Enhanced Solar Fuel H2 Generation over g-C3N4 Nanosheet

Nov 22, 2017 - The g-C3N4-Co2P powders were synthesized by a simple grinding method. The g-C3N4 powders (200 mg) with 1 wt % Co2P were grounded in an ...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 816−826

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Enhanced Solar Fuel H2 Generation over g‑C3N4 Nanosheet Photocatalysts by the Synergetic Effect of Noble Metal-Free Co2P Cocatalyst and the Environmental Phosphorylation Strategy Rongchen Shen,†,‡ Jun Xie,†,‡ Hongdan Zhang,†,§ Aiping Zhang,†,‡ Xiaobo Chen,*,§ and Xin Li*,†,‡ †

College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants Resource and Utilization, Ministry of Agriculture, South China Agricultural University, Guangzhou 510642, People’s Republic of China ‡ College of Materials and Energy, South China Agricultural University, Guangzhou 510642, People’s Republic of China § Department of Chemistry, University of Missouri−Kansas City, Kansas City, Missouri 64110, United States ABSTRACT: Highly active and durable earth-abundant cocatalysts and photocatalysts are under increasing demand in developing practical hydrogen-evolution reaction (HER) systems. Especially, exploiting noble-metal-free Pt-like HER cocatalysts still remains a significant challenge. In this study, the synergistic effect of metal−organic framework (MOF) derived earth-abundant Co2P cocatalysts and the robust bioinspired environmental phosphorylation strategy in boosting photocatalyic H2 generation over the graphitic carbon nitride (g-C3N4) nanosheets was thoroughly investigated and revealed. The maximum H2-evolution rate of the ternary g-C3N4-Co2P-K2HPO4 photocatalytic systems could reach 27.81 μmol h−1, which was approximately 556 times higher than that of pure g-C3N4 nanosheets. The loaded earthabundant Co2P nanoparticles with a good electrical conductivity could not only improve visible-light absorption and decrease the recombination of the electron−hole pairs, but also mainly serve as an efficient cocatalyst to lower the H2-evolution overpotentials. Furthermore, K2HPO4 could generate an additional H2-evolution pathway through proton-reduction cycle and enhance the oxidation ability of TEOA by effectively consuming the holes, thus significantly boosting photocatalytic hydrogen evolution of the binary g-C3N4-Co2P heterojunctions. It could be anticipated that this work will open a new pathway to rationally fabricate a noble metal-free, low-cost and high-activity metal phosphide cocatalyst for efficient photocatalytic hydrogen evolution over gC3N4 nanosheet photocatalysts under visible-light illumination. KEYWORDS: Photocatalytic hydrogen evolution, Noble metal-free Co2P cocatalysts, g-C3N4 Nanosheet, Solar fuel, Environmental phosphorylation strategy

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erojunctions31−36 and Z-scheme systems37,38 have been available. Apparently, loading noble-metal-free cocatalysts such as metallic Ni,39−41 Ni(OH)2,42,43 WC,44 WS2,45 CoO46 and NiSx27,39,47−49 has been proven to be one of the most appealing approaches to boost the photocatalytic HER over gC3N4. However, it is still challenging to synthesize and anchor earth-abundant cocatalysts with high activity onto g-C3N4 in an effective and simple manner. Particularly, the earth-abundant transition metal phosphides (TMP), including CoP,50 Ni2P51−54 and Ni12P5,55 have been found to be active HER cocatalysts over g-C3N4. It is postulated that P as base sites can trap the positively charged protons and restrict the electrons delocalization in the metal, thus effectively improve the HER.56 However, the traditional incorporation and deposition−precipitation procedures generally lead to the low

INTRODUCTION Since Fujishima and Honda pioneerly integrated Pt cathode and TiO2 photoanode for photoelectrochemical H2-evolution reaction (HER) in 1972,1 photocatalytic HER over various heterogeneous semiconductors, such as TiO2,2−5 SiC,6,7 CdS,8−10 Cd1−xZnxS11,12 and g-C3N413−18 has been recognized as the most safe, sustainable, economical and environmentally friendly technology in conversing renewable solar energy to clean fuels.19,20 Among them, the nontoxic, stable and low-cost 2D g-C3N4 seems to be more promising for the photocatalytic HER, owing to its suitable conduction band (CB) level and band gap (2.7 eV). Nevertheless, many unfavorable factors strongly restrict the scale-up applications of g-C3N4 in photocatalytic HER, including fast electron−hole recombination, insufficient valence band edge for water oxidation and ineffective utilization of broadband light. To address these concerns, enormous modification routes, such as doping,21,22 fabricating nanostructures,23 dye sensitization,24 coupling with nanocarbons,25,26 loading Co-catalysts,27−30 constructing het© 2017 American Chemical Society

Received: September 8, 2017 Revised: October 26, 2017 Published: November 22, 2017 816

DOI: 10.1021/acssuschemeng.7b03169 ACS Sustainable Chem. Eng. 2018, 6, 816−826

Research Article

ACS Sustainable Chemistry & Engineering

Synthesis of Co2P. The Co2P powders were synthesized by a solid-state phosphorization reaction. The as-prepared Co-MOFs (1 g) and Na2HPO2 (5 g) were mixed in a porcelain boat. The mixture was annealed at 300 °C for 1 h under N2 atmosphere with a heating rate of 3 °C min−1. The resulting black Co2P powders were filtrated and washed with DI water and ethanol three times, respectively, and dried at 80 °C for 4 h under vacuum. Synthesis of g-C3N4-Co2P. The g-C3N4-Co2P powders were synthesized by a simple grinding method. The g-C3N4 powders (200 mg) with 1 wt % Co2P were grounded in an agate mortar for 2 h. This resulting sample was denoted as g-C3N4-1%-Co2P. Other composite photocatalysts with different weight percent of Co2P were similarly labeled. Materials Characterization. The X-ray powder diffraction (XRD) measurement was done by a MSAL-XD2 diffractometry. TEM and HRTEM were evaluated by JEM-2100HR (200 kV, Japan). The diffuse reflection spectra were determined by Shimadzu UV-2550 UV−vis spectrometer (in the wavelength range 200−800 nm using an integrating sphere accessory). XPS data were performed with a VG ESCALAB250 surface analysis system. All the binding energies were calibrated based on the C 1s level at 284.8 eV. The Brunauer− Emmett−Teller (BET) specific surface area and pore structures were determined by nitrogen adsorption−desorption isotherm measurements at 77 K (ASAP 2010). The steady-state PL spectra were collected on a LS 50B (PerkinElmer, Inc., USA) at 385 nm. The timeresolved decay curves of the as-fabricated samples were recorded with a FLS920 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) under the excitation of a hydrogen flash lamp with the wavelength at 325 nm (nF900; Edinburgh Instruments). Photocatalytic Hydrogen Evolution. Photocatalytic H2-evolution experiment was performed in a 100 mL three-neck Pyrex flask under the irradiation of 300W Xe lamp (PLS-SXE300, Beijing Perfect light Technology Co. Ltd. China). In a typical experiment, the asprepared catalyst (50 mg) was dispersed in 80 mL of aqueous solution containing K2HPO4, distilled water (68 mL) and TEOA (12 mL). Before irradiation, the suspensions were sonicated for 40 min and purged with N2 for 40 min to ensure anaerobic conditions. After illuminating for 1 h, 0.4 mL of gas was analyzed by an online chromatograph (GC-9500, TCD, using Ar as a carrier gas). Photoelectrochemical Measurements. The working electrodes were prepared by dispersing as-prepared binary photocatalysts (2.5 mg/mL) in 2 mL of ethanol and 20 μL of 0.25% Nafion solution. After ultrasonication for 2 h, the working electrodes were fabricated by drop casting 50 μL of the solution onto a 2 × 3.5 cm fluorine-doped tinoxide (FTO) glass substrate and dried under the infrared lamp (repeating the process ten times). The resulting FTO glass was calcined at 150 °C for 1 h under N2 gas flow in a tube furnace. Transient Photocurrent Tests. Transient photocurrent test was measured with an electrochemical analyzer CHI 660 (ChenHua Instruments Co., Shanhhai, China) through a three-electrode system. The as-prepared electrodes were used as the working electrodes. The Ag/AgCl (saturated KCl) and Pt plate were used as a reference electrode and a counter electrode, respectively. 0.1 M Na2SO4 and 0.1 mM K2HPO4 aqueous solutions were used as the electrolyte, respectively. A Xe lamp (300 W) with a UV cutoff filter (λ > 420 nm) was used as a light source Electrochemical Impedance Spectra (EIS) Tests. The electrochemical impedance spectra (EIS) of above-mentioned working electrodes in a standard three-electrode system were also analyzed via an electrochemical analyzer CHI 660 over a frequency range of 0.01−105 Hz with an ac amplitude of 2 mV in the dark. 0.1 M Na2SO4 and 0.1 mM K2HPO4 aqueous solution were used as the electrolyte. Electrocatalytic Hydrogen Evolution. The electrocatalytic hydrogen evolution was tested using a three-electrode cell. The Ag/ AgCl (saturated KCl) and Pt plate were used as a reference electrode and a volunteer electrode, respectively. The tests were performed in 0.5 M H2SO4 electrolyte solution with a 5 mV−1 scan rate. The obtained potentials were normalized to reverse hydrogen electrode (RHE). In a 0.5 M H2SO4 and K2HPO4 aqueous solution, E (RHE) is equal to the sum of E (Al/AgCl) and 0.202 V. The working electrodes

surface areas and nonuniform nanoparticle size of cocatalysts, thereby hindering the charge separation and subsequent HER. Recently, various TMP electrocatalysts or cocatalysts with high surface area and HER activity were found to be readily fabricated through direct phosphorization of corresponding MOFs,57,58 in comparison with the phosphorization of metal hydroxides. Consequently, it is highly desirable that the noble metal-free Co2P cocatalysts could be simply and effectively fabricated by the direct phosphorization of Co-based MOFs, whose positive role in boosting photocatalytic HER over gC3N4 will be further thoroughly investigated. Meanwhile, to mimic the natural photosynthetic environment, the robust bioinspired environmental phosphorylation strategy has been recently employed to boost the photocatalytic HER of g-C3N4-Pt Schottky junctions.59,60 In natural photosynthesis, the phosphate mediators are the primate components of thylakoid membranes immobilizing the photosynthetic centers, which can promote migration of both electrons and protons. Inspired by the phosphate-involving natural photosynthesis, Ye’s group first proposed an environmental “phosphorylation” strategy and achieved the highest H2evolution rate of 947 μmol h−1 in the optimized ternary K2HPO4-g-C3N4-Pt systems under visible light,59 clearly indicating the key roles of low-cost and nontoxic K2HPO4 as a proton carrier in significantly promoting proton transport/ reduction and improving hole oxidation. Unfortunately, in these photocatalytic systems containing the cheap K2HPO4 or H3PO4, noble-metal Pt cocatalysts limit their practicability. In this regard, combining the earth-abundant cocatalysts and robust environmental phosphorylation strategy should be appealing for the sustainable photocatalytic HER, which, however, has not been achieved yet. Herein, the interesting environmental phosphorylation strategy was employed to boost the photocatalytic HER over g-C3N4 decorated by the metal−organic framework (MOF) derived earth-abundant Co2P cocatalysts. The maximum HER rate of the ternary g-C3N4-Co2P-K2HPO4 photocatalytic systems was 27.81 μmol h−1, which was about 556 times higher than that of pure g-C3N4 nanosheets. The synergism of g-C3N4-Co2P heterojunctions and environmental phosphorylation strategy was strongly responsible for the significantly boosted electrons-hole separation and accelerated HER and TEOA oxidation kinetics. It is expected that this study will provide a strategy to construct the environment-friendly, lowcost and earth-abundant g-C3N4-based systems for the improved photocatalytic HER.

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EXPERIMENTAL SECTION

Synthesis of g-C3N4 Nanosheets. The g-C3N4 nanosheets were prepared by the thermal oxidation etching of the bulk g-C3N4 powders. Bulk g-C3N4 powder was first synthesized through the thermal polycondensation of urea at a temperature of 550 °C for 4 h in static air (with a heating rate of 5 °C min−1). After that, the g-C3N4 nanosheets were further fabricated by thermal oxidation etching of asobtained bulk g-C3N4 powders at 500 °C for another 2 h at the same condition. The resulting products were filtrated, washed with distilled (DI) water for three times and then dried at 80 °C for 8 h. Synthesis of Co-MOFs. A clear solution was obtained through dissolving 0.87 g of Co(NO)3·6H2O into 30 mL of methanol. Subsequently, the as-prepared solution was mixed with 20 mL of methanol solution contaning 1.97 g of 2-methylimidazole by strongly magnetic stirring for 1 h. After 24 h of standing in a dark, the final products were centrifuged and washed with DI water and methanol three times. 817

DOI: 10.1021/acssuschemeng.7b03169 ACS Sustainable Chem. Eng. 2018, 6, 816−826

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ACS Sustainable Chemistry & Engineering were prepared as follows: 6 mg of photocatalysts power were added into 2 mL of DI water to make a solution, and then the mixture was sonicated for 2 h. The resulting samples were deposited on glassy carbon electrode with 3 μL as prepared solution. After drying under the infrared lamp, 3 μL of Nafion solution (0.5%) was added on the catalyst layer and dried under the infrared lamp. Mott−Schottky Tests. The Mott−Schottky (MS) measurements were performed in the dark by scanning the electrode potential from −0.5 to +1.5 V at a scan rate of 25 mV/s. The impedance-potential characteristics were recorded at a frequency of 1 kHz. 0.1 M Na2SO4 aqueous solution was used as the electrolyte.

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RESULTS AND DISCUSSION Structures and Compositions of Photocatalysts. The chemical composition and phase structure of the as-prepared photocatalysts were researched by XRD measurements. Figure 1 displays the XRD patterns of pure g-C3N4, g-C3N4-Co2P and

Figure 2. TEM (A) and (B); HRTEM (C) and (D) image of g-C3N410%Co2P sample.

the g-C3N4-Co2P photocatalysts are presented in Figure 2C,D. From Figure 2C, the observed interplanar spacings of 0.208 and 0.221 nm correspond to the (211) and (112) planes of orthorhombic Co2P (JCPDS No. 89-3030), respectively. The TEM and HRTEM results clearly prove that the Co2P nanoparticles were successfully loaded on the g-C3N4 nanosheets. The EDX spectra of the g-C3N4-2%Co2P sample are shown in Figure 3. As displayed in Figure 3, the main signals of C, N, Co and P can be detected in the spectra of g-C3N4-2% Co2P composite, suggesting the coexistence of all these elements. Moreover, the contents of the elements could be calculated to be 58.46, 41.22, 0.23 and 0.09 wt %, respectively. Figure 3B−F shows the elemental mapping pictures, indicating that all the elements are homogeneously distributed in the binary photocatalysts. These results suggest that g-C3N4-2% Co2P are successfully fabricated. XPS were performed to detect the bonding configuration and chemical components of the binary g-C3N4-2%Co2P photocatalysts. Figure 4A shows the XPS survey spectrum of the gC3N4-2%Co2P sample. It could be easily found the existence of Co, P, C, N and little amount of O elements. In Figure 4B, two peaks located at 288.2 and 284.8 eV were related to the sp2 carbon in the NCN aromatic nuclei and sp2-bond graphitic carbon groups, respectively.44,55 Whereas, in the N 1s XPS spectrum (Figure 4C), the peaks located at 398.5, 399.5, 400.8 and 404.5 eV could be assigned to the sp2-bonded of the CNC, tertiary nitrogen NC3, nitrogen atoms in CNH and π-excitation, respectively.61,62 In addition, four distinct peaks at 780.4, 786.3, 796.5 and 802.2 eV could be observed in Figure 4D. Two strong peaks of Co 2p are located at 780.4 and 796.5 eV, corresponding to Co 2p2/3 and Co 2p1/2, while two weak peaks at 786.3 and 802.2 eV could be attributed to the Co 2p2/3 satellite and Co 2p1/2 satellite, respectively. With respect to P 2p XPS spectrum (Figure 4E), the peak at 129.3 eV corresponds to P 2p2/3 in the TMP, whereas the peak at 133.1 eV might be assigned to the oxidized phosphorus species (P2O5 or PO43−).50 The little amount O 1s peak in the Figure 4A might be caused by adsorbed H2O on the surface of the sample. The XPS results further confirmed that the g-C3N4Co2P composites have been well fabricated. To further understand the porous structures of the photocatalysts, the g-C3N4 and g-C3N4-2%Co2P photocatalysts were validated by N2 adsorption−desorption isotherms. Figure 5 shows the pore size distribution and the N2 adsorption− desorption isotherms of the samples. It can be seen that both

Figure 1. Powder XRD patterns of pristine g-C3N4 nanosheets, Co2P and various composite photocatalysts.

pure Co2P. Two main diffraction peaks at around 13.04° and 27.5° for pure g-C3N4 nanosheets, could be assigned to the (100) and (002) lattice planes (JCPDS No. 87-1526), respectively. The diffraction peak at 13.04° could be considered as an in-plane structural motif of heptazine, whereas the diffraction peak at 27.5° could be assigned to the interlayer stacking.39,47 These two diffraction peaks indicate that the structures of g-C3N4 nanosheets were well retained after loading Co2P. Besides these two peaks, it can also be found in Figure 1 that the diffraction peaks at 40.7°, 43.3°, 48.7°, 52.1° and 56.2°, could be indexed to the (112), (211), (013), (020) and (302) lattice planes of the orthorhombic Co2P (JCPDS No. 89-3030). These diffraction peaks indicate the formation of Co2P with good crystallinity. Moreover, no obvious change for the diffraction peak at around 27.5° and other detectable impurities were observed in those binary g-C3N4-Co2P composite samples, indicating that the loading of Co2P nanoparticles has no influence on the crystal structure of the g-C3N4 nanosheets. The diffraction intensity of the orthorhombic phase Co2P tended to become much stronger with increasing its content. In a word, the XRD pattern strongly proved that the binary g-C3N4-Co2P composites have been successfully synthesized. To further investigating structure and morphology of the gC3N4-Co2P, TEM and HRTEM were measured. Figure 2A,B shows the microstructure of the g-C3N4-Co2P photocatalysts. Clearly, the g-C3N4 has a typical layered platelet-like structure. Furthermore, it could be also found that the Co2P nanoparticles were uniformly dispersed on the surface of g-C3N4 nanosheets with the size of 50−100 nm. Moreover, the HRTEM images of 818

DOI: 10.1021/acssuschemeng.7b03169 ACS Sustainable Chem. Eng. 2018, 6, 816−826

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Figure 3. EDX spectra of the g-C3N4-Co2P sample.

Figure 4. XPS survey spectrum (A), and high-resolution XPS spectra of the C 1s region (B), N 1s region (C), Co 2p region (D) and P 2p region (E) of a g-C3N4-2%Co2P sample.

diffuse reflectance spectra. As shown in Figure 6A, all the samples have a similar absorption edge at around 460 nm, corresponding to the band gap excitation of pure g-C3N4 nanosheets. Moreover, it could be easily found that all the samples showed an obvious increase in light-absorb absorption, compared with that of pure g-C3N4 nanosheets, indicating that the loading of Co2P nanoparticles has a favorable optical absorption. Furthermore, no obvious shift of the edge absorption could be found in the as-prepared binary photocatalysts, suggesting that the Co2P was loaded on the surface of the g-C3N4 nanosheets. The band gap of the as-prepared photocatalytic could be calculated by the following eq 1:

samples show a classical type-IV adsorption−desorption isotherm with a H3 hysteresis loop according to the IUPAC classification, indicating the presence of typical mesoporous structures. The mesoporous structures of both samples are further confirmed in Table 1. Notably, the surface area of (15.573 m2 g−1) and pore volume (0.1175 cm3 g−1) of the gC3N4-2% Co2P were much smaller than those of pure g-C3N4 (46.8803 m2 g−1 and 0.3856 cm3 g−1), respectively. These results clearly suggested that the surface area should be not the key factor determining the overall photocatalytic H2 evolution over g-C3N4-Co2P hybrid heterostructures. Optical Properties of Photocatalysts. The optical properties of g-C3N4-Co2P were measured by the UV−vis 819

DOI: 10.1021/acssuschemeng.7b03169 ACS Sustainable Chem. Eng. 2018, 6, 816−826

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Figure 5. N2 adsorption−desorption isotherms at 77 K and the corresponding pore size distribution curves (inset) of g-C3N4 and gC3N4-2%Co2P.

Table 1. Pore Structure Parameters of g-C3N4 and g-C3N42%Co2P photocatalysts

BET surface area (m2 g−1)

mean pore diameter (nm)

pore volume (cm3 g−1)

g-C3N4 g-C3N4-2%Co2P

46.8803 15.573

30.819 23.62

0.3856 0.1175

Figure 7. (A) Time-dependent photocatalytic H2 evolution and (C) the average rate of H2 evolution over (a) g-C3N4 (b) g-C3N4-0.5% Co2P (c) g-C3N4-1%Co2P (d) g-C3N4-2%Co2P (e) g-C3N4-3%Co2P, (f) g-C3N4-5%Co2P and (g) g-C3N4-Pt. (B) Time-dependent photocatalytic H2 evolution and (D) the average H2-evolution rate over different photocatalysts over (a) g-C3N4-K2HPO4 and g-C3N4-2% Co2P with the K2HPO4 content of (b) 0.025 (c) 0.05 (d) 0.075 (e) 0.1 (f) 0.125 and (g) 0.15 mM.

obviously boosted after loading the orthorhombic Co2P nanoparticles. Moreover, all samples exhibit a linear growth of the H2-evolution rates as increasing the irradiation time, implying that the binary photocatalysts have excellent photostability under illumination. As shown in Figure 7C, the average rates of hydrogen-evolution were 0.05, 4.51, 5.54, 6.42, 5.34, 2.42 and 11.53 μmol h−1 for pure g-C3N4, g-C3N4-0.5%Co2P, gC3N4-1%Co2P, g-C3N4-2%Co2P, g-C3N4-3%Co2P, g-C3N4-5% Co2P and g-C3N4-Pt, respectively. Especially, the highest average rate of hydrogen evolution was 6.415 μmol h−1 during the photocatalytic H2-evolution reaction, at the optimized loading content of 2 wt % Co2P, which was about 120 times higher than that of pure g-C3N4 nanosheets. The rates of hydrogen evolution increased at the beginning, then decreased with increasing the contents of orthorhombic Co2P nanoparticles, because the excess Co2P cocatalysts might reduce light absorption and block the active sites of the pure g-C3N4. Furthermore, as displayed in Figure 7B,D, it was found that the performance of the photocatalysts was improved significantly after adding K2HPO4. Clearly, the average rates of hydrogen evolution were 0, 20.99, 23.38, 23.96, 27.81, 24.62 and 23.11 μmol h−1 for the g-C3N4-0.1 mM K2HPO4 and g-C3N4-2% Co2P with the K2HPO4 content of 0, 0.025, 0.05, 0.075, 0.1, 0.125 and 0.15 mM, respectively, indicating the positive roles of the environmental phosphorylation strategy in boosting the photocatalytic H2 evolution over the binary g-C3N4-Co2P heterojunctions. Notably, through a rough comparison in Table 2, despite this activity (27.81 μmol h−1) is much lower than those of Pt,59,60,70 NiS,33,63,64 CoP50 and Ni(OH)232 loaded gC3N4(/CdS) photocatalysts, it is much better than those reported results for most g-C3N4 photocatalytic systems decorated by different earth-abundant cocatalysts, such as NiS,49,66 Ni,41 Ni(OH)2,42 MoS2,77 Ni12P555 and Ni2P,52 indicating the excellent synergetic effect of noble metal-free Co2P cocatalyst and the environmental phosphorylation strategy. Additionally, no obvious synergetic effect of the loading of Ni2P and the addition of the K2HPO4 could be

Figure 6. (A) UV−vis absorption spectra of all the photocatalysts. (B) Tauc plots of the UV−vis spectra.

a = A(hν − Eg )n /2 /hν

(1)

where A, h, v and Eg represent the proportionality constant, Planck’s constant, frequency of the incident light and band energy, respectively. The n value is related to the type of the semiconductor. The n value for g-C3N4 is 1 due to its direct transition. In Figure 6B, the band gaps of all the photocatalysts are estimated to be 2.61 eV, which is consistent with the previous study.39 As enhancing the loading content of Co2P nanoparticles, the binary photocatalysts color gradually turn from yellow to black, further suggesting that loading Co2P could boost the visible-light absorption. All these results indicate that the loading of Co2P nanoparticles is beneficial for improving the visible-light absorption, which partially favors the enhancement of photocatalytic activity toward HER over the pure g-C3N4 nanosheets. Activities and Stabilities of Photocatalysts. Figure 7 shows the photocatalytic H2-generation activities of the asprepared photocatalytsts. The TEOA was used as a scavenger to consume holes. No H2 was detected without irradiation or catalysts, indicating that H2 was mainly generated from the photocatalytic process. As displayed in Figure 7A, it could be easily found that the pure g-C3N4 nanosheets show a low photocatalytic H2-generation activity, due to the fast recombination of the electron−hole pairs. Furthermore, the photocatalytic H2-evolution activity of g-C3N4 nanosheets is 820

DOI: 10.1021/acssuschemeng.7b03169 ACS Sustainable Chem. Eng. 2018, 6, 816−826

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ACS Sustainable Chemistry & Engineering Table 2. Summary of the Photocatalytic H2 Evolution on g-C3N4-Based Photocatalystsa photocatalysts

Co-catalysts/mass ratio

g-C3N4 g-C3N4 g-C3N4 g-C3N4 g-C3N4 e-C3N4 g-C3N4 g-C3N4 mpg-C3N4 g-C3N4 g-C3N4 g-C3N4 g-C3N4 g-C3N4 mg-C3N4 g-C3N4 g-C3N4 g-C3N4 g-C3N4 g-C3N4 CsTaWO6/g-C3N4 g-C3N4 CdS/g-C3N4 CdS/g-C3N4 g-C3N4 g-C3N4 g-C3N4 nanosheets P-P C3N4 g-C3N4 nanosheets g-C3N4 sg-C3N4

Co2P/2 wt % Co2P/2 wt % (0.1 mM K2HPO4) Ni/10 wt % NiS/1.25 wt % NiS/1.5 mol % NiS/1.0 wt % Ni(OH)2/0.5 wt % Ni/NiO/2 wt % CNT/6.8 wt %, NiS/1 wt % CB/0.5 wt %, NiS/1.0 wt % Ni(dmgH)2/3.5 wt % Pt/3 wt % Pt/3 wt % Pt/1 wt % Pt/3 wt % MCNTs/2.0 wt % CF/10 wt %, Pt/1.0 wt % CQDs, Pt/3 wt % S-doped, Pt/1 wt % AuPd/0.5 wt % Au/0.5 wt % Pt/0.6 wt % NiS/9 wt % Ni(OH)2/4.76 MoS2/0.5 wt %, Pt/1.0 wt % Ni12P5/2 wt % Pt/3.0 wt % (0.1 mM K2HPO4) Pt/3.0 wt % CoP/3.0 wt % Ni2P/0.48 wt % Ni2P/2 wt %

power (Xe lamp), wavelength 300 300 500 300 300 150 350 350 300 300 300 300 300 300 300 300 350 400 300 300 300 350 300 300 300 350 300 500 300 300 300

W, W, W W, W, W, W, W, W, W, W, W, W, W, W, W, W, W, W, W, W W, W, W, W, W, W W W, W, W,

λ > 420 nm λ > 420 nm λ λ λ λ λ λ λ λ λ λ λ λ λ λ λ λ λ

> 420 nm > 420 nm > 400 nm > 400 nm > 420 nm ≥ 420 nm ≥ 420 nm > 420 nm > 420 nm > 420 nm > 420 nm > 400 nm > 400 nm > 420 nm > 420 nm > 400 nm ≥ 400 nm

λ λ λ λ λ

≥ 420 nm ≥ 420 nm > 420 nm > 400 nm > 400 nm

λ > 400 nm λ > 400 nm λ > 420 nm

activity (μmol h−1)

reference (year)

6.42 27.81 8.41 46 44.77 4.2 7.6 10 26.05 49.6 1.18 41.7 1.80 34 272 1.15 54 5.805 12.16 16.3 0.458 207.6 128.2 115.18 23.1 6.33 947 195.8 96.2 5.67 8400 μmol m2 h−1

this work this work 41 (2015) 63 (2013) 64 (2014) 49 (2015) 42 (2013) 65 (2015) 66 (2015) 27 (2015) 67 (2014) 68 (2017) 69 (2015) 45 (2015) 70 (2015) 71 (2012) 25 (2015) 72 (2016) 73 (2013) 74 (2015) 75 (2015) 76 (2013) 33 (2015) 32 (2016) 77 (2013) 55 (2016) 59 (2015) 60 (2016) 50 (2017) 52 (2017) 53 (2017)

a

The sacrificial agent is TEOA for all above photocatalytic systems. The CNT, CB and CF represent carbon nanotubes, carbon black and carbon fiber, respectively.

observed. The detailed reason is still unclear. In a conclusion, this result indicated that the synergetic effect of the loading of Co2P and the addition of the K2HPO4 could effectively boost the photocatalytic H2-evolution performance over g-C3N4 nanosheets. Furthermore, the photostability is an important consideration for practical photocatalytic applications. The photocatalytic stability of the g-C3N4-2%Co2P and g-C3N4-2%Co2P0.1 mM K2HPO4 was tested by the repeated photocatalytic experiment for five times. Each cycle was performed for 3 h. As displayed in Figure 8A,B, almost 25% of activity lost after 5 cycles. It might be caused by the Co2P spalling from the surface of the g-C3N4 nanosheets, indicating that the as-prepared binary photocatalysts were relatively stable for the photocatalytic HER in the TEOA solution. Charge-sepration Performancs of Photocatalysts. To investigate the roles of Co2P in promoting the separation of charge−hole pairs, the as prepared photocatalysts were measured by PL spectra. PL spectra were usually used to study the separation and recombination performances of photoinduced electron−hole pairs.78 The PL spectra of as-prepared pure g-C3N4 and others binary g-C3N4-Co2P photocatalysts are displayed in Figure 9A. All samples exhibit similar emission peak at 450−475 nm, corresponding to the band gap excitation of pure g-C3N4 nanosheets. Obviously, the PL intensity of all the binary samples is much lower than that of pure g-C3N4

Figure 8. (A) Repeated cycles of photocatalytic H2 evolution over the g-C3N4-2%Co2P. (B) Repeated cycles of photocatalytic H2 evolution over the g-C3N4-2%Co2P-0.1 mM K2HPO4.

nanosheets, indicating the positive roles of Co2P in promoting the charge separation. Especially, the g-C3N4-2%Co2P photo821

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Figure 9. (A) Steady state PL spectra of different samples. EIS spectra (B) and photocurrent response (C) of g-C3N4, g-C3N4-2%Co2P and g-C3N42%Co2P-K2HPO4. Time-resolved transient PL decay of g-C3N4 (D) and g-C3N4-2%Co2P (E).

average charge lifetimes of the g-C3N4 and g-C3N4-2%Co2P were 11.451 and 16.650 ns, respectively. The calculated results showed that, after loading Co2P, the average charge lifetimes of the photocatalysts was increased from 11.451 to 16.650 ns. This is because that the loaded Co2P could accept photogenerated electrons from the surface of the g-C3N4 nanosheets, which are beneficial for enhancing photocatalytic activity of pure g-C3N4. Proposed Photocatalytic Mechanism. Figure 10 shows the MS plots of three photocatalyst film electrodes. The g-C3N4

catalyst exhibits the lowest PL intensity among all the composites, implying the highest charge-separation rate in this sample. Moreover, the order of PL intensity is almost consistent with that of H2-evolution activity. Therefore, the PL spectra clearly verified that the loading of Co2P could effectively improve the separation of electron−hole pairs under visible light. EIS is also a technique for revealing the interfacial charge transfer and recombination process.44,47 The Nyquist plots of pure g-C3N4 nanosheets, g-C3N4-2%Co2P and g-C3N4-2% Co2P-0.1 mM K2HPO4 are shown in Figure 9B. It could be easily found the g-C3N4-2%Co2P and g-C3N4-2%Co2P-0.1 mM K2HPO4 have an obvious smaller dimeter compared with that of pure g-C3N4 nanosheets, indicating that the g-C3N4-2%Co2P and g-C3N4-2%Co2P-0.1 mM K2HPO4 exhibit a more effective separation and transfer of photoinduced charge-hole pairs. Accordingly, loading Co2P and adding K2HPO4 could effectively decrease the recombination of the photoinduced charge−hole pairs on the surface of g-C3N4 nanosheets, thus improving the photocatalytic H2 generation. To more comprehensively understand roles of Co2P and K2HPO4 in accelerating separation of electron−hole pairs, the pure g-C3N4 nanosheets, g-C3N4-2%Co2P and g-C3N4-2% Co2P-0.1 mM K2HPO4 were studied by the transient photocurrent−time (I−t) curves. As shown in Figure 9C, it could be easily found that both the binary composite photocatalysts show higher photocurrent than the pure gC3N4 nanosheets, suggesting that the K2HPO4 and Co2P nanoparticles could powerfully improve the separation of photogenerated hole−electron pairs, thus boosting the photocatalytic H2 generation To further understand the process of hole−electron separation, the photocatalysts were tested by the fluorescence lifetime. The average lifetime of the photogenerated hole− electron pairs could be calculated through the following eq 2: =

A1τ12 + A 2 τ2 2 + A3τ32 A1τ1 + A 2 τ2 + A3τ3

Figure 10. MS plots of three photocatalyst film electrodes.

nanosheets have a positive slope, indicating that the g-C3N4 is an n-type semiconductor. It is known that the intersection points of the tangential and X-axial represent the flat band potentials of the corresponding photocatalysts. The calculated flat-band potentials of the g-C3N4, g-C3N4-2%Co2P, g-C3N4-2% Co2P-0.1 mM K2HPO4 were estimated to be −1.12, −1.05 and −1.01 eV (vs Ag/AgCl), respectively. Apparently, a positive shift was observed for the flat-band potentials of binary and ternary systems as compared to that of pure g-C3N4. Generally, the CB position of an n-type semiconductor is more negative than its flat-band potential by −0.2 V. Meanwhile, all the binary photocatalysts exhibit a similar band gap of 2.61 eV. Thus, the VB levels of the g-C3N4, g-C3N4-2%Co2P and g-C3N4-2%Co2P0.1 mM K2HPO4 were calculated to be 1.29, 1.36 and 1.4 V (vs Ag/AgCl), respectively. Usually, the positive VB means more efficient oxidation of TEOA. So that, both loading of Co2P and addition of K2HPO4 could enhance the VB levers of pure g-

(2)

where A is the corresponding amplitude and τ is the PL emission lifetime of a given state. As shown in Figure 9D,E, the 822

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ACS Sustainable Chemistry & Engineering C3N4 nanosheets, which could further strengthen the oxidation of TEOA on the g-C3N4 surface. These results suggest that the loading of Co2P and addition of K2HPO4 could improve photocatalytic activity of the pure g-C3N4 toward TEOA oxidation. In order to deeply understand the key roles of Co2P in boosting the photocatalytic H2-generation process, the g-C3N4, g-C3N4-2%Co2P and Co2P were measured by the polarization curves. As displayed in Figure 11A, the onset potentials of g-

Scheme 1. Schematic Charge-Separation Mechanism for Photocatalytic H2 Evolution in the Ternary g-C3N4-Co2PK2HPO4 Photocatalytic Systems

Figure 11. Polarization curves of (a) g-C3N4, (b) g-C3N4-K2HPO4, (c) g-C3N4-2%Co2P, (d) g-C3N4-2%Co2P-0.1 mM K2HPO4, (e) Co2P and (f) Co2P-0.1 mM K2HPO4 were measured at a scan rate of 5 mV S−1 in 0.1 M Na2SO4 and 0.1 mol K2HPO4 solution. (B) Voltammograms obtained for TEOA solutions with and without K2HPO4. (a) g-C3N4, (b) g-C3N4-0.1 mM K2HPO4, (c) g-C3N4-2%Co2P and (d) g-C3N4-2% Co2P-0.1 mM K2HPO4.

At the same time, the photoexcited holes would be consumed by TEOA, which could be also boosted by adding the K2HPO4 and loading the Co2P. Consequently, the loading of Co2P and addition of K2HPO4 could strongly improve the charge-carrier separation, decrease the H2-evolution overpotentials and enhance the oxidation of TEOA, thus leading to the significantly boosted photocatalytic H2 evolution over the gC3N4 nanosheets.

C3N4-2%Co2P photocatalysts are obvious smaller than that of pure g-C3N4 nanosheets, suggesting the loading of Co2P could decrease the onset potential and boost the H2-generation kinetics over the pure g-C3N4 nanosheets, which could fundamentally improve the photocatalytic H2 evolution over g-C3N4 nanosheets. Furthermore, when the K2HPO4 was added, the current density of the sample increased, thus suggesting that added K2HPO4 could effective boost the photocatalytic hydrogen evolution of the photocatalysts. The influence of K2HPO4 on TEOA oxidation was also tested by voltammetry. As shown in Figure 11 B, an obvious oxidation peaks was observed at about 1.3 V (vs Ag/AgCl), corresponding to the oxidation of TEOA. Furthermore, with the addition of K2HPO4, the oxidation peaks shifted to 1.0 V, indicating that K2HPO4 could boost the oxidation of the TEOA. The TEOA-oxidation results are completely consistent with those on the basis of MS plots. All these results clearly confirmed that adding K2HPO4 could fundamentally promote the TEOA oxidation. According to the above analysis, the photocatalytic mechanism of the ternary g-C3N4-Co2P-K2HPO4 photocatalysts is proposed and displayed in Scheme 1. The g-C3N4 nanosheets will be excited after absorbing visible light, and then the photogenerated electrons and holes will be formed. Due to the fast rate of the electron−hole recombination, pure g-C3N4 nanosheets exhibit a negligible photocatalytic HER. In contrast, after loading suitable amount of Co2P nanoparticles on the surface of the g-C3N4 nanosheets, the photoexcited electrons (excited from the surface of the g-C3N4 nanosheets) will rapidly move to the surface of Co2P cocatalysts due to suitable Fermi level and good electrical conductivity of Co2P, thus driving the photocatalytic H2 production over Co2P cocatalysts. Meanwhile, HPO4− could provide H+ and then produce H2 and PO4− on the surface of Co2P cocatalysts in the TEOA solution. Next, the PO4− would combine with H+ from water to regenerate HPO4−. The proton-reduction cycle was finished.59

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CONCLUSIONS In this work, the cobalt-based MOFs derived Co2P nanoparticles were first demonstrated to be an efficient cocatalyst to improve the photocatalytic H2 evolution over the g-C3N4 nanosheets. The synergetic effect of noble metal-free Co2P and the low-cost K2HPO4 were demonstrated to be an efficient strategy to significantly improve their photocatalytic H2 production over the g-C3N4 nanosheets under visible light illumination. The maximum H2-evolution rate of the g-C3N4Co2P-0.1 mM K2HPO4 photocatalysts was 27.81 μmol h−1, at the optimized contents of 2 wt % Co2P and 0.1 mmol K2HPO4, which was about 561 times higher than that of pure g-C3N4 nanosheets. More importantly, it was revealed that the loading of Co2P and addition of K2HPO4 could improve electron−hole separation, lower the H2-evolution overpotentials and boost the oxidation of TEOA, thus achieving the enhanced photocatalytic H2-evolution activities. It is postulated that this work will provide some helpful design concepts for developing earthabundant, low-cost and high-activity g-C3N4-based composite photocatalysts.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 20 85282633; fax: +86 20 85285596. E-mail address: [email protected] (X. Li). *E-mail address: [email protected] (X. Chen). ORCID

Hongdan Zhang: 0000-0003-2535-4131 Xin Li: 0000-0002-4842-5054 Notes

The authors declare no competing financial interest. 823

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(17) 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. (18) Cui, Y. J.; Wang, Y. X.; Wang, H.; Cao, F.; Chen, F. Y. Polycondensation of ammonium thiocyanate into novel porous gC3N4 nanosheets as photocatalysts for enhanced hydrogen evolution under visible light irradiation. Chinese. J. Catal. 2016, 37, 1899−1906. (19) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503− 6570. (20) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering heterogeneous semiconductors for solar water splitting. J. Mater. Chem. A 2015, 3, 2485−2534. (21) Panneri, S.; Ganguly, P.; Mohan, M.; Nair, B. N.; Mohamed, A. A. P.; Warrier, K. G.; Hareesh, U. Photoregenerable, Bifunctional Granules of Carbon-Doped g-C3N4 as Adsorptive Photocatalyst for the Efficient Removal of Tetracycline Antibiotic. ACS Sustainable Chem. Eng. 2017, 5, 1610−1618. (22) Chuang, P.-K.; Wu, K.-H.; Yeh, T.-F.; Teng, H. Extending the pi-Conjugation of g-C3N4 by Incorporating Aromatic Carbon for Photocatalytic H2 Evolution from Aqueous Solution. ACS Sustainable Chem. Eng. 2016, 4, 5989−5997. (23) Fang, L. J.; Li, Y. H.; Liu, P. F.; Wang, D. P.; Zeng, H. D.; Wang, X. L.; Yang, H. G. Facile Fabrication of Large-Aspect-Ratio g-C3N4 Nanosheets for Enhanced Photocatalytic Hydrogen Evolution. ACS Sustainable Chem. Eng. 2017, 5, 2039−2043. (24) Zhang, X.; Peng, T.; Song, S. Recent advances in dye-sensitized semiconductor systems for photocatalytic hydrogen production. J. Mater. Chem. A 2016, 4, 2365−2402. (25) Zhang, J.; Huang, F. Enhanced visible light photocatalytic H2 production activity of g-C3N4 via carbon fiber. Appl. Surf. Sci. 2015, 358, 287−295. (26) Li, X.; Yu, J.; Wageh, S.; Al-Ghamdi, A. A.; Xie, J. Graphene in Photocatalysis: A Review. Small 2016, 12, 6640−6696. (27) Wen, J.; Li, X.; Li, H.; Ma, S.; He, K.; Xu, Y.; Fang, Y.; Liu, W.; Gao, Q. Enhanced visible-light H2 evolution of g-C3N4 photocatalysts via the synergetic effect of amorphous NiS and cheap metal-free carbon black nanoparticles as co-catalysts. Appl. Surf. Sci. 2015, 358, 204−212. (28) Liang, S.; Xia, Y.; Zhu, S.; Zheng, S.; He, Y.; Bi, J.; Liu, M.; Wu, L. Au and Pt co-loaded g-C3N4 nanosheets for enhanced photocatalytic hydrogen production under visible light irradiation. Appl. Surf. Sci. 2015, 358, 304−312. (29) Han, C. C.; Lu, Y.; Zhang, J. L.; Ge, L.; Li, Y. J.; Chen, C. F.; Xin, Y. J.; Wu, L. E.; Fang, S. M. Novel PtCo alloy nanoparticle decorated 2D g-C3N4 nanosheets with enhanced photocatalytic activity for H2 evolution under visible light irradiation. J. Mater. Chem. A 2015, 3, 23274−23282. (30) Chen, F.; Yang, H.; Wang, X. F.; Yu, H. G. Facile synthesis and enhanced photocatalytic H2-evolution performance of NiS2-modified g-C3N4 photocatalysts. Chinese. J. Catal. 2017, 38, 296−304. (31) Low, J.; Yu, J.; Jaroniec, M.; Wageh, S.; Al-Ghamdi, A. A. Heterojunction Photocatalysts. Adv. Mater. 2017, 29, 1601694. (32) Yan, Z.; Sun, Z.; Liu, X.; Jia, H.; Du, P. Cadmium sulfide/ graphitic carbon nitride heterostructure nanowire loading with a nickel hydroxide cocatalyst for highly efficient photocatalytic hydrogen production in water under visible light. Nanoscale 2016, 8, 4748−4756. (33) Yuan, J.; Wen, J.; Zhong, Y.; Li, X.; Fang, Y.; Zhang, S.; Liu, W. Enhanced photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/g-C3N4 heterojunctions. J. Mater. Chem. A 2015, 3, 18244−18255. (34) Zhang, X.; Peng, B.; Zhang, S.; Peng, T. Robust Wide VisibleLight-Responsive Photoactivity for H2 Production over a Polymer/ Polymer Heterojunction Photocatalyst: The Significance of Sacrificial Reagent. ACS Sustainable Chem. Eng. 2015, 3, 1501−1509. (35) Zhou, M. J.; Hou, Z. H.; Chen, X. B. Graphitic-C3N4 nanosheets: synergistic effects of hydrogenation and n/n junctions

ACKNOWLEDGMENTS X. Li thanks National Natural Science Foundation of China (51672089), the Science and Technology Planning Project of Guangdong Province (2015B020215011) and the State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology) (2015-KF-7) for their support. X. Chen appreciates the financial support from the U.S. National Science Foundation (DMR-1609061), the College of Arts and Sciences, University of Missouri− Kansas City and University of Missouri Research Board.

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