Bifunctional Cu3P Decorated g-C3N4 Nanosheets as a Highly Active

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Bi-functional Cu3P Decorated g-C3N4 Nanosheets as a Highly Active and Robust Visible-Light Photocatalyst for H2 Production Rongchen Shen, Jun Xie, Xinyong Lu, Xiaobo Chen, and Xin Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04403 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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ACS Sustainable Chemistry & Engineering

Bi-functional Cu3P Decorated g-C3N4 Nanosheets as a Highly Active and Robust Visible-Light Photocatalyst for H2 Production

Rongchen Shen,a,b Jun Xie,a,b Xinyong Lu,a,b Xiaobo Chen,c* Xin Li a,b* a 5

College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants Resource and

Utilization, Ministry of Agriculture, South China Agricultural University, Guangzhou 510642, PR China b

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, PR China

c

Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO, 64110, USA.

*

Corresponding author at: College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, P. R. China. Tel.: +86 20 85282633; fax: +86 20 85285596. E-mail address: [email protected] (X. Li), [email protected] (X. Chen).

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Abstract: The rational design of sustainable noble-metal-free heterojunctions remains a key challenge for highly efficient and durable photocatalytic H2 production. In this study, it was revealed that the robust copper phosphide (Cu3P) nanoparticles may serve as a cocatalyst and a p-type semiconductor at the low (1.5 5

wt%) and high (10 wt%) loading contents, respectively. Both Cu3P cocatalyst and semiconductor could evidently boost the visible-light-driven photocatalytic H2 production over the graphitic carbon nitride (g-C3N4) nanosheets. Comparably speaking, the heterojunction effects between p-type Cu3P and n-type g-C3N4 are speculated to play a more prominent role in dramatically boosting the photocatalytic H2 production than the electron-sink roles of surface Cu3P cocatalysts. Impressively, among all the

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as-fabricated photocatalysts, the high quality 10 wt% g-C3N4-Cu3P could achieve the highest photocatalytic H2-production rate of 159.41 µmol g-1h-1, which is approximately 1014 times higher than that of pristine g-C3N4. In cycling experiments, g-C3N4-10 wt% Cu3P exhibited an acceptable photostablity. More importantly, it was further demonstrated that the earth-abundant dual-functional Cu3P nanoparticles could markedly facilitate the separation of electron-hole pairs and H2-evolution

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kinetics, thus achieving the distinctly boosted photocatalytic H2 generation. This work will provide new

insights

into

the

rationally

designing

environment-friendly

g-C3N4-based

hybrid

nanoheterojunctions for visible-light-responsive photocatalytic H2 generation through loading the noble-metal-free bifunctional cocatalysts or semiconductors. Keywords: Photocatalytic Hydrogen Evolution, charge carrier separation, g-C3N4 nanosheets, noble 20

metal-free copper phosphide (Cu3P), p-n heterojunctions.

Introduction Hydrogen, as a green and environment-friendly energy source, has received a great deal of attention.1-3 Since the innovative report on the photoelectrochemical H2 production over the Pt


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attached to titanium dioxide (TiO2) photoanode by Fujishima and Honda in 1972,4 enormous efforts have been devoted to the renewable and sustainable visible-light-driven photocatalytic H2 generation over a variety of semiconductors during the past 40 years, such as TiO2,5-8 ZnxCd1-xS9-11 and CdS12-15. In particular, metal-free graphic carbon nitride (g-C3N4), as one of the best semiconductors, was first 5

reported by Wang’s group for the photocatalytic H2 generation in 2009.16 Since then, it has captured much attention in the fields of photocatalytic CO2 reduction, H2 production and environmental remediation, due to its non-toxicity, low cost, favorable conduction band (CB) level and relatively narrow band gap (2.7ev), and outstanding structural/optoelectronic designability.17,18 However, the solar energy conversion efficiency, electrical conductivity, separation of electron–hole pairs and

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water-oxidation ability of pristine g-C3N4 are usually relatively poor. Consequently, many effective methods and techniques, such as Z-scheme system,19-21 improving crystallinity,22,23 nanocarbon coupling,24-29 modulating electronic structure30-32 surface and vacancy engineering33,34, heterojunction construction,35-40 nanostructure design41-44 and loading cocatalysts,45-47 have been employed to improve the thermodynamics and kinetics of g-C3N4, thus achieving the substantially boosted photoactivities.

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In consideration of the solar energy utilization, the two most appealing strategies, namely, loading cocatalysts and constructing semiconductor heterojunctions, have been readily available for significantly boosting the photocatalytic H2 evolution over g-C3N4. On the one hand, loading cocatalysts can not only increase the g-C3N4 active sites, but also can effectively promote the separation of holes and electrons.1,48 Suitable cocatalysts need to have both high conductivity and high

20

effcient electrocatalytic H2-generation activity. So far, various noble-metal-free electrocatalysts such as MoS2,49 WS2,50 WC,51 Ni(OH)2,19,52 NiSx,53-59 CoP,60,61 NiCoP62,63 and NixP64,65 have been demonstrated to be the excellent earth-abundant cocatalysts to boost the photocatalytic H2 evolution over g-C3N4. Furthermore, constructing p-n heterojunctions has been found to be effective in improving photocatalytic H2 evolution over semiconductors though retarding recombination of the

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electron-hole pairs. The formation of p-n heterojunctions can alter the band gap of the g-C3N4, thus ACS Paragon Plus Environment


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improving the visible-light adsorption ability and electron-hole separation efficiency.66,67 As might be expected, under irradiation, the electrons will transfer to the conduction band (CB) of the n-type g-C3N4 whereas the holes will transfer to the valence band (VB) of the p-type semiconductor. Accordingly, the photoactivity of the g-C3N4 can be thoroughly improved. Nevertheless, despite all 5

these exciting results, it remains a demanding scientific challenge in exploiting the earth-abundant co-catalysts and p-type semiconductors for designing and fabricating highly efficient and durable g-C3N4-based photocatalytic H2-evolution systems. Cu is one of the most abundant elements preserved in the earth. Especially, Cu3P, as a semiconducting (p-type) and plasmonic material, has attracted considerable research with respect to

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optical and electronic applications in the past three years.68,69 Fascinatingly, the p-type Cu3P has been also proven to be an excellent electrocatalyst for reducing the overpotentials for electrocatalytic H2 generation.70-72 To date, Cu3P nanowires and nanotubes have been successfully applied in electrocatalytic H2 generation.71,73,74 Importantly, it is also noted that the p-type Cu3P has been also coupled with p-type TiO2 or CdS to fabricate the p-n heterojunctions for highly efficient photocatalytic

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H2 evolution.75,76 Accordingly, the deep insights into on the multi-functional roles of Cu3P seem to be important and promising for developing the nonprecious-metal HER systems. Importantly, the fascinating multi-functional roles of Cu3P in boosting the photocatalytic H2 generation gain little attention. Strongly motivated by these interesting previous accomplishments, herein, we aim to rationally

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design the noble metal-free bifunctional copper phosphide (Cu3P) modified g-C3N4 photocatalysts and investigate the component ratio-dependent sensitization roles of Cu3P in boosting the photocatlytic H2 evolution. Then, the photocatalytic H2-production performances of the bifunctional g-C3N4 nanosheets-Cu3P nanoheterojunctions are carefully tested. The highest photocatalytic H2-production rate of the bifunctional g-C3N4-Cu3P nanoheterojunctions could reach 159.41 µmolg-1h-1 in the

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presence of triethanolamine (TEOA) as a sacrificial reducing agent. The component ratio-dependent ACS Paragon Plus Environment

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sensitization roles of Cu3P loaded on the surface of g-C3N4 in improving the photocatalytic performance are thoroughly revealed in detail. It is expected that this study could provide a promising and general approach to synthesize environment-friendly noble-metal-free g-C3N4-based composite photocatalysts for highly efficient visible-light hydrogen generation. 5

Experimental section Preparation of photocatalysts Synthesis of g-C3N4 nanosheets The g-C3N4 nanosheets were obtained though heating urea at 550 °C for 4 h with a heating rate of 5 °C min−1 under air in a muffle furnace. Then, the g-C3N4 powders were heated to 500 °C for 2 h after

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grinding. Next, the as-obtained g-C3N4 nanosheets were treated with ultrasonic machine for 10 h in 1 L of 0.5 M hydrochloric acid (HCl), washed with deionized (DI) water until the pH closed to neutral. The resulting samples were collected by filtration and dried at 60 °C for 12 h under vacuum. Preparation of Cu3P Cu3P nanoparticles were synthesized as follows: aqueous solution of copper nitrate (100 mL, 0.05

15

M) and NaOH (20 mL，0.25 M) were mixed by magnetic stirring for 2 h. Afterwards, the resulting precipitates were collected by filtration and dried at 60 °C for 6 h under vacuum to obtain the Cu(OH)2. Next the as prepared Cu(OH)2 (0.5 g) and NaH2PO2 (2.5 g) were mixed on a porcelain boat. The mixtures were calcined at ca. 300 °C for 1 h under N2 atmosphere. The resulting black Cu3P samples were filtrated and washed with DI water and ethanol three times, respectively, and dried at 80 °C for 4

20

h under vacuum. Preparation of g-C3N4-Cu3P The 225 mg of g-C3N4 and 25 mg of Cu3P (10 wt % Cu3P) were ground in an agate mortar. The resulting sample was denoted as g-C3N4-10%Cu3P. The other composites with different amount of Cu3P were similarly prepared and labeled.



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Characterization The structures of g-C3N4-Cu3P were analyzed by XRD using Cu Kα radiation (MSAL-XD2 diffractometer) with a scan rate of 4° min−1. The morphology and structures of the photocatalysts were analyzed by TEM and HRTEM (JEM-2100HR 200 kV, Japan). The diffuse reflection spectra were 5

determined in the wavelength range 200–800 nm using a Shimadzu UV-2550 UV–vis spectrometer equipped with the integrating sphere accessory. (X-ray photoelectron spectroscopy) XPS data were performed with a VG ESCALAB250 surface analysis.The steady-state PL spectra were tested on a LS 50B (Perkin Elmer, Inc., USA) at 385 nm. Nitrogen adsorption–desorption isotherms were measured on a Gemini-2360 analyzer (Micromeritics Co., USA) at 77 K. The specific surface area and pore size

10

distribution were analyzed by Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH), respectively. The time-resolved 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). The inductively coupled plasma (ICP) measurements were tested by a PerkinElmer Optima 3300DV (ICP)

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spectrometer for elemental analysis.

Photocatalytic test Photocatalytic H2 evolution experiment was tested in a 100ml three-neck Pyrex flask under ambient temperature. The flask was illuminated by a 300W Xe lamp (PLS-SXE300, Beijing Perfect 20

Light Technology Co., Ltd, with the intensity ca. 160 mVcm-2). In a typical experiment, 50mg of as-prepared samples were dispersed in a mixed solution of distilled water (68 mL) and triethanolamine (TEOA) (12 mL). The suspensions were subject to ultrasound for 40 min and evacuated with N2 for 40 min to remove the dissolved oxygen. After illuminating for 1 h, 0.4 mL gas was extracted from the three-neck Pyrex flask analyzed using a gas chromatograph (GC-9500, TCD, using Ar as carrier gas).

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Photoelectrochemical measurements ACS Paragon Plus Environment

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The working electrodes were prepared as follows: 5 mg of as-prepared photocatalysts were added into liquor containing ethanol (2 mL) and Nafion solution (20 µl 0.25%). After ultrasonication, 50 µL of the solution was injected onto a 2×3.5 cm2 fluorine-doped tin-oxide (FTO) glass substrate and dried under the infrared lamp (repeating the process ten times). The resulting electrodes were then dried in 5

an oven and calcined at 150 °C for 1 h in a N2 gas flow. Transient photocurrent tests. Transient

photocurrent

test

was

measured

with

an

electrochemical

analyzer CHI 660 (CH Instruments, Shanghai, China) in a three-electrode system. The as-prepared electrode, Ag/AgCl and Pt plate were used as the working electrode, a reference electrode and the 10

counter electrode, respectively. 0.1M Na2SO4 was used as the electrolyte. A Xe lamp (300 W) with a UV cut-off filter (λ>420nm) was used as a light source. Electrochemical impedance spectra (EIS) tests. The electrochemical impedance spectra (EIS) were also tested in a three-electrode system over a frequency range of 0.01–105 Hz with an ac amplitude of 2 mV in the dark. 0.1 M Na2SO4 aqueous

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solution was used as the electrolyte. Mott−Schottky (MS) tests The measurements were performed by scanning the electrode potential from −0.5 to 1.5 V at a scan rate of 25 mV/s in the dark and under visible light irradiation, respectively. The impedance-potential characteristics were recorded at a frequency of 1 kHz.

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The electrocatalytic hydrogen evolution. The electrocatalytic hydrogen evolution was tested using a three-electrode cell, using Ag/AgCl as a reference electrode and Pt plate as the counter electrode. The test was performed in 0.5 M H2SO4 electrolyte solution with a 5 mV-1 scan rate. The working electrodes were prepared as follows: 6 mg of photocatalysts power were added into 2 mL of DI water and sonicated for 2 h. The resulting samples

25

were deposited on glassy carbon electrode with 3 µL as prepared solution. After drying under the ACS Paragon Plus Environment


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infrared lamp, 3 µL of Nafion solution (0.5%) was added on the catalyst layer and dried under the infrared lamp. Results and discussion The structures and compositions of photocatalysts

5

Figure 1. Powder XRD patterns of pristine g-C3N4 and various composite photocatalysts. XRD measurements were used to study the chemical composition and crystal structures. As showed in Figure 1, all identified diffraction peaks of Cu3P are well in accordance with the standard pattern(JCPDS 71–2261),73,76 but the intensities of peaks were changed. For g-C3N4, two 10

sharp intense peaks at around 13.04o and 27.5o were detected, corresponding to the (100) and (002) lattice planes of graphite-like carbon nitride layers (JCPDS 87–1526) respectively. The diffraction peak at 13.04o could be assigned to an in-plane structural motif of the continuous heptazine network, whereas the diffraction peak at 27.5o corresponds to the interlayer stacking. Clearly, for the g-C3N4-Cu3P binary composite, the intensity of (300) diffraction peak remarkably increases with

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increasing the content of Cu3P, suggesting the formation of binary composites. Obviously, there is no observed change for two diffraction peaks of g-C3N4, indicating that the crystal structure of g-C3N4 still retained after loading Cu3P nanoparticles. The XRD results clearly confirmed that the binary g-C3N4-Cu3P composites have been successfully constructed.


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Figure 2. TEM (A), (B) and (C); HRTEM (D) image of g-C3N4-10%Cu3P sample.

Figure 3. (A) FESME of g-C3N4-10%Cu3P sample and (B-E) the corresponding elemental mapping of 5

C, N, Cu and P elements. The structure and morphology of the g-C3N4-10% Cu3P were further observed by TEM and HRTEM, which were showed in Figure 2. As displayed in Figure 2A and B, the TEM image of the ACS Paragon Plus Environment


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g-C3N4 exhibits a typical 2D nanosheet structure. Furthermore, Cu3P nanoparticles with an average diameter of about 20 to 50 nm were well dispersed on the g-C3N4 nanosheets in Figure 2C. The lattice spacing of 0.249 nm could be found in the HRTEM (Figure 2D), corresponding to the (112) plane of hexagonal Cu3P (JCPDS #71-2261). The TEM image of g-C3N4-Cu3P strongly verified that the Cu3P 5

were well loaded on the surface of g-C3N4 nanosheets. Furthermore, Figure 3A-E showed the FESEM and elemental mapping pictures, suggesting that the co-existence of C, N, Cu, and P elements in g-C3N4-10% Cu3P. It could be also found that the similar location and content of C and N fundamentally confirmed the presence of g-C3N4 nanosheets, whereas the content of Cu is much higher than that of P, due to the lower content of P in the Cu3P. The content of Cu and P was further

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tested by the ICP-MS, which showed that the molar ratio of Cu and P were almost 2.5:1. The results further indicated that the binary g-C3N4-Cu3P photocatalysts were well fabricated

Figure 4. XPS survey spectrum (A), and high-resolution XPS spectra of the C 1s region (B), N 1s region (C), Cu 2p region (D) and P 2p region (E) of a g-C3N4-10% Cu3P sample. ACS Paragon Plus Environment

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To further study surface chemical composition and chemical status of the g-C3N4-Cu3P, the samples were tested by XPS. The XPS survey spectrum of the g-C3N4-Cu3P and corresponding high-resolution XPS spectra of various elements are shown in Figure 4A. As observed in Figure 4A, it could be easily found that the peaks at around 288, 400, 933,134 and 530 eV represent the C 1s, N 1s, P 2p, Cu 2p and 5

O 1s in the g-C3N4-Cu3P photocatalyst, respectively. In the Figure 4B, two high-resolution C 1s peaks at 284.8 and 288.2 eV could be obviously observed. The peak at 284.8 eV could be assigned to the sp2-bond graphitic carbon groups, while the peak at 288.2 eV is attributed to sp2 carbon in the N-C=N aromatic nuclei. From Figure 4C, four high-resolution N 1s peaks at 398.8, 399.9, 400.9 and 404.5eV were detected. The peak at 398.8 eV is ascribed to the sp2-hybridized nitrogen atoms in the C=N-C,

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demonstrating the successful synthesis of graphite-like g-C3N4.77 Moreover, the peaks at 399.9 and 400.0 eV are assigned to the nitrogen atoms in N-C3 and C-N-H, whereas the peak at 404.5 eV is attributed to π-excitation in the polymeric g-C3N4 structures. From Figure 4D, the XPS spectrum of Cu 2p could be deconvoluted into four independent peaks, which were attributed to the following groups: 933.0 (Cu 2p3/2), 953.2 (Cu 2p1/2) and two-week peaks 944.0, 936.4 eV (both Cu1+ satellite)

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respectively. Lastly, the Figure 3E shows the P 2p3/2 binding energy at 134.4 eV. The peak of 134.4 eV is usually assigned to the probable species (P2O5 or PO43-) produced by air oxidation of Cu3P nanoparticles on g-C3N4 nanosheets.78 The O 1s peak in the Figure 4A might be caused by the adsorbed H2O on the surface of the sample. In a conclusion, the XPS results provide powerful proof to further confirm that Cu3P nanoparticles have been well loaded on the surface of g-C3N4 nanosheets.



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Figure 5. (A) N2 adsorption–desorption isotherms and the corresponding pore size distribution curves (B) BET surface areas and pore volume of g-C3N4, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P. The pore structures and BET surface areas for g-C3N4, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P 5

were further investigated by N2 adsorption–desorption experiments. The Nitrogen adsorption– desorption isotherms and the corresponding pore size distribution curves of the g-C3N4, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P were showed in the Figure 5A. Clearly, it could be found that all as-prepared samples display the type IV profiles with H3 hysteresis loops, suggesting the presence of mesopores. It can also be found in Figure 5 that the corresponding pore size distribution curves of all

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three photocatalysts exhibit the broad peaks at about 2-35 nm, further verifying that the macropores and mesopores are presented. The results of the BET surface areas and pore volume for g-C3N4, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P were showed in Figure 5B. Obviously, the BET surface areas and pore volume were significantly reduced with increasing the content of Cu3P. Accordingly, the grind method may make some Cu3P nanoparticles fill and block the mesopores or macropores of the

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g-C3N4 nanosheets, thus leading to the evident decrease in the surface areas and pore volume. These results fully indicate that the surface areas have little influence on the photocatalytic H2 evolution. The optical properties of photocatalysts The optical properties of g-C3N4 with different amounts of Cu3P could be determined by the UV-vis diffuse reflectance spectra. Figure 6A shows the UV–vis absorption spectra of pristine g-C3N4, Cu3P ACS Paragon Plus Environment

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and binary g-C3N4-Cu3P nanocomposites. Clearly, all samples except for Cu3P exhibit a similar absorption edge at about 450 nm with a band gap of about 2.62eV. Notably, Cu3P exhibit a very broad absorption at about 700 nm with a band gap of about 1.55eV. The band gap of Cu3P is consistent with those previously reported results.76 No obvious absorption edge shift of g-C3N4 could be found in the 5

binary composites, suggesting that the Cu3P had no impact on the structure of the pristine g-C3N4 nanosheets. Furthermore, as displayed in Figure 6A, as increasing the Cu3P content, the visible-light absorption of the g-C3N4-Cu3P was significantly enhanced, due to the favorable optical absorption of Cu3P. All these results suggest that the loading of Cu3P nanoparticles could significantly improve the visible-light absorption of g-C3N4 photocatalyst, thus partially favoring the enhancement of

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photocatalytic H2-evolution activity.

Figure 6. (A) UV–vis absorption spectra of all photocatalysts (a) g-C3N4 (b) g-C3N4-1%Cu3P, (c) g-C3N4-1.5%Cu3P,

(d)

g-C3N4-2%Cu3P,

(e)

g-C3N4-9%Cu3P,


(f)

g-C3N4-10%Cu3P,

(g)


Page 14 of 33

g-C3N4-12%Cu3P, (h) Cu3P. Tauc plots of the UV–vis spectra of g-C3N4 (B) and Cu3P (C), respectively.

Photocatalytic activities and stabilities The photocatalytic H2-generation activities of bifunctional g-C3N4-Cu3P were evaluated under 5

visible light irradiation. No obvious H2 could be detected without photocatalysts or irradiation. Figure 7 shows the visible-light photocatalytic H2 evolution over different photocatalysts. Obviously, it can be found from Figure 7 that the photocatalytic rate of hydrogen evolution over pure g-C3N4 was about 0.05 µmolg-1h-1, suggesting that the rapid recombination of charge carriers resulted in the negligible H2-evolution activity over the bare g-C3N4 nanosheets. Whereas, when g-C3N4-Cu3P was used as a

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photocatalysts, the rate of photocatalytic hydrogen evolution increased and reached approximately 159.41µmolg-1h-1. The linearly increasing time-dependent amounts of H2 evolution for all samples clearly conﬁrmed their relatively excellent photostabilities. It was clear that incorporating g-C3N4 with Cu3P can significantly enhance the photocatalytic H2-generation activities. The effect of the weight ratio of g-C3N4:Cu3P were showed in Figure 7A, the average rates of H2

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production over various photocatalysts were further calculated and presented in Figure 7B. During the 3 h photocatalytic reaction process, the average rates of H2 evolution were 0.05, 60.57, 104.34, 44.9, 37.23, 55.36, 62.55, 159.41, 142.05 and 115.87µmolg-1h-1 for pristine g-C3N4, g-C3N4-1%Cu3P, g-C3N4-1.5%Cu3P,

g-C3N4-2%Cu3P,

g-C3N4-4%Cu3P,

g-C3N4-6%Cu3P,

g-C3N4-8%Cu3P,

g-C3N4-10%Cu3P, g-C3N4-12.5%Cu3P, g-C3N4-15%Cu3P and g-C3N4-1%Pt, respectively. It could be 20

seen that all bifunctional g-C3N4 nanosheets-Cu3P nanoheterojunctions exhibit much better photo-activity than the pristine g-C3N4. Especially, the optimum loading content of 10 wt % Cu3P could achieve the highest H2-evolution rate of 159.41 µmolg-1h-1. It was also noted from Figure 7B that the average rate of H2 evolution of g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P were about 643 and 1014 times higher than that of pristine g-C3N4, respectively. As shown in Table 1, although the average

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H2-evolution rates over these bifunctional g-C3N4 nanosheets-Cu3P nanoheterojunctions are ACS Paragon Plus Environment

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significantly lower than those of various cocatalysts modified g-C3N4-based composite photocatalysts, such as NiCoP (534.2/1643 µmolg-1h-1),62,63 NixP(8528 µmolg-1h-1)65 and CoP nanodots (1924 µmolg-1h-1)78, they are much higher than those previously reported results for g-C3N4-0.01wt%WS2 (101µmolg-1h-1),50 g-C3N4-15wt%WC (146.2µmolg-1h-1),51 g-C3N4-0.5mol% Ni(OH)2 (152µmolg-1h-1)79 5

and g-C3N4-2.0 wt%Ni12P5 (126.2µmolg-1h-1)64. These results demonstrated that the g-C3N4-based semiconductor modified by Cu3P nanoparticles should be the potential photocatalysts for efficient H2 evolution. Table 1 Summary of the photocatalytic H2 evolution on g-C3N4-based photocatalysts. Photocatalystsa

Co-catalysts/Mass

Power

(Xe

lamp), Activity

ratio

wavelength

(µmol·g-1h-1)

(year)

g-C3N4

Cu3P/1.5wt%

300 W, λ>420nm

104.34

This work

g-C3N4

Cu3P/1.0wt%

300 W, λ>420nm

159.41

This work

g-C3N4

Ni12P5/2wt%

350 W, λ>400nm

126.6

64

(2016)

g-C3N4

CoP/3.0 wt%

300 W, λ>400nm

1924

78

(2017)

sg-C3N4

Ni2P/2wt%

300 W, λ>420nm

8400µmol·m2h

80

(2017)

g-C3N4

CoP/3.4wt%

300 W, λ>420nm

420

60

(2017)

g-C3N4-CdS

CoP/5 wt%

300 W, λ>400nm

23536

38

(2017)

g-C3N4

Ni2P/3.5wt%

300 W, λ>420nm

474.7

81

(2018)

g-C3N4

Ni2P/1wt%

300 W, λ>420nm

362.4

82

(2017)

g-C3N4

Ni12P5/5wt%

300 W, λ>420nm

535.7

83

(2017)

g-C3N4

NiCoP/0.5wt%

300 W, λ>420nm

534.2

62

(2017)

g-C3N4

NixP

300 W, λ>420nm

8528

65

(2017)

g-C3N4

NiCoP

300 W, λ>420nm

1643

63

(2017)

g-C3N4

Ni3P/5wt%

300 W, λ>420nm

120

84

(2017)


Reference


Page 16 of 33

g-C3N4

Ni2P/5wt%

300 W, λ>420nm

160

84

(2017)

g-C3N4

Ni2P/2wt%

300 W, λ>420nm

82.5

85

(2017)

g-C3N4

Ni2P/0.48wt%

300 W, λ>420nm

567

86

(2017)

g-C3N4

CoP/0.25wt%

300 W, λ>420nm

474.4

61

(2017)

Figure 7. (A) Time-dependent photocatalytic H2 evolution and (B) the average rate of H2 evolution over (a) g-C3N4, (b) g-C3N4-1%Cu3P, (C) g-C3N4-1.5%Cu3P, (d) g-C3N4-2%Cu3P, (e)g-C3N4-4%Cu3P, 5

(f) g-C3N4-6%Cu3P (g) g-C3N4-8%Cu3P (h) g-C3N4-10%Cu3P, (i) g-C3N4-12.5%Cu3P, (j) g-C3N4-15%Cu3P and (k) g-C3N4-1% Pt


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Figure 8. Repeated cycles of photocatalytic H2-evolution over the g-C3N4-1.5%Cu3P (red line) and g-C3N4-10%Cu3P samples (blue line). Moreover, a good photocatalyst should have a highly durable for practical photocatalytic 5

applications. Thus, taking the g-C3N4-1.5%Cu3P and g-C3N4-10%Cu3P as examples, the repeating photocatalytic H2 experiment was tested under visible light for five cycles. Each cycle was performed for 3 h. The stable and reproducible H2-evolution activities of the g-C3N4-1.5%Cu3P and g-C3N4-10%Cu3P samples were showed in Figure 8. Almost 25% of activity lost after 5 cycles. It is believed that the decreased activity might be attributed to a small quantity of Cu3P nanoparticles

10

releasing from the binary composite, which might be caused by the un-intimate contact of the g-C3N4-Cu3P interfaces fabricated by the grinding method. Nevertheless, this above result clearly shows that the g-C3N4-10%Cu3P composite photocatalysts are relatively stable for the practical H2



Page 18 of 33

generation from the TEOA solution. Charge separation properties

Figure 9. (A) Photoluminescence spectra of g-C3N4, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P 5

photocatalysts with an excitation wavelength of 385 nm. (B) Nyquist plots of different electrodes in 0.1 M Na2SO4 aqueous solutions in the dark. (C) Transient photocurrent responses (I–t curves) of photocatalysts in 0.1 M Na2SO4 aqueous solution under visible light irradiation at 0.2 V vs Ag/AgCl. To understand the roles of Cu3P in promoting charge carrier separation, PL spectra was carried out to study the photoinduced interfacial charge dynamics. In general, the PL spectra were used to reveal

10

the information of separation processes, transfer and migration of the photoexcited holes and electrons separation in the excited semiconductors.87 The PL spectra of g-C3N4, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P were measured at the excitation wavelength of 385 nm. As observed in the Figure 9A, all photocatalysts display the similar PL emission peak at about 450 nm, corresponding to the ban-gap excitation of pristine g-C3N4 sample. Notably, the intensity of g-C3N4-10% Cu3P and ACS Paragon Plus Environment

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g-C3N4-1.5% Cu3P are weaker than that of the pristine g-C3N4, indicating that the loading of Cu3P could greatly retain the recombination of charge carriers and enhance the interfacial charge transfer under illumination. The PL intensity of three samples is consistent with the corresponding photocatalytic H2-evolution activities. As a result, the improved separation of hole–electron pairs will 5

favor the increase in photoactivity and quantum yield. The charge transfer and recombination process could be also revealed by the EIS Nyquist plots. As displayed in Figure 9B, through comparing the EIS Nyquist plots of the different photocatalysts, it was clearly showed that the diameters of the g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P were much smaller than that of pure g-C3N4, owing to the effective charge-carrier separation and transfer of photoexcited

10

charges. Therefore, it was demonstrated that the loading of Cu3P could effectively improve the charge transfer and separation of g-C3N4, thus achieving the enhanced H2 generation. Additionally, the roles of Cu3P in accelerating charge carrier separation were further verified by the photocurrent−time responses of different photocatalysts. The transient photocurrent response could manifest the separation and collection efficiency of hole-electron pairs occurring on the photocatalyst

15

surface. The transient visible-light photocurrent responses (I−t curves) for several samples under the same condition (λ>420 nm) are shown in Figure 9C. It showed that the modification of Cu3P nonoparticles causes a significant enhancement in the photocurrent intensity in the following order: g-C3N4-10% Cu3P>g-C3N4-1.5% Cu3P>g-C3N4 (consistent with the photocatalytic performance results reported above). These results demonstrate that Cu3P loaded on the g-C3N4 could strongly improve the

20

separation and collection efficiency of hole-electron pairs, thus facilitating the enhancement in H2-production activity.



Page 20 of 33

Figure 10. Time-resolved transient PL decay of (a) g-C3N4, (b) g-C3N4-1.5%-Cu3P and c g-C3N4-10%-Cu3P. To further reveal the separation and transfer processes of photo-generated electrons and holes, the 5

time-resolved transient fluorescence lifetime was showed in the Figure 10. The average lifetime could be calculated according to the following equation:88 ࡭ ࣎૛ ା࡭ ࣎૛ ା࡭ ࣎૛

൏ ࣎ ൐ൌ ࡭૚ ࣎૚ ା࡭૛ ࣎૛ ା࡭૜ ࣎૜ ૚ ૚

૛ ૛

૜ ૜

Where τ1, τ2 and τ3 are the emission lifetimes, and A1, A2 and A3 are the corresponding amplitudes. 10

Clearly, the calculated average lifetime of g-C3N4, g-C3N4-1.5%-Cu3P and

g-C3N4-10%-Cu3P were 11.45, 12.91 and 14.062 ns, respectively. It could be easily found that both the average lifetime of the binary photocatalysts were longer than that of pure g-C3N4. The result further shows that after loading the Cu3P on the surface of the g-C3N4, the average fluorescence lifetime was increased from 11.451 to 14.062 ns. The increased average fluorescence lifetime indicated that the


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efficient electron transfer should occur between the Cu3P and g-C3N4, which are advantageous for the improved photocatalytic H2 evolution. Proposed photocatalytic mechanism

5

Figure 11. (A) Polarization curves of g-C3N4, Cu3P, g-C3N4-1.5% Cu3P and g-C3N4-10% Cu3P photocatalysts (at a scan rate of 5 mVS-1 in 0.5 M H2SO4 solution). (B) MS plots of the photocatalysts film electrodes (at a frequency of 1 kHz in a 0.1 M Na2SO4 aqueous solution). To further identify the key roles of Cu3P in improving the H2 evolution during the photocatalytic process, the polarization curves of Cu3P and different photocatalysts were also performed. The

10

cathodic current ranging from 0 to −0.9 V vs Ag/AgCl could be attributed to the electrocatalytic hydrogen evolution. As show in Figure 11A, pure g-C3N4 nanosheet electrode shows the worst hydrogen evolution reaction performance compared with other electrodes. After loading Cu3P, the much lower overpotential indicated the enhanced photocatalytic activity over g-C3N4 nanosheets. In order to understand the intrinsic electronic properties of the film electrode in contact with the

15

electrolyte solution, the Cu3P, g-C3N4 and g-C3N4-10% Cu3P film electrodes were tested by the MS measurements. Figure 11B showed the MS plots, 1/C

2

versus E, for the photocatalysts. The g-C3N4

nonosheets 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



Page 22 of 33

corresponding photocatalysts. The CB levels of the g-C3N4, g-C3N4-10% Cu3P in dark state and in light state were estimated to be -1.46, -1.31 and -1.21eV, respectively. Meanwhile, all the binary photocatalysts exhibit a similar band gap of 2.6 eV. Thus, the VB levels of the g-C3N4 and g-C3N4-10% Cu3P in dark state and in light state were calculated to be 1.14, and 1.29 and 1.39eV. Moreover, from 5

the Figure 11B, the VB level of the Cu3P could be estimated to be 1.63 eV. Based on its band gap of 1.55 eV, the Cu3P should have the CB level of 0.08 eV. It was envisaged that the Fermi level of the n-type semiconductor g-C3N4 was closed to the CB, while the Fermi level of the p-type semiconductor Cu3P was closed to the VB. Notably, the CB level of g-C3N4-10% Cu3P under the light illumination shows a positive shift in comparison with those of the pristine g-C3N4 and g-C3N4-10% Cu3P in the

10

dark, whereas the CB level of Cu3P in g-C3N4-10% Cu3P under the light illumination exhibits a negative shift compared with that of the Cu3P in the dark, suggesting the formation of p-n junction through the effective interface band alignment.


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Figure 12. (A) Schematic of the charge separation and H2 evolution over g-C3N4 modified by Cu3P cocatalyst. (B) Schematic of the relative energy band levels of g-C3N4 and Cu3P before irradiation. (C) Schematic of the charge separation and H2 evolution over g-C3N4-Cu3P p-n heterojunction. Based on the above analysis, it is proposed that the loaded Cu3P on the surface of g-C3N4 could, at the low content, serve as active sites and an electron sink to trap the phohoto-generated electrons and drive the photocatalytic H2 production (Figure 12 A). whereas, at the high content, we envisage that Cu3P could mainly play the semiconductor role in boosting the photocatalytic H2 evolution, which can act as a p-type semiconductor to donate electrons for g-C3N4 through the p–n g-C3N4-Cu3P heterojunction, thus leading to the formation of competitive electron transfer on the surface of Cu3P (Figure 12C), due to the significantly boosted both bulk charge transport and surface reaction kinetics. More specifically, after band alignment under light irradiation, the photo-generated electrons on the surface of Cu3P semiconductor could readily transfer to the surface of g-C3N4 nanosheets, and then achieve the



photocatalytic H2 evolution over g-C3N4. At the same time, the holes will easily migrate from the VB of g-C3N4 to the VB of Cu3P, thus decreasing the oxidation capacity of TEOA. Similarly, in the previous report, the bifunctional roles of CoO as a cocatalyst and p-type semiconductor have been successfully employed to explain the enhanced photocatalytic H2 evolution over g-C3N4.67 Nevertheless, bifunctional roles of Cu3P as both cocatalysts and semiconductors, synergistically achieve the evidently boosted separation of the electron-hole pairs, the increased light absorption, and the improved H2-evolution kinetics, thus leading to the fundamentally enhanced photocatalytic H2 generation over binary hybrid systems. Conclusions In this work, the bifunctional g-C3N4 nanosheets-Cu3P nanoheterojunctions are first synthesized via a grinding method. The component ratio-dependent sensitization roles of the bifunctional copper phosphide (Cu3P) as both cocatalysts and p-type semiconductors at different loading contents are thoroughly revealed, both of which could significantly boost the charge-carrier separation and photocatalytic H2 production over g-C3N4 nanosheets using triethanolamine (TEOA) as a sacrificial reducing agent. Impressively, the binary g-C3N4-Cu3P photocatalytic systems were found to exhibit two optimum H2-generation rates of 104.34 and 159.41 µmolg-1h-1, at the Cu3P loading contents of 1.5 and 10 wt%, respectively. At the low loading content, the Cu3P as noble metal-free H2-evolution active sites could dominantly fabricate the Cu3P cocatalysts/g-C3N4 heterojunctions, which was verified by the markedly decreased electrocatalytic hydrogen-evolution potentials. By contrary, at the high


Page 24 of 33

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loading content, it was supposed that Cu3P as a p-type semiconductor could achieve the p-n g-C3N4-Cu3P heterojunctions, which was observed by the shifted conduction/valance band potentials under light illumination. This work will offer new ideas to design environment-friendly noble-metal-free g-C3N4 modified by the semiconducting cocatalysts for highly efficient visible-light hydrogen generation.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X. Li) , [email protected] (X. Chen).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT X. Li would like to thank National Natural Science Foundation of China (51672089)， Specical

funding

on

Applied

Science

and

technology

in

Guangdong

(2017B020238005) 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-Kansan City and University of Missouri Research Board. References



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For Table of Contents Use Only

The bi-functional noble metal-free Cu3P as both cocatalyst and semiconductor could significantly enhance the photocatalytic H2 evolution over the g-C3N4 nanosheets.


Bifunctional Cu3P Decorated g-C3N4 Nanosheets as a Highly Active

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