2P or Co@Co2P/RGO and (b) P (2p) in Co2P or Co@Co2P/RGO. Figure 3. (a) The amount of hydrogen evolution by EY photosensitized systems catalyzed loaded on the graphene sheets. (b) Effect of pH on photocatalytic hydrogen generation activity of Co@Co2P/RGO. (c) Apparent quantum efficiency (AQE) of Co@Co2P/RGO, Co2P and Pt dye sensitized system under different wavelengths of visible light irradiation. (d) Stability testing and comparison of hydrogen production over EY-sensitized Co@Co2P/RGO, Co/RGO and Pt/RGO nanohybrid. Figure 4. (a) Photoluminescence and (b) Time-resolved photoluminescence (TRPL) spectra of EY, Co2P/EY, and Co@Co2P/RGO/EY at an excitation wavelength of 464 nm. (c) Transient photocurrent-time profile of EY photosensitized Co@Co2P/RGO, Co2P and Co/RGO coated on ITO glass. (d) Linear sweep voltammetry (LSV) curves of bare ITO glass, Co2P, Co@Co2P/RGO coated ITO electrodes. Scheme 1. Formation mechanism of Co@Co2P/RGO nanocomposite. Scheme 2. Hydrogen evolution mechanism of Co@Co2P/RGO/EY system

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Scheme 1.

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Scheme 2.

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TOC Graphic

The photocatalytic hydrogen evolution reaction rate is highly dependent on Co surface state, indicating by binding energy data. The key process of hydrogen generation, Co2P-H species formation, follows lower hydrogen adsorption free energies (∆GH) route. This could explain why Co@Co2P/RGO reached high hydrogen generation rate, 1068 µmol•h-1, much higher than that of Pt/RGO catalyst (822 µmol•h-1), while a high AQE (33.3%) was achieved at 520 nm. Moreover, it opens a design strategy for development of co-catalyst with enhanced efficiencies through change of surface H species formation.

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