Unique photocatalytic activities of transition metal phosphide for hydrogen evolution

Journal of Colloid and Interface Science 541 (2019) 287–299

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Regular Article

Unique photocatalytic activities of transition metal phosphide for hydrogen evolution Yanbing Li a,b,c, Zhiliang Jin a,b,c,⇑, Hai Liu a,b,c,d, Haiyu Wang a,b,c, Yupeng Zhang a,b,c, Guorong Wang a,b,c,⇑ a

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 c Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu University, Yinchuan 750021, PR China d State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, PR China b

g r a p h i c a l a b s t r a c t Fig. 10 H2 evolution mechanism of EY-sensitized samples under visible light irradiation. The two n-type single transition metal phosphides with excellent photo-catalytic activity are reported, which is of great significance for the discovery and synthesis of novel and more efficient photo-catalysts. And completely controllable synthesis will greatly reduce application costs. Furthermore, the fairly high activity of H2 evolution of the resulting CoP and WP having low crystallinity and extremely strong optical properties is reveled in detail.

a r t i c l e

i n f o

Article history: Received 21 December 2018 Revised 20 January 2019 Accepted 23 January 2019 Available online 24 January 2019 Keywords: CoP WP H2 evolution Characterization High photo-catalytic activity

a b s t r a c t Economical, efficient and stable single photo-catalysts are vital for realizing enhanced photo-catalytic H2 evolution activity. In this study, we report two n-type single transition metal phosphide with excellent photo-catalytic activity under visible light irradiation, which is of great significance for the discovery and synthesis of novel and more efficient photo-catalysts. And completely controllable synthesis will greatly reduce application costs. Furthermore, the fairly high activity of H2 evolution of the resulting catalysts having low crystallinity and extremely strong optical properties is reveled in detail via investigating the dynamics of hydrogen generation of CoP and WP. Moreover, it is found that CoP and WP have low band-gap energy and increased optical absorption properties through studies. Meanwhile, low crystallinity state for they also is get by XRD technology. These may be the main reasons why CoP and WP exhibit efficient H2 evolution activity. In addition, the possible mechanism showing high photocatalytic activity of hydrogen evolution for CoP and WP is proposed by a series of other characterizations,

⇑ Corresponding authors at: School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China. E-mail addresses: [email protected] (Z. Jin), [email protected] (G. Wang). https://doi.org/10.1016/j.jcis.2019.01.101 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

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such as SEM, TEM, XPS, BET, transient photo-current response, steady-state fluorescence, transient-state fluorescence and Mott-Schottky studies etc. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction It is universally acknowledged that energy is the material basis for people to survive, and renewable and clean energy will become the core of energy consumption in the future [1]. Consequently, it is impatient to explore the potential and sustainable environmentfriendly energy along with the continuous progress of industrialization [2]. However, solar energy containing enormous energy is a clean and sustainable energy. How to make better use of solar energy to solve the various problems of people are facing in the development has become the focus followed. Decades ago, heterogeneous photocatalysis of semiconductor has been regard as a potential technology to solving the increasingly serious energy shortage and environmental pollution problems, thus it continues to attract more and more attention [3]. Meanwhile, photocatalytic and electrocatalysis water splitting is considered to be an advanced energy conversion technology to generate H2 or O2 regarded as a promising and sustainable fuel [4,5]. It includes the following two half reactions [6]: hv

2H2 O ! 2H2 þ O2 2H2 O ! 2H2 þ 4Hþ þ 4e 4Hþ þ 4e ! 2H2 Up to now, hydrogen is considered one of the best options because its overwhelmingly high utilization value and nonpolluting nature. Over the past decades, hundreds of photocatalysts have been studied for hydrogen evolution, for instance, TiO2 [7], g-C3N4 [8,9], CdS [10,11] and graphene [12–14]. Regretfully, although it has never stopped looking for new photocatalysts with expected catalytic properties, no catalyst can be used as a single catalyst to show its ideal catalytic property. In recent years, in order to enhance the activity of photocatalysts for H2 evolution, almost all catalysts with enhanced catalytic activity are composite materials attributed to an efficient electron transfer [15]. Meanwhile, the photocatalytic activity of semiconductors can also be increased by compounding the metal organic frame (MOFs) with increased surface area and electrochemical energy storage [16]. As an alternative pathway for efficient H2 evolution, dye sensitization has been proved to be a likely way for extending the range of captured light, especially photocatalysts with large bandgap such as TiO2 [17,18]. So far, EY has been used as a sensitizer for most catalysts, which also indicates that eosin is a very good sensitizer [19,20]. And Eosin Y-sensitized semiconductor has been found to be highly active for H2 evolution under visible light irradiation because the Eosin Y-sensitized photocatalyst can greatly promote the separation of electrons and holes. Many catalysts sensitized by EY have been reported for efficient H2 evolution. For example, the highest apparent quantum yield (12.14%) of Eosin Y-sensitized multiwalled carbon nanotube (MWCNT)/Pt catalyst for photocatalytic water reduction has been obtained under visible light irradiation (k  420 nm) [21]. Min et al. [22] reported Eosin Ysensitized graphene oxide (RGO) to achieve efficient photochemical energy conversion and the highest apparent quantum yield (9.3%) for H2 evolution at 520 nm. Therefore, dye sensitization is still a valuable technology in the field of photocatalysis.

In most previous reports, transition metal phosphides were mainly used as co-catalysts to improve the separation and transfer efficiency of electron-hole pairs. Qin et al. [23] reported for the first time that p-type Cu3P loaded on n-type g-C3N4 forms a p-n junction to accelerate charge separation and transfer for improving photocatalytic activity of H2 evolution. And p-type copper phosphide, an efficient promoter to enhance photocatalytic H2 generation from water when coupled with n-type cadmium sulphide nanorods, also reported by Sun et al. [24]. Furthermore, most of the transition metal phosphides (Ni2P/g-C3N4 [25], Ni2P/CdS [26], CoP [27,28], FeP [29], MoP [30], WP [31], Fe2P [32] etc.) have been researched and reported in the field of photoelectrochemical hydrogen evolution. It can be seen that the hydrogen evolution properties of pure transition metal phosphide have hardly been reported in the field of photocatalysis. In this paper, we successfully synthesize pure CoP and WP and investigate their performance of photocatalytic hydrogen production. It is found that pure CoP and WP exhibit excellent photocatalytic property under dye sensitization conditions, mainly due to their narrow band gap structure and strong absorption ability to light. Meanwhile, their excellent property of hydrogen evolution is further demonstrated by a series of characterizations, such as XRD, SEM, TEM, BET, XPS, UV–vis DRS, Transient photocurrent, Transient fluorescence and Mott-Schottky studies etc. In addition, the photocatalytic activity of complex W-P-Co is also investigated. It is discovered that the photocatalytic activity of W-P-Co is only slightly improved when CoP and WP are combined with a mass of 1:1, otherwise its photocatalytic activity will be inhibited. This shows that the recombination of CoP and WP does not accelerate the rapid transfer of electrons, but rather they will each other become the recombination centers of electrons. When CoP and WP are combined at the same mass, the performance of hydrogen evolution of W-P-Co increases slightly due to their mutual synergy, which indicates that the combination of CoP and WP does not fit the matching theory of valence band. 2. Experimental section All reagents were of analytical grade and could be used without further purification. 2.1. The synthesis of Co-MOFs Co-MOFs is synthesized by previous report [33]. In a typical process, 0.118 g of benzimidazole and 0.14 g of Co(NO)36H2O is dissolved into 25 mL of ultrapure water. Subsequently, 5 mL of ammonia water is added to the as-prepared solution after stirring 15 min, and then the obtained solution is continuously stirred for 45 min. After keeping a 6 h standing in a dark, the final products are centrifuged and washed with DI water and ethanol three times. 2.2. The synthesis of CoP In a typical preparation, 1 g of Co-MOFs and 5 g sodium hypophosphite are mixed evenly in a porcelain boat. Subsequently, the resulting mixture is calcined at 300 °C for 2 h in N2 atmosphere at a heating rate of 4 °C/min. The resulting black CoP samples are washed with distilled water and dried at 80 °C for over 6 h.

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2.3. The synthesis of WP 0.66 g of Na2WO42H2O and 1 g of NaPO2H2H2O is dissolved into 100 mL of ultrapure water, and the resulting solution is completely dissolved in ultrasonic cleaner. Whereafter, the mixed systems is kept at 80 °C until water in the solution is dried, then it is ground evenly and calcined at 300 °C for 2 h in N2 atmosphere at a heating rate of 4 °C/min. Finally, the resulting back catalyzer is washed with distilled water and dried at 80 °C for more than 6 h. 2.4. The preparation of W-P-Co WP and CoP with different ratio of mass (mCoP:mWP = 5:1, 5:2, 5:3, 5:4, 1:1, 1:5, 2:5, 3:5, 4:5) are strongly grinded to self assemble, and the obtained W-P-Co is collected for further using.

as a simulated light-source under magnetic stirring in a PCX50A Discover 5 W nine-channel system of photo-catalytic reaction. The amount of H2 evolution is recorded by operating a Tianmei GC7900 gas chromatograph (TCD, 13Xcolumn) with N2 as carrier per 1 h. The total illumination time of each reaction bottle is designed to be 5 h and the evaluated experiment of hydrogen evolution is implemented at room temperature. The apparent quantum efficiency (AQE) under the 300 W Xe lamps and different wavelengths is measured. The output intensity is obtained by taking PL-MW2000 photoradiometer. And AQE calculated according to the following equation:

AQE ¼ 2 

the number of evolved hydrogen molecules  100% the number of incident photon

The turnover number (TON) of dye Eosin Y for H2 evolution is calculated by taking the following equation:

2.5. Characterization of samples The surface morphology of the obtained catalyzers is unveiled by the scanning electron microscope (SEM, JSM-6701F, JEOL, 50 kV) and TEM images of CoP and WP are studied by the transmission electron microscope (TEM, JEM1200EX, JEOL, 100 kV). The Xray diffraction (XRD) patterns of the resulting semiconductors are revealed by the X-ray diffractometer (Rigaku RINT-2000) taking Cu Ka radiation (tube current 40 mA, tube voltage 40 kV), and the scanning range from 5° to 80° at a scanning rate of 8° per minute. The surface element and the valence states of the samples are detected by operating ESCALAB 250Xi X-ray photo-electron spectrometer. Photo-luminescence spectra are investigated by using FLUOROMAX-4 fluorescence spectrophotometer at normal atmospheric temperature. The fluorescence decay times are tested by taking the Horiba Jobin Yvon Data Station HUB at room temperature. The Brunauer-Emmett-Teller (BET) specific surface area (SBET), pore structure and pore size distributions of as-prepared CoP and WP are calculated by nitrogen adsorption-desorption isotherms at 77 K. UV–vis diffuse reflectance spectra of the prepared catalyzators are performed on a Shimadzu UV-2550 by taking BaSO4 as the reference. The photoelectrochemical tests (containing I-T, LSV, EIS and MS measurements) are recorded on the VersaStat4-400 electrochemical workstation at room temperature and a standard three-electrode systems is taken. Meanwhile, a Pt electrode serve as the counter electrode and a saturated calomel electrode is used as the reference electrode, and the electrolytic solution is 0.2 mol/L Na2SO4 aqueous solution. Furthermore, the 300 W xenon lamp is chosen as a light source of getting photocurrent response. In addition, the I-T measurement of samples with on/off cycles is tested at a fixed bias of 0.3 V vs. SCE. Electrochemical impedance spectroscopy (EIS) of CoP, WP and Co-P-W are performed at a forward bias of 0.3 V with AC amplitude of 10 mV and a frequency range of 10,000–1 Hz in the absence of light irradiation. Meanwhile, the Mott-Schottky studies of CoP and WP are carried out from 0.6 to 0.6 V with AC amplitude of 10 mV at a frequency of 100 Hz. 2.6. Photo-catalytic H2 evolution experiments Photo-catalytic H2 evolution experiments of all as-prepared samples are operated in a quartz-glass reactor (62 mL). The specific steps are taken as follows: 10 mg of the ready photocatalysts and 20 mg EY (eosin Y) are added to a reactor with 30 mL of mixture aqueous solution system containing 15%v/v triethanolamine (TEOA, pH = 10) as a hole sacrificial reagent. Subsequently, the mixture suspension is plenarily scattered for 10 min by taking an ultrasonic cleaner and stirred for another 10 min. Then air in the reaction system must be removed by ready N2. Then, a quartzglass reactor containing catalyst is illuminated by 5 W LED served

289

TON ¼ 2 

number of evolved hydrogen molecules number of dye Eosin Y molecules

3. Results and discussion 3.1. Morphology and structure The scanning electron microscope (SEM) and the highresolution transmission electron microscope (TEM) are used to investigated the morphology and structure of samples. Fig. 1A reveals a magnified scanning electron microscopy (SEM) image of as-prepared CoP, indicating that the resulting CoP is stacking structure of fragment having different sizes. And as-synthesized WP presents morphology of aggregated nanoparticles with different sizes as shown in Fig. 1B. The SEM image having a magnification displays that morphology and structure of complexes (W-P-Co) of CoP and WP as shown in Fig. 1G, demonstrating W-P-Co is a larger nanosheets structure with a markedly irregular shape than pure CoP. This may indicate an increase in the specific surface area that helps improve photocatalytic activity. Meanwhile, the increased specific surface area is more conducive to developing the synergistic effect of CoP and WP for H2 evolution. TEM analysis is an extremely powerful tool to further investigate the individual morphology and structure of samples. The TEM images of pure CoP at a magnification exhibits that stacking structure of nanosheets with different sizes as shown in Fig. 1C, which is exactly the same as the result of SEM images. The TEM image in Fig. 1D shows that the pure WP consists of slightly aggregated nanoparticles having diameters of different microns. It can be seen from the heavy stacking area and the lightly stacked area that the diameters of pure WP nanoparticles are relatively uniform, which is consistent with SEM images reveals. Figs. 1E and F is an X energy dispersive region of the spectrum. It demonstrate the presence of Co and P elements in CoP, and there are W and P elements in WP sample, which indicates that CoP and WP may be successfully synthesized. However, Cu element is also exist in the spectrum, which mainly from copper carriers used during testing [34]. In addition, no other elements exist in them, which indicates the high purity of CoP and WP. 3.2. XRD and XPS studies The X-ray diffraction (XRD) analysis is an important method for structural research and plays a momentous role in the structural research of catalytic materials, biomaterials, etc. Fig. 2A exhibits XRD patterns of pure CoP and pure WP. All the typical peaks located at 2h = 31.6°, 36.4°, 48.4°, 56.8° can be indexed to the corresponding the crystal surface of (0 1 1), (1 1 1), (2 0 2) and (3 0 1)

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Fig. 1. (A) SEM images of pure CoP and (B) pure WP, (C) TEM images of pure CoP and (D) pure WP, (E) EDX of pure CoP and (F) pure WP, (G) SEM images of W-P-Co.

of unprocessed CoP (PDF#89-4862). The intensity of all peaks indicate that the degree of crystallization of crude CoP is relatively low and the wide diffraction peaks expose that crystallite sizes of CoP are relatively small. The XRD pattern of as-prepared WP displays relatively stronger diffraction peaks compared to CoP at 2h = 21.0°, 28.7°, 31.2°, 43.3°, 44.6°, 46.6°, 56.7°, 68.9°, 73.8° and 76.5°, attributed to (1 0 1), (0 0 2), (2 0 0), (1 1 2), (2 1 1), (1 0 3), (0 2 0), (1 1 4), (4 1 1) and (1 2 3) plane of WP (PDF#80-238). It can be seen from the XRD pattern that the degree of crystallization of (2 0 0), (1 1 2) and (2 1 1) crystal surface of WP are higher compared to other plane, and the peak width of other crystal surface are relatively broad, demonstrating that different sizes of WP particles and low crystallization. And no other impurity peak can be observed in the XRD patterns, implying the high purity of CoP and WP products. This result is consistent with results detected by EDX. We believe that semicrystal state can provide more active

sites compared to highly crystallized state for the interface of electron transfer. This may be one of the reasons why pure CoP and WP exhibit high catalytic activity in the sensitization system. It can be observed from the XRD pattern of W-P-Co that WP does not exist and the change of peak intensity of CoP is not obvious, which indicates the structure of WP is more easily destroyed. X-ray Photoelectron Spectroscopy (XPS) is used to investigate the composition and surface chemical status of as-synthesized samples. The quite remarkable XPS spectrum of Co 2p can be fitted by five main peaks at 779.3, 782.8, 794.2, 798.8 and 803.0 eV, respectively. And the high-resolution XPS spectrum of P 2p displays two peaks at 129.7 and 133.6 eV in Fig. 4D. The peak at 129.7 eV for P and the peak at 779.2 eV attributed to the Co 2p3/2 are close to the binding energies for Co and P in CoP [42], and the binding energy peaks at 794.2 and 803.0 eV can be observed from the Fig. 2B, which can be assigned to Co 2p1/2. However, the

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291

Fig. 2. (A) XRD patterns of samples and (B, C, D) XPS spectra of samples.

Fig. 3. N2 adsorption-desorption isotherm and pore size distribution (inset) of CoP and WP.

Sample

SBET/(m2/g)

Pore volume/(cm3/g)

Average pore size/nm

phorus (P2O5 or PO3 4 ) [38,39,42]. The obviously strong peaks at 35.39 and 37.59 eV are corresponded to W4f7/2 and W4f5/2 of W6+ [40].

CoP WP

10 21

0.05 0.08

19 19

3.3. BET study

Table 1 The SBET, pore volume and pore diameter for CoP and WP.

XPS diffraction peaks at 782.8 and 798.8 eV are designated as two shakeup satellites of Co 2p3/2 and Co 2p1/2, they caused by the oxidized Co species of CoP on the surface of sample [35–37]. And the peak at 133.6 eV may be ascribed to the oxidized species of phos-

As shown in Fig. 3, the Brunauer-Emmett-Teller (BET) specific surface area (SBET), pore structure and pore size distributions of as-prepared CoP and WP are calculated by nitrogen adsorptiondesorption isotherms at 77 K and the results are shown in Table 1, which is characteristic of mesoporous materials and the small pore

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distribution demonstrates a great deal of pores are approximately 5 nm in the both CoP and WP samples. It can be seen from Fig. 3 that they belong to type II isotherm, designated as the adsorption isotherms of multi molecular layers. This indicates the adsorption heat of first layer of CoP and WP samples are larger than that of their condensation heat. So the following BET formula can also express their isotherms:

Va cðp=pÞ ¼ V am ð1  p=pÞ½1 þ ðc  1Þp=p Here the V a represent the adsorption amount under pressure p, V am is the saturated adsorption amount of the monolayer, p is the saturated vapor pressure of the adsorbed liquid at the adsorption temperature, and c is the adsorption constant related to the adsorption heat. It can be obtained from Table 1 that SBET, pore volume and average pore size of CoP is 10 m2/g, 0.05 cm3/g and 19 nm, respectively. Meanwhile, that of WP is 21 m2/g, 0.08 cm3/g and 19 nm, respectively. The dynamics performance of H2 evolution of WP is a little better than that of CoP as shown in Fig. 7A, which may be due to the fact that WP with larger SBET and pore volume can provide more active sites compared to CoP in the same reaction system. 3.4. Optical properties of the photocatalysts The photo-absorption performance of photo-catalysts is an essential prerequisite for insuring more efficient photo-catalytic activity. Fig. 4A displays the UV–Vis diffuse reflectance spectra (DRS) of pure CoP, WP and W-P-Co samples. All three products show similar band gap absorption at 200–800 nm. The band gap absorption in the ultraviolet and near infrared regions imply that electron transitions may be realized at higher and lower energies. And it can be seen clearly from Fig. 4A that all samples exhibit different band gap absorption at the range of 200–800 nm, which indicates that the probability of electron transfer is enhanced significantly under light irradiation. This is quite advantageous for more efficient activity of H2 evolution. Meanwhile, both CoP and WP exhibit stronger optical absorption density, and it continue to increase with the increase of wavelength, implying the reducing in the reflectivity of absorbed light, that is, the light absorbed by the sample is more utilized. This may be an extremely essential reason for the efficient photocatalytic activity of pure CoP and WP. However, the optical absorption density of W-P-Co sample is smaller than CoP and WP, which may be the cause of its decreased activity of hydrogen evolution. All these favorable factors for photo-catalytic activity of H2 evolution may be closely related to

the color (black) of the samples. In addition, the band gap of CoP and WP can be estimated by the intercept of the tangent lines to the plots according to the equation [41]: ahʋ = A(hʋ  Eg)n/2, where a, ʋ, Eg and A represented absorption coefficient, light frequency, bandgap energy, and a constant, respectively. But the band gap of CoP and WP cannot derived by Tauc plot because of extremely strong optical absorption intensity and multiple absorption as shown in Fig. 4B. The band gap energy of pure CoP (Eg = 1.73 eV) is obtained by previous reports [42,49]. However, it can be inferred from Mott-Schottky study that the band gap of WP is close to and greater than 1.23 eV. It is certain that the band gap of CoP and WP is relatively small, which may be a non-negligible cause of they showing high catalytic activity. 3.5. Photo-catalytic H2-evolution property The photo-catalytic hydrogen-evolution performance of the resulting samples are evaluated under visible light irradiation taking 15%(v/v) triethanolamine aqueous solution as the reaction circumstance. Fig. 5A displays the photo-catalytic stability of H2 evolution of pure CoP (10 mg) materials under visible light irradiation for 20 h (per 5 h is executed as a cycle). It can be observed from the Fig. 5A that H2-evolution yield of quantitative CoP has not overt decrease after visible light irradiation for 20 h, implying CoP has fairly good stability in the long-term run. However, the slightly drop at 15–20 h may be caused by superfluous EY added, which can be confirmed by the results of Fig. 5C and D revealed. Meanwhile, this same result also is observed in Fig. 6A. The effect of pH on the activity of H2 evolution of the resulting catalytic material is showed in Figs. 5B and 6B. CoP and WP exhibit different selectivity to their respective reaction environments. CoP and WP have the best catalytic activity under weak alkaline circumstance (pH = 8) and a little strong alkaline environment (pH = 10), respectively. A mild alkaline environment is the most beneficial factor for system of hydrogen evolution, mainly due to a result from the protonation of triethanolamine with the increase of acidity, which lead to a less effective electron donor. And the content of H+ is prominently reduced with the increase of alkalinity, which is unfavorable thermodynamically to H2 evolution of photo-catalyst in the reaction system. Figs. 5C and 6C exhibit the influence of different amount of EY on the catalytic performance of the samples. It can be seen from the figures that EY has remarkable effect on the catalytic activity of hydrogen evolution. The yield of hydrogen production is obviously increased with the amount of EY added in the reaction system. It demonstrate that the amount of EY added cannot satisfy

Fig. 4. (A) UV–Vis diffuse reflectance spectroscopy of CoP, WP and W-P-Co (B) The plot of (ahv)1/2 versus energy for the band gap energy of CoP and WP.

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Fig. 5. (A) The stability test of CoP, (B) pH effect for photocatalytic activity of CoP, (C) the effect of different amount of EY for photocatalytic activity of CoP and (D) Catalytic duration test of CoP.

all the active sites exposed by the catalysts and promote the antenna effect. So it causes an increase of activity and stability of samples. The amount of hydrogen generation is observably decreased with the increase of EY added sequentially, implying that some of the active sites provided by the photocatalysts are covered by redundant EY, and excess dyes can be used as filters for input light, which can cause self-quenching of excited dye molecules, even lead to light loss and dye decomposition [43]. Such proper amount of dyes is very important in dye sensitization system to show the catalytic activity of catalysts. The time of catalytic kinetic duration of quantitative samples (10 mg) are estimated in the same reaction system as shown in Figs. 5D and 6D. It can be observed that 10 mg samples can react continuously for 60 h, and its amount of hydrogen evolution can reach the maximum 601.1 and 483.9 lmol, respectively, which indicates very high stability and a more efficient catalytic activity of CoP and WP. Pure CoP and crude WP can become a candidate having more efficient catalytic performance. In this study, the number of EY molecules is 2.89  105 in the reaction system and the amount of H2 evolution of CoP and WP photocatalysts reach to 601.1 and 483.9 respectively lmol in 60 h of visible light irradiation. Consequently, the calculated TON of dye Eosin Y for H2 evolution is 41.45 and 33.37, respectively. This further confirms the effect of EY on the photocatalytic activity of CoP and WP samples. As shown in Fig. 7A, when CoP and WP are mechanically compounded with the mass ratio of 1:1, the catalytic activity of the composite slightly increased compared to single CoP and WP. However, the photo-catalytic activity of W-P-Co decreased evi-

dently when the ration is changed as shown in Fig. 7B, implying that the combination of CoP and WP with high photo-catalytic activity cannot effectively enhance hydrogen production activity. This may be caused by decreased light absorption density as shown in Fig. 4A. In addition, manual mixing of CoP and WP may lead to more possibilities of leaching, which can be proved via the stability measurement for W-P-Co photocatalyst as shown in Fig. 7C. It can be quite clearly seen from the result of stability test for it that W-PCo sample still shows highly efficient photo-catalytic activity in the first cycle. However, hydrogen production declined sharply n the second cycle and beyond, which is distant compared with the stability of W-P-Co catalyst. This result is fully illustrates that the stability, contrary to expectation, is caused by weak binding forces of CoP and WP, demonstrating that unique photo-catalytic activities CoP and WP. Moreover, they exhibit absolute advantage for the activity of H2 evolution over that of other reported single photocatalysts as displayed in Table 2 and CoP and WP is either co-catalysts or rarely reported in dye-sensitized systems for photocatalytic hydrogen production, implying that the novelty as well as superiority of the reported catalysts. 3.6. Apparent quantum efficiency (AQE) test Fig. 8 display the photo-catalytic H2 evolution of CoP and WP under light irradiation of different wavelengths taking various cutoff filters (k  400, 420, 450, 475, 500, 550 and 600 nm). It can be expressly observed from Fig. 8A that the CoP catalyst can efficiently produce hydrogen gas under visible light irradiation of k  420 nm, and the largest AQE is obtained under visible light

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Fig. 6. (A) The stability test of WP, (B) pH effect for photocatalytic activity of WP, (C) the effect of different amount of EY for photocatalytic activity of WP and (D) Catalytic duration test of WP.

irradiation at 475 nm. These AQE values demonstrate that CoP has better photo-response in the visible region and it can be excited at a lower optical quantum density. Meanwhile, the largest AQE of WP is about 1.1 under visible light irradiation at 475 nm as shown in Fig. 8B. It can be clearly seen that WP has broad absorption range on light compared with CoP, implying relative small band gap of WP and strong light response capability. And the AQE of WP shows remarkably decrease at 550–600 nm, which may be caused by defect of light absorption. In order to design a more efficient catalysts of hydrogen evolution, these results provide a good basis for the modification of CoP and WP.

3.7. The photo-luminescence (PL) experiments In order to deeply further investigate the interaction of electron migration between EY and catalysts for hydrogen evolution, the assynthesized samples are performed by taking steady-state fluorescence spectra and transient-state fluorescence spectra, because it is generally believed that the density of the fluorescent signals can be used to reflect the separation efficiency of charge carriers. And the decrease of peak density implies facilitating electron capture and efficient separation, while growing peak density represent the decreased electron-hole pairs separation. PL spectra of the resulting photo-catalysts are exhibited as shown in Fig. 9A under excitation wavelength 420 nm, and all samples display similar photo-luminescence emission peak at 537 nm. The blazing reduction of PL intensity of CoP and WP indicate intense interactions occur between EY and catalyzers. This interaction is mainly

manifested in the fact that the recombination of electrons and holes is greatly inhibited. Meanwhile, because of the suitable band gap and mesoporous structure of CoP and WP, they can be used as a promising host matrix to the equipped EY molecules or other co-catalysts for more efficient activity of hydrogen evolution. Furthermore, the time-resolved fluorescence decay spectra of as-prepared samples are implemented to further confirm the transfer behavior of photo-produced electrons and holes and investigate the quenching mechanism of excited state EY. It can be evidently seen from the Table 3 that the fluorescence density of all samples is single exponential decay corresponding to dynamic quenching mechanism, which accords with one radiative lifetimes: IðtÞ ¼ A1 et=s1 [44]. And the attenuation rates of radiation or nonradiative transition are defined as c and knc: s = (c + knc)1 [45]. The excited state EY has a lifetime of 0.2899 ns, and the average lifetime is 0.73 ns, 0.004 ns, 0.03 ns respectively after joining CoP, WP and W-P-Co samples. These results demonstrate that an additional nonradiative decay channel may exist between photocatalysts and dye EY, and recombination of the electrons and holes is effectively suppressed. 3.8. Photoelectrochemical experiments (I-T, LSV, EIS and MottSchottky) In order to further study the separation and transfer of photoproduced electrons and holes, the transient photo-current responses of as-synthesized WP and CoP samples are recorded by several on/off cycles under visible light irradiation (Fig. 10A). The

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Fig. 7. H2-evolution performance of W-P-Co.

Table 2 The catalytic activity comparison of pure catalysts under EY-sensitized condition and the composite catalyst with CoP as a co-catalyst. Photocatalyst Pure rGO Pure g-C3N4 Co-Mo-P/g-C3N4 CoP/g-C3N4 2D-2D CoP/g-C3N4 P-Co-N/g-C3N4 CoP/CdS La, Cr:SrTiO3/CoP CdxZn1-xSe/CoP Pure Mo-S Pure Co-S Pure Ni4S3 Pure g-C3N4 Pure Bi7.53Co0.47O11.92 Pure Ni2P Pure MoO3

Co-catalyst

Co-Mo-P CoP CoP/Pt CoP CoP CoP CoP

Light Source

H2 Evolution Rate

Refs.

k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm 300 W xenon arc lamp k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm k > 420 nm

<1.3 mmol/h 1.9 mmol/h 129.3 mmol/h 840 mmol/h CoP > Pt 96.2 mmol/h 2120 mmol/h 198.4 mmol/h 120 mmol/h <15 mmol/h <10 mmol/h 4 mmol/h 2 mmol/h 2 mmol/h 6.9 mmol/h 0.4 mmol/h

[12] [35] [35] [36] [38] [39] [49] [50] [51] [52] [52] [53] [54] [54] [55] [55]

density of transient photo-current responses of WP is the strongest compared with CoP and W-P-Co catalysts under exactly the same test conditions, indicating that the mobility of electrons for WP is more efficient and implying that suppressed recombination of electrons and holes. Meanwhile, it can be seen from the photocurrent responses curves that the density of WP and CoP is more stable than W-P-Co complex, which also imply that the stability of catalytic kinetics of WP and CoP samples. In addition, Fig. 10B displays the polarization curves of the resulting samples having representativeness. The cathodic currents intensity can be

evidently found in the range from 1.0 to 0.4 V vs reversible hydrogen electrode, which is mainly assigned to the hydrogen evolution reaction [46], and the highest current density is observed for the WP electrode, which remarkably demonstrates the fast-speed electron migration at sample interface. The inter-facial charge-migration resistances in the electrolyte solution can be reflected by EIS Nyquist plot. The electron migration process can be performed by the arc at high frequency region. And the smaller the radius of the arc, the smaller the resistance of electron transfer at the interface. The arc of WP representing is

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Fig. 8. AQE of CoP and WP under different wavelengths from 400 to 600 nm.

Fig. 9. (A) Steady-state fluorescence, (B) and transient fluorescence measurements of samples.

Table 3 Decay parameters of EY in TEOA aqueous solution when introduction of CoP, WP and W-P-Co (the excited and emission wavelength are 454 nm and 538 nm). System

EY

CoP

WP

W-P-Co

Lifetime, s (ns) s Rel(%) A Average Lifetime (ns)

0.48 100 0.48

0.73 100 0.73

0.004 100 0.004

0.03 100 0.03

relatively smaller compared with CoP and W-P-Co catalyst, signifying that charge carriers are more easily transferred on the WPelectrode/electrolyte interface. The flat-band potential (Efb) of CoP and WP catalysts are evaluate by Mott–Schottky curves in 0.2 M Na2SO4 electrolyte at the frequency of 100 Hz. Both CoP and WP exhibit positive slopes of E-C2 plots as shown in Fig. 10D, demonstrating that they are n-type semiconductors [47]. The Efb of CoP and WP are 0.75 and 1.07 V versus SCE, respectively. It is generally known that the conduction band potentials (ECB) for n-type semiconductor is more negative about 0.1 or 0.2 V than its Efb [48]. So the ECB of CoP and WP are about ECB = 0.95 and ECB = 1.27 V versus SCE. Consequently, samples corresponding normal hydrogen electrode potentials (ENHE) are obtained by the formula ENHE = ESCE + 0.241 V: ENHE = 0.71 (CoP) and ENHE = 1.03 V (WP), respectively. The band gap (Eg) of CoP and WP are Eg = 1.74 and Eg = 1.43 V. The valence band potentials (EVB) can be calculated by the equation EVB = ECB + Eg. The EVB of CoP and WP are EVB = 0.79 V and

EVB = 0.16 V, respectively. It can be observed from Fig. 10D that the Efb of WP is more negative than that of CoP, signifying that WP has a higher Fermi level. And from the side of solution, the sample can more easily trap electrons on conduction band. Thermodynamically speaking, the catalyst has a stronger reduced ability after lighting. For aqueous systems, the photo-catalyst is more likely to generate photo-electrons, which is in favor of activity of hydrogen evolution. 3.9. Photo-catalytic mechanism of H2 evolution Based on the above much researches, the mechanism of EYsensitized WP and CoP for hydrogen evolution under visible light irradiation is proposed as shown in Fig. 11. From the point of view of H2 evolutionary kinetics, both pure CoP and WP exhibit efficient catalytic activity. On the one hand, relatively low band-gap makes CoP and WP catalysts more easily excited by visible light. On the other hand, after samples and EY are stimulated simultaneously by light, the electrons jumping from the valence band of photo-catalyst to the conduction band and that from HOMO (highest-occupied molecular orbital) to LUMO (lowestunoccupied molecular orbital) of EY molecules. In detail, the EY molecules are adsorbed to the active sites of CoP and WP and excited to form EY1* after light irradiation. Then EY3*, more stable than EY1*, are shaped by ISC. Whereafter EY- having reducing power are produced with reduction and quenching of EY3*. The potential of the HOMO levels of EY is 5.60 eV and LUMO levels

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297

Fig. 10. (A) Photocurrent response of CoP, WP and W-P-Co (B) LSV curses of CoP, WP and W-P-Co (C) The electrochemical impedance spectroscopy (EIS) of CoP, WP and W-PCo (D) Mott-Schottky curses of CoP and WP.

Fig. 11. H2 production mechanism of EY-sensitized samples under visible light illumination.

of EY* is 3.45 eV [22], which is more negative compared with the ECB of CoP and WP catalysts, which strongly drives the electrons transfer from excited EY to the conduction band of CoP and WP samples (duo to different energy levels are formed between EY and catalysts). Meanwhile, the holes having oxidation capacity are consumed by triethanolamine (15%v/v TEOA) used as sacrificial reagent, which leads to the inability of the excited electrons to recombine with the holes, thus the more excited electrons have more opportunities to participate in the water splitting reaction.

4. Conclusion To sum up, pure CoP and WP with the dominant and excellent performance for highly efficient hydrogen evolution are successfully synthesized and revealed in detail. It can be found from the kinetics of hydrogen evolution that they exhibit very excellent photo-catalytic property for water splitting reaction and they show unique photo-catalytic performance compared with other same work reported under visible light irradiation. And the 10 mg of

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CoP and WP can be continuously involved in the water splitting reaction for 60 h, with hydrogen production reaching 601.1 and 483.9 lmol respectively, which shows again their excellent catalytic property and photocatalytic stability. In addition, it can be obtained by several studies of characterization that CoP and WP has low band-gap energy and strong optical absorption properties, which satisfies the conditions desired for achieving highly photocatalytic activity.

Author contributions Yanbing Li, Zhiliang Jin and Guorong Wang conceived and designed the experiments; Yanbing Li performed the experiments; Zhiliang Jin, Hai Liu, Haiyu Wang and Yupeng Zhang contributed reagents/materials and analysis tools; Yanbing Li wrote the paper. Conflicts of interest The authors declare that they have no competing interests. Acknowledgements This work was financially supported by the Chinese National Natural Science Foundation (21862002 and 41663012), the project of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, the new technology and system for clean energy catalytic production, Major scientific project of North Minzu University (ZDZX201803), the key scientific research projects of North Minzu University (No. 2017KJ16) and the 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. References [1] Lihua Lin, Yu Zhiyang, Xinchen Wang, Crystalline carbon nitride semiconductors for photocatalytic water splitting, Angew. Chem. Int. Ed. (2018). [2] Min Zheng, Xiaohu Cao, Yong Ding, Tian Tian, Junqi Lin, Boosting photocatalytic water oxidation achieved by BiVO4 coupled with ironcontaining polyoxometalate: analysis the true catalyst, J. Catal. 363 (2018) 109–116. [3] Yu. Jiaguo, Yang Hai, Bei Cheng, Enhanced photocatalytic H2-production activity of TiO2 by Ni(OH)2 cluster modification, J. Phys. Chem. C 115 (2011) 4953–4958. [4] Xichen Zhou, Zhen Liu, Yifan Wang, Yong Ding, Facet effect of Co3O4 nanocrystals on visible-light driven water oxidation, Appl. Catal. B 237 (2018) 74–84. [5] Fangyuan Song, Yong Ding, Baochun Ma, Changming Wang, Qiang Wang, Du. Xiaoqiang, Fu. Shao, Jie Song, K7[CoIIICoII(H2O)W11O39]: a molecular mixedvalence keggin polyoxometalate catalyst of high stability and efficiency for visible light-driven water oxidation, Energy Environ. Sci. 6 (2013) 1170–1184. [6] Min Zheng, Yong Ding, Yu. Li, Du. Xiaoqiang, Yukun Zhao, In situ grown pristine cobalt sulfide as bifunctional photocatalyst for hydrogen and oxygen evolution, Adv. Funct. Mater. 27 (2017) 1605846. [7] Jing Zhang, Xu. Qian, Zhaochi Feng, Meijun Li, Can Li, Importance of the relationship between surface phases and photocatalytic activity of TiO2, Angew. Chem. Int. Ed. 47 (2010) 1766–1769. [8] Jie Chen, Shaohua Shen, Penghui Guo, Meng Wang, Wu. Po, Xixi Wang, Liejin Guo, In-situ reduction synthesis of nano-sized Cu2O particles modifying gC3N4 for enhanced photocatalytic hydrogen production, Appl. Catal. B 152–153 (2014) 335–341. [9] Yu. Huijun, Run Shi, Tierui Zhang, Alkali-assisted synthesis of nitrogen deficient graphitic carbon nitride with tunable band structures for efficient visible light-driven hydrogen evolution, Adv. Mater. 29 (2017) 1605148. [10] Qizhao Wang, Yanbiao Shi, Du. Zhongyi, Jijuan He, Junbo Zhong, Lianchun Zhao, Houde She, Gang Liu, Su. Bitao, Synthesis of rod-like g-C3N4/ZnS composites with superior photocatalytic activities for degradation methyl orange, Eur. J. Inorg. Chem. 24 (2015) 4108–4115. [11] Baojun Ma, Xu. Haojie, Keying Lin, Jie Li, Haijuan Zhan, Wanyi Liu, Can Li, Mo2C as non-noble metal co-catalyst in Mo2C/CdS composite for enhanced photocatalytic H2 evolution under visible light irradiation, Cheminform 9 (2016) 820–824.

[12] Weiying Zhang, Yuexiang Li, Shaoqin Peng, Template-free synthesis of hollow Ni/reduced graphene oxide composite for efficient H2 evolution, J. Mater. Chem. A 5 (2017) 13072–13078. [13] Alejandra Garcia, Josep Albero, Hermenegildo Garcia, Multilayer n-doped graphene films as photoelectrodes for H2 evolution, Chemphotochem 1 (2017) 9. [14] Ameerunisha Begum, Aasif Hassan Sheikh,, Golam Moula, Sabyasachi Sarkar, Fe4S4 cubane type cluster immobilized on a graphene support: a high performance H2 evolution catalysis in acidic water, Sci. Rep. 7 (2017) 16948. [15] Weiying Zhang, Yuexiang Li, Shaoqin Peng, Xiang Cai, Enhancement of photocatalytic H2 evolution of eosin Y-sensitized reduced graphene oxide through a simple photoreaction, Beilstein J. Nanotechnol. 5 (2014) 801–811. [16] Yan Zhang, Jingwei Huang, Yong Ding, Porous Co3O4/CuO hollow polyhedral nanocages derived from metal-organic frameworks with heterojunctions as efficient photocatalytic water oxidation catalysts, Appl. Catal. B 198 (2016) 447–456. [17] W. Justin Youngblood, Seung-Hyun Anna Lee, Kazuhiko Maeda, Thomas E. Mallouk, Visible light water splitting using dye-sensitized oxide semiconductors, Acc. Chem. Res. 42 (12) (2009) 1966–1973. [18] Ryu Abe, Koujirou Hara, Kzuhiro Sayama, Kazunari Domen, Hironori Arakawa, Steady hydrogen evolution from water on Eosin Y-fixed TiO2 photocatalyst using a silane-coupling reagent under visible light irradiation, J. Photochem. Photobiol., A 137 (2000) 63–69. [19] Xing Liu, Yuexiang Li, Shaoqin Peng, Lu. Gongxuan, Shuben Li, Photosensitization of SiW11O839-modified TiO2 by Eosin Y for stable visiblelight H2 generation, Int. J. Hydrogen Energy 38 (2013) 11709–11719. [20] Yuexiang Li, Guo Miaomiao, Shaoqin Peng, Lu Gongxuan, Shuben Li, Formation of multilayer-Eosin Y-sensitized TiO2 via Fe3+ coupling for efficient visiblelight photocatalytic hydrogen evolution, Int. J. Hydrogen Energy 34 (2009) 5629–5636. [21] Qiuye Li, Liang Chen, Lu. Gongxuan, Visible-light-induced photocatalytic hydrogen generation on dye-sensitized multiwalled carbon nanotube/Pt catalyst, J. Phys. Chem. C 111 (30) (2007) 11494–11499. [22] Shixiong Min, Lu. Gongxuan, Dye-sensitized reduced graphene oxide photocatalysts for highly efficient visible-light-driven water reduction, J. Phys. Chem. C 115 (28) (2011) 13938–13945. [23] Zhixiao Qin, Menglong Wang, Rui Li, Yubin Chen, Novel Cu3P/g-C3N4 p-n heterojunction photocatalysts for solar hydrogen generation, Sci. China Mater. 61 (2018) 861–868. [24] Zijun Sun, Qiudi Yue, Jingshi Li, Xu. Jun, Huafei Zheng, Du. Pingwu, Copper phosphide modified cadmium sulfide nanorods as a novel p-n heterojunction for highly efficient visible-light-driven hydrogen production in water, J. Mater. Chem. A 3 (2015) 10243–10247. [25] Ping Ye, Xinling Liu, James Iocozzia, Yupeng Yuan, Gu. Lina, Xu. Gengsheng, Zhiqun Lin, A highly stable non-noble metal Ni2P co-catalyst for increased H2 generation by g-C3N4 under visible light irradiation, J. Mater. Chem. A 5 (2017) 8493–8498. [26] Junfang Wang, Peifang Wang, Jun Hou, Jin Qian, Chao Wanga, Yanhui Ao, In situ surface engineering of ultrafine Ni2P nanoparticles on cadmium sulfide for robust hydrogen evolution, Catal. Sci. Technol. 8 (2018) 5406–5415. [27] He Li, Xiaoqing Yan, Bo Lin, Mengyang Xia, Jinjia Wei, Bolun Yang, Guidong Yang, Controllable spatial effect acting on photo-induced CdS@CoP@SiO2 ballinball nano-photoreactor for enhancing hydrogen evolution, Nano Energy 47 (2018) 481–493. [28] Xiulin Yang, Lu. Ang-Yu, Yihan Zhu, Mohamed Nejib Hedhili, Shixiong Min, Kuo-Wei Huang, Yu Han, Lain-Jong Li, CoP nanosheet assembly grown on carbon cloth: a highly efficient electrocatalyst for hydrogen generation, Nano Energy 15 (2015) 634–641. [29] Xu. You, Wu. Rui, Jingfang Zhang, Yanmei Shia, Bin Zhang, Anion-exchange synthesis of nanoporous FeP nanosheets as electrocatalysts for hydrogen evolution reaction, Chem. Commun. 49 (2013) 6656–6658. [30] Peng Xiao, Mahasin Alam Sk, Larissa Thia, Xiaoming Ge, Rern Jern Lim, JingYuan Wang, Kok Hwa Lim, Xin Wang, Molybdenum phosphide as an efficient electrocatalyst for the hydrogen evolution reaction, Energy Environ. Sci. 7 (2014) 2624–2629. [31] Joshua M. McEnaney, J. Chance Crompton, Juan F. Callejas, Eric J. Popczun, Carlos G. Read, Nathan S. Lewis, Raymond E. Schaak, Electrocatalytic hydrogen evolution using amorphous tungsten phosphide nanoparticles, Chem. Commun. 50 (2014) 11026–11028. [32] Yan Zhang, Huijuan Zhang, Yangyang Feng, Yu Li, Liu Wang, Unique Fe2P nanoparticles enveloped in sandwichlike graphited carbon sheets as excellent hydrogen evolution reaction catalyst and lithium-ion battery anode, ACS Appl. Mater. Interface 7 (48) (2015) 26684–26690. [33] Haiyu Wang, Zhiliang Jin, Design and synthesis of polymeric carbon nitride@zeolitic imidazolate frameworks (CoWS) semiconductor junction nanowires for efficient photocatalytic hydrogen evolution, New J. Chem. 42 (2018) 17396–17406. [34] Hao Yang, Zhiliang Jin, Hu. Hongyan, Yingpu Bi, Lu. Gongxuan, Ni-Mo-S nanoparticles modified graphitic C3N4 for efficient hydrogen evolution, Appl. Surf. Sci. 427 (2018) 587–597. [35] Yupeng Zhang, Zhiliang Jin, Su. Yanfang, Guorong Wang, Bingzhen Ma, Co-MoP modulation g-C3N4 photocatalyst for charge separation and electron transfer routes, Mole. Catal. 462 (2019) 46–55. [36] Hui Zhao, PingPing Jiang, Wen Cai, CoP cocatalyst decorated graphitic C3N4 with enhanced and stable photocatalytic H2 evolution activity from water under visible light irradiation, Chem. Asian J. 12 (2017) 361–365.

Y. Li et al. / Journal of Colloid and Interface Science 541 (2019) 287–299 [37] Rongchen Shen, Jun Xie, Hongdan Zhang, Aiping Zhang, Xiaobo Chen, Xin Li, 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, ACS Sustain. Chem. Eng. 6 (2018) 816–826. [38] Xiaojing Wang, Xiao Tian, Yingjie Sun, Jiayu Zhu, Fa-tang Li, Mu. Huiying, Jun Zhao, Enhanced schottky effect of 2D–2D CoP/g-C3N4 interface for boosting photocatalytic H2 evolution, Nanoscale 10 (2018) 12315–12321. [39] Chunmei Li, Du. Yonghua, Danping Wang, Shengming Yin, Tu. Wenguang, Zhong Chen, Markus Kraft, Gang Chen, Xu. Rong, Unique P-Co-N surface bonding states constructed on g-C3N4 nanosheets for drastically enhanced photocatalytic activity of H2 evolution, Adv. Funct. Mater. 27 (2017) 1604328. [40] Kelin He, Jun Xie, Xingyi Luo, Jiuqing Wen, Song Ma, Xin Li, Yueping Fang, Xiangchao Zhang, Enhanced visible light photocatalytic H2 production over Zscheme g-C3N4 nansheets/WO3 nanorods nanocomposites loaded with Ni (OH)x cocatalysts, Chin. J. Catal. 38 (2017) 240–252. [41] Jing Jiang, Shaowen Cao, Hu. Chenglong, Chunhua Chen, A comparison study of alkali metal-doped g-C3N4 for visible-light photocatalytic hydrogen evolution, Chin. J. Catal. 38 (2017) 1981–1989. [42] Jingqi Tian, Qian Liu, Abdullah M. Asiri, Xuping Sun, Self-supported nanoporous cobalt phosphide nanowire arrays: an efficient 3D hydrogenevolving cathode over the wide range of pH 0–14, J. Am. Chem. Soc. 136 (2014) 7587–7590. [43] Xiaojie Zhang, Zhiliang Jin, Yuexiang Li, Shuben Li, Lu. Gongxuan, Efficient photocatalytic hydrogen evolution from water without an electron mediator over Ptrose bengal catalysts, J. Phys. Chem. C 113 (2009) 2630–2635. [44] Xu. Quanlong, Bicheng Zhu, Chuanjia Jiang, Bei Cheng, Yu. Jiaguo, Constructing 2D/2D Fe2O3/g-C3N4 direct Z-scheme photocatalysts with enhanced H2 generation performance, Solar Rrl. 2 (2018) 1800006. [45] Fu. Junwei, Chuanbiao Bie, Bei Cheng, Chuanjia Jiang, Yu. Jiaguo, Hollow CoSx polyhedrons act as high-efficiency cocatalyst for enhancing the photocatalytic hydrogen generation of g-C3N4, ACS Sustain. Chem. Eng. 6 (2018) 2767–2779.

299

[46] Pengfei Xia, Mingjin Liu, Bei Cheng, Yu. Jiaguo, Liuyang Zhang, Dopamine modified g-C3N4 and its enhanced visible-light photocatalytic H2-production activity, ACS Sustain. Chem. Eng. 6 (2018) 8945–8953. [47] Fu. Junwei, Xu. Quanlong, Jingxiang Low, Chuanjia Jiang, Yu. Jiaguo, Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst, Appl. Catal. B 243 (2019) 556–565. [48] Xuqiang Hao, Jun Zhou, Zhiwei Cui, Yicong Wang, Ying Wang, Zhigang Zou, Znvacancy mediated electron-hole separation in ZnS/g-C3N4 heterojunction for efficient visible-light photocatalytic hydrogen production, Appl. Catal. B 229 (2018) 41–51. [49] Dan Zhao, Bing Sun, Xiangqing Li, Lixia Qin, Shizhao Kang, Dong Wang, Promoting visible light-driven hydrogen evolution over CdS nanorods using earth-abundant CoP as a cocatalyst, RSC Adv. 6 (2016) 33120. [50] Pengfei Tan, Anquan Zhu, Yi Liu, Yongjin Ma, Wenwen Liu, Hao Cui, Jun Pan, Insights into the efficient charge separated and transferred La, Cr-codoped SrTiO3 modified with CoP as noble-metal-free co-catalyst for superior visiblelight driven photocatalytic hydrogen generation, Inorg. Chem. Front. 5 (2018) 679–686. [51] Bocheng Qiu, Qiaohong Zhu, Mingyang Xing, Jinlong Zhang, A robust and efficient catalyst of CdxZn1-xSe motivated by the CoP for the photocatalytic hydrogen evolution under the sunlight irradiation, Chem. Commun. 53 (2017) 897–900. [52] Duanduan Liu, Zhiliang Jin, Yingpu Bi, Charge transmission channel construction between MOF and rGO by means of Co-Mo-S modification, Catal. Sci. Technol. 7 (2017) 4478–4488. [53] Duanduan Liu, Zhiliang Jin, Yongke Zhang, Guorong Wang, Bingzhen Ma, Light harvesting and charge management by Ni4S3 modified MOF and rGO in the process of photocatalysis, J. Colloid Interface Sci. 529 (2018) 44–52. [54] Duanduan Liu, Zhiliang Jin, Hongxuan Li, Lu. Gongxuan, Modulation of the excited-electron recombination process by introduce g-C3N4 on Bi-based bimetallic oxides photocatalyst, Appl. Surf. Sci. 423 (2017) 255–265. [55] Yanbing Li, Haiyu Wang, Yupeng Zhang, Zhiliang Jin, Effect of electron-hole separation in MoO3@Ni2P hybrid nanocomposite as highly efficient metal-free photocatalyst for H2 production, J. Colloid Interface Sci. 537 (2019) 629–639.