Synthesis of novel CoxMo1-xS-Cd0.5Zn0.5S composites with significantly improved photocatalytic hydrogen evolution performance under visible-light illumination

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Synthesis of novel CoxMo1-xS-Cd0.5Zn0.5S composites with significantly improved photocatalytic hydrogen evolution performance under visible-light illumination Aixia Wang a,b, Dongsheng Dai b, Songsong Li b, Nan Xiao b, Boran Xu b, Yangqin Gao b, Lei Ge a,b,* a State Key Laboratory of Heavy Oil Processing, College of New Energy and Materials, China University of Petroleum Beijing, 102249, PR China b Department of Materials Science and Engineering, College of New Energy and Materials, China University of Petroleum Beijing, 102249, PR China

article info

abstract

Article history:

Recently, MoS2 incorporates with Co2þ (or Ni2þ) was found to increase the photocatalytic

Received 5 January 2019

performance of semiconducting materials more effectively. In this study, novel CoxMo1-xS

Received in revised form

was effectively deposited on the surface of Zn0.5Cd0.5S semiconductors as an efficient

31 January 2019

promotor using in-situ hydrothermal process. The as-prepared CoxMo1-xS-Zn0.5Cd0.5S

Accepted 4 February 2019

composites are examined by the following techniques: XRD, TEM, DRS, XPS, PL and TRPL.

Available online 26 February 2019

The photocatalytic hydrogen evolution performance under visible illumination over

Keywords:

xS-Zn0.5Cd0.5S

Photocatalytic water splitting

formance with a homologous hydrogen generation rate of 188.65 mmol h1, which is esti-

Zn0.5Cd0.5S is remarkably increased by adding cheap CoxMo1-xS as promotor. The CoxMo1hybrid specimen with 10% molar amount illustrates the best catalytic per-

CoxMo1-xS

mated to be 14.5 folds than that of unmodified Zn0.5Cd0.5S specimen in the presence of

Hydrogen production

visible light. The apparent quantum yield of Co0.3Mo0.7eZn0.5Cd0.5S sample is determined

Zn0.5Cd0.5S photocatalyst

to be 16.72% at monochromatic light of 420 nm. The experimental outcomes indicate that the synergistic action between CoxMo1-xS and Zn0.5Cd0.5S obviously promotes transfer of photo-induced charge carriers in the hybrid sample. A reasonable catalytic mechanism for the increased photocatalytic performance of CoxMo1-xS promotor was presented and authenticated by TRPL measure, which would present a new notion for the design of ideal semiconductors with plummy photocatalytic capability. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. State Key Laboratory of Heavy Oil Processing, College of New Energy and Materials, China University of Petroleum Beijing, 102249, PR China. E-mail address: [email protected] (L. Ge). https://doi.org/10.1016/j.ijhydene.2019.02.018 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Results and discussion Characterization of CoxMo1-xS-Cd0.5Zn0.5S hybrid samples The crystallographic textures of the composites were detected by XRD technique. The XRD outcomes of the Zn0.5Cd0.5S and CoxMo1-xS-Cd0.5Zn0.5S composites are presented in Fig. 1. The information of the normative card file of CdS in JCPDS No.10454 and ZnS in JCPDS No.12-688 are illustrated for comparison. The diffraction peaks at 26.1 , 46.0 and 54.6 can be normally attributed to the (100), (110) and (112) crystal faces of Zn0.5Cd0.5S [38], respectively. However, no distinct diffraction peaks from CoxMo1-xS phase could be observed. Nevertheless, the presence of CoxMo1-xS species in hybrid samples can be expediently confirmed in the following XPS and TEM discussions. To obtain perspicacity into the microstructures of the asprepared pure Zn0.5Cd0.5S and CoxMo1-xS-Zn0.5Cd0.5S composites, the morphologies were detected by HRTEM and TEM techniques. As illustrated in Fig. 2a and c, the pure Zn0.5Cd0.5S specimen presents a spherical appearance with diameters of 20e30 nm. After the in-situ chemical deposition of CoxMo1-xS on Zn0.5Cd0.5S materials, highly dispersed co-catalysts emerge on Zn0.5Cd0.5S particles which should arise from the deposition of CoxMo1-xS co-catalysts, and no notable variation in the microstructure of Zn0.5Cd0.5S could be discovered. Fig. 2b and d illustrate the HRTEM images of pure Zn0.5Cd0.5S specimen as well as CoxMo1-xS-Zn0.5Cd0.5S hybrid sample, different crystal

ZnS PDF#12-688

(112)

(110)

(002)

The constantly consuming of non-renewable energies of natural gas, coal and oil, and the caused issues of inadequate energy resources and environmental deterioration, are seriously impacting and constraining the mankind progress along with industrial growth [1,2]. Photocatalytic water splitting to produce clean H2 with the help of semiconductor under light illumination has been regarded as a potential approach to resolve the world energy issue [3e9]. In the last of couple decades, tremendous efforts have been focused on exploiting new semiconductors for photocatalytic H2 evolution, including oxides [10e13], metal sulfides [14e17], and oxynitrides [18e20]. However, developing a novel visible light induced catalyst with superior electron-hole segregation capability and outstanding catalytic steadiness to cater to the demands of practical utilizations still reserves enormous challenge. Among those newly developed semiconductor photocatalysts, zinc cadmium sulfide (ZnxCd1-xS) has attracted significant concern because of its compatible band-gap and good catalytic performance [21e23]. As candidate of perspective semiconductor catalysts, ZnxCd1-xS illustrates a lot of preeminent properties for actual utilizations, such as good steadiness, elemental richness and easy to synthesize [24,25]. However, some inherent disadvantages, especially its less specific surface area, rapid electron hole reunion and restricted optical absorption, still remained unresolved. To enhance the photocatalytic activity of ZnxCd1-xS, decorating appropriate promotors to catch photo-generated charge carriers and supply delicately designated active spots for interface redoxing reactivity is a promising method [26e29]. Although the cost of noble metals is quite expensive, their outstanding catalytic activities and high physiochemical stability ensure them as the most effective co-catalyst today [30e32]. Therefore, considering from the economic perspective, exploring low-cost co-catalysts as effective as the noble one is a significative task for practical application of photocatalytic hydrogen evolution. In recent years, as a typical laminated structured metal sulfide, molybdenum disulfide (MoS2) has received tremendous concern due to its unique electro and photo features [33e35]. Interestingly, recent investigations revealed that substituting the cations in the MoS2 with transition metal ions such as Co2þ or Ni2þ can effectively increase the catalytic activity of MoS2. Sun's group reported a noble promotor which MoS2 nanosheet was incorporated with Co2þ (or Ni2þ) ions to facilitate the production of many unsaturated reactive spots for H2 evolution, then resulted in a promoted onset potential and inhibited charge carriers recombination [36]. Zheng’ group prepared 1D CdS@NiMoS core-shell nanowire composites that presented improved photocatalytic performance for the hydrogen evolution [37]. Motivated by the above research, herein we have synthesized CoxMo1-xS-Zn0.5Cd0.5S composites via in-situ hydrothermal method for photocatalytic H2 release via water splitting. The catalytic performance of Zn0.5Cd0.5S was remarkably increased after introducing the CoxMo1-xS as a cocatalyst. The visible light induced catalytic hydrogen production steadiness for CoxMo1-xS-Zn0.5Cd0.5S was also performed.

On account of the experimental outcomes, a reasonable catalytic mechanism for improved hydrogen release performance was presented. The outcome illustrates that CoxMo1-xS can play as an efficient promotor in Zn0.5Cd0.5S materials, and it provides us inspiration that the promotor should be potentially utilized in other catalytic materials for hydrogen production.

Intensity(a.u.)

Introduction

Zn0.5Cd0.5S

CdS PDF#10-454

20

30

40

50

60

70

2θ/degree Fig. 1 e XRD patterns of Zn0.5Cd0.5S-samples.

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Fig. 2 e TEM images of Zn0.5Cd0.5S sample (a), CoxMo1-xS-Zn0.5Cd0.5S composite (c); HRTEM image of Zn0.5Cd0.5S sample (b), CoxMo1-xS-Zn0.5Cd0.5S composite (d).

orientations and crystal plane spacing spacings are revealed. The (002) crystal plane with lattice stripe of 0.32 nm in Zn0.5Cd0.5S phase can be distinguished [39,40]. After the introduction of CoxMo1-xS species, a new lattice stripe of 0.29 nm is detected that should be attributed to (200) crystallographic face of CoxMo1-xS species. Fig. 3 displays TEM mapping of Co0.05Mo0.95eZn0.5Cd0.5S sample, from which the characteristic signals of Cd, Co Zn, Mo and S is evidently detected. The Cd, Co Zn, Mo and S elements are distributed evenly on the surface of Co0.05Mo0.95eZn0.5Cd0.5S sample, which further proved that the Co0.05Mo0.95SeZn0.5Cd0.5S photocatalytic materials were successfully synthesized during the hydrothermal process. The TEM outcomes illustrate that tight boundaries are produced between Zn0.5Cd0.5S and CoxMo1-xS co-catalyst in the hybrid sample. The intimate interfaces play an important role for efficacious increasement of segregation behaviors of photo-induced charge carriers. To detect the light harvesting performance, the obtained specimens were tested on a Shimadzu UV-4100 spectrophotometer. Fig. 4 presents that the pure Zn0.5Cd0.5S specimen shows significant light absorption with a sharp decrease at ~550 nm, which is attributed to the band gap transition of charge carriers. The band gap width of pure Zn0.5Cd0.5S is estimated to be 2.2 eV calculated from Tauc's plot which is depicted in the inset of Fig. 4, and the outcome has been consistent with the result presented in the literature [31].

After presence of Co0.05Mo0.95S as co-catalyst, the Co0.05Mo0.95eZn0.5Cd0.5S hybrid samples exhibit a homologous DRS curve in appearance, and illustrate significantly increased optical absorbance strength in visible light area in comparison to Zn0.5Cd0.5S. The optical absorbance intensity of the asprepared samples was further promoted by increasing the decorating amount of Co0.05Mo0.95S co-catalyst, the outcome has been consistent with the colour variety from buff to charcoal grey. Nevertheless, when the loading amount of Co0.05Mo0.95S exceeds 10%, the hydrogen production efficiency starts to decrease, the possible reason is the competition of optical absorption between Zn0.5Cd0.5S and Co0.05Mo0.95S. As the depositing content of Co0.05Mo0.95S increases, Co0.05Mo0.95S blocks the surface of zinc cadmium sulfides, which inhibits optical absorption of zinc cadmium sulfide and then reduces the quantity of photo-generated charge carriers. The Co0.3Mo0.7eZn0.5Cd0.5S hybrid sample was conducted by the XPS technique to analyze the chemical binding environment of elements. It can be discovered that the binding energies of Zn 2p1/2 and Zn 2p3/2 (Fig. 5a) are found at 1044.9 eV and 1021.9 eV, illustrating that chemical valency of Zn component is determined to be þ2. Fig. 5b presents that the XPS peaks for Cd 3d3/2 and Cd 3d5/2 are ascertained to be 411.7 eV as well as 404.8 eV, and the result affirms emerge of Cd2þ ions. The experimental outcome is consistent with our previous studies [39,40]. As presented in Fig. 5c, three

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Fig. 3 e TEM mapping of 10% Co0.05Mo0.95SeZn0.5Cd0.5S composite.

photoelectron peaks with binding energies of 161.6 eV, 162.1 eV and 163.1 eV for the S 2p orbital are separated, and the XPS peaks are ascribed to S2 valence [38]. Fig. 5d illustrates that Co 2p1/2 and Co 2p3/2 orbital is consistent with binding energies at 793.7 eV and 778.3 eV in the Co0.3Mo0.7S specimen. Moreover, Fig. 5e presents XPS peaks at 232.09 eV and 235.4 eV, which belong to the Mo (3d) species in Co0.3Mo0.7S cocatalysts, can be distinctly distinguished [37]. According to the testing outcome, the CoxMo1-xS species has been

Fig. 4 e UVevisible adsorption spectra of pure Zn0.5Cd0.5S, and Co0.05Mo0.95eZn0.5Cd0.5S composites with different molar ratios of Co0.05Mo0.95S.

successfully synthesized and decorated on surface of zinc cadmium sulfide.

Photocatalytic hydrogen production activity The visible light induced catalytic performance of obtained specimens was evaluated in Na2S/Na2SO3 sacrificial reagent solution; the outcomes were illustrated in Fig. 6. Catalytic test of hydrogen production assessment over Co0.05Mo0.95S specimen was performed, and no H2 evolution activity could be detected for the pure Co0.05Mo0.95S sample. The pure Zn0.5Cd0.5S sample without Co0.05Mo0.95S presents a poor catalytic hydrogen releasing rate of 13.02 mmol h1. By adding Co0.05Mo0.95S co-catalyst, the catalytic H2 evolution performance is remarkably improved. As illustrated in Fig. 7, the hydrogen releasing performances of Co0.05Mo0.95eZn0.5Cd0.5S hybrid samples are conducted. After depositing 2% Co0.05Mo0.95S on Zn0.5Cd0.5S surface, the hydrogen production activity is evidently enhanced to 47.19 mmol h1. The hydrogen production activity is continuously improved with increasing the loading content of Co0.05Mo0.95S species. The 10% Co0.05Mo0.95eZn0.5Cd0.5S sample depicts the highest hydrogen releasing performance with a hydrogen production speed of 103.62 mmol h1; and the activity is 8 folds than zinc cadmium sulfide sample. Nevertheless, continue enhance of decorating amount of Co0.05Mo0.95S results in decrease of hydrogen releasing speed. The drop of catalytic performance should be induced by increased decorating ratio of Co0.05Mo0.95S cocatalysts, which would prevent absorption of incident light of Zn0.5Cd0.5S, resulting in decline of quantity of photoinduced electron-hole pairs. The DRS result also presents

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Fig. 5 e XPS spectra of Co0.3Mo0.7eZn0.5Cd0.5S samples: (a) Zn 2p; (b) Cd 3d; (c) S 2p; (d) Co 2p; (e)Mo 3d

that CoxMo1-xS would compete with Zn0.5Cd0.5S in adsorbing the incident light. Accordingly, the rivalry of optical absorption between Zn0.5Cd0.5S and Co0.05Mo0.95S should be another key parameter which prevents the further enhancement in the catalytic hydrogen releasing performance. Fig. 8 gives catalytic hydrogen production performance of the CoxMo1-xSeZn0.5Cd0.5S (x ¼ 0, 0.05, 0.3, 0.5, 0.7, 0.9, 1) samples with diverse Co and Mo ratios. The Co0.3Mo0.7Se Zn0.5Cd0.5S specimen (the mole ratio of Co to Mo is 3:7) illustrates optimal hydrogen releasing performance with

hydrogen production speed of 188.65 mmol h1, and the catalytic activity is 14.5 folds when compare to zinc cadmium sulfide (Zn0.5Cd0.5S). The improved photocatalytic performance of CoxMo1-xS-Zn0.5Cd0.5S samples may originate from several reasons. First, CoxMo1-xS co-catalyst has high electron catching capability. The Zn0.5Cd0.5S semiconductor and CoxMo1-xS co-catalyst have different work functions, and leads to formation of built-in electric field at interfaces, which plays as promoting power to boost the efficient segregation of photo-excited charge carriers. Moreover, the close contacts

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Fig. 6 e Photocatalytic H2 evolution over Co0.05Mo0.95e Zn0.5Cd0.5S composite samples with different Co0.05Mo0.95S contents under visible light.

between CoxMo1-xS and Zn0.5Cd0.5S obviously extend the period of electron-hole pairs in the CoxMo1-xS decorated specimens. The abundant interfaces generate in the period of in-situ preparing procedure could offer “super highway” to favor rapid conveying of photo-generated charges pass through the interfaces. Therefore, a suitable molar ratio of Co to Mo in the CoxMo1-xS species is also vital for optimizing catalytic performance of hybrid samples. To reveal the stability of CoxMo1-xS-Zn0.5Cd0.5S specimens, the catalytic activity of 10% Co0.3Mo0.7SeZn0.5Cd0.5S and 10% Co0.05Mo0.95eZn0.5Cd0.5S samples were continuously performed by cycle tests of catalytic hydrogen releasing. Fig. 9 gives the hydrogen production performance in cycle tests. The hydrogen releasing rate does not show significant decline after reacted for 4 cycles, and the result illustrates that 10%-

Fig. 7 e Rate of H2 evolution over Co0.05Mo0.95eZn0.5Cd0.5S composite samples with different Co0.05Mo0.95S (Co:Mo ¼ 1:19) contents.

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Fig. 8 e Rate of H2 evolution 10% CoxMo1-xSeZn0.5Cd0.5S composite samples. CoxMo1-xS-Zn0.5Cd0.5S hybrid sample shows sufficient steadiness for hydrogen evolution via catalytic water splitting. The apparent quantum yield of H2 production for 10% Co0.3Mo0.7eZn0.5Cd0.5S hybrid sample was conducted. In the experiment, monochromatic light source comes from 300 W xenon light with 420 nm band pass filter. The AQY experiment lasted for 5 h. In the period the experiment, the optical intensity of the homogeneous light was tested by irradiometer for every 1 h. The AQY result was measured based on Eq. (1) as presented in Experimental part and the outcomes are depicted in Table 1. Table 1 presents that the AQY of hybrid specimen is determined to be 16.72% in the initial 1 h and maintains steady for the next 4 h.

Photocatalytic mechanism discussion According to the characterization results of catalytic experiments of CoxMo1-xS-Zn0.5Cd0.5S, a reasonable mechanism for

Fig. 9 e Cycling runs for the photocatalytic H2 evolution in the presence of the 10% Co0.05Mo0.95S/Zn0.5Cd0.5S and 10% Co0.3Mo0.7eZn0.5Cd0.5S composite sample under visible light irradiation.

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Table. 1 e Apparent quantum yield (AQY) and light intensity over 10% Co0.3Mo0.7eZn0.5Cd0.5S composite sample at different time. Irradiation Time (h) Light intensity (mW/cm2) AQY (%)

1

2

3

4

5

5.1 16.72

5.0 15.89

4.9 15.23

5.0 14.95

5.1 15.31

catalytic hydrogen releasing of CoxMo1-xS-Zn0.5Cd0.5S hybrid sample is presented. Fig. 10 gives the detailed catalytic mechanism; Zn0.5Cd0.5S semiconductor can absorb visible light and produce charge carriers under xenon lamp illumination. Nevertheless, if there is no CoxMo1-xS species, the photo-induced charge carriers are inclined to reunite fleetly, leading to lower catalytic hydrogen releasing performance. After depositing CoxMo1-xS on the Zn0.5Cd0.5S surface, CoxMo1xS shows effective electron trapping capability, in which way the photo-induced electrons conveying from Zn0.5Cd0.5S to CoxMo1-xS species is promoted and attend in the Hþ to H2

Fig. 10 e Schematic illustration for the charge transfer and separation in CoxMo1-xS-Zn0.5Cd0.5S system and proposed mechanism for photocatalytic H2 production under visible light irradiation.

reducing reaction, resulting in effective segregation of charge carriers. Consequently, CoxMo1-xS species could play as electron accumulator and conveyor to extend the longevity of electron hole pairs, then enhancing the probability of photoinduced electrons to partake in the redox process to reduce Hþ to hydrogen. The photo-generated holes would be gathered in Zn0.5Cd0.5S component and are consumed by Na2S and Na2SO3 sacrificial agentia. Therefore, the reunion procedure of charge carriers is effectually prevented, leading to distinctly enhancement of the hydrogen releasing performance for the CoxMo1-xS-Zn0.5Cd0.5S hybrid catalysts. To prove the mechanism of improved catalytic activity in hybrid samples, the Zn0.5Cd0.5S and 10% Co0.3Mo0.7e Zn0.5Cd0.5S samples were tested by the photoluminescence (PL) spectra, and the result was useful to probe the recombination strength of photo-induced electron-holes. Higher reunion of photo-induced charge carriers usually leads to

Fig. 11 e (a) PL spectra of Zn0.5Cd0.5S and 10% Co0.3Mo0.7Se Zn0.5Cd0.5S samples. (b) The time-resolved fluorescence spectra of Zn0.5Cd0.5S and 10% Co0.3Mo0.7eZn0.5Cd0.5S samples.

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lower catalytic performance. Fig. 11a illustrates that the PL intensity of Zn0.5Cd0.5S sample is obviously decreased after introducing of Co0.3Mo0.7S co-catalyst. The PL characterization outcomes clearly indicate that surface decoration of Co0.3Mo0.7S is propitious to inhibiting the reunion procedure of photo-excited charge carriers, which would lead to enhanced hydrogen releasing performance [40]. To gain further proof on the presented catalytic mechanism, the dynamical action of photo-induced electron-hole pairs was also characterised by TRPL (time-resolved fluorescence spectroscopy) technique (as shown in Fig. 11b). The TRPL decline lifetime of obtained samples improves in the following sequence: Zn0.5Cd0.5S < Co0.3Mo0.7eZn0.5Cd0.5S, the testing result indicates that the obtained lifetime was 1.01 ns and 1.24 ns. The TRPL outcomes distinctly prove that the reunion of photoinduced electron-hole pairs is effectively inhibited in Co0.3Mo0.7eZn0.5Cd0.5S hybrid samples [41,42].

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Conclusions In this study, a novel CoxMo1-xS-Zn0.5Cd0.5S hybrid samples were prepared via an in-situ hydrothermal process. The CoxMo1-xS is evenly decorated on Zn0.5Cd0.5S particles and produce close contacting areas. An optimal photocatalytic hydrogen production rate is determined to be 188.65 mmol h1 over 10% Co0.3Mo0.7eZn0.5Cd0.5S hybrid specimen; and it is 14.5 folds than Zn0.5Cd0.5S. Finally, a reasonable catalytic mechanism is presented. The enhancement of hydrogen releasing performance over CoxMo1-xS-Zn0.5Cd0.5S hybrid sample is explained as the inhibited charge reunion and boosted charge conveying at the CoxMo1-xS-Zn0.5Cd0.5S contacting area. The proposed photocatalytic mechanism was proved by PL and TRPL techniques. Hence, the experimental outcomes illustrate that CoxMo1-xS is promising species for Zn0.5Cd0.5S semiconductor, and it could be also potentially applied to increase the catalytic hydrogen releasing performance of other semiconductors.

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Acknowledgements This work was financially supported by the National Science Foundation of China (Grant No. 51572295, 21273285 and 21003157), Beijing Nova Program (Grant No. 2008B76), and Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-06-20 and 2462016YXBS05).

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2019.02.018.

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