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Enhanced photocatalytic hydrogen evolution over semi-crystalline tungsten phosphide
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 8 4 8 e2 6 8 6 2
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Enhanced photocatalytic hydrogen evolution over semi-crystalline tungsten phosphide Zhiliang Jin a,c,d,*, Qiyan Jian a,c,d,**, Qingjie Guo b,*** a
School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan, 750021, PR China c Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu University, Yinchuan 750021, PR China d Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu University, Yinchuan 750021, PR China b
highlights The tungsten phosphide (SC-WP) has a unique semi-crystalline structure and excellent optical properties. The SC-WP cocatalyst improved the visible light absorption ability and the utilization of absorbed light. Highly efficient and stable photocatalytic hydrogen evolution systems were constructed.
article info
abstract
Article history:
Increasing the separation efficiency and transfer rate of photogenerated charges is the
Received 2 June 2019
dominant factor for improving photocatalytic activity. Herein, we successfully prepared
Received in revised form
semi-crystalline WP (SC-WP) with good optical properties and as a cocatalyst to modify CdS
3 August 2019
nanorods (CdS NRs) to construct SC-WP/CdS (PD) composite catalyst by simple electrostatic
Accepted 10 August 2019
self-assembly method for photocatalytic hydrogen evolution. Two high-efficiency and
Available online 20 September 2019
stable photocatalytic hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10 vol% lactic acid solution as sacrificial agents, respectively. Surpris-
Keywords:
ingly, the maximum photocatalytic H2 production rate of 15446.21 mmol h1 g1 is obtained
Tungsten phosphide
over 10PD composite, which is 10.58 times greater than that of pure CdS. The improved
CdS NRs
photocatalytic activity can be attributed to the fact that the SC-WP nanoparticles provides
Semi-crystalline
a large number of exposed active sites on the surface of CdS for hydrogen evolution re-
Photocatalytic hydrogen evolution
action, which can efficiently capture photogenerated electrons from CdS nanorods and promotes the transport and separation of light-induced charges. And the introduction of SC-WP nanoparticles with excellent optical properties can efficiently improve the visible light absorption range and the utilization rate of the absorbed light of the PD composite. In addition, the SC-WP nanoparticles show semi-crystalline state, which is also conducive to enhancing the photocatalytic activity. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China. ** Corresponding author. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China. *** Corresponding author. E-mail addresses: [email protected] (Z. Jin), [email protected] (Q. Jian), [email protected] (Q. Guo). https://doi.org/10.1016/j.ijhydene.2019.08.080 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction As a kind of green energy with good combustion performance, convenient transportation and high utilization rate, hydrogen energy has stood out from a variety of energy sources and is recognized as the best choice to replace fossil fuels [1,2]. However, the mature industrial methods of hydrogen production now available are demanding, costly and require the consumption of fossil fuels, which cause damage to the environment [3e5]. In recent decades, as one of the effective ways to convert solar energy into chemical energy, semiconductor photocatalytic decomposition of water to hydrogen has been extensively studied [6e9]. Therefore, a large number of semiconductor materials have been developed and applied in the research field of photocatalytic technology, such as TiO2 [10,11], WO3 [12,13], C3N4 [14,15], ZnS [16,17], WS2 [18,19], MoSe [20] and so on. At present, due to the problem that the photocatalytic material still has less light absorption, poor stability of the system itself, too high or too low band edge, and the like, it can not meet the requirements of industrial production. Therefore, further research is needed to develop efficient photocatalytic decomposition of water and hydrogen evolution systems and photocatalysts [21]. Due to their excellent photocatalytic activities, nonlinear optical properties and photoelectric conversion properties, metal sulfides have been widely used in photocatalytic degradation of organic pollutants, photocatalytic reduction of CO2, photocatalytic decomposition of water to hydrogen and other fields. CdS has a band gap of approximately 2.4 eV and excellent photocatalytic performance in the visible range [22e29]. However, due to the severe photo-corrosion phenomenon of CdS in photocatalytic decomposition of water, the hydrogen evolution efficiency of CdS is low and the stability is poor, which greatly limits the photocatalytic activity of CdS [30e32]. Tungsten phosphide WP has attracted extensive attention from researchers at home and abroad because of its excellent hydrotreating activity and excellent sulfur poisoning resistance. It has been gradually applied to hydrodesulfurization, hydrodenitrogenation, electrocatalysis, photocatalysis, etc [33]. In recent years, WP has become a research hotspot, but it has few applications in photocatalytic decomposition of water for hydrogen evolution. In the previous reports, the preparation of WP is complicated, the preparation conditions are harsh, the equipment requirements are high, and the cost is large [34]. Our research group simplified the preparation of WP, introduced it into photocatalytic decomposition of water and hydrogen evolution system, and did a lot of research, and found that it exhibits high activity and stability, and has great research value. In this work, we successfully prepared semi-crystalline WP (SC-WP) and introduced SC-WP as a cocatalyst to modify CdS NRs. Two different high-efficiency and stable composite photocatalytic decomposition hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10 vol% lactic acid solution as sacrificial reagents. In addition, the photocatalytic activity of the composite catalyst in this
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work was compared with that of other CdS-based photocatalysts, and the results show that this work has obvious advantages. As SC-WP nanoparticles provide a large number of exposed active sites for hydrogen evolution reaction, these active sites can capture photogenerated electrons on CdS nanorods quickly and participate in the rapid evolution of hydrogen, promote the transmission and separation of photogenerated charges, inhibit the recombination of photogenerated electrons and hole pairs, thereby improving the utilization of photogenerated electrons generated by visible light excitation. Moreover, because of the excellent optical properties of SC-WP nanoparticles, the visible light absorption rate and the utilization rate of absorbed light of the composite catalysts are enhanced, and the SC-WP nanoparticles are semi-crystalline, which is conducive to improving the photocatalytic activity.
Experimental section Catalysts preparation All chemicals are reagent grade and used without further purification.
Preparation of CdS CdS NRs was synthesized by solvothermal method. 2.3750 g CdCl2$2.5H2O and 2.3750 g NH2CSNH2 were dissolved in 60 mL ethylenediamine, then held in an autoclave at 160 C for 36 h, and the product was washed with deionized water and absolute ethanol and dried at 60 C.
Preparation of SC-WP The SC-WP was synthesized via calcination. Typically, dissolve 3.30 g Na2WO4$2H2O and 4.24 g NaPO2H2$H2O in distilled water and mix well, then recrystallize the above solution, and finally, the precursor powder formed by recrystallization was calcined at 300 C for 30 min under N2 protection, and the obtained product was washed with distilled water and dried at 60 C.
Preparation of SC-WP/CdS composites SC-WP/CdS (PD) composites were prepared by simple electrostatic self-assembly method. Typically, 0.15 g of CdS NRs and calculated amount of SC-WP sample were weighed, and the composite catalysts with SC-WP content of 5%, 10%, 15% and 20% were prepared by self-assembly method, and recorded as XPD (X ¼ 5, 10, 15, 20). The specific steps for preparing XPD composite catalyst are shown in Fig. 1.
Preparation of working electrodes Firstly, 5 mg sample powder was added to 200 mL of ethanol containing 40 mL l5% Nafion solution and dispersed by ultrasonic treatment to form a paste. Then the paste catalyst was dripped onto the surface of the FTO glass (1 cm2). Finally, the FTO working electrode loaded with catalyst was obtained by drying at room temperature.
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Characterization of composite photocatalyst X-ray diffraction (XRD) spectra of all samples were obtained (Model Rigaku Rint-2000 CuK alpha X-ray diffractometer, Testing conditions: the tube current and tube voltage is set at 40 mA and 40 kV respectively, and at a scanning rate of 5 /min in the 10e80 2q range). The morphology of the samples was observed by scanning electron microscopy (SEM, JSM-6701F JEOL, acceleration voltage 5000 V) and transmission electron microscopy (TEM, JEM1200EX JEOL, acceleration voltage 10000 V). The surface chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The BET specific surface area of the sample was determined by the nitrogen adsorption-desorption isotherm of the ASAP 2020 M analyzer at 77 K. Ultravioletevisible diffuse reflectance spectroscopy (UVevis DRS) used BaSO4 as a reference for recording solid powders on a UV-2550 spectrophotometer. The photoluminescence (PL, containing steady-state fluorescence and transient fluorescence) spectra of the samples were recorded on a FLUOROMAX-4 fluorescence spectrometer at room temperature. The photoelectrochemical tests (containing Chronoamperometry, Linear Scan Voltammetry, Potentiostatic EIS and Mott-Schottky) are performed on an VersaStat4-400 electrochemical workstation in a standard three-electrode system with platinum plate and calomel electrode as counter electrode and reference electrode, respectively. Meanwhile, 0.2 M Na2SO4 aqueous solution and 300 W xenon lamp used as the electrolytic solution and a solar simulator illumination source (XMM65e63, C16). And I-T and LSV curves are recorded at a stationary bias of 0.2 V vs. SCE. EIS curves of samples are recorded at a stationary bias of 0.2 V with AC amplitude of 10 mV and a frequency range of 10,000e1 Hz. The Mott-Schottky curves are recorded from 1.5e1.5 V with AC amplitude of 10 mV at a frequency of 1 KHz.
Photocatalytic hydrogen production kinetics test Hydrogen evolution activity of photocatalyst was tested under PCX-50C multi-channel photochemical reaction system (5 W LED white light, 420 nm). Specifically, 10 mg catalyst was added to 62 mL quartz reaction bottle and 30 mL sacrificial
agent (1.0 M ammonium sulfite solution or 10 vol% lactic acid solution) was added. After ultrasonic dispersion, the air in the bottle was removed by nitrogen gas. The hydrogen produced was detected regularly by using Tianmei GC 7900 gas chromatograph (TCD, 13X column) with N2 as carrier gas (once every 0.5 h for a total time of 5 h). In addition, the apparent quantum efficiency (AQE) of 10PD in the range of 400e600 nm was measured and calculated. The formula is as follows: AQE ¼
2 the number of evolved hydrogen molecules 100% the number of incident photon
Results and discussion Crystal structure and morphology To understand the bulk structure of the prepared samples, powder X-ray diffraction (XRD) measurements were performed on pure CdS, pure SC-WP, 5PD, 10PD, 15PD and 20PD samples. Fig. 2(a) shows the XRD pattern of pure SC-WP (scanning speed: 1 /min and 5 /min), it can be observed that three characteristic diffraction peaks appear at 31.05, 42.84 and 44.60, corresponding to the (011), (202) and (211) crystal planes of WP (PDF#29e1364), respectively. In addition, it can also be observed that very weak diffraction peaks correspond to the (103), (114), (411) and (123) crystal faces of WP, respectively. The characteristics of XRD diffraction peak of SC-WP: having arched sharp peaks, the width of the peak is wide, and the intensity of the peak is weak. Indicating that the prepared SC-WP sample is semi-crystalline. The characteristics of the XRD pattern of SC-WP are consistent with the characteristics of the XRD pattern in the related reports of semi-crystalline materials [35]. It can be seen from Fig. 2(b) that the characteristic diffraction peaks of pure CdS, 5PD, 10PD, 15PD and 20PD samples belong to CdS, and the composite catalysts have no characteristic peaks of SC-WP. This is because the content of SC-WP is relatively small and the dispersion is high [36]. At the same time, we can observe that the diffraction peak intensity of the composite catalyst decreases as the content of SC-WP increases, which can be attributed to the shielding effect of SC-WP, because the SC-WP
Fig. 1 e Schematic diagram of the steps for preparing XPD composites.
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Fig. 2 e XRD patterns of (a) pure SC-WP at different scanning rates and (b) pure CdS, 5PD, 10PD, 15PD and 20PD.
is loaded on the surface of CdS, and the incident and diffracted X-rays are weakened [37]. In order to further understand the morphology and structure of the samples, Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) were used for characterization. Fig. 3(a) and (b) are SEM images of pure SC-WP, and it can be seen that the SC-WP sample shows the state of the aggregated nanoparticles with different particle sizes.
Fig. 3(c) shows the HRTEM image of the pure SC-WP at a resolution of 2 nm, it can be observed that most of the regions in the HRTEM image have no lattice fringe, and there are some lattice fringe in a small part of the regions, and the d spacing value is 0.289 nm, corresponding to the plane (011) of SC-WP. The SAED pattern of pure SC-WP is shown in Fig. 3(d), and the d spacing values calculated from the pattern correspond to the (011) and (202) planes of SC-WP. Fig. 3 (e) and (f) are the
Fig. 3 e SEM images of (a), (b) pure SC-WP and (e) 10PD, (c) HRTEM imags of pure SC-WP, (d) the SAED pattern of pure SC-WP, (f) TEM images, (g) HRTEM imags of 10PD and (h) EDX images and element mapping for 10PD.
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Fig. 4 e XPS spectra of (a) W 4f, (b) P 2p, (c) Cd 3d and (d) S 2p.
SEM and TEM images of 10PD composite samples respectively. From Fig. 3 (e), it can be clearly observed that SC-WP nanoparticles (as shown in the red circles marker) are loaded uniformly on CdS nanorods. TEM images of 10PD composite shows that some black spots (as shown in the red circles marker) are evenly distributed on CdS nanorods. These black spots are SC-WP nanoparticles. The results of SEM and TEM images are consistent. Fig. 3(g) is an HRTEM image of a 10PD composite sample. Only one type of lattice fringe was observed in the entire HRTEM image with a lattice spacing of 0.316 nm, corresponding to the (101) crystal plane of CdS. In addition, a region without lattice fringes (as shown in the area marked by the red curve) can be clearly observed in the HRTEM image, and we speculate that this region belongs to SC-WP nanoparticles. At the same time, the presence of Cd, S, W and P elements in 10PD composite catalyst can be clearly observed from Fig. 3 (h), and the distribution of these elements is uniform, which is consistent with the results of scanning electron microscopy and transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) can qualitatively analyze the elemental composition, chemical state and molecular structure of the sample surface, which is an important characterization method for analyzing materials. Fig. 4 shows the XPS high resolution spectrum of elements in the 10PD composite photocatalyst. As we observed form Fig. 4 (a), the W
4f shows two good resolution spin orbital splitting peaks at 37.52 and 35.39 eV, corresponding to the characteristic peak of W 4f5/2 and W 4f7/2, respectively. The spin energy separation is 2.23 eV, which is consistent with the W6þ in the previous reports [11]. As can be seen from Fig. 4(b), the P 2p exhibits a binding energy characteristic peak belonging to P2p1/2 at 133.07 eV [11,38]. As shown in Fig. 4(c), the Cd 3d shows two characteristic peaks at 411.53 and 404.77 eV, which are attributed to the characteristic peaks of Cd 3d5/2 and Cd 3d3/2, respectively. It proves that there are Cd2þ in the samples. Moreover, the spin orbit separation of 6.76 eV between Cd 3d5/ 2þ is present on the 2 and Cd 3d3/2 further indicates that the Cd surface of 10PDcomposite catalyst [39,40]. In Fig. 4(d), the binding energies of S 2p at 162.22 and 161.11 eV correspond to the characteristic peaks of the S 2p1/2 and S 2p3/2 spin orbital components of S2, respectively, indicating that the S element mainly exists in the form of S2 on the surface of 10PD sample [41].
BET characterization The Brunauer Emmett Teller specific surface areas (SBET) and the pore diameter distribution of pure CdS and 10PD were determined via nitrogen adsorption-desorption isotherms at 77 K, and the BET equation is shown in formula (1):
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Table 1 e SBET, Pore volume and Average pore size for CdS and10PD. Samples SBET/(m2/ g) CdS 10PD
Pore volume/ (cm3/g)
Average pore size/ nm
0.18 0.17
26 24
30 31
Va cðp=p* Þ a ¼ * Vm ð1 p=p Þf1 þ ðc 1Þp=p* g Va the absorption amount under pressure p; Vam the saturated adsorption capacity of monolayer;
(1)
p* the saturated vapor pressure; c Adsorption constant; The results are shown in Table 1 and Fig. 5. It can be seen from Fig. 5 (a) that the isotherms of CdS and 10PD samples are IV-type curves and have H3 hysteresis loops, which is attributed to the capillary condensation of mesoporous materials in the process of multilayer adsorption [42,43]. It can be observed from Fig. 5(b) that the pore sizes of CdS and 10PD samples are widely distributed between 2 and 50 nm, and only a small number of pore sizes are distributed in the range of more than 50 nm or less than 2 nm, and the average pore sizes of CdS and 10PD samples are 26 and 24 nm, respectively (as shown in Table 1). The above results show that there are a large number of mesopores in the samples, which is consistent with the results in Fig. 5 (a). Compared with CdS, the SBET of the 10PD sample increased slightly, and the pore volume decreased slightly. This can be attributed to the successful coating of SCWP on the surface of CdS, which is beneficial to provide more active sites for photocatalytic decomposition of water.
Optical properties of the photocatalysts UVevis DRS is an important characterization for measuring the optical properties of semiconductor materials. As is known to all, the optical properties of semiconductor materials have some influence on the activity of photocatalytic decomposition of water for hydrogen evolution [44]. Fig. 6(a)
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shows the UVevis DRS of pure SC-WP, CdS and XPD samples. It can be clearly observed that pure SC-WP has a strong optical absorption density, and its optical absorption density increases with the increase of wavelength, that is to say, the utilization of light absorbed by SC-WP is high. The excellent optical properties of SC-WP are largely due to its black color. Compared with pure CdS, the light absorption density of XPD composite catalyst is significantly increased and increases with the increase of SC-WP content. This may be because the color of the XPD composite samples deepens as the SC-WP content increases (as shown in Fig. 6(c) physical drawings of samples). The characterization results are not in perfect agreement with the hydrogen evolution activity test results. This indicates that the optical properties of the photocatalyst have a certain influence on its photocatalytic activity, but it is not the dominant factor determining the photocatalytic activity. In addition, the UVevis DRS of SC-WP-coated composites are redshifted to higher wavelength, which meant that their band gap is narrowed. The band gaps of CdS was estimated by formula (2) [45]. As shown in Fig. 6(b), the band gaps of CdS is 2.38 eV [39,46]. 1=n ðahvÞ ¼ A hv Eg a the absorption coefficient; h the Plank's constant ðincident photon energyÞ; v the frequency of the radiation; A a constant; Eg band gap width;
(2)
Photocatalytic hydrogen generation properties Fig. 7 shows the hydrogen production kinetic test results for all samples under visible light irradiation. It has been found that the composite catalysts have hydrogen production activity not only in the 10 vol% lactic acid solution used as a sacrificial agent, but also in a system in which a 1.0 M ammonium sulfite solution is used as a sacrificial agent. We can see from Fig. 7(a) and (b) that the composite catalyst exhibits high efficient hydrogen production activity in a 10 vol% lactic acid solution system. The average hydrogen production rate of pure CdS in 5 h was 1460.29 mmol h1g1. The hydrogen production rate of the composite catalyst was significantly increased after introduction of SC-WP. When the amount of
Fig. 5 e (a) N2 adsorption-desorption isotherm (b) and pore diameter distribution of CdS and 10PD samples.
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Fig. 6 e (a) DRS of CdS, 5PD, 10PD, 15PD and 20PD samples; (b) the plot of (ahv)1/2 versus energy for the band gap energy of CdS and (c) the color of samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
SC-WP added was 10 wt%, the highest hydrogen production rate was achieved, and the average hydrogen production rate at 5 h reached 15446.21 mmol h1 g1, which was 10.58 times higher than that of pure CdS. In addition, Fig. 7(c) and (d) show the results of the photocatalytic activity of the composite catalysts for photocatalytic water splitting for hydrogen evolution in a 1 M ammonium sulfite solution system. It can be seen that the hydrogen production activity of the composite catalysts in 1 M ammonium sulfite solution system was lower than that in 10 vol% lactic acid solution system. The average hydrogen production rate of the composite catalyst with the best activity in 5 h can reach 4908.84 mmol h1g1. Moreover, the stability of the 10PD composite catalyst for photocatalytic hydrogen production was carried out, and the results are shown in Fig. 7(e) and (f). Five cycles of testing were performed and the test time for each cycle was 5 h. At the end of each cycle, the H2 in the reaction system was exhausted by nitrogen and then the next cycle was tested. The entire test time was 25 h. It was observed that the activity of the composite catalyst did not decrease in the five cycle tests, which indicated that the 10PD catalyst has an excellent photocatalytic stability. In addition, the AQE of 10PD in the range of 400e600 nm was measured. As can be seen from Fig. 8, with the increase of wavelength, AQE of 10PD increases first and then decreases. At a wavelength of 475 nm, AQE reached a maximum of 12.81%. At the same time, the photocatalytic activity of the composite catalysts in this work was compared with that of other CdS-based photocatalysts, and the results are shown in Table 2. Furthermore, we studied the catalyst dosage effect on photocatalytic hydrogen production and the sacrificial donor
concentration on photocatalytic hydrogen generation. As shown in Fig. 9 (a), the high hydrogen evolution rate can be affected by the concentration of catalyst in the aqueous solution according to our experiment results. The highest hydrogen evolution rate can be obtained when the catalyst dosage at 20 mg in 30 mL, as displayed in Fig. 9(b). We found that in the reaction system in this paper, both too low and too high concentration of sacrificial agent solution is not conducive to the reaction, and it is beneficial to keep the concentration of sacrificial agent solution within the range of 10 vol %-20 vol%. Simultaneously, the photocatalytic hydrogen evolution rate under aerobic condition was measured. As shown in Fig. 9(c) the photocatalytic hydrogen evolution rates under aerobic condition is similar to the anaerobic condition. The photocatalytic hydrogen evolution process accompanying oxygen consumption at the beginning. Due to the more negative CB of pure CdS NRs (0.53 eV) than the potential of O2/∙O 2 (0.33 eV), the O2 more easier to get electrons formation ∙O 2 . SC-WP photocatalyst produces large number of electrons under light irradiation and then rapid consumption of oxygen in the reactor in the first hour. Therefore, the photocatalytic hydrogen evolution rates under aerobic condition is similar to the anaerobic condition.
Characterization of fluorescence spectra The efficiency of photogenerated charge carrier capture, separation, migration, transfer and recombination is the most important factor affecting the photocatalytic activity of semiconductor materials. Steady-state and transient
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Fig. 7 e (a), (b) The H2 evolutionary activity of SC-WP, CdS, 5PD, 10PD, 15PD and 20PDt samples in 10 vol% lactic acid solution system and (c), (d) 1 M ammonium sulfite solution system, (e) the photo-catalytic stability test of 10PD sample in 10% lactic acid solution system and (f) 1 M ammonium sulfite solution system.
fluorescence spectroscopy is an important characterization method for studying the capture, separation, migration, transfer and recombination of photogenerated charge carriers [47]. Fig. 10(a) shows the steady-state fluorescence spectra of pure CdS and XPD composite catalysts. The fluorescence intensity of XPD composite catalysts is lower than that of pure CdS, and the fluorescence intensity of 10PD is the lowest among all XPD composite catalysts. The decrease of
fluorescence intensity indicates the quenching of fluorescence, which proves the effective separation of photogenerated electron-hole pairs [48]. Fig. 10(b) is a transient fluorescence spectrum of the samples. It can be observed that the XPD composite catalysts have a slower fluorescence decay than pure CdS. This indicates that the XPD composite catalysts have a longer life and better stability [45]. And the stability of 10PD is the best among all the composite catalysts,
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¼
A1 t21 þ A2 t22 þ A3 t23 A1 t1 þ A2 t2 þ A3 t3
< t > the average lifetime;
(4)
A1 ; A2 ; A3 the corresponding percentage; t1 ; t2 ; t3 the the fluorescence decay lifetime; As can be seen from Table 2, pure CdS shows a slower decay component and two faster decay components, 153.07, 4.98 and 0.46 ns, respectively. The decay of the composite catalysts slowed down in all three components. In addition, the average lifetime of pure CdS was 1.87 ns. After the addition of cocatalyst, the average lifetime of the composite catalyst increased to 2.98, 3.37, 3.10 and 3.07 ns, respectively. This indicates that the introduction of cocatalyst is helpful to the separation and transfer of charges, inhibits the recombination of electron-hole pairs and increases the lifetime of the catalyst.
The electrochemical characterization Since charge separation and transfer efficiency have a great influence on the activity of the photocatalyst, all photocatalysts were electrochemically tested to further investigate the intrinsic mechanism of enhanced PD photocatalytic activity for hydrogen evolution. First of all, transient photocurrent response tests and linear sweep voltammetry (LSV) tests
This work [2] [4] [21] [22] [23] [24] [32] [39] [47] 15446 5543 5840 3104 17727 12304 5330 172.7 838 62 12.81 (475 nm) e e e e e e e 3.89 (430 nm) 0.11 (430 nm) nm) nm) nm) nm)
nm) nm) nm) nm)
5 W LED (l 420 nm) 300WXe-lamp (l 420 300WXe-lamp (l 420 300WXe-lamp (l 420 300WXe-lamp (l 420 5 W LED (l 420 nm) 300WXe-lamp (l 420 300WXe-lamp (l 400 300WXe-lamp (l 420 300WXe-lamp (l 420
The data obtained after fitting is shown in Table 3, and the average life of each sample is calculated by formula (4) [50].
SC-WP Mo2C@C nitrogen doped carbon matrix WP (crystalline state) CoP@SiO2 NiSe2 MoS2 g-C3N4-Pt Ni2P Al2O3
(3) Table 2 e Comparison of hydrogen evolution activity of photocatalysts based on CdS.
I the normalised emission intensity; t the time after the pulsed laser excitation; Ai the amplitude ðpre exponential factorÞ; ti the fluorescence decay lifetime;
CdS CdS CdS CdS CdS CdS CdS CdS CdS CdS
Ai expð t=ti Þ
i¼1;2;3
Light source
X
Co-catalyst
IðtÞ ¼
Photocatalyst
which corresponds to the hydrogen production kinetic test results. Transient fluorescence data was fitted to a three-level exponential fit using equation (3) [49].
Sacrificial reagent
AQE (%)
Fig. 8 e AQE of hydrogen evolution for 10PD under different wavelength irradiation.
10 vol% lactic acid 10 vol% lactic acid lactic acid 1.0 M (NH4)2SO3 5 vol% lactic acid 10 vol% lactic acid 10 vol% lactic acid 25 vol% CH3OH H2O (artificial gill) H2O (artificial gill)
Activity (mmol$h1$g1)
Ref.
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Fig. 9 e The H2 evolutionary activity of (a) different amounts of 10PD catalyst, (b) 10PD catalyst in different concentrations of sacrificial agent solutions and (c) 10PD catalyst under aerobic condition and anaerobic condition.
were performed on these photocatalysts. Fig. 11 (a) shows the transient photocurrent-time curves of the working electrode for different samples, it can be observed that CdS shows a weak photocurrent response. After introduction of SC-WP, the photocurrent response of the composite photocatalysts is significantly enhanced, and the photocurrent response of the
10PD composite photocatalyst is the strongest. It is well known that a stronger photocurrent response represents a higher photocurrent intensity, and a higher photocurrent intensity corresponds to a more efficient separation and transfer of photogenerated charges, indicating that the recombination of photogenerated electron-hole pairs is more
Fig. 10 e (a) Stable fluorescence spectra and (b) transient fluorescence spectra of CdS, 5PD, 10PD, 15PD and 20PD.
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Table 3 e Decay parameters of CdS, 5PD, 10PD, 15PD and 20PD. Parameter Sample CdS 5PD 10PD 15PD 20PD
B1 (%)
t1 (ns)
B2 (%)
t2 (ns)
B3 (%)
t3 (ns)
(ns)
c2
23.72 22.97 23.19 23.24 24.02
4.98 5.52 5.56 5.52 5.45
22.18 20.37 18.95 19.66 19.69
0.46 0.70 0.75 0.71 0.71
54.10 56.66 57.86 57.09 56.29
153.07 157.53 157.74 158.59 156.76
1.87 2.98 3.37 3.10 3.07
1.86 1.78 1.74 1.86 1.83
effectively suppressed. It is further demonstrated that the addition of SC-WP enhances the activity and lifetime of the photocatalyst. Fig. 11(b) shows the linear sweep voltammetry curves of the catalysts, it can be clearly seen that the PD composite catalyst has a higher cathode current density than the CdS, which is attributed to the reduction reaction of decomposing water for hydrogen evolution. It indicates that the PD composite catalysts have a low overpotential, which further proves that the PD composite catalyst has excellent photocatalytic activity. In addition, the interfacial charge transport behavior of these photocatalysts was investigated by electrochemical impedance spectroscopy (EIS). As shown in Fig. 12, it can be seen from the Nyquist curves of the EIS that the radius of the curves corresponding to the electrode of the PD samples is significantly smaller than that of the CdS, wherein the radius of the curve corresponding to the electrode of the 10PD sample is the smallest. In the Nyquist curve of the EIS, the impedance of the electrode corresponding to the smaller diameter is smaller, that is, the corresponding interface charge transfer rate is faster. The results show that the photocatalytic activity of 10PD is the best among all photocatalysts. The electrochemical test results are consistent with the hydrogen production kinetic results and the other characterization experiments described above. Fig. 13 shows Mott-Schottky curves of pure SC-WP and pure CdS samples, respectively. It can be clearly observed that Mott-Schottky curves of SC-WP and pure CdS show positive slope of C2-E plots, proving that they are n-type semiconductor [51]. From the figures, we can get that the flat-band potentials (Efb) of SC-WP and CdS are 0.32 and 0.57 V vs. SCE, respectively. The n-type semiconductor conduction band
potential (ECB) is 0.2 or 0.1 V more negative than the flat band potential [52]. Thus the ECB of SC-WP and CdS are about 0.52 and 0.77 V vs. SCE, respectively. And according to the following formula ENHE ¼ ECB þ 0.241 V, the normal hydrogen electrode potential (ENHE) of SC-WP and CdS can be estimated to be about 0.28 and 0.53 V vs. NHE, respectively. From the previous analysis (in Fig. 6(a)), we can get the Eg of CdS sample is 2.38 eV, and the literature shows that the Eg of SC-WP is about 1.43 eV [53], so the valence band potential (EVB) of CdS and SC-WP sample can be calculated by the formula EVB¼ ECB þ Eg, So the EVB of CdS and SC-WP are about 1.85 and 1.15 V, respectively.
Fig. 12 e EIS plots of pure CdS, 5PD, 10PD, 15PD and 20PD.
Fig. 11 e (a) Transient photocurrent responses curves and (b) LSV curves of pure CdS, 5PD, 10PD, 15PD and 20PD.
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Fig. 13 e Mott-Schottky curses of (a) pure SC-WP and (b) pure CdS NRs.
Speculation on the mechanism of photocatalytic hydrogen production Based on the above experimental results, taking 10% lactic acid solution as a sacrificial agent as an example, we proposed the reaction mechanism of photocatalytic decomposition of water-hydrogen evolution system of XPD composite catalyst, as shown in Fig. 14. The reason for the enhanced activity of XPD composite catalyst in photocatalytic decomposition of hydrogen from water under visible light can be summarized as the following two points: 1) SC-WP nanoparticles have excellent optical properties, improve the visible light absorption rate and absorption light utilization rate of the composite catalyst, which is favorable for improving the photocatalytic activity; 2) The introduction of SC-WP nanoparticles provides a large number of exposed active sites for hydrogen evolution reaction, and is beneficial to the transport and separation of photogenerated charges in the catalytic system, inhibiting the recombination of photogenerated electron-hole pairs, thus improving the utilization of photogenerated electrons produced by visible light excitation.
Fig. 14 e Mechanism diagram of photocatalytic hydrogen production for PD composite.
In detail, under visible light irradiation, the CdS NRs can be stimulated to produce photo-generated electrons and holes. Electrons in the VB of CdS NRs transit to the CB of CdS, Then the photoelectrons on the CB of CdS NRs are quickly captured by SC-WP NPs and are rapidly involved in the hydrogen evolution reduction reaction. At the same time, the holes remain on the VB of CdS and are consumed by lactic acid. This greatly promotes the transmission and separation of photogenerated charges, inhibits the recombination of photogenerated electron-hole pairs, thereby increasing the utilization of photogenerated electrons and greatly increasing the activity of the photocatalytic system. The related charge transfer process is as follows: 8 þ Þ CdS þ hv/CdSðe > CB þ hVB > < CdSðeCB /SC WPðeCB > 2e þ 2H2 O/H2 þ 2OH > : þ CdSðhVB Þ þ LA/LAþ
Conclusions In summary, we successfully prepared semi-crystalline WP and as a cocatalyst to modify CdS NRs to construct SC-WP/CdS Composite catalyst by simple electrostatic self-assembly method for photocatalytic hydrogen evolution. The prepared SC-WP nanoparticles was found has semi-crystalline character and excellent optical properties by low temperature thermal decomposition a mixture of recrystallized Na2WO4$2H2O and NaPO2H2$H2O as precursors. Two high-efficiency and stable composite photocatalytic decomposition hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10% lactic acid solution as sacrificial agents, respectively. Surprisingly, the maximum photocatalytic H2 production rate of 15446.21 mmol h1 g1 is obtained over 10PD composite, which is 10.58 times greater than that of pure CdS. The improved photocatalytic activity can be attributed to the fact that the SC-WP nanoparticles provides a large number of exposed active sites on the surface of CdS for hydrogen evolution reaction, which can efficiently capture photogenerated electrons from CdS nanorods and
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promotes the transport and separation of light-induced charges. And the introduction of SC-WP nanoparticles with excellent optical properties can efficiently enhance the visible light absorption range and the utilization rate of the absorbed light of the PD composite. In addition, the SC-WP nanoparticles show semi-crystalline state, which is also conducive to enhancing the photocatalytic activity.
Author contributions Qiyan Jian and Zhiliang Jin conceived and designed the experiments; Qiyan Jian performed the experiments; Zhiliang Jin and Qingjie Guo contributed reagents/materials and analysis tools; Qiyan Jian wrote the paper.
Acknowledgments This work was financially supported by the Open Project of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University (2019-KF-36), the Chinese National Natural Science Foundation (21862002 and 41663012), the new technology and system for clean energy catalytic production, Major scientific project of North Minzu University (ZDZX201803).
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Available online at www.sciencedirect.com
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Enhanced photocatalytic hydrogen evolution over semi-crystalline tungsten phosphide Zhiliang Jin a,c,d,*, Qiyan Jian a,c,d,**, Qingjie Guo b,*** a
School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan, 750021, PR China c Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu University, Yinchuan 750021, PR China d Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu University, Yinchuan 750021, PR China b
highlights The tungsten phosphide (SC-WP) has a unique semi-crystalline structure and excellent optical properties. The SC-WP cocatalyst improved the visible light absorption ability and the utilization of absorbed light. Highly efficient and stable photocatalytic hydrogen evolution systems were constructed.
article info
abstract
Article history:
Increasing the separation efficiency and transfer rate of photogenerated charges is the
Received 2 June 2019
dominant factor for improving photocatalytic activity. Herein, we successfully prepared
Received in revised form
semi-crystalline WP (SC-WP) with good optical properties and as a cocatalyst to modify CdS
3 August 2019
nanorods (CdS NRs) to construct SC-WP/CdS (PD) composite catalyst by simple electrostatic
Accepted 10 August 2019
self-assembly method for photocatalytic hydrogen evolution. Two high-efficiency and
Available online 20 September 2019
stable photocatalytic hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10 vol% lactic acid solution as sacrificial agents, respectively. Surpris-
Keywords:
ingly, the maximum photocatalytic H2 production rate of 15446.21 mmol h1 g1 is obtained
Tungsten phosphide
over 10PD composite, which is 10.58 times greater than that of pure CdS. The improved
CdS NRs
photocatalytic activity can be attributed to the fact that the SC-WP nanoparticles provides
Semi-crystalline
a large number of exposed active sites on the surface of CdS for hydrogen evolution re-
Photocatalytic hydrogen evolution
action, which can efficiently capture photogenerated electrons from CdS nanorods and promotes the transport and separation of light-induced charges. And the introduction of SC-WP nanoparticles with excellent optical properties can efficiently improve the visible light absorption range and the utilization rate of the absorbed light of the PD composite. In addition, the SC-WP nanoparticles show semi-crystalline state, which is also conducive to enhancing the photocatalytic activity. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China. ** Corresponding author. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China. *** Corresponding author. E-mail addresses: [email protected] (Z. Jin), [email protected] (Q. Jian), [email protected] (Q. Guo). https://doi.org/10.1016/j.ijhydene.2019.08.080 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction As a kind of green energy with good combustion performance, convenient transportation and high utilization rate, hydrogen energy has stood out from a variety of energy sources and is recognized as the best choice to replace fossil fuels [1,2]. However, the mature industrial methods of hydrogen production now available are demanding, costly and require the consumption of fossil fuels, which cause damage to the environment [3e5]. In recent decades, as one of the effective ways to convert solar energy into chemical energy, semiconductor photocatalytic decomposition of water to hydrogen has been extensively studied [6e9]. Therefore, a large number of semiconductor materials have been developed and applied in the research field of photocatalytic technology, such as TiO2 [10,11], WO3 [12,13], C3N4 [14,15], ZnS [16,17], WS2 [18,19], MoSe [20] and so on. At present, due to the problem that the photocatalytic material still has less light absorption, poor stability of the system itself, too high or too low band edge, and the like, it can not meet the requirements of industrial production. Therefore, further research is needed to develop efficient photocatalytic decomposition of water and hydrogen evolution systems and photocatalysts [21]. Due to their excellent photocatalytic activities, nonlinear optical properties and photoelectric conversion properties, metal sulfides have been widely used in photocatalytic degradation of organic pollutants, photocatalytic reduction of CO2, photocatalytic decomposition of water to hydrogen and other fields. CdS has a band gap of approximately 2.4 eV and excellent photocatalytic performance in the visible range [22e29]. However, due to the severe photo-corrosion phenomenon of CdS in photocatalytic decomposition of water, the hydrogen evolution efficiency of CdS is low and the stability is poor, which greatly limits the photocatalytic activity of CdS [30e32]. Tungsten phosphide WP has attracted extensive attention from researchers at home and abroad because of its excellent hydrotreating activity and excellent sulfur poisoning resistance. It has been gradually applied to hydrodesulfurization, hydrodenitrogenation, electrocatalysis, photocatalysis, etc [33]. In recent years, WP has become a research hotspot, but it has few applications in photocatalytic decomposition of water for hydrogen evolution. In the previous reports, the preparation of WP is complicated, the preparation conditions are harsh, the equipment requirements are high, and the cost is large [34]. Our research group simplified the preparation of WP, introduced it into photocatalytic decomposition of water and hydrogen evolution system, and did a lot of research, and found that it exhibits high activity and stability, and has great research value. In this work, we successfully prepared semi-crystalline WP (SC-WP) and introduced SC-WP as a cocatalyst to modify CdS NRs. Two different high-efficiency and stable composite photocatalytic decomposition hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10 vol% lactic acid solution as sacrificial reagents. In addition, the photocatalytic activity of the composite catalyst in this
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work was compared with that of other CdS-based photocatalysts, and the results show that this work has obvious advantages. As SC-WP nanoparticles provide a large number of exposed active sites for hydrogen evolution reaction, these active sites can capture photogenerated electrons on CdS nanorods quickly and participate in the rapid evolution of hydrogen, promote the transmission and separation of photogenerated charges, inhibit the recombination of photogenerated electrons and hole pairs, thereby improving the utilization of photogenerated electrons generated by visible light excitation. Moreover, because of the excellent optical properties of SC-WP nanoparticles, the visible light absorption rate and the utilization rate of absorbed light of the composite catalysts are enhanced, and the SC-WP nanoparticles are semi-crystalline, which is conducive to improving the photocatalytic activity.
Experimental section Catalysts preparation All chemicals are reagent grade and used without further purification.
Preparation of CdS CdS NRs was synthesized by solvothermal method. 2.3750 g CdCl2$2.5H2O and 2.3750 g NH2CSNH2 were dissolved in 60 mL ethylenediamine, then held in an autoclave at 160 C for 36 h, and the product was washed with deionized water and absolute ethanol and dried at 60 C.
Preparation of SC-WP The SC-WP was synthesized via calcination. Typically, dissolve 3.30 g Na2WO4$2H2O and 4.24 g NaPO2H2$H2O in distilled water and mix well, then recrystallize the above solution, and finally, the precursor powder formed by recrystallization was calcined at 300 C for 30 min under N2 protection, and the obtained product was washed with distilled water and dried at 60 C.
Preparation of SC-WP/CdS composites SC-WP/CdS (PD) composites were prepared by simple electrostatic self-assembly method. Typically, 0.15 g of CdS NRs and calculated amount of SC-WP sample were weighed, and the composite catalysts with SC-WP content of 5%, 10%, 15% and 20% were prepared by self-assembly method, and recorded as XPD (X ¼ 5, 10, 15, 20). The specific steps for preparing XPD composite catalyst are shown in Fig. 1.
Preparation of working electrodes Firstly, 5 mg sample powder was added to 200 mL of ethanol containing 40 mL l5% Nafion solution and dispersed by ultrasonic treatment to form a paste. Then the paste catalyst was dripped onto the surface of the FTO glass (1 cm2). Finally, the FTO working electrode loaded with catalyst was obtained by drying at room temperature.
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Characterization of composite photocatalyst X-ray diffraction (XRD) spectra of all samples were obtained (Model Rigaku Rint-2000 CuK alpha X-ray diffractometer, Testing conditions: the tube current and tube voltage is set at 40 mA and 40 kV respectively, and at a scanning rate of 5 /min in the 10e80 2q range). The morphology of the samples was observed by scanning electron microscopy (SEM, JSM-6701F JEOL, acceleration voltage 5000 V) and transmission electron microscopy (TEM, JEM1200EX JEOL, acceleration voltage 10000 V). The surface chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The BET specific surface area of the sample was determined by the nitrogen adsorption-desorption isotherm of the ASAP 2020 M analyzer at 77 K. Ultravioletevisible diffuse reflectance spectroscopy (UVevis DRS) used BaSO4 as a reference for recording solid powders on a UV-2550 spectrophotometer. The photoluminescence (PL, containing steady-state fluorescence and transient fluorescence) spectra of the samples were recorded on a FLUOROMAX-4 fluorescence spectrometer at room temperature. The photoelectrochemical tests (containing Chronoamperometry, Linear Scan Voltammetry, Potentiostatic EIS and Mott-Schottky) are performed on an VersaStat4-400 electrochemical workstation in a standard three-electrode system with platinum plate and calomel electrode as counter electrode and reference electrode, respectively. Meanwhile, 0.2 M Na2SO4 aqueous solution and 300 W xenon lamp used as the electrolytic solution and a solar simulator illumination source (XMM65e63, C16). And I-T and LSV curves are recorded at a stationary bias of 0.2 V vs. SCE. EIS curves of samples are recorded at a stationary bias of 0.2 V with AC amplitude of 10 mV and a frequency range of 10,000e1 Hz. The Mott-Schottky curves are recorded from 1.5e1.5 V with AC amplitude of 10 mV at a frequency of 1 KHz.
Photocatalytic hydrogen production kinetics test Hydrogen evolution activity of photocatalyst was tested under PCX-50C multi-channel photochemical reaction system (5 W LED white light, 420 nm). Specifically, 10 mg catalyst was added to 62 mL quartz reaction bottle and 30 mL sacrificial
agent (1.0 M ammonium sulfite solution or 10 vol% lactic acid solution) was added. After ultrasonic dispersion, the air in the bottle was removed by nitrogen gas. The hydrogen produced was detected regularly by using Tianmei GC 7900 gas chromatograph (TCD, 13X column) with N2 as carrier gas (once every 0.5 h for a total time of 5 h). In addition, the apparent quantum efficiency (AQE) of 10PD in the range of 400e600 nm was measured and calculated. The formula is as follows: AQE ¼
2 the number of evolved hydrogen molecules 100% the number of incident photon
Results and discussion Crystal structure and morphology To understand the bulk structure of the prepared samples, powder X-ray diffraction (XRD) measurements were performed on pure CdS, pure SC-WP, 5PD, 10PD, 15PD and 20PD samples. Fig. 2(a) shows the XRD pattern of pure SC-WP (scanning speed: 1 /min and 5 /min), it can be observed that three characteristic diffraction peaks appear at 31.05, 42.84 and 44.60, corresponding to the (011), (202) and (211) crystal planes of WP (PDF#29e1364), respectively. In addition, it can also be observed that very weak diffraction peaks correspond to the (103), (114), (411) and (123) crystal faces of WP, respectively. The characteristics of XRD diffraction peak of SC-WP: having arched sharp peaks, the width of the peak is wide, and the intensity of the peak is weak. Indicating that the prepared SC-WP sample is semi-crystalline. The characteristics of the XRD pattern of SC-WP are consistent with the characteristics of the XRD pattern in the related reports of semi-crystalline materials [35]. It can be seen from Fig. 2(b) that the characteristic diffraction peaks of pure CdS, 5PD, 10PD, 15PD and 20PD samples belong to CdS, and the composite catalysts have no characteristic peaks of SC-WP. This is because the content of SC-WP is relatively small and the dispersion is high [36]. At the same time, we can observe that the diffraction peak intensity of the composite catalyst decreases as the content of SC-WP increases, which can be attributed to the shielding effect of SC-WP, because the SC-WP
Fig. 1 e Schematic diagram of the steps for preparing XPD composites.
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Fig. 2 e XRD patterns of (a) pure SC-WP at different scanning rates and (b) pure CdS, 5PD, 10PD, 15PD and 20PD.
is loaded on the surface of CdS, and the incident and diffracted X-rays are weakened [37]. In order to further understand the morphology and structure of the samples, Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) were used for characterization. Fig. 3(a) and (b) are SEM images of pure SC-WP, and it can be seen that the SC-WP sample shows the state of the aggregated nanoparticles with different particle sizes.
Fig. 3(c) shows the HRTEM image of the pure SC-WP at a resolution of 2 nm, it can be observed that most of the regions in the HRTEM image have no lattice fringe, and there are some lattice fringe in a small part of the regions, and the d spacing value is 0.289 nm, corresponding to the plane (011) of SC-WP. The SAED pattern of pure SC-WP is shown in Fig. 3(d), and the d spacing values calculated from the pattern correspond to the (011) and (202) planes of SC-WP. Fig. 3 (e) and (f) are the
Fig. 3 e SEM images of (a), (b) pure SC-WP and (e) 10PD, (c) HRTEM imags of pure SC-WP, (d) the SAED pattern of pure SC-WP, (f) TEM images, (g) HRTEM imags of 10PD and (h) EDX images and element mapping for 10PD.
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Fig. 4 e XPS spectra of (a) W 4f, (b) P 2p, (c) Cd 3d and (d) S 2p.
SEM and TEM images of 10PD composite samples respectively. From Fig. 3 (e), it can be clearly observed that SC-WP nanoparticles (as shown in the red circles marker) are loaded uniformly on CdS nanorods. TEM images of 10PD composite shows that some black spots (as shown in the red circles marker) are evenly distributed on CdS nanorods. These black spots are SC-WP nanoparticles. The results of SEM and TEM images are consistent. Fig. 3(g) is an HRTEM image of a 10PD composite sample. Only one type of lattice fringe was observed in the entire HRTEM image with a lattice spacing of 0.316 nm, corresponding to the (101) crystal plane of CdS. In addition, a region without lattice fringes (as shown in the area marked by the red curve) can be clearly observed in the HRTEM image, and we speculate that this region belongs to SC-WP nanoparticles. At the same time, the presence of Cd, S, W and P elements in 10PD composite catalyst can be clearly observed from Fig. 3 (h), and the distribution of these elements is uniform, which is consistent with the results of scanning electron microscopy and transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) can qualitatively analyze the elemental composition, chemical state and molecular structure of the sample surface, which is an important characterization method for analyzing materials. Fig. 4 shows the XPS high resolution spectrum of elements in the 10PD composite photocatalyst. As we observed form Fig. 4 (a), the W
4f shows two good resolution spin orbital splitting peaks at 37.52 and 35.39 eV, corresponding to the characteristic peak of W 4f5/2 and W 4f7/2, respectively. The spin energy separation is 2.23 eV, which is consistent with the W6þ in the previous reports [11]. As can be seen from Fig. 4(b), the P 2p exhibits a binding energy characteristic peak belonging to P2p1/2 at 133.07 eV [11,38]. As shown in Fig. 4(c), the Cd 3d shows two characteristic peaks at 411.53 and 404.77 eV, which are attributed to the characteristic peaks of Cd 3d5/2 and Cd 3d3/2, respectively. It proves that there are Cd2þ in the samples. Moreover, the spin orbit separation of 6.76 eV between Cd 3d5/ 2þ is present on the 2 and Cd 3d3/2 further indicates that the Cd surface of 10PDcomposite catalyst [39,40]. In Fig. 4(d), the binding energies of S 2p at 162.22 and 161.11 eV correspond to the characteristic peaks of the S 2p1/2 and S 2p3/2 spin orbital components of S2, respectively, indicating that the S element mainly exists in the form of S2 on the surface of 10PD sample [41].
BET characterization The Brunauer Emmett Teller specific surface areas (SBET) and the pore diameter distribution of pure CdS and 10PD were determined via nitrogen adsorption-desorption isotherms at 77 K, and the BET equation is shown in formula (1):
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Table 1 e SBET, Pore volume and Average pore size for CdS and10PD. Samples SBET/(m2/ g) CdS 10PD
Pore volume/ (cm3/g)
Average pore size/ nm
0.18 0.17
26 24
30 31
Va cðp=p* Þ a ¼ * Vm ð1 p=p Þf1 þ ðc 1Þp=p* g Va the absorption amount under pressure p; Vam the saturated adsorption capacity of monolayer;
(1)
p* the saturated vapor pressure; c Adsorption constant; The results are shown in Table 1 and Fig. 5. It can be seen from Fig. 5 (a) that the isotherms of CdS and 10PD samples are IV-type curves and have H3 hysteresis loops, which is attributed to the capillary condensation of mesoporous materials in the process of multilayer adsorption [42,43]. It can be observed from Fig. 5(b) that the pore sizes of CdS and 10PD samples are widely distributed between 2 and 50 nm, and only a small number of pore sizes are distributed in the range of more than 50 nm or less than 2 nm, and the average pore sizes of CdS and 10PD samples are 26 and 24 nm, respectively (as shown in Table 1). The above results show that there are a large number of mesopores in the samples, which is consistent with the results in Fig. 5 (a). Compared with CdS, the SBET of the 10PD sample increased slightly, and the pore volume decreased slightly. This can be attributed to the successful coating of SCWP on the surface of CdS, which is beneficial to provide more active sites for photocatalytic decomposition of water.
Optical properties of the photocatalysts UVevis DRS is an important characterization for measuring the optical properties of semiconductor materials. As is known to all, the optical properties of semiconductor materials have some influence on the activity of photocatalytic decomposition of water for hydrogen evolution [44]. Fig. 6(a)
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shows the UVevis DRS of pure SC-WP, CdS and XPD samples. It can be clearly observed that pure SC-WP has a strong optical absorption density, and its optical absorption density increases with the increase of wavelength, that is to say, the utilization of light absorbed by SC-WP is high. The excellent optical properties of SC-WP are largely due to its black color. Compared with pure CdS, the light absorption density of XPD composite catalyst is significantly increased and increases with the increase of SC-WP content. This may be because the color of the XPD composite samples deepens as the SC-WP content increases (as shown in Fig. 6(c) physical drawings of samples). The characterization results are not in perfect agreement with the hydrogen evolution activity test results. This indicates that the optical properties of the photocatalyst have a certain influence on its photocatalytic activity, but it is not the dominant factor determining the photocatalytic activity. In addition, the UVevis DRS of SC-WP-coated composites are redshifted to higher wavelength, which meant that their band gap is narrowed. The band gaps of CdS was estimated by formula (2) [45]. As shown in Fig. 6(b), the band gaps of CdS is 2.38 eV [39,46]. 1=n ðahvÞ ¼ A hv Eg a the absorption coefficient; h the Plank's constant ðincident photon energyÞ; v the frequency of the radiation; A a constant; Eg band gap width;
(2)
Photocatalytic hydrogen generation properties Fig. 7 shows the hydrogen production kinetic test results for all samples under visible light irradiation. It has been found that the composite catalysts have hydrogen production activity not only in the 10 vol% lactic acid solution used as a sacrificial agent, but also in a system in which a 1.0 M ammonium sulfite solution is used as a sacrificial agent. We can see from Fig. 7(a) and (b) that the composite catalyst exhibits high efficient hydrogen production activity in a 10 vol% lactic acid solution system. The average hydrogen production rate of pure CdS in 5 h was 1460.29 mmol h1g1. The hydrogen production rate of the composite catalyst was significantly increased after introduction of SC-WP. When the amount of
Fig. 5 e (a) N2 adsorption-desorption isotherm (b) and pore diameter distribution of CdS and 10PD samples.
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Fig. 6 e (a) DRS of CdS, 5PD, 10PD, 15PD and 20PD samples; (b) the plot of (ahv)1/2 versus energy for the band gap energy of CdS and (c) the color of samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
SC-WP added was 10 wt%, the highest hydrogen production rate was achieved, and the average hydrogen production rate at 5 h reached 15446.21 mmol h1 g1, which was 10.58 times higher than that of pure CdS. In addition, Fig. 7(c) and (d) show the results of the photocatalytic activity of the composite catalysts for photocatalytic water splitting for hydrogen evolution in a 1 M ammonium sulfite solution system. It can be seen that the hydrogen production activity of the composite catalysts in 1 M ammonium sulfite solution system was lower than that in 10 vol% lactic acid solution system. The average hydrogen production rate of the composite catalyst with the best activity in 5 h can reach 4908.84 mmol h1g1. Moreover, the stability of the 10PD composite catalyst for photocatalytic hydrogen production was carried out, and the results are shown in Fig. 7(e) and (f). Five cycles of testing were performed and the test time for each cycle was 5 h. At the end of each cycle, the H2 in the reaction system was exhausted by nitrogen and then the next cycle was tested. The entire test time was 25 h. It was observed that the activity of the composite catalyst did not decrease in the five cycle tests, which indicated that the 10PD catalyst has an excellent photocatalytic stability. In addition, the AQE of 10PD in the range of 400e600 nm was measured. As can be seen from Fig. 8, with the increase of wavelength, AQE of 10PD increases first and then decreases. At a wavelength of 475 nm, AQE reached a maximum of 12.81%. At the same time, the photocatalytic activity of the composite catalysts in this work was compared with that of other CdS-based photocatalysts, and the results are shown in Table 2. Furthermore, we studied the catalyst dosage effect on photocatalytic hydrogen production and the sacrificial donor
concentration on photocatalytic hydrogen generation. As shown in Fig. 9 (a), the high hydrogen evolution rate can be affected by the concentration of catalyst in the aqueous solution according to our experiment results. The highest hydrogen evolution rate can be obtained when the catalyst dosage at 20 mg in 30 mL, as displayed in Fig. 9(b). We found that in the reaction system in this paper, both too low and too high concentration of sacrificial agent solution is not conducive to the reaction, and it is beneficial to keep the concentration of sacrificial agent solution within the range of 10 vol %-20 vol%. Simultaneously, the photocatalytic hydrogen evolution rate under aerobic condition was measured. As shown in Fig. 9(c) the photocatalytic hydrogen evolution rates under aerobic condition is similar to the anaerobic condition. The photocatalytic hydrogen evolution process accompanying oxygen consumption at the beginning. Due to the more negative CB of pure CdS NRs (0.53 eV) than the potential of O2/∙O 2 (0.33 eV), the O2 more easier to get electrons formation ∙O 2 . SC-WP photocatalyst produces large number of electrons under light irradiation and then rapid consumption of oxygen in the reactor in the first hour. Therefore, the photocatalytic hydrogen evolution rates under aerobic condition is similar to the anaerobic condition.
Characterization of fluorescence spectra The efficiency of photogenerated charge carrier capture, separation, migration, transfer and recombination is the most important factor affecting the photocatalytic activity of semiconductor materials. Steady-state and transient
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Fig. 7 e (a), (b) The H2 evolutionary activity of SC-WP, CdS, 5PD, 10PD, 15PD and 20PDt samples in 10 vol% lactic acid solution system and (c), (d) 1 M ammonium sulfite solution system, (e) the photo-catalytic stability test of 10PD sample in 10% lactic acid solution system and (f) 1 M ammonium sulfite solution system.
fluorescence spectroscopy is an important characterization method for studying the capture, separation, migration, transfer and recombination of photogenerated charge carriers [47]. Fig. 10(a) shows the steady-state fluorescence spectra of pure CdS and XPD composite catalysts. The fluorescence intensity of XPD composite catalysts is lower than that of pure CdS, and the fluorescence intensity of 10PD is the lowest among all XPD composite catalysts. The decrease of
fluorescence intensity indicates the quenching of fluorescence, which proves the effective separation of photogenerated electron-hole pairs [48]. Fig. 10(b) is a transient fluorescence spectrum of the samples. It can be observed that the XPD composite catalysts have a slower fluorescence decay than pure CdS. This indicates that the XPD composite catalysts have a longer life and better stability [45]. And the stability of 10PD is the best among all the composite catalysts,
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A1 t21 þ A2 t22 þ A3 t23 A1 t1 þ A2 t2 þ A3 t3
< t > the average lifetime;
(4)
A1 ; A2 ; A3 the corresponding percentage; t1 ; t2 ; t3 the the fluorescence decay lifetime; As can be seen from Table 2, pure CdS shows a slower decay component and two faster decay components, 153.07, 4.98 and 0.46 ns, respectively. The decay of the composite catalysts slowed down in all three components. In addition, the average lifetime of pure CdS was 1.87 ns. After the addition of cocatalyst, the average lifetime of the composite catalyst increased to 2.98, 3.37, 3.10 and 3.07 ns, respectively. This indicates that the introduction of cocatalyst is helpful to the separation and transfer of charges, inhibits the recombination of electron-hole pairs and increases the lifetime of the catalyst.
The electrochemical characterization Since charge separation and transfer efficiency have a great influence on the activity of the photocatalyst, all photocatalysts were electrochemically tested to further investigate the intrinsic mechanism of enhanced PD photocatalytic activity for hydrogen evolution. First of all, transient photocurrent response tests and linear sweep voltammetry (LSV) tests
This work [2] [4] [21] [22] [23] [24] [32] [39] [47] 15446 5543 5840 3104 17727 12304 5330 172.7 838 62 12.81 (475 nm) e e e e e e e 3.89 (430 nm) 0.11 (430 nm) nm) nm) nm) nm)
nm) nm) nm) nm)
5 W LED (l 420 nm) 300WXe-lamp (l 420 300WXe-lamp (l 420 300WXe-lamp (l 420 300WXe-lamp (l 420 5 W LED (l 420 nm) 300WXe-lamp (l 420 300WXe-lamp (l 400 300WXe-lamp (l 420 300WXe-lamp (l 420
The data obtained after fitting is shown in Table 3, and the average life of each sample is calculated by formula (4) [50].
SC-WP Mo2C@C nitrogen doped carbon matrix WP (crystalline state) CoP@SiO2 NiSe2 MoS2 g-C3N4-Pt Ni2P Al2O3
(3) Table 2 e Comparison of hydrogen evolution activity of photocatalysts based on CdS.
I the normalised emission intensity; t the time after the pulsed laser excitation; Ai the amplitude ðpre exponential factorÞ; ti the fluorescence decay lifetime;
CdS CdS CdS CdS CdS CdS CdS CdS CdS CdS
Ai expð t=ti Þ
i¼1;2;3
Light source
X
Co-catalyst
IðtÞ ¼
Photocatalyst
which corresponds to the hydrogen production kinetic test results. Transient fluorescence data was fitted to a three-level exponential fit using equation (3) [49].
Sacrificial reagent
AQE (%)
Fig. 8 e AQE of hydrogen evolution for 10PD under different wavelength irradiation.
10 vol% lactic acid 10 vol% lactic acid lactic acid 1.0 M (NH4)2SO3 5 vol% lactic acid 10 vol% lactic acid 10 vol% lactic acid 25 vol% CH3OH H2O (artificial gill) H2O (artificial gill)
Activity (mmol$h1$g1)
Ref.
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Fig. 9 e The H2 evolutionary activity of (a) different amounts of 10PD catalyst, (b) 10PD catalyst in different concentrations of sacrificial agent solutions and (c) 10PD catalyst under aerobic condition and anaerobic condition.
were performed on these photocatalysts. Fig. 11 (a) shows the transient photocurrent-time curves of the working electrode for different samples, it can be observed that CdS shows a weak photocurrent response. After introduction of SC-WP, the photocurrent response of the composite photocatalysts is significantly enhanced, and the photocurrent response of the
10PD composite photocatalyst is the strongest. It is well known that a stronger photocurrent response represents a higher photocurrent intensity, and a higher photocurrent intensity corresponds to a more efficient separation and transfer of photogenerated charges, indicating that the recombination of photogenerated electron-hole pairs is more
Fig. 10 e (a) Stable fluorescence spectra and (b) transient fluorescence spectra of CdS, 5PD, 10PD, 15PD and 20PD.
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Table 3 e Decay parameters of CdS, 5PD, 10PD, 15PD and 20PD. Parameter Sample CdS 5PD 10PD 15PD 20PD
B1 (%)
t1 (ns)
B2 (%)
t2 (ns)
B3 (%)
t3 (ns)
c2
23.72 22.97 23.19 23.24 24.02
4.98 5.52 5.56 5.52 5.45
22.18 20.37 18.95 19.66 19.69
0.46 0.70 0.75 0.71 0.71
54.10 56.66 57.86 57.09 56.29
153.07 157.53 157.74 158.59 156.76
1.87 2.98 3.37 3.10 3.07
1.86 1.78 1.74 1.86 1.83
effectively suppressed. It is further demonstrated that the addition of SC-WP enhances the activity and lifetime of the photocatalyst. Fig. 11(b) shows the linear sweep voltammetry curves of the catalysts, it can be clearly seen that the PD composite catalyst has a higher cathode current density than the CdS, which is attributed to the reduction reaction of decomposing water for hydrogen evolution. It indicates that the PD composite catalysts have a low overpotential, which further proves that the PD composite catalyst has excellent photocatalytic activity. In addition, the interfacial charge transport behavior of these photocatalysts was investigated by electrochemical impedance spectroscopy (EIS). As shown in Fig. 12, it can be seen from the Nyquist curves of the EIS that the radius of the curves corresponding to the electrode of the PD samples is significantly smaller than that of the CdS, wherein the radius of the curve corresponding to the electrode of the 10PD sample is the smallest. In the Nyquist curve of the EIS, the impedance of the electrode corresponding to the smaller diameter is smaller, that is, the corresponding interface charge transfer rate is faster. The results show that the photocatalytic activity of 10PD is the best among all photocatalysts. The electrochemical test results are consistent with the hydrogen production kinetic results and the other characterization experiments described above. Fig. 13 shows Mott-Schottky curves of pure SC-WP and pure CdS samples, respectively. It can be clearly observed that Mott-Schottky curves of SC-WP and pure CdS show positive slope of C2-E plots, proving that they are n-type semiconductor [51]. From the figures, we can get that the flat-band potentials (Efb) of SC-WP and CdS are 0.32 and 0.57 V vs. SCE, respectively. The n-type semiconductor conduction band
potential (ECB) is 0.2 or 0.1 V more negative than the flat band potential [52]. Thus the ECB of SC-WP and CdS are about 0.52 and 0.77 V vs. SCE, respectively. And according to the following formula ENHE ¼ ECB þ 0.241 V, the normal hydrogen electrode potential (ENHE) of SC-WP and CdS can be estimated to be about 0.28 and 0.53 V vs. NHE, respectively. From the previous analysis (in Fig. 6(a)), we can get the Eg of CdS sample is 2.38 eV, and the literature shows that the Eg of SC-WP is about 1.43 eV [53], so the valence band potential (EVB) of CdS and SC-WP sample can be calculated by the formula EVB¼ ECB þ Eg, So the EVB of CdS and SC-WP are about 1.85 and 1.15 V, respectively.
Fig. 12 e EIS plots of pure CdS, 5PD, 10PD, 15PD and 20PD.
Fig. 11 e (a) Transient photocurrent responses curves and (b) LSV curves of pure CdS, 5PD, 10PD, 15PD and 20PD.
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Fig. 13 e Mott-Schottky curses of (a) pure SC-WP and (b) pure CdS NRs.
Speculation on the mechanism of photocatalytic hydrogen production Based on the above experimental results, taking 10% lactic acid solution as a sacrificial agent as an example, we proposed the reaction mechanism of photocatalytic decomposition of water-hydrogen evolution system of XPD composite catalyst, as shown in Fig. 14. The reason for the enhanced activity of XPD composite catalyst in photocatalytic decomposition of hydrogen from water under visible light can be summarized as the following two points: 1) SC-WP nanoparticles have excellent optical properties, improve the visible light absorption rate and absorption light utilization rate of the composite catalyst, which is favorable for improving the photocatalytic activity; 2) The introduction of SC-WP nanoparticles provides a large number of exposed active sites for hydrogen evolution reaction, and is beneficial to the transport and separation of photogenerated charges in the catalytic system, inhibiting the recombination of photogenerated electron-hole pairs, thus improving the utilization of photogenerated electrons produced by visible light excitation.
Fig. 14 e Mechanism diagram of photocatalytic hydrogen production for PD composite.
In detail, under visible light irradiation, the CdS NRs can be stimulated to produce photo-generated electrons and holes. Electrons in the VB of CdS NRs transit to the CB of CdS, Then the photoelectrons on the CB of CdS NRs are quickly captured by SC-WP NPs and are rapidly involved in the hydrogen evolution reduction reaction. At the same time, the holes remain on the VB of CdS and are consumed by lactic acid. This greatly promotes the transmission and separation of photogenerated charges, inhibits the recombination of photogenerated electron-hole pairs, thereby increasing the utilization of photogenerated electrons and greatly increasing the activity of the photocatalytic system. The related charge transfer process is as follows: 8 þ Þ CdS þ hv/CdSðe > CB þ hVB > < CdSðeCB /SC WPðeCB > 2e þ 2H2 O/H2 þ 2OH > : þ CdSðhVB Þ þ LA/LAþ
Conclusions In summary, we successfully prepared semi-crystalline WP and as a cocatalyst to modify CdS NRs to construct SC-WP/CdS Composite catalyst by simple electrostatic self-assembly method for photocatalytic hydrogen evolution. The prepared SC-WP nanoparticles was found has semi-crystalline character and excellent optical properties by low temperature thermal decomposition a mixture of recrystallized Na2WO4$2H2O and NaPO2H2$H2O as precursors. Two high-efficiency and stable composite photocatalytic decomposition hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10% lactic acid solution as sacrificial agents, respectively. Surprisingly, the maximum photocatalytic H2 production rate of 15446.21 mmol h1 g1 is obtained over 10PD composite, which is 10.58 times greater than that of pure CdS. The improved photocatalytic activity can be attributed to the fact that the SC-WP nanoparticles provides a large number of exposed active sites on the surface of CdS for hydrogen evolution reaction, which can efficiently capture photogenerated electrons from CdS nanorods and
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promotes the transport and separation of light-induced charges. And the introduction of SC-WP nanoparticles with excellent optical properties can efficiently enhance the visible light absorption range and the utilization rate of the absorbed light of the PD composite. In addition, the SC-WP nanoparticles show semi-crystalline state, which is also conducive to enhancing the photocatalytic activity.
Author contributions Qiyan Jian and Zhiliang Jin conceived and designed the experiments; Qiyan Jian performed the experiments; Zhiliang Jin and Qingjie Guo contributed reagents/materials and analysis tools; Qiyan Jian wrote the paper.
Acknowledgments This work was financially supported by the Open Project of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University (2019-KF-36), the Chinese National Natural Science Foundation (21862002 and 41663012), the new technology and system for clean energy catalytic production, Major scientific project of North Minzu University (ZDZX201803).
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