Dynamically Optimized Multi-Interface Novel BiSI-Promoted Redox

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Kinetics, Catalysis, and Reaction Engineering

Dynamically Optimized Multi-Interface Novel BiSIPromoted Redox Sites Spatially Separated N-p-n Double Heterojunctions BiSI/MoS2/CdS for Hydrogen Evolution chengxin zhou, Ruilin Wang, Chunping Jiang, Jinwei Chen, and Gang Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00234 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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Industrial & Engineering Chemistry Research

Dynamically Optimized Multi-Interface Novel BiSI-Promoted Redox Sites Spatially Separated N-p-n Double Heterojunctions BiSI/MoS2/CdS for Hydrogen Evolution Chengxin Zhoua, Ruilin Wanga*, Chunping Jianga,b*, Jinwei Chena, Gang Wanga aCollege

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

The People’s Republic of China. bWest

China school of Public health No 4 west China teaching hospital, Chengdu

610041, The People’s Republic of China.

ABSTRACT Novel BiSI promoted n-p-n double heterojunctions multi-interface photocatalyst BiSI/MoS2/CdS was constructed. BiSI is applied to photocatalytic hydrogen evolution. It possesses a small band gap and a strong optical absorption coefficient, therefore, the optical absorption scope and coefficient of MoS2/CdS have been effectively enhanced by compounding with BiSI. The continuous heterojunctions strengthened the function of single junction and guided the carriers’ transfer direction, thus the redox reactions occur at spatially separated sites. Built-in electric field along the radial direction of BiSI nanorod and MoS2 interlayer helps to transport carriers within lifetime. Carrier dynamics is optimized by multi-interface structure. In general, a new material BiSI is introduced to construct a multi-interface structure to optimize carrier dynamics, which resulted in a 46-fold increase in hydrogen production efficiency. *Corresponding authors. Emails: [email protected] & [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEY WORDS: BiSI, multi-interface, dynamics optimization, n-p-n continuous heterojunctions, enhanced optical absorption, photocatalytic water splitting. 1. Introduction With the rapid development of industrialization, traditional fossil fuel cannot meet the increasing demand for society development anymore, what’s more, the harmful gases released from burning fossil fuels seriously polluted the environment. Therefore, alleviating the energy and environmental problems by developing clean and efficient new energy is becoming more and more imminent1-7. After decades of research, scientists have found that using inexhaustible solar energy to produce hydrogen by water decomposing has a broad prospect. In 1972, Fujishima firstly found that hydrogen evolution can be realized by Pt/TiO2 electrode photoelectric conversion, since then, TiO2 series photocatalysts have been widely studied8-12. The basic principles of producing hydrogen by water splitting using semiconductor material as photocatalyst can be generally summarized as follows: When the energy of light radiation (hν) is no less than the energy gap (Eg) of semiconductor, electrons will be excited and holes remain in valence band, leading to generation of free electrons and holes, respectively. Then H2O molecules will undergo redox reactions with electrons or holes at different positions of photocatalyst13,14. However, after decades of development, photocatalytic hydrogen evolution by water splitting is still at the stage of theoretical research, as its efficiency is far below the requirement of industrial application. The main factors can be summed up as follows: While some of the semiconductors are photochemically stable, their band gaps may 2


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be too wide to effectively absorb visible light. Instead, the ones with narrower gaps can match the solar spectrum very well, but may have the problems of photocorrosion, toxic and so on. What’s more, the yield of light quantum is too low for most of the materials, which is no more than 4% in general14. Among them, the most serious problem is that a high proportion of carriers will compound before successfully arrive at the surface and react with reactants to carry out redox reactions. Therefore, our main task is to solve these problems above. Among the numerous photocatalysts, chalcogenide n-style semiconductor CdS has achieved widely interest due to its quite appropriate forbidden bandwidth (Eg≈2.42 eV). What’s more, its conduction band potential is very suitable for reducing water. However, it also has serious shortcomings, for example, the carriers are easy to compound and the photocorrosion phenomenon is quite serious15-21. To solve these problems, usually a cocatalyst is introduced into the CdS system. The research findings show that cocatalyt has the ability of promoting carriers’ separation, providing more active reaction sites and reducing the overpotential of hydrogen generation16,17,21-28,33. Precious metals are usually utilized as cocatalysts (such as Pt, Ag), however, their expensive prices and scarce storage limit their wide range of applications17,33, therefore, exploring inexpensive non-precious cocatalyst is very meaningful. For example, as a stable non-precious metal sulfide, the p-type semiconductor MoS2 has been widely studied21-25. Studies show that it can work as an efficient cocatalyst due to its lesser Eg and unique samdwich conformation. Previous studies have shown that 3



it is constructed by three parallel S-Mo-S layers26,29-32. The experimental results also showed that the photoactivity CdS was improved after combination of MoS221,23, 34-39. However, the efficiency still can not meet the requirements of industrial production, therefore, it is very meaningful to search for semiconductors have lesser forbidden bandwidths and appropriate energy band positions to continue upgrading activation of MoS2/CdS40-42. In a large number of semiconductor materials, we conjecture that BiSI may be an ideal candidate due to the following reasons. BiSI is constructed by atomic links pertain to D162h space grouping. The atoms in BiSI all locate on minute surface vertical to the c-axis43. BiSI possesses dual links [(BiSI)∞]2 associated by a double helicalaxis and connected by Bi-S bonds. The dual links may be deemed to be no interaction and one dual link can be regarded as the calculation model of BiSI, as can be seen in Scheme 144. BiSI displays many important performances. The existing researches prove that BiSI is a kind of indirect-type material43, that is to say, nadir of conduction band (CB) and vertex of valence band (VB) are not seated in the same point in Brillouin District44. It can be seen from the BiSI atomic structure schematic diagram that the Bi atom is connected to two S atoms and an I atom. Bi3+ arranges in a stable d10s2p0 electron arrangement, the compound comprising bismuth element will show enhanced relativistic effect, therefore, the material will exhibit antioxidant properties and extend the breadth of conduction band by remarkably reducing the minimum of the conduction band. By hybridizing S and I p states, there will be a conduction band diffusion along the ribbon texture, therefore, the compound will display fine conductivity along the ribbon. The analyses show that the electron 4


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construction of BiSI is a bit complicate as there are a lot of bond extrema, which commonly seated apart from higher symmetry dots in the Brillouin District43. The effective mass of electron is the lightest in the orientation along the ribbons (along [010]), while the effective mass of hole is the lightest in the orientation contrary to [001] orientation. Also worth mentioning is that the effective mass of electron is lighter than that of hole in BiSI, as a result, the electron will migrate faster44. For example, the researchers have found that BiSI/MoS2 can be used in the degradation of crystal violet (CV)53. Considering the above factors, we predict that it will be suitable for photocatalytic hydrogen evolution because of its small band gap, strong light absorption capacity, stable photochemical properties, low cost, safety and non-toxicity. What’s more, because BiSI contains the late-transition metal of Bi, so it shows higher absorption performance and effective filtering of defects of live electricity and increased spin-orbit coupling effect, which will bring about the reduction of effective mass. Considering the above factors, in this work, we introduce the V-VI-VII group semiconductor material bismuth sulfo iodide (BiSI) to MoS2/CdS system to construct an unique n-BiSI/p-MoS2/n-CdS double heterojunction multi-interface structure to further improve the photocatalysis activation of CdS dynamically. For this designed construction, there will be two built-in electric fields as well two interfaces in the double continuous heterojunctions, thus amplifying the advantages of traditional pn heterojunction by further promoting carriers migration. What’s more, the internal electrical field along the radial of BiSI nanorod helps to transport the carriers in 5



effective lifetime successfully, as well the MoS2 interlayer can transport the carriers rapidly within lifetime. That is to say, the directional transport of carriers is further optimized dynamically. Also to be note, the oxidation and reduction reaction can occur at spatially separated active sites as the electrons and holes severally migrate to different material surfaces. In good agreement with the theoretical analysis, we found that the photocatalytic hydrogen evolution tests showed that the obtained n-BiSI/p-MoS2/n-CdS shows a much increased photocatalytic H2 productive efficiency by 46-fold as much as that of MoS2/CdS, as well it is stable and recyclable under visible light. Furthermore, we have characterized the new material BiSI by XRD, SEM, XPS, UV-Vis, Kubelka-Munk simulation, PL, Mott-Schottky tests, EIS, photocurrent tests, photocatalytic hydrogen evolution tests and so on to clearly explain its properties. Therefore, we can have an overall understanding of its future application in photocatalytic H2 production by H2O decomposing.

Scheme 1 Crystallographic texture of BiSI. 2. Experimental section All chemical medicines are reagent grade and used as initial purity. They are bismuth 6


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nitrate (Bi(NO3)3·5H2O), acetic acid (CH3COOH), thioacetamide (C2H5NS), sodium iodide (NaI), sodium molybdate (Na2MoO4·2H2O), cadmium chloride (CdCl2·2.5H2O) and hyposulphite (Na2S2O3·5H2O). 2.1 Synthesis of the materials Synthesis of BiSI 0.728 g Bi(NO3)3·5H2O was added to 30 mL CH3COOH and stirred to colorless, then mixed with 0.113 g thioacetamide and agitated until milk white, 15 minutes later, 0.374 g sodium iodide was added. After stirring for 3 hours, heated for 10 hours at 180 ℃. Finally centrifuged and drid at 60 ℃ for several hours. Synthesis of BiSI/MoS2 An appropriate amount of as prepared BiSI was added to the beaker containing 60 mL deionized water and dispersed for half an hour by ultrasonic treatment. Then, sodium molybdate and thioacetamide were mixed simultaneously under vigorously agitating for several hours. Then, the suspension was heated at 160 ℃ for 12 h. Finally centrifuged and drid at 60 ℃ for several hours. MoS2 was synthesized under the identical condition except for the adjunction of BiSI. Synthesis of BiSI/MoS2/CdS A given mass of BiSI/MoS2 was added to water under the action of ultrasound, then cadmium chloride and sodium thiosulfate were dissolved in the suspension Afterwards, the suspension was heated under 100 ℃ for two hours. Finally centrifuged and drid at 60 ℃ for several hours. CdS was synthesized under the same conditions except for the adjunction of 7



BiSI/MoS2. BiSI/CdS was synthesized in the identical manner except mixing with BiSI instead of BiSI/MoS2. What’s more, for comparative experiments, MoS2/CdS was also prepared in the identical manner except mixing with MoS2 instead of BiSI/MoS2. Scheme 2 displays the approximate synthetic route of the photocatalyst.

Scheme 2 Synthetic route of BiSI/MoS2/CdS. 2.2 Characterization The crystallographic construction of as constructed specimens were investigated using X-ray diffraction (XRD; Empyeran) equiped with copper Kα emission, which were ranging from 10° to 80°. The X-ray photoemission spectrometer (XPS) was afterwards applied to verify the elements contented in the photocatalyst and the chemical states of them. The scanning electron microscope (SEM) pictures were obtained through Hitachi S-4800. We also utilized the Quantachrome instrument to test the pore properties. The spectrophotometer (UV-3600) was applied to study UV-visible diffusion reflex spectrums of the specimens. The fluorometer (F-7000, Hitachi) was applied to implement photoluminescence spectrums (PL) of the photocatalysts. As to the photoelectrical properties of the photocatalysts, we used the electrochemical workstation (CHI660E, Shanghai Chenhua Instruments). The three electrodes used in the electrochemical test are severally the tin oxide doped with 8


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fluoride glass with photocatalyst coated on, carbon and silver/silver chlorid. In the whole photoelectric test, the 0.2 mol L-1 of Na2SO4 aqueous solution operated as electrolyte. 2.3 Photocatalytic reaction The photocatalytic hydrogen evolution tests were measured on photocatalytic hydrogen evolution system of Perfectlight Labsolution. 50 mg as prepared specimen was added to 0.1 L solution comprising 10% volume percentage 2-hydroxypropanoic acid used as hole sacrificial reagent. The 300 W Xenon light was applied, while the wave filter was applied to filter out the UV ray. Hydrogen evolution properties were tested by the online gas chromatograph (SHIMADZU GC-2014). By way of removing oxygen, it has been introduced high purity nitrogen to the reaction device for a short period of time prior to starting test. The environment of the reaction device remained at about 10±0.5 ℃ during the whole photocatalytic period using the thermostatic waterbath. Apparent quanta efficiency (AQE) obtained at 420 nm can be estimated by the formula below: AQE(%)=[2N(H2)/N(p)]×100%=[2n(H2)×NAhc]/[S×P×t×λ]×100%

(1)

in which N(H2) is hydrogen molecules’number, N(p) is photons’ number, n(H2) is molar number of produced hydrogen, NA is Avogadro constant, c is light velocity, S is radiating area, P is the average strength of incident ray, t is radiating time, λ is wavelengh of incident light. The calculated AQE of BiSI/MoS2/CdS is about 66.2%. 2.4 Electrochemical tests Electrochemical experiments were implemented through the CHI660E device 9



(Shanghai Chenhua Instruments). It has been used the dispending means to fabricate the testing sample and maintained the area to be about 1 cm × 1 cm, afterwards, drying in the vacuum drying oven for several hours. Mott-Schottky tests wre performed at three different frequencies, they are 1 kHz, 2 kHz and 5 kHz, respectively. As to the transient light current experiment, the xenon light was applied as the light source. The electrochemical impedance spectrum (EIS) experimentations were conducted at the region of 10-2 to 106 hertz. In addition, as to all the electrochemical measurements, the Na2SO4 solution was severed as electrolyte. 3. Analysis 3.1 Structural Analysis Crystallographic construction of BiSI (before and after irradiation), CdS, MoS2, BiSI/MoS2, MoS2/CdS and BiSI/MoS2/CdS (before and after irradiation) are as shown in Figure 1. Seen from Figure 1(a), the three primary diffraction summits of BiSI are situated at 20.2°, 29.5° and 32.7° consult to PDF#43-0652, which severally match with (120), (121) and (310) lattice planes of orthorhombic (pnam) BiSI. Figure 1(b) is the XRD pattern of BiSI/MoS2. Slight deviations were existed maybe owing to the coupling effect between diverse materials. The results showed that the as prepared MoS2 belongs to hexagonal crystal system and corresponds to PDF#37-1492, as displayed in Figure 1(c). As it should be, compared with single MoS2, there were minor peak offsets in BiSI/MoS2 as a result of interaction force among diverse components. Consulting to PDF#75-1546, the principal diffraction summits situated at 2θ=26.4°, 43.8° and 52.0° severally correspond to the cubic CdS lattice planes of 10


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(111), (220) and (311), as displayed in Figure 1(c). For the diffraction summits of CdS in BiSI/MoS2/CdS, the three principal diffraction peaks situated at 2θ=26.5°, 44.0° and 52.2° severally, the slight migrations verified the interaction inhere in CdS and the materials in connection with it. The above test results demonstrated the generation of BiSI, MoS2 and CdS. In addition, we can obviously see that the summits of BiSI are very sharp, indicating that its crystallinity is quite high. As we know, the crystal with better crystallization have fewer lattice defects and larger specific surface area. However, the broadening of the diffraction peak of MoS2 indicates that its crystallinity is relatively low. For MoS2, the lower crystallinity is beneficial to the formation of irregular morphology and to expose more abundant marginal active reaction sites, which is beneficial to the photocatalytic reaction45-48. Also to be note, in order to prove the stability of these photocatalysts, the XRD spectra of BiSI and BiSI/MoS2/CdS have been used were also tested, as Figure 1(a, c) shows. We collected the used photocatalysts by centrifugation. The results show that the crystal form and structure of photocatalyst have not changed after irradiation, which proves that the properties of these materials are quite stable.

11



Figure 1. XRD patterns of BiSI (before and after irradiation) (a), BiSI/MoS2 (b), CdS, MoS2, MoS2/CdS and BiSI/MoS2/CdS (before and after irradiation) (c). 3.2 Valence analysis of elements We carried out the XPS experiment to verify the elements contented in the photocatalyst and the chemical states of them. Figure 2(a, b, c, d) are the outcomes of MoS2/CdS, while Figure 3(a, b, c, d, e, f) are the outcomes of BiSI/MoS2/CdS for comparison. Seen from the XPS outcome in Figure 3(a), the summits located at 161.2 and 157.9 eV severally match with the specific bound energies of Bi3+ 4f5/2 and Bi3+ 4f7/2. The XPS spectrum in Figure 3(b) shows peaks severally located at 630.2 and 618.3 eV, which severally correspond to I- 3d3/2 and I- 3d5/2. The XPS outcome in 12


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Figure 3(c) displays summits sited at 411.6 and 404.8 eV severally match with the specific bound energies of Cd2+ 3d3/2 and Cd2+ 3d5/2. Comparing to Cd peaks located at 411.8 eV and 405.0 eV in MoS2/CdS, shown in Figure 2(a), Cd 3d bound energies deviate to the smaller side verify that CdS got more electrons in the composite BiSI/MoS2/CdS than in MoS2/CdS. That is to say, after compounding with BiSI, the efficiency of electrons-holes separation and migration was improved, more electrons will participate in reduction. Meanwhile, the division of energy level of 6.8 eV verifies the successful synthesis of CdS. In Figure 3(d), the summits situated at 231.9 and 225.5 eV match to the bound energies of Mo4+ 3d3/2 and Mo4+ 3d5/2, respectively. In comparison with Mo peaks located at 231.5 eV and 225.2 eV in MoS2/CdS, shown in Figure 2(b), the positive shifts of Mo 3d bound energies verify the effect existed between BiSI and MoS2, also of note, it verifies that MoS2 lost more electrons finally. In Figure 3(e), the summits situated at 161.2 and 160.6 eV severally match with the bound energies of S2- 2p1/2 and S2- 2p3/2. As shown in Figure 3(f) is the full spectrogram, which verifies the photocatalyst comprise Bi3+, I-, Cd2+, Mo4+ and S2-. Based on the above analysis and combined with XRD test results, the formation of BiSI, MoS2 and CdS, as well the interactions and actions between the three substances were proved.

13



Figure 2. XPS outcomes of Cd 3d (a), Mo 3d (b), S 2p (c) and MoS2/CdS (d).

14


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Figure 3. XPS outcomes of Bi 4f (a), I 3d (b), Cd 3d (c), Mo 3d (d), S 2p (e) and BiSI/MoS2/CdS (f). 3.3 Morphologic Properties The SEM imagines were used to investigate the morphologies and sizes of the specimens. By analysising SEM patterns in Figure 4, we can have a clear and intuitive understanding of the morphologies of the photocatalysts. As shown in Figure 4(a, b), 15



we can see that BiSI holds a typical rod-like structure around 100-200 nm in diameter and several microns in length. It has a smooth surface and the end of the rod is slightly sharpened. The morphology of BiSI/MoS2 is shown in Figure 4(c), we can see that there are irregular nanosheet-like MoS2 attached to the surface of rod-like BiSI. Combined with XRD test results, it is known the summits of MoS2 are broadened, indicating that the crystallinity of MoS2 is relatively low. The irregular shape of MoS2 proves that it is amorphous furthermore. Combining with the existing experimental outcomes, we know that amorphous MoS2 has more abundant marginal reactive sites, which is beneficial to the photocatalytic performance47,48. Figure 4(d, e, f) show the morphology of BiSI/MoS2/CdS. Compared with the morphology of BiSI-MoS2 displayed in Figure 4(c), there are nanoparticle shaped CdS attached and the particle size is smaller than 100 nm. In order to visually describe the specific substances corresponding to each morphology, Figure 5 was added. Figure 5(a1, a2) displayed the SEM and XRD figures of pure BiSI, Figure 5(b1, b2) represent the SEM and XRD figures of BiSI-MoS2, while Figure 5(c1, c2) represent the SEM and XRD figures of BiSI/MoS2/CdS. It can be seen that the SEM results are match to XRD analyses, thus verifying specific substances corresponding to each morphology.

16


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Figure 4. SEM patterns of BiSI (a, b), BiSI/MoS2 (c) and BiSI/MoS2/CdS (d, e, f).

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(a2) (121)

(a1)

BiSI

(120) (310)

PDF#43-0652

100 nm

(b1)

(b2)(121) (120) (002)

BiSI-MoS2

(310) (103)(105)

100 nm

(c1)

(c2) (121) (120) (310) (111) (002)

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

100 nm

Figure 5. SEM and XRD images of BiSI (a1, a2), BiSI-MoS2 (b1, b2) and BiSI-MoS2/CdS (c1, c2). 3.4 Surface and pore structure analysis The N2 adsorption-desorption characteristics of the specimens appear in Figure 6, while the test data of superficial properties and aperture properties of the specimens are as listed in Table 1. Seen from the plots in Figure 6(a), we can concluded that all specimens with obvious hysteresis loops are pertain to Style IV consult to the IUPAC assortment49-52. Therefore, it can be considered that these materials all belong to 18


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mesopore structures. In combination with Figure 6(b), we can find out the apertures of the materials all located at the range of 2 to 50 nm, therefore, it further proves they are mesopore substances. These obtained data also show that BiSI/MoS2/CdS possesses a lager specific area and pore volume, which illustrates that the compounding of BiSI can optimize the pore structure of MoS2/CdS and it’s beneficial to the improvement of adsorption properties.

(a)

(b)

Figure 6 N2 adsorption-desorption plots (a) and aperture scatter plots (b) of the specimens. Table 1 Surface and pore structure properties.

Specimens

S (m2 g-1)

CdS MoS2/CdS BiSI/MoS2/CdS

3.660 14.836 78.723

Pore diameter (nm) 3.661 3.732 2.974

Pore volume (cm3 g-1) 0.012 0.032 0.127

3.5 Optical Properties UV-vis spectra, PL spectrums and Mott-Schottky tests were utilized to characterize the optic and dynamical performances of the specimens. Figure 7(a) appears that BiSI has a higher optical absorption intensity, even much higher than that of binary 19



composite. Optical absorption ability of semiconductor material is crucial to its photocatalytic performance, therefore, it can explain why the photocatalyst exhibits much improved photocatalytic activity after compounding with BiSI. Because of the introduction of BiSI, the optical absorption intensity of BiSI/MoS2/CdS is greatly enhanced compared with that of MoS2/CdS. Even though the light absorptivity of BiSI is even exceed BiSI/MoS2/CdS in the wave range of 300-500 nm and 750-800 nm, the visible light water splitting mainly uses the light in 420-740 nm range, therefore, BiSI/MoS2/CdS showed much higher photoactivity. Combined with the results of hydrogen production tests, we found that BiSI alone had no photocatalytic activity, therefore, the advantage of its high optical absorption ability cannot be utilized directly, however, it can play the role of modifier to strengthen performance indirectly. It can also be found that photo absorptivity of BiSI/MoS2/CdS obviously increased in the range of vis-NIR region, which is possibly due to the composition of narrow forbidden bandwidth material BiSI. With the enhancement of optical absorption capacity, more photons can be effectively utilized, thus more photogenerated electrons will be stimulated, therefore, the efficiency of light quantum will be improved. The PL spectrums appear in Figure 8 was utilized to describe the photogenerated carriers’ trapping, immigrating, transferring efficiency and the mechanism for increased H2 production efficiency. It can be seen that the CdS alone holds a quite high peak for the reason of serious compounding of photo-induced carriers, as it is the same to BiSI alone. That is to say, most of the photogenerated carriers are recombined 20


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rather that arriving at the surface of the photocatalyst to carry out redox reactions. An obvious peak was also observed in the MoS2/CdS spectrum, however, it is much reduced compared with CdS as the recombination of carriers was suppressed by compounding with MoS2. As to BiSI/MoS2/CdS, there was an obvious fluorescence quenching even in comparation with MoS2/CdS, which proves that after the compounding of BiSI, the migration efficiency of carriers was greatly enhanced. From the point of view of dynamics, the multi-interface structure can optimize the dynamic performance of carriers. Essentially, the intensity of PL emission displays the opposite development of the carriers’ separation and transport ability, as well contrary to the development of the photocatalyst’s hydrogen evolution performance. Therefore, we can draw a conclusion from the above description that after the compounding of BiSI, the carriers’ separation and transmission efficiency in the BiSI/MoS2/CdS system was greatly increased. In other words, the interface can optimize the dynamics of carriers. The multi-interface structure makes the carrier migration more smooth than the single interface structure, which may be the effect of multiple internal electrical fields exist in the heterojunctions. The outcomes of UV-Vis along with PL results above explained the reasons why BiSI/MoS2/CdS showed much increased photoactivity from the perspective of optical and dynamic properties. For the purpose of describing band structures of the specimens more intuitively, the following formula was utilized to calculate their band gap energies:

αhν=A(hν–Eg)n/2

21


(2)


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in which hν, α, A, and Eg are respectively the energy of photons, absorption coefficient, constant value of proportionality, band gap energy. To be note, n is relevant to the material’s style, generally speaking, n is regarded as 1 when the material is direct style semiconductor, such as CdS and MoS2, while n is equal to 4 when the material is indirect transition semiconductor, such as BiSI. The simulation test results are as described in Figure 7(b, c, d), the energy band gaps of BiSI, MoS2 and CdS are severally 1.60 eV, 1.50 eV and 2.42 eV. As for specific energy level positions, we adopted the Mott-Schottky test to simulate the flat band potentials (EFB). Electrochemical workstation was utilized to carry out this series of tests. According to the existing theory, as to n-type semiconductor, the EFB is on the verge of CB, while for p-type material, it is on the verge of VB. The basic formulas for Mott-Schottky theory are as follows： 1/C2=2(E-Efb-kBT/e)/εε0eND ND=(2/εε0e)[d(E)/d(1/C2)]=(2/εε0e)(1/slope)

(3) (4)

As described in Figure 9(a, b, c), the simulated EFB of BiSI, MoS2 and CdS are labeled as -0.20 V, 1.21 V and -0.93 V vs NHE, while the test environment is neutral. As shown in Figure 9(d, e) are the simulated EFB values of MoS2/CdS and BiSI/MoS2/CdS, they are about -0.82 V and -0.86 V vs NHE, respectively. It’s worth noting that the test value will be affected by pH value of electrolyte based on the Nerst formula, therefore, the same electrolyte was used throughout the whole experiment. The potential values versus silver/silver chloride electrode were translated into these versus NHE basing on following conversion formula: 22


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ENHE, pH=EAg/AgCl+E0+0.05916pH

(5)

where E0 is approximately equal to 0.1976 V. The outcomes appear that the EFB of BiSI, MoS2 and CdS are severally 0.21 eV, 1.62 eV and -0.52 eV, while the pH value is zero. Then, combining with following equation: EVB=ECB+Eg

(6)

we can draw a conclusion that the CB and VB for BiSI are -0.20 eV and 1.40 eV, for MoS2 are -0.29 eV and 1.21 eV, for CdS are -0.93 eV and 1.49 eV (vs NHE, pH value is 7). It is known that when the semiconductors with different energy levels contact with each other, the carriers tend to migrate to the energy levels that reduce their energy. Therefore, based on these test data, the schematic diagram of the energy level of the photocatalyst system was drawn, as shown in Scheme 3. It can be clearly seen that the electrons will migrate from MoS2 to BiSI and CdS, at the same time, holes will migrate from BiSI and CdS to MoS2, therefore, the effective separation of photogenerated electrons-holes can be realized. What’s more, combined formula (4) and Figure 9(d, e), it is obvious that the tangent slope of BiSI/MoS2/CdS is smaller compared with MoS2/CdS, appearing that carrier density (ND) in the BiSI/MoS2/CdS system is higher. Moreover, the tangent slope is positive, indicating that BiSI/MoS2/CdS is n-type conduction and the main conductive carriers are electrons.

23



Figure 7. UV-vis spectrums (a) and Tauc-plots of BiSI (b), MoS2, CdS (c) and BiSI/MoS2/CdS (d).

Figure 8. PL patterns of the specimens.

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(a)

(b)

-0.20

1.21

(d)

(c)

-0.82

0.93

(e)

-0.86

Figure 9. Mott-Schottky curves of BiSI (a), MoS2 (b), CdS (c), MoS2/CdS (d) and BiSI/MoS2/CdS (e) in Na2SO4 aqueous solution (pH=7).

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Scheme 3 Diagrammatic sketch of experimental energy level potentials of BiSI, MoS2 and CdS. For the sake of investigating the migration and surface transfer property of the electrons-holes, EIS experiment of different samples were carried out to verify the surface interaction among different components. It can be seen from Figure 10(a) that BiSI/MoS2/CdS holds a smaller radius of arc in comparison with MoS2/CdS, which verifies that surface interaction was existed between BiSI and MoS2/CdS, and it promoted the carriers’ transfer. That is to say, carriers suffer less resistance and can migrate more smoothly, in other words, the carrier transport dynamics is optimized. This is well combined with the PL test results described above, they all proved that the mobility of carrier is enhanced in this multi-interface system. What’s more, 2.5% BiSI/MoS2/CdS holds the smallest radius of arc, which is in accordance with the results of hydrogen production tests. Appeared in Figure 10(b) is photocurrent performance of the specimens. The current density of BiSI/MoS2/CdS is obviously 26


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increased in comparison with that of MoS2/CdS and there is no obvious reduction. The outcomes of EIS and photocurrent all can explain why BiSI/MoS2/CdS displayed an enhanced hydrogen evolution efficiency.

(b)

(a)

2.5%BiSI/MoS2/CdS

3%BiSI/MoS2/CdS 1%BiSI/MoS2/CdS MoS2/CdS

Figure 10. EIS patterns (a) and photocurrent patterns (b) of the specimens. The inset shows the equivalent circuit model for fitting the EIS plots. 3.6 Photocatalytic Properties Photocatalysis properties were measured utilizing solar simulated photo-source (λ>420 nm, AM=1.5) to estimate the photocatalytic performance of the photocatalysts. As described in Figure 11 are the hydrogen production efficiencies of the specimens, to be note, they were all carried out in the same reactivity environment. As it can be seen, BiSI and MoS2 alone can't decompose water into hydrogen for the reason of too positive CB potentials compared with the reduction potential of water. The single CdS showed extremely poor performance for the reason of severe photocorrosion and serious combina of photogenerated electrons-holes. Also to be note, neither BiSI/MoS2 or BiSI/CdS showed hydrogen production activity. It is also worth noting that when BiSI and MoS2/CdS are combined in a physical approach, the H2 27



production efficiency of MoS2/CdS was not increased, which indicates that the greatly enhanced photoactivity of BiSI/MoS2/CdS is caused by the heterojunctions formed between BiSI and MoS2/CdS in the system. Appeared in Figure 12(a, b) are the photocatalytic performances of 1% BiSI/MoS2/CdS (9 mmol g-1 h-1), 2.5% BiSI/MoS2/CdS (21 mmol g-1 h-1), 3% BiSI/MoS2/CdS (13 mmol g-1 h-1). It can be concluded that while weight percentage of BiSI/MoS2 in BiSI/MoS2/CdS was 2.5%, in other words, 2.5% BiSI/MoS2/CdS exhibited the highest H2 production efficiency. The weight percentages of BiSI and MoS2 have important impacts on photoactivities. The hydrogen production efficiency decreased while weight percentage of BiSI/MoS2 is 3%, which perhaps due to the superfluous BiSI/MoS2 would obstruct the light absorption of CdS, in other words, the “shielding effect”. What’s worse, redundant BiSI/MoS2 may cause extra compounding locus and enhance carriers’ migration resistance, therefore exacerbating the carriers’ compounding. For the reason of further explore the influence of component mass percentage, we carried out a more series of tests and kept the mass percentage of BiSI/MoS2 was 2.5%, meanwhile, changing the mass percentage of BiSI in BiSI/MoS2 from 5% to 10% as well 15%. It can be concluded from Figure 12(c, d) that along with the addition of BiSI, there was a tendency of first increase and then decrease for hydrogen production efficiency and the highest point was located at 10% BiSI. When the mass percentage of BiSI is up to 15%, it resulted in a activity reduction to a small extent. The reason for the decrease may be that CB position of BiSI is relatively positive and it has no ability to restore water. Even though it holds a quite small forbidden bandwidth, the excessive loading 28


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will reduce the reducibility of the BiSI/MoS2/CdS. The photoactivity of BiSI/MoS2/CdS system obey this sequence: BiSI(10%)/MoS2/CdS(21 mmol g-1 h-1)>BiSI(15%)/MoS2/CdS(15 mmol g-1 h-1)>BiSI(5%)/MoS2/CdS(12 mmol g-1 h-1). On account of confirming the stability and recyclability of the photocatalysts, it has been implemented four consecutive H2 evolution rounds. As described in Figure 12(e), every run was carried out for four hours and re-tested for four times. It is also worth noting that the hydrogen production activity was hardly decreased for 12 h at the fewest, as described in Figure 12(f). The above hydrogen production test results verified that the photocatalysts are steady and repeatable. The outcomes are in coincidence with the above mentioned outcomes as well.

Figure 11. Photoactivity of the specimens.

29



Figure 12. Photoactivity (a, c) and steadiness (b, d, e, f) of the specimens. 4. Discussion of Mechanism By analyzing the experimental data, we know that BiSI/MoS2/CdS carried out a much increased photocatalytic performance than that of MoS2/CdS. In order to discover the mechanisms in this system, may be it can interpret from the following several points of view: (1) These three semiconductors sequentially compounded to form an unique n-p-n type double heterojunction multi-interface structure, in other words, two continuous pn heterojunctions. It is known that the pn type composite photocatalyst can regulate the band structure and expand the optical absorbance range, as well facilitate carriers’ 30


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transfer at the pn junction and prohibit their recombination. Therefore, when designing composite photocatalysts, it’s a tendency to construct pn heterojunctions. In this paper, the continuous double heterojunctions with two built-in electrical fields were constructed, thus amplifying the advantages of pn heterojunction. According to the schematic diagram of the energy band structure and the XPS test results, it can be seen that the electrons will transfer from MoS2 to BiSI and CdS, while the holes will transfer from BiSI and CdS to MoS2. Therefore, the oxidation reaction and reduction reaction can be realized at spatially separated active sites, as appeared in Scheme 4(a, b). Combining the calculation results above, we can see that the M-S slope of BiSI/MoS2/CdS is gentler, in other words, the carrier density (ND) in BiSI/MoS2/CdS system is higher, that is to say, more effective charge carriers, and it’s match well to theoretical analyses. (2) The designed nanorod-nanosheet-nanoparticle construction can increase the electrons-holes separation efficiency. The internal electrical field along the nanorods’ radial helps to transport carriers in the effective lifetime successfully, as well the middle nanosheet layer can transport carriers rapidly within lifetime. Therefore, the electrons will arrive at the surface of CdS nanoparticles and occur reduction reaction effectively. As is known, for the reason of carriers’ migration in the heterojunction interface, the surface potential of the heterojunction region exists gradient. The disparity between potentials of the contacted surfaces will cause spatial separation of redox reactions loci and formation of the self-built electrical field. (3) It is known from the existing research results that BiSI has many outstanding 31



properties53. It is a promising optoelectronic material, but it rarely has been applied to photocatalytic hydrogen production by water splitting according to our knowledge. From the above test results, we can see that the forbidden bandwidth of BiSI is quite small, which is 1.60 eV. Therefore, the scope of wavelength absorbance of the composite photocatalyst can be effectively broadened. As mentioned in the text, BiSI is a kind of indirect transition material, that is to say, electrons have to go through K electron layer to reach VB, therefore, possibility of electrons-holes recombination is reduced. At the same time, its chemical properties and photochemical properties are quite stable. Another important advantage has been found is that its optical absorption performance is particularly good, may be the result of comprehensive results of its own properties and the high uniform orientation of the rod like structure and the rod axis. (4) In this system, MoS2 is working as a non-precious metal cocatalyst. The narrow forbidden bandwidth and unique stratiform internal structure make it an effective cocatalyst. From the results described above, we can see that MoS2 has an irregular morphology and a low crystallinity, therefore, there will be more edge active reaction sites, thus improving its performance47,48.

32


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Scheme 4 Diagrammatic sketch of carriers’ transition. Diagrammatic sketches of longitudinal section (a) and band structure (b). 5. Conclusion In conclusion, the multi-interface novel BiSI-promoted n-p-n double heterojunctions photocatalyst BiSI/MoS2/CdS with spatially separated redox reaction sites was synthesized by hydrothermal and solvothermal methods. It showed the substantially increased hydrogen evolution activity in comparision with MoS2/CdS. Considering the characterization outcomes of XRD, SEM, BET, UV-Vis, PL, Mott-Schottky tests, EIS, I-t and so on, it can be found that the excellent performance of BiSI-MoS2/CdS can be attributed to its unique n-p-n double heterojunctions formed in the designed nanorod-nanosheet-nanoparticle structure, which strengthened the function of conventional pn heterojunction and artificially controlled the migration directions of the carriers, which realized the oxidation and reduction reactions carried out at spatially separated sites. The built-in electrical fields along the radial direction of BiSI nanorod and MoS2 interlayer help to transport the carriers within lifetime. Carrier dynamics is optimized due to the multi-interface structure. In general, a new material 33



BiSI is introduced to construct a multi-interface structure to optimize carriers dynamics. The small forbidden bandwidth and high light absorption coefficient of BiSI exerted vital function in enhancing photocatalyst’s optical absorption range and intensity. The BiSI has been applied in the field of photocatalysis H2 evolution by water splitting, these findings may take a place in developing less costly effective H2 evolution photocatalysts.

Acknowledgments We sincerely thank the following funds for their support: National Nature Science Foundation of China (51602209), Provincial Nature Science Foundation of Sichuan (2018FZ0105, 2017CC0017, 2016GZ0423).

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83x45mm (96 x 96 DPI)


Dynamically Optimized Multi-Interface Novel BiSI-Promoted Redox

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