Phase-Modificate Defects Engineering CdS Sphalerite-Wurtzite

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Kinetics, Catalysis, and Reaction Engineering

Phase-modificate Defects Engineering CdS Sphalerite-wurtzite System for Efficient Photocatalytic H2 Evolution Under Visible Light Irradiation chengxin zhou, Haowei Yang, Jinwei Chen, Gang Wang, Chunping Jiang, and Ruilin Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00512 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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Industrial & Engineering Chemistry Research

Phase-modificate Defects Engineering CdS Sphalerite-wurtzite System for Efficient Photocatalytic H2 Evolution Under Visible Light Irradiation Chengxin Zhoua, Haowei Yanga, Jinwei Chena, Gang Wanga, Chunping Jianga,b*, Ruilin Wanga* a

College of Materials Science and Engineering, Sichuan University, Chengdu 610065,

The People’s Republic of China. b

West China school of Public health No 4 west China teaching hospital, Sichuan

University, Chengdu 610041, The People’s Republic of China.

ABSTRACT According to quantum size effect and interface defects theory, the phase-modificate defects engineering photocatalyst MoS2/CdS-1/CdS-2 with smaller lattice mismatch degree was successfully designed through simple hydrothermal and solvothermal methods, in which sphalerite CdS-1 was nanoparticle (NP) while wurtzite CdS-2 was nanosheet (NS) shaped. The results showed H2 evolution activity was increased, which was about 20 times and 30 times than that of MoS2/CdS-1 and MoS2/CdS-2, respectively. Also of note, 600 times and 900 times compare to CdS-1 and CdS-2 severally. According with calculation results, there were less interface states because of smaller lattice mismatch degree. The absorbance was enhanced by realizing more than once reflects and absorbs of ray. The scope of ray absorption was extended on account of the quantum scale effect. It is hoped to fundamentally enhance the photocatalytic activity of CdS by self-modifying and defects engineering. *Corresponding authors. Emails: [email protected] & [email protected]

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KEY WORDS: CdS sphalerite-wurtzite; Phase-modificate; Defect engineering; Quantum size effect; Multiple reflection; Photocatalytic H2 Evolution. 1. Introduction Environmental problems and clean energy seem to be a pair of paradox nowadays, it is necessary to obtain sustainable and clean energy. Conversion solar energy to chemical bond energy with photocatalyst has received much attention1,2. Many materials have been studied, however, it is still far from the requirements of industrial application because of the disappointing photocatalytic activity, which is mainly caused by the narrow solar spectral response and high electron-hole recombination rate3,4. Therefore, exploring catalyzer with broad light absorption scope and effective carriers’ severance performance is highly necessary. As an n-type II-VI semiconductor, CdS has a band gap of 2.4 eV, which can absorb solar light up to 520 nm5-7. The conduction band (CB) potential of CdS stays at a lower position compared to that the voltage of hydrogen ions reduce to hydrogen, making it proper for hydrogen evolution. The photocatalytic activity of pure CdS is nevertheless negligible due to the fast electron-hole recombination and serious photocorrosion8-10. The photocorrosion process is as shown in the following equations: CdS + hν

h+ + e -

(1)

2h+ + H2O

1/2O2 + 2H+

(2)

e - + H+

1/2H2

2h++ CdS

(3)

Cd2+ + S

(4)

As shown above, cadmium sulfide is labile and easy to be decomposed into sulphur, 2


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then release poisonous cadmium ions under irradiation11-13. Many methods have been investigated to inhibit the rates of both CdS photocorrosion and the recombination photogenerated carriers, such as constructing heterojunctions, introducing cocatalysts and so on14,15. Precious metals, for instance, platinum, aurum are utilized to be cocatalysts, however, they are too expensive to be widely used, therefore, it’s also significant to develop inexpensive and earth-abundant efficient cocatalysts16-18. MoS2, has intercalated construction of trilaminar atomic shell (S-Mo-S) fitted together through intermolecular force, already has been applied to many aspects19-22. Simulation by density functional theory indicated that the Helmholtz function of H bond to MoS2 is on the brink of nought, which can be mentioned in the same breath with platinum and make it a kind of prospective inexpensive and massive cocatalyst. Various research results testified that it is hopeful to become an effective cocatalyst on account of the quite small forbidden bandwidth and representative laminated construction23-25. However, the photocatalytic activity for hydrogen evolution by MoS2 embellished CdS is far below meeting the needs of industrial application26-29. Many researchers keep on looking for materials with smaller forbidden bandwidths and suitable positions to modify MoS2/CdS to further strengthen its property30-33. For example, Yu et al. reported firstly a facile one-pot strategy with biomolecule assistance to fabricate novel hollow spheres, which led to a superior high photoactive activity for water splitting by visible light without noble metals26. Qin et al. successfully prepared the TiO2 supported MoS2/CdS composite photocatalyst, it revealed that the titanium dioxide nanomaterial can be applied to enhance the scatter 3



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degree of other substances28. Zn0.5Cd0.5S(ZCS) nanorods with MoS2/RGO cocatalysts were reported by Guo et al. and synthesized by simultaneous reduction reactions. The photocatalysts showed the best hydrogen evolution rate up to activity of 2310 µmol/h30. However, the methods mentioned above still cannot solve the CdS photocorrosion problem fundamentally. Therefore, we aim to construct a CdS self-modified defects-engineering system with fewer surface states. Based on the quantum

effect,

the forbidden bandwidth of a semiconductor material can be changed by nanoparticle size, the smaller the size, the larger the band gap21,25,34-36. Therefore, the transformation of CdS from block to nanoscale leads to a deviation of absorb scope towards shortwave direction. Thus, it may be possible to realize effective separation of carriers in CdS phase-modificate composite system because of the existence of energy level differences. What’s more, there will be less surface states because of the lower lattice mismatch degree. To confirm this conjecture, we designed CdS self-modified defects engineering structure MoS2/CdS-1/CdS-2, in which CdS-1 was nanoparticle (NP) and CdS-2 was nanosheet (NS) shaped, in this paper we’ll collectively refer to as CdS-1 and CdS-2. 2. Material and methods 2.1 Materials The materials used in this paper are of analytical grade without further treatment, they are cadmium chloride hemipentahydrate (2CdCl2·5H2O) (Chengdu Kelong Chemical Reagent Factory), sodium thiosulfate (Na2S2O3·5H2O) (Chengdu Kelong Chemical 4


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Reagent Factory), sodium sulfide (Na2S·9H2O) (Chengdu Kelong Chemical Reagent Factory),

sodium

molybdat

dihydrat

(Na2MoO4·2H2O)

(JDC

MOLY

SCIENCE&TECHNOLOGY CO.LTD MOLY CHEM DIVISION), thiacetamide (C2H5NS) (Chengdu Kelong Chemical Reagent Factory) and ethylenediamine (C2H8N2) (Chengdu Kelong Chemical Reagent Factory). 2.2 Preparation of the specimens 2.2.1 Preparation of MoS2 0.01 mol sodium molybdate and 0.01 mol thioacetamide were added into 80 ml deionized water, then agitated vigorously for half an hour. Afterwards, the suspension was put into a 100 ml Teflon-lined stainless-steel autoclave and reacted at 180 °C for 16 hours. MoS2 was obtained after centrifugation and washing with deionized water and ethanol for several times, then dried at 80 °C for several hours. 2.2.2 Preparation of MoS2/CdS-1 The previously synthesized MoS2 was scattered in 60 ml deionized water. 0.012 mol 2CdCl2·5H2O was added into the above suspension, then 0.012 mol sodium thiosulfate was mixed and last agitating for several hours. Afterwards, the suspension was sealed in a 100 ml Teflon-lined stainless-steel autoclave and reacted at 80 °C for 2 hours. The precipitate was cleaned repeatedly and dried at 80 °C for several hours. CdS-1 was constructed follow the identical method above without adding MoS2. 2.2.3 Preparation of MoS2/CdS-1/CdS-2 The as prepared MoS2/CdS-1 was scattered in 40 ml ethylenediamine and ultrasound treatment for half an hour to make it evenly spread. Then 0.91 g cadmium chloride 5



hemipentahydrate and 0.96 g sodium sulphide were dissolved in the suspension with agitating. A few hours later, the suspension was sealed in a 50 ml Teflon-lined stainless-steel autoclave and heated for 12 hours. The finally outcome was obtained after centrifuging, then cleaned repeatedly, followed by drying for several hours. CdS-2 was prepared in the same way above without adding MoS2/CdS-1. 2.3 Characterization The X-ray diffraction equipment (XRD; D/MAX-2000/PC, Rigaku Corp.) possesses a monochromatic copper Kα radiation (λ=0.15406 nm) and a scanning range of 10−60° was applied to test the specimens’ crystal structures. Then the X-ray photoelectron spectrometer (XPS) was used to characterize the surface state of the composite. SEM images were achieved by field-emission SEM (Hitachi S-4800) tests. The specific surface area and porosity properties were carried out utilizing the Gemini VII 2390 equipment. The spectrophotometer (UV3600) was used to investigate the specimens’ ultraviolet-visible diffusing reflection spectrometry patterns. The fluorometry (F-7000, Hitachi) was used to investigate the photoluminescence spectra (PL). Then,the computer controlled impedance measurement unit (CHI660E, Shanghai Chenghua Instruments) was used to carry out the specimens’ photoelectrochemistry performances. 2.4 Photocatalytic reaction Photocatalytic H2 evolution experiments were executed by Perfectlight Labsolution (Peking). 50 mg photocatalyst was added into a 100 ml of mixing liquid including 10% volume fraction C3H6O3 as hole sacrificial reagent. An optical filter (λ>420 nm, 6


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AM=1.5) was used to cut off the ultraviolet light. The hydrogen production efficiency was monitored utilizing the chromatographic instrument (SHIMADZU GC-2014) per hour. For the sake of moving away O2, the whole system including solution were deggased by N2 for several minutes prior to illumination. The equipment’s environment maintained at about 10 °C by thermostatic waterbath. 2.5 Electrochemical tests Electrochemical tests were carried out via the equipment of CHI660E (Shanghai Chenghua Instruments). The specimens to be tested were prepared through point spreading method, and the coated region is approximately 1 cm × 1 cm, at last, drying at 80 °C for 3 h. The carbon electrode was used as the counter electrode, while the Ag/AgCl electrode as the reference electrode. For transient light current tests, we utilized the 0.2 mol･L-1 sodium sulphate aqueous solution to be electrolyte. The xenon light source (300 W) was used. For electrochemical impedance spectroscopy (EIS) experiments, the frequency domain was chosen as 10-2-106 Hz, while ac amplitude was 0.005 V. Here, as for the electrolyte, the certain concentration mixing solution of C3H6O3 was used. For Mott–Schottky measurements, 0.2 mol･L-1 of sodium sulphate aqueous was used to be electrolyte. 3. Results and discussion 3.1 Catalyst structure The XRD results of the specimens are as shown in Figure 1. As shown in Figure 1(a), consulted to standardized data PDF#37-1492, the key diffraction maximums situated in 14.4°, 39.5° and 49.7° belong to hexangular MoS2. Just like exhibited in Figure 7



1(b), consulted to standardized data PDF#75-1546, the three key diffraction maximums located in 26.5°, 44.0° and 52.2° belong to cubical CdS. It can be seen in Figure 1(d), the three main diffraction maximums located in 24.8°, 28.2° and 47.9° belong to hexangular CdS, consulted to standardized data PDF#77-2306. As for the result of MoS2/CdS-1, just like exhibited in Figure 1(c), referred to standardized data PDF#75-1546, there were three main diffraction maximums severally situated in 2θ=26.5°, 44.0° and 52.2°, which in consistent with (111), (220) and (311) crystallographic faces of cubical CdS, the crystal lattice parameters are a=b=c=0.582 nm. Through analyzing the main phase of the samples, the three lines located in 26.4°, 43.8° and 52.0° correspond well with the standard diffraction peaks of cubical CdS, while there were slight misregistrations as the result of coupling effect between MoS2 and CdS-1. Meanwhile, the three key diffraction maximums situated in 2θ=15.5°, 39.8° and 49.0° are in consistent with (002), (103) and (105) crystallographic faces of hexangular MoS2 (PDF#37-1492), the deviations were exist compared with the standardized angles situated in 14.4°, 39.5° and 49.7° due to the coupling influence among various substances. As for the XRD result of MoS2/CdS-1/CdS-2 shown in Figure 1(e), we can see that the diffraction peaks of MoS2 and CdS-1 are corresponding well with the peaks of MoS2 and CdS-1 in MoS2/CdS-1, respectively. The three more additional maximums situated in 2θ=24.9°, 28.1°, and 47.9° are match well with (100), (101) and (103) crystallographic faces of hexangular CdS (PDF#77-2306). The crystal lattice parameters are a=b=0.414 nm, c=0.671 nm. The XRD results indicate the successful formation of MoS2, CdS-1 and CdS-2. The 8


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diffraction peaks of MoS2 and CdS-1 in MoS2/CdS-1/CdS-2 have slight misregistrations compare to the diffraction peaks in MoS2/CdS-1, which indicates the contact formed between CdS-1 and CdS-2.

PDF#37-1492 PDF#75-1546 PDF#77-2306

(111) (100)

(e)

(101)

(002)

(d)

(100)

(101)

(220) (103)(311) (103) (105) (103)

(c) (002)

(220) (311) (105) (103) (220) (311)

(111)

(b) (111)

(a)(002)

(103)

(105)

Figure 1. XRD figures of (a)MoS2, (b)CdS-1, (c)MoS2/CdS-1, (d)CdS-2 and (e)MoS2/CdS-1/CdS-2. 3.2. Composition of the catalyst In this section, XPS was utilized to ulteriorly measure what element and what valence they are in the photocatalysts. The XPS pattern of Figure 2 (c) proves that it contains the Cd, S and C. Figure 3 (d) indicates that the Cd, Mo, S and C exist. The pattern revealed in Figure 2 (a) indicates maximums at 411.2 and 404.4 eV severally matching with the representative bound energies of Cd2+ 3d3/2 and Cd2+ 3d5/2. The fabrication of CdS can be indicated by the division of energy of 6.8 eV. Figure 2 (b) exhibits the S 2p located at 161.7 and 160.6 eV, which is matching well with the 9



bound energies of S 2p1/2 and S 2p3/2. Figure 3 (a) indicates maximums at 411.3 and 404.5 eV severally matching with the representative bound energies of Cd2+ 3d3/2 and Cd2+ 3d5/2. XPS spectrum in Figure 3 (b) displays the S 2p located at 161.9 and 160.8 eV, which is matching well with the bound energies of S 2p1/2 and S 2p3/2. The characteristic bound energies of Cd and S have slight deviations compare to that of in CdS-2, maybe caused by the interfacial contacts formed between the three substances. The XPS peaks in Figure 3 (c) displays maximums at 231.4 and 225.0 eV severally matching well with the representative bound energies of Mo 3d3/2 and Mo 3d5/2. The carbon peak is from exogenous carbon. As we know, S2- (VS) will be oxidized into elemental S in the photocorrosion process. When there are sulphur vacancies, S2- are tending to be absorbed on the active sites of sulphur vacancies, then realize recrystallization of CdS. Seen from the XPS results of CdS-2 shown in Table 1, the ratio of atomic percentage of S/Cd=16.66/16.71420 nm, AM=1.5) as illuminant. The properties of CdS-1(15 µmol/h), CdS-2 (10 µmol/h) MoS2/CdS-1 (460 µmol/h), MoS2/CdS-2 (290 µmol/h), the mechanical mixing of MoS2/CdS-1 and MoS2/CdS-2 (380 µmol/h) and MoS2/CdS-1/CdS-2 (9000 µmol/h) are as exhibited in Figure 11(a). The hydrogen evolution was 9000 µmol/h as the mass fraction of CdS-2 was 25% without utilizing any noble metal, which was almost twenty-fold higher than MoS2/CdS-1 and thirty-fold higher than MoS2/CdS-2. As we know, pure CdS showed negligible photocatalytic performance on account of the serious photocorrosion and facile compounding of carriers. When mixing MoS2/CdS-1 and MoS2/CdS-2 by mechanical means, the photocatalytic hydrogen evolution was far below than that of MoS2/CdS-1/CdS-2. The result indicated that superficies combination was formed between CdS-1 and CdS-2 rather than simple mechanical mixing, it also illustrated the importance of the formation of surface contacts. The CdS-1/CdS-2 showed negligible photocatalytic activity, therefore, we 23



can say that MoS2 worked as a pivotal ingredient. Displayed in Figure 11(b) are the photocatalytic properties of 20 wt% MoS2/CdS-1/CdS-2 (4900 µmol/h), 25 wt% MoS2/CdS-1/CdS-2 (9000 µmol/h), 30 wt% MoS2/CdS-1/CdS-2 (4800 µmol/h). We can conclude from the above results that while the mass percentage of CdS-2 in MoS2/CdS-1/CdS-2 was 25 wt%, the photocatalyst put up higher photocatalytic activity. When the content of CdS-2 is appropriate, there will retain a lot of space between different materials to ensure sufficient contact with reactants, so the catalytic performance can be improved by realizing multiple reflection and absorption of light. However, too much CdS-2 will prevent the light absorption of CdS-1, and the decrease of interspace is not conducive to the contact between catalyst and reactant. And as displayed in Figure 11 (c,d) are the specimens’ stability in at least four hours, we can conclude that they are stable. We can draw a conclusion from Figure 11 (e,g) that there was no significant decline of photocatalytic performance for at the fewest twelve hours. For the sake of further testify the catalysts’ stabilization, we executed four successive H2 production rounds in the same experimental environment. Every round last for four hours and test for four rounds repeatedly, as Figure 11 (f,h) displayed. The outcomes prove that both MoS2/CdS-1, MoS2/CdS-2 and MoS2/CdS-1/CdS-2 are all repeatable and steady. And these outcomes also show great agreement with other characterization results.

24


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(a)

(b)

(c)

(d)

(f)

(e)

(g)

(h)

Figure 11. Photocatalytic hydrogen production efficiency of CdS-1, CdS-2, MoS2/CdS-1, MoS2/CdS-2, mechanical mixing of MoS2/CdS-1 and MoS2/CdS-2, MoS2/CdS-1/CdS-2 (a, c). H2 evolution performance (b) and stabilization (d) of MoS2/CdS-1/CdS-2 with different mass fractions. Stability (e) and cyclicity (f) of 25



MoS2/CdS-1 and MoS2/CdS-2. Stability (g) and cyclicity (h) of MoS2/CdS-1/CdS-2. 3.7 Analysis of the principles of increased photocatalytic performance According to the outcomes, we can draw a conclusion that MoS2/CdS-1/CdS-2 processes an enhanced photocatalytic activity. Maybe the principles can be explained as below: (1) MoS2 is a kind of prospective non-precious metal cocatalyst to take the place of precious metal with lower activation potential and more active hydrogen evolution sites. It processes a small forbidden bandwidth to extend the scope of ray absorb. (2) According to the Mott-Schottky test results, we can draw a conclusion that the potentials of bottom of the conduction band and top of the valence band in CdS-1 and CdS-2 are different, therefore, the carriers can realize migration between different materials. The abruption of carriers can be facilitated on account of the superficies combination formed between CdS-1 and CdS-2. CdS-1 and CdS-2 are all n-type semiconductor, therefore, we can regard the system as n-N type homogeneous structure. Combining basic knowledge, the narrow band gap side is electronic accumulation layer, the wide band gap side is depletion layer (n stands for the smaller forbidden bandwidth n-style substance and N stands for the bigger forbidden bandwidth n-style substance). Therefore, electrons accumulate in CdS-2, which can effectively restrain the compounding of carriers. (3) For the CdS self-modified system, their contact surface is (111) crystal plane, so it can be simplified as a two-dimensional system, and further combining the transformation formula of interplanar spacing and lattice constant, we can consider 26


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that the interplanar spacing(d) can be used to replace the lattice constant(a) to calculate the lattice mismatch. Therefore, the lattice mismatch degree can be calculated by the following formula38,39:

δ=2|d1-d2|/(d1+d2)

(10)

Where d1 and d2 stand for the interplanar spacings of CdS-1 and CdS-2, respectively. Seen from the XRD and TEM results, d1=0.335 nm, d2=0.336 nm, so the lattice mismatch degree of CdS-1 and CdS-2 is calculated to be 0.29 %, which is much smaller than that of different substances. According to the calculation and TEM results, we can see that CdS-1 and CdS-2 with smaller lattice mismatch degree and the defect density is lower. As we know, defects will be formed at the interface due to lattice mismatch and too many defects will become carriers’ recombination center. In order to obtain more ideal heterojunctions, materials with smaller lattice mismatch degree should be chosen firstly. Also of note, the interaction force between the same kind of materials is stronger. When there are defects in the surface of semiconductor materials, electronic states will appear in the forbidden band, namely, the surface states. The dangling bond and defect level may be existing in the forbidden band to form interface state. Interface states may also be formed because the thermal expansion coefficients are different for disparate materials and the inter diffuse of the elements in the compound semiconductors. The surface states will be charged by the photogenerated carriers, thereby limit the water splitting reaction. Theoretically, there will be less interface states in the system. Therefore, it may explain why MoS2/CdS-1/CdS-2 showed the 27



enhanced performance. The growth mechanism of MoS2/CdS-1/CdS-2 is as shown in Scheme 2. Displayed in Scheme 3(a,b) are the morphology structure diagram of MoS2/CdS-1/CdS-2 and the charge transfer progress diagram.

Scheme 2 Schematic diagrams for growth mechanism of MoS2/CdS-1/CdS-2.

(a)

(b) CB EF VB

Scheme 3 Schematic diagram of MoS2/CdS-1/CdS-2 (a) and the carriers migrate in MoS2/CdS-1/CdS-2 (b). 28


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4. Conclusions In

summary,

the

phase-modificate

defects

engineering

composite

system

MoS2/CdS-1(NP)/CdS-2(NS) has been developed through facile hydrothermal and solvothermal approaches. The hydrogen production results showed it could greatly increase the hydrogen production performance, which was approximately twenty-fold and thirty-fold compare to that of MoS2/CdS-1 and MoS2/CdS-2, respectively. The increased activity may be on account of the unique contact formed between CdS-1 and CdS-2, which reduced the surface states density to a great extent and was more likely to form a close contact. What’s more, there were sulphur vacancies in CdS-2, which can reduce the photocorrosion to a certain degree. Therefore, carriers’ separation efficiency was increased and added carriers participated in redox process of water. The system improved the performance of CdS fundamentally by self-modifying and defects engineering without compounding with other materials.

Acknowledgments The authors greatly acknowledge the financial support by the National Nature Science Foundation of China (NSFC, 51602209), the Provincial Nature Science Foundation of Sichuan (2018FZ0105, 2017CC0017, 2016GZ0423).

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Phase-Modificate Defects Engineering CdS Sphalerite-Wurtzite

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