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Disentangling the Photocatalytic Hydrogen Evolution Mechanism of One Homogeneous CobaltCoordinated Polymer Lin-Feng Gao, Zhi-Yuan Zhu, Wan-Shu Feng, Qiang Wang, and Hao-Li Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09767 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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The Journal of Physical Chemistry

Disentangling the Photocatalytic Hydrogen Evolution Mechanism of One Homogeneous CobaltCoordinated Polymer Lin-Feng Gao,a, b, c Zhi-Yuan Zhu,b Wan-Shu Feng,b Qiang Wang*a, b, c and Hao-Li Zhang*a, c a

State Key Laboratory of Applied Organic Chemistry (SKLAOC), Lanzhou University,

Lanzhou, 730000, China. b

Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province,

College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China. c

Key Laboratory of Special Function Materials and Structure Design, Ministry of Education,

China.

Abstract

One novel cobalt-coordinated graphitic carbon-nitride-type polymer (Co-g-CN) integrating the advantages of both molecular catalytic efficiency and nano semiconductor stability was fabricated, which served as homogeneous photocatalyst exhibiting superior hydrogen evolution efficiency (ca. ~ 12.3 mmol g-1 h-1) under visible light irradiation in the absence of noble metal cocatalyst. Various techniques including laser photolysis and electron paramagnetic resonance

ACS Paragon Plus Environment

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were combined to disentangle the underlying photocatalytic mechanism, which suggested that unlike nano semiconducting catalysis, the multivalent Co metal center of the polymer mediated electron transfer process, directly got involved in the proton reduction by sequentially exchanging electrons in a way similar to those molecular coordinated catalysts. These findings provide useful insight into the photocatatlytic mechanism of metal center–mediated water splitting process, and the employment of economical non-noble metal-coordinated polymer as highly efficient catalyst may open new avenue for long-term conversion of sunlight into sustainable hydrogen energy.

Introduction Artificial photocatalytic water splitting, which enables the production of sustainable and environmentally benign hydrogen energy alternative to traditional fossil fuels, has been a promising strategy to handle currently urgent energy and environment issues worldwide.1-4 Through reviewing previous literatures, typical photocatalytic hydrogen evolution systems could be categorized into two types: molecular catalyst- and nano semiconductor catalyst-based reactions.5 For the former, the system usually consists of a molecular catalyst, photosensitizer and sacrificial electron donor.6-7 Although the hydrogen evolution rate in molecular catalyst based systems is oftentimes high partially due to the homogeneous reaction environment, the instability of the molecular catalysts plagues their practical applications. By contrast, nano semiconductor catalyst-based systems are well-known for their enduring catalytic performance, 2, 4, 8

but the rate of hydrogen evolution still leaves much room to be improved. Therefore, one


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catalyst simultaneously possesses the nano semiconductor-like stability and homogeneous molecular-like efficiency is greatly desired. Carbon-nitride-type polymers have attracted ever increasing interest for photocatalytic hydrogen production from water reduction due to their stability and low production cost.2, 4, 8-9 Our group recently reported a novel graphitic carbon nitride-type polymer (g-CN) by preorganizing formamide and citric acid precursors into supramolecular structures and then polycondensed them at a lower temperature than that for traditional C3N4.10 Polymers synthesized by this method intrinsically possessed rich hydrophilic groups (-OH, -COOH and NHx) and hence rendered them conveniently dissoluble in aqueous solutions as homogeneous water splitting photocatalysts. Considering the transition metals in molecular catalysts usually account for their high H2 evolution rate, we are inspired to mimic those catalysts by introducing such metal centers into the carbon-nitride-type polymers. Fortunately, with abundant intrinsic “nitrogen pots”,10 cobalt could conveniently coordinate with the sites and form a cobaltcoordinated graphitic carbon nitride-type polymer (Co-g-CN) (Scheme 1). Remarkably, unlike our previously reported Iron-coordinated polymer (Fe-g-CN) which relied on Pt nanoparticles as co-catalyst for efficient water reduction,10 herein the as-prepared Co-g-CN displayed superior hydrogen generation rate in the absence of noble metal, reflecting the Co could be the crucial catalytic center.


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Scheme 1. Synthesis strategy of Co-g-CN, where formamide, citric acid and cobalt salts first preorganized into polymer-like structures and further polycondensed into 2D cobaltcoordinated carbon-nitride-type polymers. We then employed various techniques including laser photolysis and electron paramagnetic resonance (EPR) and UV-vis absorption measurements in order to disentangle the mechanism of H2 production. The investigation pictured the multistep electron transfer processes during water reduction and disclosed the cobalt-coordinating played a key role. Unlike nano semiconducting catalysis, the multivalent Co metal center of the polymer directly mediated electron transfer process and accomplished the proton reduction by sequentially exchanging electrons. This work thus introduced the concept of metal-coordination into a nano semiconducing material and endowed it with molecular catalytic behaviors such as homogeneity and high efficiency while maintaining durability. The exploration of underlying photocatalytic mechanism also shed light on the rational design of carbon nitride-type polymers with even higher H2 evolution efficiency.


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Experimental Section Materials Citric acid, sodium acetate and cobalt(II) chloride hexahydrate (CoCl2•6H2O) were purchased from Tianjin GuangFu Technology Development Co. Ltd. Eosin Y alcohol soluble (EY) and formamide were from Aladdin Chemistry Co. Ltd. Triethylamine (TEA) and N,Ndimethylformamide (DMF, dried), ethanol (EtOH) was purchased from Rianlon Bohua (Tianjin) Pharmaceutical& Chemical Co. Ltd. All solvents and chemicals were used without any further purification except for DMF, which was dried with CaH2 and distilled under reduced pressure. Deionized water (resistivity: 18.3 MΩ•cm) was used to prepare aqueous solution throughout the experiment. Syntheses of Co-g-CN In a typical procedure, a mixture of 10 mL of formamide, 0.70 g citric acid, 0.70 g sodium acetate and 0.50 g CoCl2•6H2O were heated to 230 ℃ in a 50 mL autoclave for 3 hours. Subsequently, 30 mL of ethanol was poured into the mixture to precipitate the products. The precipitation was then separated by centrifugation at 11000 rpm for 5 minutes and washed by deionized H2O for at least 3 times. Characterization Spectroscopic measurement The FT-IR spectra were recorded using a VERTEX 70V/80V Fourier transformed infrared spectrometer (Bruker, Germany) by means of the KBr pellet technique. Raman spectra were


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measured with a Renishaw in via Raman Microscope System at the excitation of 633 nm. The collection of UV/Vis absorption spectra was performed on a TU-1810 Spectrophotometer (Beijing Purkinje General Instrument, China). Laser photolysis experiments were performed on a LP920 (Edinburgh Instrument) transient absorption spectrometer available with kinetic (PMT) and spectral (ICCD) dual detection modes. The OPO laser (Opolette HE 355 LD UVDM, Optotek Inc.) with output wavelength from 236 nm-2400 nm was employed as the excitation source and the samples were purged with nitrogen to remove oxygen in the solution before measurements. Other characterizations The size and morphology of samples were characterized using field-emission scanning electron microscopy (FE-SEM, Hitachi S4800) at an accelerating voltage of 5.0 kV. The samples for SEM analysis were prepared by dripping the solution of sample onto silicon wafer and dried under 60°C. The TEM micrographs were obtained using A Tecnai G2 F30 Field Emission Transmission Electron Microscope at an operating voltage of 300 kV. Samples were dispersed onto holey carbon grids with the evaporation of excess solvent. Powder X-ray diffraction (XRD) patterns of the products were recorded on a Panalypical X‟ Pert PRO diffractometer using Cu K α X-rays between 5°and 90°. X-ray photoelectron spectroscopy (XPS) was performed on AXIS Ultra. Elementary analysis was performed by using a Vario EL cube and Co element content was determined via inductively coupled plasma atomic emission spectroscopy (ICP-AES) on IRIS ER/S.


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Electrochemical measurements CHI 660b electrochemical workstation (Shanghai Chenhua) was used to record cyclic voltammograms (CVs) of samples: Platinum as working electrode, platinum wire as counter electrode, Ag/AgNO3 (0.01 M) electrode as the reference and ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal standard. 0.3 g/L Co-g-CN was dissolved in DMF. Electron paramagnetic resonance measurements The Electron paramagnetic resonance spectra (EPR) were obtained from Bruker ER200DSRC10/12 apparatus. The test solution included Co-g-CN (0.25 g/L), EY (0.25 g/L) and 10% TEA (v/v) in ethanol/H2O (v/v=1:1) solution at a pH value of 11.0. Procedure for Photocatalytic H2 Evolution Photocatalytic hydrogen evolution experiments were performed by irradiating an ethanol/water solution of samples in a 25-ml round-bottom flask, in the presence of Eosin Y (EY) and triethylamine (TEA) using a white-light LED (light spectrum: 400-750 nm; energy density: 13.7 mW/cm2 at a distance of 6 cm) at room temperature. Prior to irradiation, the solution was sealed with a silicone stopper, degassed, flushed with dry nitrogen, and sonicated in dark for ~1-2 min to ensure an adsorption-desorption equilibrium between the photocatalyst and reactant. Hydrogen was detected by a calibrated Varian GC-3380 Gas Chromatograph with a thermal conductivity detector and nitrogen as the carrier gas. The optimized reaction conditions are used for H2 production with suitable amount of catalyst (0.025 g/L) in ethanol/water (v/v=1:1) solution in the presence of EY (4.0×10-4 M) and TEA (10 % v/v), at a pH value of 11.0 adjusted by hydrochloric acid.


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Results and Discussion

c

b

1 mm -1

1570 cm

d

-1

Transmittance (a.u.)

1360 cm

500 nm

20 nm

807 cm 2168 cm

-1

-1

1690 cm

-1

1610 cm

-1

e

1000 1500 2000 2500-1 3000 4000 Raman Shift (cm )

3000 2000 1000 Wavenumber (cm-1)

Intensity (a.u.)

a

Raman intensity ( a.u.)

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f 20

40 60 2()

80

Figure 1. The SEM image (a), TEM and HRTEM images (b, c), Raman spectra (d), FT-IR spectra (e) and XRD patterns (f) of Co-g-CN. The weight ratio of the as-prepared Co-g-CN was 33.07% (C), 27.43% (N), 18.32% (O), 2.40% (H) through elementary analysis and 18.78% (Co) via inductively coupled plasma atomic emission spectroscopic (ICP-AES) measurement, respectively. A N/C molar ratio of ~0.71 was smaller than that of traditional C3N4, reflecting the formation of a new member of the carbon nitride family.10-11 The cobalt content confirmed that the cobalt element was successfully coordinated in the polymer. Moreover, the O and H content indicated possible formation of -OH, -COOH functional groups on the surface.


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As shown in Figure 1a, the scanning electron microscopic (SEM) image demonstrated that Co-g-CN had a partially rolled and stacked lamellar structure. TEM images confirmed the polymers mainly exist as planar structures with lateral size of several hundred nanometers (Figure 1b). High-resolution TEM (HRTEM) images of the Co-g-CN in Figure 1c provided further persuasive evidence as to the 2D planar structure of Co-g-CN and excluded the existence of aggregated Co particles in the sample. More HRTEM images of the Co-g-CN were shown in Figure S1. Raman spectra of Co-g-CN (Figure 1d) displayed two obvious peaks at 1360 cm-1 and 1570 cm-1, attributed to D band (disordered amorphous carbon) and G band (graphite),12 respectively, which implied that Co-g-CN exhibited graphitic properties. The large range of bands from about 2500 to 3200 cm−1 however implied the second-order bands arising from graphitic lattice vibration modes and overtones as well.13 In the FT-IR spectra (Figure 1e), the intense broad bands from 3000 to 3600 cm-1 corresponded to the stretching mode of -NHx and OH groups, 14 in good agreement with the elementary analysis of Co-g-CN. Meanwhile, the bands at 1690 and 1610 cm-1 were attributed to C=O and C=N stretching, respectively; the 11001640 cm-1 region was ascribed to the stretching mode of C-N heterocycles, and the peak at 807 cm-1 was due to the breathing mode of s-triazine rings.15 In addition, the distinguished band at 2168 cm−1 stemmed from the residual nitrile species because the coordination of Co prevented the complete polymerization of g-CN.16 The observation was further verified by the XRD patterns of Co-g-CN (Figure 1f). The new peaks located at approximately 17.3°, 24.5°, 34.8°, 39.1°, 50.2°, 53.2°and 58.3°account for cobalt cyanide species,17-18 which provided strong evidence for the formation of cobalt-coordinated polymer.


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O

N

a

1200

900 600 300 Binding energy (eV) 534.6

0

398.5

b 405 402 399 396 Binding Energy (eV)

533.1

531.1

c

540

Intensity (a.u.)

Co

400.7 399.6

401.9

537 534 531 528 Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

C

Intensity (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

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784.4

799.4 804.7

782.7

798.1 788.0

d 808 800 792 784 Binding Energy (eV)

776

Figure 2. XPS survey spectra of Co-g-CN (a), high-resolution binding energy spectra for N 1s (b), O 1s (c) and Co 2p (d) of Co-g-CN. We then carried out X-ray photoemission spectroscopic (XPS) measurements to further reveal the chemical constitutions and the electronic structures of the Co-g-CN. The survey spectrum of Co-g-CN in Figure 2a distinctly indicated the element peaks of C, N, O, Co, in accordance with the results of elementary analysis and ICP-AES above. Figure 2b showed the high-resolution N 1s spectra of Co-g-CN. In the literatures, a broad peak from ~397-405 eV was usually deconvoluted into four or five components related to different nitrogen types, such as pyridinic-type, nitrile or metal-Nx, pyrrolic- or graphitic-type, or oxidized N.19-21 In our case, the binding energy at 401.9 eV, 400.7 eV and 398.5 eV were ascribed to N atoms corresponding to amino functions (N-H), pyrrolic type and pyridinc type, respectively.

15, 22-24

The peak at 399.6

eV was strong evidence of nitrogen coordinated to cobalt, in agreement with the previous report where peak at 399.4 ± 0.2 eV was assigned to pyridyl N associated with metal (Co and Fe).19


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Olson et al. also attributed the 399.2 eV in the N 1s spectrum to the nitrogen in Co–Nx centers,25 same as the result found by Artyushkova and co-workers.26 More specifically, nitrile functional groups were suggested to account for the N in the Co-N coordination according to the literatures.20-21 The conclusion was in good accordance with the above FT-IR analysis (Figure 1e) and XRD measurement (Figure 1f). The high-resolution O 1s spectra (Figure 2c) displayed three peaks: The one at 531.1 eV implied the existence of -C=O,

27

and the peaks at 533.1 eV and 534.6 eV were attributed to

ketonic oxygen and hydroxyl oxygen in carboxyl groups.28 Remarkably, the decomposed Co 2p spectrum demonstrated two states of cobalt in Co-g-CN. Among the six characteristic peaks shown in the spectrum (Figure 2d), the binding energy at 782.7 eV and 798.1 eV was ascribed to CoII species, whereas the 784.4 eV and 799.4 eV were the binding energy of Co III species. 788.0 eV and 804.7 eV were the satellite peaks. As we show in the following part, the mixed valence states of Co played crucial roles in the photocatalysis. In addition, the high-resolution C 1s

80 60

a

Co-g-CN

40 20 g-CN

0 0

1

2 3 4 Time (h)

5

Amount of H2 (mmolg-1)

spectra of Co-g-CN was displayed in Figure S2 and discussed in the Supporting Information.

Amount of H2 (mmolg-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


60 50 40 30 20 10 0

b

1

2

3

4

Figure 3. Comparisons of rates of photocatalytic H2 evolution for Co-g-CN and g-CN (a), repeated phocatalytic cycles for Co-g-CN (b).


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The as-prepared Co-g-CN was applied as a catalyst for visible-light-driven hydrogen evolution in a homogeneous system. In Figure 3a, the hydrogen production rates of Co-g-CN and g-CN were shown under the same photocatalytic conditions. The total amount of H2 evolution was ~61.5 mmol g-1 for Co-g-CN in 5 hours, with a calculated rate of hydrogen generation of 12.3 mmol g-1 h-1. Quantum Efficiency (QE) was determined for the system in a broad range of 400-750 nm (see the section on QE calculation procedures in Supporting Information) and found to be 0.6%. The as-prepared Co-g-CN was also very robust and could be repeatedly used for several cycles (Figure 3b). No structural changes for the polymers were observed in the corresponding XPS spectra and XRD patterns before and after photocatalytic reactions (Figure S3). These results emulated or surpassed previous H2 production efficiency carbon nitride-type catalyst-based photocatalysis.2-3, 29 In comparison, no H2 was detected by gas chromatography when g-CN was used as a control catalyst. Clearly, the Co center in Co-g-CN is a key factor to promote H2 generation from water. Control measurements demonstrated that the catalyst, light source and photosensitizer were indispensable constituents for this photocatalytic water splitting system (Figure S4). Moreover, two other Co-g-CNs with different initial mass amount of cobalt dichloride (0.4 g and 0.6 g) were synthesized for comparison (Figures S5 and S6a). The one with 0.5 g of initial mass amount of cobalt dichloride showed the highest H2 evolution amount and represented Co-g-CN in the whole experiment. The pH value for the photocatalytic reactions was also optimized to be at 11.0 (Figures S6b).


12

O.D.

5 ms 10 ms 15 ms 20 ms 30 ms 40 ms

-0.12 -0.24

50 ms 60 ms 70 ms 80 ms 90 ms 100 ms

400 500 600 Wavelength (nm)

0.04

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d

0.00

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e

0.005

200 400 600 800 1000 Time (ms)

120 ms 140 ms 160 ms 180 ms 200 ms

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700

2.0 1.5

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f

1.0

0 1 2 3 42 5 6 [Cat.]10 g/L

0.02 0.00

0.000 0

-0.1

700

EY+TEA 560 nm EY+TEA 403 nm fitting

c

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0.010

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0/

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O.D.

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EY

0

200 400 600 800 1000 Time (ms)

0

EY+Cat.

fitting

200 400 600 800 1000 Time (ms)

Figure 4. Transient absorption spectra of EY (a), EY+Cat. mixture (b), EY+TEA mixture (c), fitted decay curves for EY+ Cat. mixture at 560 nm and 460 nm (d), fitted decay curves for EY+ TEA mixture at 560 nm and 403 nm (e), and the decay profiles of 3EY* at 560 nm with increased Co-g-CN concentration from 0 to 0.06 g/L (f). The inset is the Stern-Volmer plot. Experimental conditions: Co-g-CN (0.03 g L-1); EY (4.0×10-5 M); TEA (0.5 % v/v); all in ethanol/water (v/v=1:1) solution at a pH value of 11. The laser excitation wavelength is 500 nm. The fact that Co-g-CN demonstrated high H2 evolution efficiency unseen by its g-CN counterpart implied the former adopted a distinct photocatalytic mechanism from traditional semiconducting carbon nitride, where the conduction band/valence band usually served as „trapping‟ sites for electron and holes for subsequent catalysis and a noble metal-cocatalyst was frequently a requisite.2-4 Herein the multivalent Co metal center might directly get involved in the proton reduction process by sequentially exchanging electrons, in a way similar to those molecular coordinated catalysts.6-7, 30 To test the hypothesis, we carried out various measurements in order to disentangle the photocatalytic mechanism of Co-g-CN. Laser


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photolysis technique adopting pump-probe strategy could picture time-resolved electron transfer dynamics and is thus a powerful tool to investigate the electron transfer process during water reduction.31-32 Figure 4a displayed the transient absorption spectra of sole photosensitizer EY, with a peak at 560 nm, which was derived from absorption of excited triplet state of EY (3EY*).33-34 With the addition of Co-g-CN into the system, the catalyst captured one electron from 3EY* upon photoexcitation and a radical cation of EY (EY+•) formed with characteristic absorption at 460 nm (Figure 4b and 4d).33-34 Note the laser photolysis experiments were performed on a LP920 (Edinburgh Instrument) transient absorption spectrometer available with kinetic (PMT, for kinetic analysis at a single wavelength) and spectral (ICCD, for spectral analysis at a given time) dual detection modes.35 Usually the former provides much more data points and thus higher sensitivity to show a decent kinetic trace but the latter, which focuses on collecting a full spectral range, providing less data points and relatively lower sensitivity for the kinetic race. In our case, the transient absorption signal of EY+• was very weak in the spectra obtained by ICCD mode (Figure 4b), but became rather apparent when recorded via PMT mode (Figure 4d). By contrast, mixing EY and TEA that is capable of donating one electron to the photosensitizer would generate a radical anion of EY-• at 403 nm after irradiation (Figure 4c).33 Hence TEA was employed as the sacrificial reagent to restore EY levels in the system for incessant photocatalytic cycles. The absorption curve of 3EY* was fitted with a monoexponential decay equation and one lifetime component of 82 µs (560 nm, Figure 4d) was extracted, which was reduced from the 146 µs of pure EY (see Supporting Information for the fitting methods, Figure S7a and Table S1). The profiles of EY+• (460 nm) however decay multiexponentially and gave to two lifetime components of 40 µs and 511 µs, respectively. The rise of 40 µs corresponded to the formation time of EY+•, which was on the similar time scale with the decay


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of 3EY* of 82 µs (Figure 4d) and thus verified the electron transfer from 3EY* to Co-g-CN. The second 511 µs was ascribed to the lifetime of EY+•, which thereafter would be restored to EY in the presence of sacrificial donors, as proved by the drastically quenched EY+• absorption in Figure S7b with the existence of TEA, and corresponding fitting results in Figure S7c and Table S1. Likewise, the lifetime of EY-• was acquired by the means of fitting the absorption curve in Figure 4e, which indicated two components (69 µs and > ms) at 403 nm, and the concomitant formation of EY-• (69 µs) and decay of 3EY*(53 µs, 560 nm) corroborated TEA as sacrificial reagent was effective electron donors for EY. As Co-g-CN was a homogeneous catalyst in the system, the catalytic concentration would influence the electron transfer process as a result of dynamic collision. Stern-Volmer plots were thus carried out to estimate the electron transfer rate from EY to Co-g-CN by using the following equation (1): 33, 36

eq. (1)

Where τ0 is the initial lifetime of sole 3EY* at 560 nm, τ the quenched lifetime of 3EY* with increased Co-g-CN concentration of [Q] and k the electron transfer rate constant from EY to Cog-CN. A slope of 23.8 g-1 L was obtained via a linear fitting for the Stern-Volmer plots and the corresponding electron transfer rate constant from EY to Co-g-CN was estimated to be ~ 1.6×105 g-1 L s-1 (Figure 4f).


15

a

8 0 Co

-8 Co

-16 -2

II

Co

III/II

I

-1 0 + E vs. Ag/Ag (V)

1.8

b

1 3000

Absorbance

16

Current(mA)

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

Intensity ( a.u.)


1.5

dark

Page 16 of 24

c

1.2

visible light

3200 3400 3600 Magnetic field (G)

Co

II

Co

0.9

0.6 3800

t=0 t = 20 min

400

I


Figure 5. The cyclic voltammogram of Co-g-CN. By subtracting 97 mV, potentials referenced to Ag/AgNO3 (0.01 M) can be converted to Fc/Fc+ (a); the electron paramagnetic resonance spectra of Co-g-CN at 115 K (b); UV-vis absorption spectra of systems containing Co-g-CN (0.3 g/L), EY (7.23 × 10-6 M), TEA (0.5 % v/v), all in ethanol/water (v/v = 1:1) solution at pH = 11 with irradiation of 3 w LED (c). The laser photolysis results unambiguously verified the production of hydrogen was accomplished through multistep electron transfer via the catalyst. To be more specific on the active sites of the catalyst, i.e., presumably the Co center, cyclic voltammetry measurements were further carried out to deduct the possible valence changes of cobalt during catalysis. As the saturated calomel electrode (SCE) has limit in nonaqueous solutions, various silver salts-based nonaqueous half-cells were more frequently used reference electrodes with the advantage of partially eliminating the junction potential.37 Herein Ag/AgNO3 (0.01 M) electrode served as the reference electrode and ferrocene/ferrocenium (Fc/Fc+) redox couple was used as an internal standard (Figure S8). Using Ag/Ag+ reference to probe possible valence changes of cobalt via cyclic voltammetry measurements has been common in previous reports.37-39 Figure 5a disclosed two successive reduction potential peaks at -0.67 V and -1.46 V (vs. Ag/Ag+) occurring with onset reduction potentials of -0.42 V and -1.16 V (vs. Ag/Ag+), respectively. Each corresponded to one electron reduction and led to respective formation of CoII and CoI species within the cobalt-coordinated catalyst. The observation was similar to those previously observed valence


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state changes in cobalt complexes during cyclic voltammetry measurements.40-41 CoIII/II was expected to be the initial valence state of catalyst as confirmed by Co 2p XPS survey spectra in Figure 2d. The appearance of CoI and CoII in the CV curve suggested the two species may also emerge in reduced intermediates of Co-g-CN. EPR and UV-vis absorption spectra of the catalyst before and after visible irradiation were then performed to verify the possibility. In Figure 5b, before irradiation, the EPR signals of the catalyst were very weak, which however dramatically increased after irradiation. According to previous literature, the signals were attributed to the formation of CoII.42-43 The observation was further confirmed by UV-vis absorption measurement. Upon irradiation by white LEDs for 20 min, a characteristic absorption of Co-gCN appeared at 450 nm, consistent with the formation of CoII species in previous report (Figure 5c and Figure S9).6-7 Meanwhile, the new broad absorption ( > 550 nm) bands were attributed to CoI species, which was also a crucial intermediate for H2 evolution.6-7

TEA

EY+•

EY

H2 H2O

1EY*

3EY*

Co

Cat.

ISC

H2

H+

2Na COO Br

Br O

O Br

EY

3EY*

CoIII

CoIII-H

O Br

CoII

CoI H+

3EY*

Scheme 2. Proposed Mechanism of visible-light-driven hydrogen evolution.


17


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On the basis of the above surveys, we proposed a possible mechanism of the hydrogen evolution process (Scheme 2). Upon irradiation of visible light, Eosin Y is immediately excited to its singlet excited state (1EY*) and quickly transforms into triplet excited state (3EY*) through intersystem crossing (ISC). One electron subsequently transfers from 3EY* to catalyst and eventually generates CoI species, which then is protonated to form CoIII−H, a generally postulated CoIII hydride intermediate.6 Herein the valance state of H is -1 and the hydride was also written in the format of “CoIII−H-” in previous literature.44 Note during metal complexcatalyzed hydrogen evolution, such metal-hydrides were well-established intermediates.39, 45 Eventually, H2 is produced by further protonation of the CoIII−H.6-7 Meanwhile, the photosensitizer of EY in the system can be replenished with electrons from the sacrificial donor TEA to reduce the EY+• radical cation and thus sustain the entire circulation. Overall, the unique 2D structure of the prepared Co-g-CN herein provides one convenient avenue for the strong electronic coupling between delocalized electrons and extrinsic metal dopants along the plane, which is expected to assist the multiple electron transport steps crucial for the photocatalytic hydrogen production. Conclusions In this work, we fabricated a novel 2D cobalt-coordinated carbon nitride-type polymers of Co-gCN integrating the advantages of both molecular catalytic efficiency and nano semiconductor stability, which served as homogeneous photocatalyst exhibiting superior hydrogen evolution efficiency (ca. ~ 12.3 mmol g-1 h-1) under visible light irradiation in the absence of noble metal cocatalyst. To explore the underlying photocatalytic mechanism, a series of techniques including laser photolysis, EPR and UV-Vis absorption were applied to thoroughly investigate the roles of Co-g-CN in H2 production. It turned out that cobalt intermediates (CoI, CoII and CoIII) played


18

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crucial roles through directly getting involved in the proton reduction by sequentially exchanging electrons, which were different from the hydrogen generation mechanism of previous carbon nitride-type catalysts. Moreover, the robustness of the catalyst, together with its merit of using economical and environmental-friendly transition metal would render Co-g-CN a prior choice for practical applications. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Q. Wang), [email protected] (H.-L. Zhang). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Supporting Information High-resolution TEM images of Co-g-CN, High-resolution binding energy spectra for C 1s of Co-g-CN, Catalyst Stability Test, Photocatalytic control experiments, Characterization of Co-gCN with different cobalt content, Influence of cobalt content and pH on H2 evolution rate, Fitting procedures and results for laser photolysis, Supplemental fitted decay curves and transient absorption spectra, Cyclic voltammogram of Fc/Fc+ redox couple internal standard and control test for UV-vis absorption spectra are included. This material is available free of charge via the Internet at http://pubs.acs.org.


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ACKNOWLEDGMENT This work is supported by National Basic Research Program of China (973 Program, No.2012CB933102), the National Natural Science Foundation of China (NSFC. 21673106, 51525303) and the Fundamental Research Funds for the Central Universities (lzujbky-2016-k08). REFERENCES 1. Marshall, J. Springtime for the Artificial Leaf. Nature 2014, 510, 22-24. 2. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76-80. 3. Lau, V. W.; Mesch, M. B.; Duppel, V.; Blum, V.; Senker, J.; Lotsch, B. V. LowMolecular-Weight Carbon Nitrides for Solar Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 1064-1072. 4. Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting Via a TwoElectron Pathway. Science 2015, 347, 970-974. 5. Willkomm, J.; Orchard, K. L.; Reynal, A.; Pastor, E.; Durrant, J. R.; Reisner, E. DyeSensitised Semiconductors Modified with Molecular Catalysts for Light-Driven H2 Production. Chem. Soc. Rev. 2015, 45, 9-23. 6. Lazarides, T.; McCormick, T.; Du, P.; Luo, G.; Lindley, B.; Eisenberg, R. Making Hydrogen from Water Using a Homogeneous System without Noble Metals. J. Am. Chem. Soc. 2009, 131, 9192-9194. 7. Zhong, J. J.; Meng, Q. Y.; Liu, B.; Li, X. B.; Gao, X. W.; Lei, T.; Wu, C. J.; Li, Z. J.; Tung, C. H.; Wu, L. Z. Cross-Coupling Hydrogen Evolution Reaction in Homogeneous Solution without Noble Metals. Org. Lett. 2014, 16, 1988-1991. 8. Martin, D. J.; Qiu, K.; Shevlin, S. A.; Handoko, A. D.; Chen, X.; Guo, Z.; Tang, J. Highly Efficient Photocatalytic H2 Evolution from Water Using Visible Light and Structure-Controlled Graphitic Carbon Nitride. Angew. Chem., Int. Ed. Engl. 2014, 53, 9240-9245. 9. Ong, W. J.; Tan, L. L.; Ng, Y. H.; Yong, S. T.; Chai, S. P. Graphitic Carbon Nitride (gC3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer to Achieving Sustainability? Chem. Rev. 2016, 116, 7159-7329. 10. Gao, L. F.; Wen, T.; Xu, J. Y.; Zhai, X. P.; Zhao, M.; Hu, G. W.; Chen, P.; Wang, Q.; Zhang, H. L. Iron-Doped Carbon Nitride-Type Polymers as Homogeneous Organocatalysts for Visible Light-Driven Hydrogen Evolution. ACS Appl. Mater. Interfaces 2016, 8, 617-624. 11. Wang, Y.; Wang, X.; Antonietti, M. Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew. Chem., Int. Ed. 2012, 51, 68-89. 12. Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron– Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47-57. 13. Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Pöschl, U. Raman Microspectroscopy of Soot and Related Carbonaceous Materials: Spectral Analysis and


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43. Hazari, A.; Kanta Das, L.; Kadam, R. M.; Bauza, A.; Frontera, A.; Ghosh, A. Unprecedented Structural Variations in Trinuclear Mixed Valence Co(II/III) Complexes: Theoretical Studies, Pnicogen Bonding Interactions and Catecholase-Like Activities. Dalton Trans. 2015, 44, 3862-3876. 44. Kellett, R. M.; Spiro, T. G. Cobalt(I) Porphyrin Catalysts of Hydrogen Production from Water. Inorg. Chem. 1985, 24, 2373-2377. 45. Baffert, C.; Artero, V.; Fontecave, M. Cobaloximes as Functional Models for Hydrogenases. 2. Proton Electroreduction Catalyzed by Difluoroborylbis(dimethylglyoximato)cobalt(II) Complexes in Organic Media. Inorg. Chem. 2007, 46, 1817-1824.


23


TOC Graphic TEA EY+•

EY

H2 H2O

1EY*

3EY*

Co

Cat.

ISC

H2 H+

2Na COO Br

Br O

O Br

EY

3EY*

CoIII

CoIII-H

O Br

CoII

CoI H+

3EY*

0.15

EY+Cat.+TEA 0.00 O.D.

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-0.15 -0.30

3

EY



Laser Photolysis 400 500 600 Wavelength (nm)

700


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Disentangling the Photocatalytic Hydrogen ... - ACS Publications

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