constructed carbon nitride with ternary cobalt phosphosulphide

Journal of Colloid and Interface Science 548 (2019) 303–311

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Regular Article

Boosting photocatalytic hydrogen evolution achieved by rationally designed/constructed carbon nitride with ternary cobalt phosphosulphide Haiyu Wang, Zhiliang Jin ⇑ School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu University, Yinchuan 750021, PR China Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu University, Yinchuan 750021, PR China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 31 January 2019 Revised 9 April 2019 Accepted 14 April 2019 Available online 15 April 2019 Keywords: Hydrogen evolution Cobalt phosphosulphide Carbon nitride Triethanolamine

a b s t r a c t Photocatalytic hydrogen production has been emerged as a promising method to solve the issue of energy shortage, however, how to design high-performance photocatalysts is an urgent problem. Recently, cobalt phosphosulfide (CoPS) has been confirmed to be an effective catalyst for energy conversion and storage. Nevertheless, combining the CoPS with other non-metallic catalysts for efficient photocatalytic hydrogen production has rarely reported. Hence, in this work, we fabricated a series of ternary CoPS and carbon nitride (CN) composite materials by using cobalt, phosphorus and sulfur as donors and CN as underlying support or substrate, respectively. As expected, the obtained CoPS(x)/CN catalysts exhibited obviously enhanced photocatalytic hydrogen evolution activity and the corresponding results of hydrogen production were measured in different pH reaction system by using offline statistics. Specifically, the CoPS(0.25)/CN catalysts reveals a remarkable hydrogen production of 14.12 mmol/g in an EY sensitized 15% (v/v) TEOA aqueous solution under visible light irradiation (k
420 nm), which attributes to the formed interfaces between CoPS and CN with strong bonding, electronic interactions or synergistic effects that can constitute more active centers than individual component. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction With the frequent occurrence of extreme weather caused by the excessive burning of fossil fuels, people are increasingly aware of ⇑ Corresponding author. E-mail address: [email protected] (Z. Jin). https://doi.org/10.1016/j.jcis.2019.04.045 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

the importance of environmental protection. Therefore, finding an environmentally friendly energy alternatives is an urgent. In recent decades, some studies have found that hydrogen is a sustainable energy carrier, and the production of hydrogen through water splitting can, directly or indirectly, address the problems that plague humans [1–5]. However, the large-scale production of hydrogen driven by sunlight requires the catalyst materials with

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highly stability, wide light response range and cheapness, which causes serious obstacles to the industrial production of hydrogen. Recently, platinum (Pt), palladium (Pd), Ruthenium (Ru), Aurum (Au) and Argentum (Ag) are the most active noble metal catalysts for the hydrogen evolution reaction, all of which are expensive and not earth-abundant materials that seriously limited their development and application [6–15]. Therefore, searching for earthabundant, efficient and stable catalysts is of great strategic significance for the large-scale production of hydrogen driven by solar energy in the future Based on this background, a series of Earthabundant transition metal compounds (TMCs) [16], transitionmetal phosphides (TMPs) [17], and transition-metal dichalcogenides (TMDCs) have been studied [18]. Interestingly, some studies have shown that the synergistic effects of TMPs and TMDCs can significantly enhance the hydrogen evolution activity and these compounds are abundant, making them attractive for a variety of applications related to energy conversion [19–25], which provides a powerful potential for the wide application of ternary transitionmetal phosphide-chalcogenides (TMPCs) in environmental protection and fuel substitution. Carbon nitride (CN), as a non-metal semiconductor, has been studied as a promising solar absorber and hydrogen evolution reaction HER catalyst due to the suitable band gap of 2.76 eV (CB = 0.83 V, VB = 1.83 V vs. NHE), which is enough to overcome the average energy of the four photons required for the water splitting reaction. Meanwhile, the surface defect sites and nitrogen pores of CN are favorable for the anchoring of electron localization and active sites. More importantly, the HOMO and LUMO levels of CN have the potential to simultaneously conduct water oxidation and reduction. However, due to the material with severe recombination of photogenerated electrons and holes recombination inside of the material, even if the noble metal Pt is used as the co-catalyst, the quantum efficiency of CN is still relatively low and far below its theoretical expection [26–31]. It is worth noting that the large number of N atoms in CN makes it have some other physical and chemical properties, such as coordination and semiconducting properties. Meanwhile, it can be applied in a wide field through the further modification of CN and changing its physical and chemical properties. For example, it can be compounded with transition-metal phosphides (TMPs) and transition-metal dichalcogenides (TMDCs), such as CoP, MoP,

Ni2P, MoS2, NiS2 and WS2, which could improve the Fermi level, electron trapping ability and photogenic carrier recombination of CN, thus enhancing its photocatalytic activity [2,32–39]. Cobalt phosphosulphide was identified in the 1960s but its properties remain to be investigate. Recently, we found that cobalt phosphosulfide (CoPS) may be a good catalyst for HER specifically, this material could have higher HER catalytic activity owing to Co octahedra in CoPS containing P2 ligands that possess higher electron-donating character than S ligands because of the weaker electronegativity of phosphorus in comparison to sulphur, and a more thermoneutral hydrogen adsorption at the active sites [32,40]. Herein, we selected a low-cost and readily available nonmetallic semiconductor photocatalyst CN, and prepared the target photocatalysts by using a simple and easy-to-operate calcination method (Scheme 1). The composite photocatalysts obtained by this method have low recombination of photoinduced electrons and holes, and higher photocatalytic activity than a single photocatalyst, which acquired a remarkable hydrogen production of 14.12 mmol/g in an EY sensitized 15% (v/v) TEOA aqueous solution under visible light irradiation (k
420 nm). 2. Experimental section 2.1. Preparation of catalysts All reagents were of analytical grade and could be used without further purification. 2.2. Preparation of CN material CN nanosheets was prepared by urea pyrolysis according to the previous reports [30,33]. Briefly, 15 g of urea was calcined at 500 °C in a muffle furnace for 2 h at a heating rate of 2 °C min1, then heated to 520 °C at the same rate and held for 6 h. 2.3. Preparation of CoPS and CoPS/CN composite materials First, an alumina boat containing sublimation sulphur and phosphorus powders, with a mass ratio of 1:1 was placed in the

Scheme 1. Synthesis of CN, CoPS and CoPS/CN.

H. Wang, Z. Jin / Journal of Colloid and Interface Science 548 (2019) 303–311

middle of the tube furnace that had a gas flow controller. The alumina boat was heated to 200 °C for 10 min with N2 gas with the heating rate of 2 °C min1. After cooled to room temperature naturally, the thiophosphate paste-like product was prepared. Afterwards, X (X = 0.15, 0.20, 0.25, 0.30 and 0.35) mmol of thiophosphate paste-like product and 0.1 g of prepared CN were placed in an alumina boat and moved to the middle of the tube furnace. The alumina boat was heated to 500 °C for 60 min with N2 gas with the heating rate of 3 °C min1, then cooled to room temperature naturally, the CoPS(X)/CN (X = 0.15, 0.20, 0.25, 0.30 and 0.35) product was prepared. The pure CoPS product was also made in the same way, but without CN. 2.4. Characterization XRD crystal structure analysis, the sample was placed on a XRD quartz sample holder, and the sample was subjected to phase analysis using a Japanese Shimadzu X-ray diffractometer. The monochromatic Cu-Ka (k = 1.5418) was used as a ray source, the tube voltage was 40 kV, the tube current was 30 mA, and the phase analysis was performed at a scanning speed of 5° min1 in the range of 2h = 5°85°.Fourier transform infrared spectroscopy (FTIR) spectroscopy was performed on a Thermo Nicolet Avatar 380 FT-IR spectrometer, after the sample and KBr were ground and compressed, the test was carried out in the range of 4000– 400 cm1 wave number. UV–vis diffuse reflectance spectroscopy was performed on a UV-2550 (Shimadzu) spectrometer by using BaSO4 as a reference. The microstructure and morphology of the prepared samples were examined using a field emission scanning electron microscope (JSM-6701F. JEOL) at an acceleration voltage of 5.0 kV. To improve conductivity, all samples were dropped onto a copper frame and plated with gold. High-resolution transmission electron microscopy (TEM) characterization was performed using a FEI Tecnai TF20 high-resolution transmission electron microscope with an accelerating voltage of 300 kV. The sample was first dispersed in ethanol and sonicated for 1 h, and then the suspension was dropped on a copper mesh, dried naturally, and subjected to TEM test. Elemental composition was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). In the data processing, C1s (binding energy = 284.8 eV) was used as an internal standard to locate the binding energy of the sample. Photoluminescence data (PL) was obtained at room temperature using a FLUOROMAX-4 spectrophotometer. 2.5. Photocatalytic hydrogen production tests Photocatalytic hydrogen production test was carried out in a quartz reaction flask (62 cm3) with a flat bottom, and irradiated with a 5 W white light under a magnetic stirring condition (PCX50A Discover 5 W). In a typical procedure, 10 mg of photocatalyst powder and 20 mg of dye Eosin Y (EY) powder were suspended in 30 mL of 15% (v/v) Triethanolamine (TEOA) aqueous solution as an electron sacrificial reagent, which was ultrasonically dispersed for 25 min. After a homogeneous solution was formed and the reaction system was purged with bubbling N2 for 25 min to purge the air from the reaction flask and to ensure a mixture of the reactants under anaerobic conditions. The amount of hydrogen evolution was measured using gas chromatography (Tianmei GC7900, TCD, 13X column, N2 as carrier). Briefly, 0.5 mL of gas was withdrawn from a quartz reaction flask (62 cm3) having a flat bottom through a dedicated chromatographic syringe at a specific time, and then injected into a gas chromatograph for detection and the obtained data was converted into a graph by a standard curve. All glassware was rigorously cleaned and carefully rinsed with distilled water prior to use.

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2.6. Photoelectrochemical measurements Working electrode preparation: A fluorine-doped tin oxide (FTO) substrate was impregnated with a cleaning agent, isopropanol, acetone solution, ethanol and water for 25 min under ultrasonic treatment. The catalyst powder (5 mg) was then added to 200 lL of absolute ethanol (containing 50 lL of a 5% Nafion solution) and ultrasonically dispersed for 25 min to form a homogeneous solution. Then, 150 lL of the suspension was uniformly applied to the pretreated FTO with a drip area of about 1 cm2. The painted electrode was dried at room temperature. All PEC measurements were performed on an electrochemical workstation (VersaStat 4–400, Advanced Measurement Technology, Inc.) in a three-electrode system. The prepared photoanode was used as a working electrode, the Pt plate was used as a counter electrode, and the saturated calomel electrode was used as a reference electrode. A 300 W xenon lamp equipped with a filter (k
420 nm) was used as an illumination source. A 0.2 M Na2SO4 aqueous solution was used as the electrolyte. 3. Results and discussion 3.1. XRD and TEM analysis The crystal structure of all samples was characterized by the XRD shown in Fig. 1a. It clearly displays that the pure CN has two characteristic peaks at 12.9° and 27.3°. The former peak can be allocated to the (1 0 0) crystal face which represented inplane packing of CN, and the peak of 27.3° is the (0 0 2) crystal planes representing the CN interfacial stacking [30,38]. In addition, the results could also reveal that all peaks of the prepared CoPS can be indexed to JCPDS No. 27-139 of CoPS. After combining the two materials, all CoPS(X)/CN samples obtained show the characteristic diffraction peaks of CN and CoPS, indicating that the composite materials have been successfully synthesized. Furthermore, the peaks of CoPS/CN at 28.5°, 32.9°, 37.0°, 40.6°, 47.3°, 56.1°, 58.8°, 61.6° and 64.1° are attributed to the (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 1 1), (2 2 2), (3 2 0) and (3 2 1) planes. Since the diffraction of the characteristic peaks coincides, the CoPS/CN composite material exhibits a peak consistent with a single catalyst, so the combination of the two does not affect its crystallinity. Interestingly, the characteristic peaks of CoPS(X)/CN shift to the left, indicating that the unit cell parameter becomes larger and the interplanar spacing becomes larger after the two materials were combined [32,33,40]. The high-resolution TEM image of the CoPS/CN shown in Fig. 1b exhibits, a lattice spacing of 0.31 nm corresponding to the (1 1 1) crystallographic plane of CoPS and an amorphous CN, further confirming that CoPS/CN composite materials are successfully obtained. Fig. 1d and e show the TEM images of CN and CoPS/CN composite material, it can be seen that CN is a sheet-like structure with holes, and the CoPS are also relatively uniformly loaded on the surface. In addition, the selected region energy dispersive EDX and elemental mapping images demonstrate that the elements of C, N, P, S, and Co uniformly distributed over CoPS/CN (Fig. 1c and f), also indicating that CoPS/CN has been successfully prepared. 3.2. SEM analysis The morphologise of the CoPS and CoPS(0.25)/CN samples were analyzed by scanning electron microscope (SEM) (Fig. 2). It displays that the obtained CoPS presents irregular particles of different sizes. Most of them are scattered, and a small amount is agglomerated. As expected, CoPS(0.25)/CN composite material

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Fig. 2. The different multiples SEM images of CoPS (a and b) and CoPS(0.25)/CN composite material (c and d).

units with NH/NH2 groups in it. A series of peaks displayed from 1200 cm1 to 1800 cm1 originate from NAC@N heterorings in the ‘‘melon” framework. The peaks at 2160 cm1 to 2180 cm1, corresponding to an asymmetric stretching vibration of cyano groups (AC„N) [30,33,38]. The broad peaks between 3000 cm1 and 3500 cm1 are incompletely condensed secondary and primary amines and their own intermolecular hydrogen bonding. With the formation of composite materials, the intensity of the relevant characteristic peaks are significantly different from the peak intensity associated with a single catalyst, indicating that the chemical structure of the composite material has changed to some extend compared with the single catalyst. It is also proved that the composite material has been successfully prepared. 3.4. UV–vis analysis

Fig. 1. (a). The XRD diffractograms of CN, CoPS, CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS (0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN composite materials. (b). HRTEM images of CoPS/CN. (c). EDX elemental mapping of the CoPS/CN composite materials refers to the signals of C, N, P, S and Co respectively as marked in (c). (d). The TEM of CN. (e). The TEM of CoPS(0.25)/CN composite material. (f). Acquire EDX of CoPS(0.25)/ CN composite material.

Moreover, to investigate the optical properties of the asprepared sample, the UV–vis diffuse reflectance spectra (DRS) were measured and the results are shown in Fig. 4a the optical absorption of as-prepared CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN and CoPS(0.35)/CN samples were enhanced compared to CN [9,26,38] indicating that it is possible for them to obtain more photons to drive the photocatalytic reaction, which may be beneficial to improve the photocatalytic performance.

retains their own single shape after compounding, the flaky CN surface is distributed with CoPS, indicating the composite material has been successfully formed and the preparation method is reasonable. In addition, both materials retain their own single shape after compounding, indicating that the preparation method is reasonable. This provide a possible method for the preparation of the catalyst in the future. 3.3. FTIR analysis After analyzing the crystal structure and morphology of the relevant samples, we further studied the structure of samples by Fourier transform infrared spectroscopy (FTIR), corresponding results are shown in Fig. 3. The curves from a to g a-g represent CoPS, CoPS (0.15)/CN, CoPS (0.20)/CN, CoPS (0.25)/CN, CoPS (0.30)/CN, CoPS (0.35)/CN and CN, respectively. For CN (the dark green curve), the peak at 808 cm1–810 cm1 is the bending mode of heptazine rings (C6N7), indicating the presence of basic melon

Fig. 3. FT-IR spectra of all samples. a–g represent CoPS, CoPS(0.15)/CN, CoPS(0.20)/ CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN and CN, respectively.

H. Wang, Z. Jin / Journal of Colloid and Interface Science 548 (2019) 303–311

The band gap of the as-prepared samples was derived using Eq. (1):

a¼

Aðhm  Eg Þ hm

n=2

ð1Þ

where A is the proportionality constant, h is Planck’s constant, m is the incident light frequency, and Eg represents the band gap energy. The value of n is related to the type of band gap. As shown in the Fig. 4b, the band gap range of all samples was estimated to be 2.87–2.75 eV using Eq. (1), and the UV–vis diffuse reflectance spectra (DRS) results indicate that the CoPS has a considerable influence on CN and leads to more efficient use of visible light. 3.5. XPS analysis In order to analyze and identify the surface chemical state of CoPS/CN composite material, XPS analysis was further executed shown in Fig. 5. Fig. 5a exhibits a survey of all relevant factors, indicating that the obtained sample possessed C, N, O, Co, P and S elements. For the cobalt 2p core level spectra of CoPS that loaded on carbon nitride. In Fig. 5b, two main peaks located at 778.3 eV and 794.4 eV are assigned to Co3+ in the octahedral site and the relatively weak peak can be attributed to the satellite. Other peak located at 800.1 eV can be ascribed to the Co2+ state, which may be originated from the surface cobalt oxide species caused by the unavoidable oxidation during the washing process [39,41,42]. The S 2p spectrum can be divided into four peaks, the peak at 159.8 eV can be ascribed to S 2p3/2. And the other two peaks attributed to S 2p1/2 are at 165.1 eV and 166.3 eV. In addition, for the P 2p region, the peaks located at 129.9, 130.4, 130.9 and 131.5 eV are corresponding to the binding energies of P 2p1/2, P 2p3/2, and PAO in phosphides and phosphorus oxide, respectively [43,44]. Therefore, the XPS results support that the obtained CoPS are indeed a distinctive ternary alloy phase. 3.6. Photocatalytic hydrogen production tests analysis The H2 evolution activities of the prepared samples are evaluated under visible light irradiationand the sacrificial reagents is 15% (v/v) triethanolamine (TEOA) aqueous solution, including dye eosin Y (EY) powder. Fig. 6a and b show the amount of hydrogen produced by the samples under continuous visible light for 5 h. The pure CN exhibits a low activity for H2 evolution due to the rapid recombination of photogenerated electrons and holes. After the CN loaded with CoPS, the hydrogen production activity was significantly increased. Specifically, the CoPS(0.25)/CN sample shows the highest hydrogen production activity that 1 g of CoPS

307

(0.25)/CN can produce 14 mmol of hydrogen under continuous visible light for 5 h demonstrating that the obtained composite material can effectively inhibit the recombination of photo-generated charges and improve the electron transport efficiency [33]. Moreover, we also optimized the pH of the reaction system. As can be seen in the Fig. 6c, the pH of the reaction system has a significant effect on the hydrogen production activity of the sample. When the pH of the reaction system is acidic, the catalytic activity of the sample can only be maintained at a lower level. The reason for this phenomenon is that TEOA can be protonated under acidic conditions, weakening its own ability to supply electrons. With the change of pH of the reaction system, the hydrogen production activity of the sample has developed remarkably, especially when the pH value is 11, its hydrogen production activity reached the highest level. However, with the increasing alkalinity of the reaction system, the hydrogen production activity of the sample beginning to decrease, which is because the decrease of proton concentration and H2 evolution became more thermodynamically unfavorable with increasing pH values, thus reducing the hydrogen production activity of the sample [45,46] According to the above discussion, CoPS(0.25)/CN can be used as a high-efficiency catalyst for catalyzing hydrogen production from TEOA solution at room temperature. In addition, stability of samples should also be considered in the future practical application. Therefore, under the same reaction conditions, the stability of the sample was also measured for 15 h (shown in Fig. 6d). It shows that there was no significant reduction for the hydrogen production activity of the sample during 15 h of consecutive operation. Thereby, it has been considered that this hydrogen production system may offer the potential to provide hydrogen supply. 3.7. PL analysis The photoluminescence (PL) spectra were further studied to discuss the electron transfer and excited state interactions between EY and CoPS/CN composite photocatalysts. As shown in Fig. 7a, CoPS/CN composite photocatalysts were introduced into the EY solution, showing different degree of fluorescence quenching. With the addition of CoPS(0.25)/CN composite material, the signal was minimized, illustrating that the recombination rate of electron-hole pairs was reduced [19,30,33]. And it is also supported with the evidence provided with time-resolved PL results (Fig. 7b). The relevant time-resolved photoluminescence (TRPL) was fitted by a tri-exponential decay model, the equation as follows:

IðtÞ ¼

X

Bi expðt=si Þ

ð2Þ

i¼1;2;3

Fig. 4. (a). UV–Visible absorption spectra of as-prepared CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN and CN. (b). The band gap of the asprepared CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN and CN.

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Fig. 5. XPS investigation of the CoPS/CN sample: (a) survey spectrum, (b) Co 2p, (c) S 2p and (d) P 2p high resolution spectra.

Fig. 6. (a and b) the H2 evolution of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN composite photocatalysts. (c). the CoPS(0.25)/CN composite photocatalyst H2 evolution in the different pH of the reaction system. (d). The stability of CoPS(0.25)/CN composite photocatalyst (pH = 10, 15%TEOA aqueous solution, containing 106 mol L1 EY).

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Fig. 7. (a). The PL spectra of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN composite photocatalysts. (b). The time-resolved photoluminescence (TRPL) spectra of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN and CN.(15%TEOA aqueous solution, containing 106 mol L1 EY).

Table 1 Fluorescence lifetime measurements of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN and CN. Samples

A1 (%)

Τ1 (s)

A2 (%)

Τ2 (s)

A3 (%)

Τ3 (s)

(s)

v2

CoPS(0.15)/CN CoPS(0.20)/CN CoPS(0.25)/CN CoPS(0.30)/CN CoPS(0.35)/CN CN

47.91 49.90 50.73 49.80 49.84 85.63

4.03E-9 4.07E-9 4.19E-9 3.87E-9 3.77E-9 1.68E-10

40.16 37.24 35.62 14.32 36.10 14.37

1.03E-9 1.01E-9 1.06E-9 1.65E-8 9.62E-10 4.00E-10

11.93 12.86 13.65 35.88 14.06 –

1.79E-8 1.81E-8 1.83E-8 9.64E-10 1.62E-8 –

1.94E-9 2.00E-9 2.15E-9 1.96E-9 1.88E-9 1.91E-10

1.39 1.37 1.36 1.33 1.37 1.56

Fig. 8. (a). The transient photocurrent response of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN composite photocatalysts. (b). The electrochemical impedance spectroscopy (EIS) of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN composite photocatalysts.

I ─ the normalized emission intensity; si ─ a decayed lifetime of luminescence; Bi ─ the corresponding weight factors. The calculated average lifetimes according to Eq. (3) is listed in Table 1:

< s >¼

X i¼1;2;3

Bi s2i =

X

Bi si

ð3Þ

i¼1;2;3

─ the average lifetime, si ─ a decayed time of the individual components; Bi ─ the corresponding weight factors. Fig. 7b shows the TRPL fitted results of samples. The long-lived and short-lived components were resulted from the g-conjugated

electronic interaction between EY and CoPS(0.25)/CN composite material and the monomeric EY revealing that the CoPS(0.25)/CN composite material could rapidly transfer the electron from excited EY which revealed that CoPS(0.25)/CN composite material possesses more efficient electron-hole pairs separation efficiency compare with others. Furthermore, from Table 1, it could be clearly seen that the average lifetime of all samples varied greatly. The average lifetime of CoPS(0.15)/CN, CoPS(0.20)/CN, CoPS(0.25)/CN, CoPS(0.30)/CN, CoPS(0.35)/CN were 1.94E-9, 2.00E-9, 2.15E-9, 1.96E-9 and 1.88E-9, respectively. Compared to that of 1.91E-10 for CN, demonstrating that the average lifetime of all CoPS(X)/CN composite materials has been greatly improved. In particular, the CoPS(0.25)/CN composite material has the maximum average lifetime, which suggests that it could rapidly enhance the electron transfer on the surface of CoPS(0.25)/CN composite material, significatly reduce the recombination of carrier and prolonged the lifetime of photogenerated charges [30,47].

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concentration of available catalytic sites. Moreover, the strongly interacting between CoPS and CN, which can highly improve the activity and facilitate electron transfer capability. These results illustrate that, apart from a high concentration of available catalytic sites, it can serve as a novel strategy to improve the catalytic activity of transition metal compounds by controlling the hydrogen adsorption energy of the active sites, for example, by tuning their electronic structure and reactivity by substituting nonmetal atomic constituents in ternary or more complex compounds. This work may open a new avenue for the designation and preparation of ultrafine ternary transition-metal phosphidechalcogenides and CN.

Scheme 2. The proposed a possible reaction mechanism of H2 evolution over CoPS/ CN.

Conflicts of interest The authors declare that they have no competing interests.

3.8. Photoelectrochemical analysis

Acknowledgments

In order to clarify the interfacial charge separation and transfer process of samples, transient photocurrent response was first tested in 0.2 M Na2SO4 aqueous solution. The corresponding result are shown in Fig. 8a. It shows that the sample has visible light response during switching cycle of the illumination. The CoPS (0.25)/CN displayed the highest transient photocurrent response, representing the CoPS could enhance the electron transfer on the surface of CN, which remarkably reduced the recombination of carrier and prolonged the lifetime of photogenerated electron, leading to excellent photocurrent. In addition, electrochemical impedance spectroscopy (EIS) was measured for all samples (Fig. 8b). According to the Nyquist plots, the sample of CoPS(0.25)CN exhibited remarkably decreased resistances in both charge transport and transfer processes compared to others, illustrating the enhanced conductivity and facilitated interface charge transfer between CoPS and CN [19,26,33]. On the basis of above results, a possible mechanism is illustrated in Scheme 2. In a stable reaction system, EY molecules can absorb photons under illumination to form an unstable singlet excited state EY1* that produces the lowestlying triplet excited state EY3* via an efficient intersystem crossing. EY3* can be quenched by TEOA to produce EY-1* and oxidative donor (TEOA+) [29]. Due to the nature of electron transport, under non-covalent stacking interactions, the e- from EY will be transferred to the CN, andelectron tunneling between the two distal carbon atoms of CN by CoPS opens up additional channels for electron hopping in CoPS/CN, which resulted that the electrons accumulated on the surface of CN that will be transferred in a straight line to the active site CoPS rather than the folding chemical bond along CN. The excellent charge transfer properties significantly reduce the recombination of carrier and prolong the lifetime of photogenerated charges [47,48].

This work was financially supported by the Chinese National Natural Science Foundation (41663012 and 21862002) and the Graduate student innovation project of North Minzu University (YCX18078). the new technology and system for clean energy catalytic production, Major scientific project of North Minzu University (ZDZX201803), the Key Laboratory for the development and application of electrochemical energy conversion technology, North Minzu University and the Ningxia low-grade resource high value utilization and environmental chemical integration technology innovation team project of North Minzu University.

4. Conclusion In summary, the insights gained from this study may be of assistance to our understanding of the effects between CoPS and CN in boosting photocatalytic hydrogen evolution. Because of the weaker electronegativity of phosphorus in comparison to sulphur, and a more thermoneutral hydrogen adsorption at the active sites, Co octahedra in CoPS containing P2 ligands that possess higher electron-donating character than S ligands. We can control the active sites by tuning their electronic structure and reactivity by substituting non-metal atomic constituents in CoPS(x)/CN [32,40]. During phosphorsulphide processes, the CoPS obtained through substrate CN are distributed evenly, which provides a high

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