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Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visible-light hydrogen production
international journal of hydrogen energy xxx (xxxx) xxx
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A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visible-light hydrogen production Xiaoming Liu a,b, Binquan Wu a, Xueya Chen a, Liushui Yan a, Huiqin Guo a, Kexin Li a,*, Luping Xu a, Jun Lin b,c,** a
School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang, 330063, PR China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China c School of Applied Physics and Materials, Wuyi University, Jiangmen, Guangdong, 529020, PR China b
highlights
graphical abstract
The Mn0.2Cd0.8S shows an excellent photocatalytic activity for H2 evolution. The Bi2MoO6/Mn0.2Cd0.8S exhibits a significantly high photocatalytic activity. A possible photocatalytic mechanism for Bi2MoO6/Mn0.2Cd0.8S has been proposed.
article info
abstract
Article history:
In this work, a series of MnxCd1-xS solid solutions as efficient photocatalysts for hydrogen
Received 16 August 2019
evolution with visible light response were synthesized via a co-precipitation method firstly.
Received in revised form
Then, the hierarchical Bi2MoO6/Mn0.2Cd0.8S heterostructured composite was prepared by
21 October 2019
combining Mn0.2Cd0.8S composite with Bi2MoO6 nanocrystalline through a hydrothermal
Accepted 26 November 2019
process. The optimized Mn0.2Cd0.8S composite’s photocatalytic activity is more than 3
Available online xxx
times of pristine CdS and the prepared Bi2MoO6/Mn0.2Cd0.8S nanocomposites exhibited a significantly improved photocatalytic activity for hydrogen evolution from water with
Keywords:
visible light response comparing with single Mn0.2Cd0.8S composite. The optimized pho-
Bi2MoO6/Mn0.2Cd0.8S composites
tocatalytic activity of Bi2MoO6/Mn0.2Cd0.8S composite is around 10 times of pristine CdS.
Heterostructure
The excellent photocatalytic activity of Bi2MoO6/Mn0.2Cd0.8S composite might be ascribed
Hydrogen production
to the well-matched energy band structures of the Bi2MoO6 and Mn0.2Cd0.8S. Furthermore,
* Corresponding author. ** Corresponding author. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China E-mail addresses: [email protected] (K. Li), [email protected] (J. Lin). https://doi.org/10.1016/j.ijhydene.2019.11.197 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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the heterojunctions in Bi2MoO6/Mn0.2Cd0.8S composite might also do some contributions to improve its photocatalytic activities to some extent. A possible photocatalytic mechanism was proposed. Due to its excellent photocatalytic activity and good stability for hydrogen evolution from water, the obtained hierarchical Bi2MoO6/Mn0.2Cd0.8S composite has potential application in photocatalytic hydrogen evolution from water by using solar power. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the 21st century, the energy crisis and environmental issues have driven human society to explore new clean, effect and renewable resources [1e4]. Hydrogen energy, due to its clean, efficiency and environmental friendliness, has been considered as a promising energy resources in the future [5]. Since TiO2 semiconductor was first utilized for photoelectronchemical water splitting with ultraviolet light excitation by Fujishima and Honda in 1972，hydrogen evolution via a photocatalytic reaction using semiconductor has been widely regarded as a promising technique to convert solar energy into chemical energy stored in hydrogen [6e8]. However, most of semiconductors photocatalysts only absorb ultraviolet light, which accounts for 3e5% of solar spectrum. On the contrary, nearly half of solar energy is in visible light region [9e16]. Therefore, achieving highly efficient and stable solar hydrogen generation from water is still a challenging and promising research topic [17e20]. Recently, the solid solution of multi-component sulfides, e.g., CoxCd1-xS, Zn1-xCdxS, (Zn0.95Cu0.05)1-xCdxS, MnxCd1-xS, etc., have attracted much more attention owing to their remarkable optical performances in visible light region, adjustable band structures and potential application in solar hydrogen generation from water [21e34]. The multicomponent sulfides can be synthesized by different methods. Usually, different synthesis methods will lead to different photocatalytic properties [31]. It is reported that the MnxCd1-xS solid solution can be modified as a type of efficient photocatalyst for hydrogen production from water with visible-light response even in the absence of cocatalyst [35e37]. However, due to its poor stability, limited surface active sites and low efficiency of charges separation, the MnxCd1-xS solid solution’s photocatalytic activity for hydrogen production from water is still low [36,37]. Therefore, the MnxCd1-xS solid solution’s photocatalytic activity need to be further improved for photochemical solar H2 production with visible light response. Bismuth molybdate (Bi2MoO6) has been regarded as a promising semiconductor photocatalyst due to its relatively narrow band gap, favorable thermal stability and excellent photocatalytic performance [38,39]. However, ascribing to the fast recombination of photo-generated charge carriers, its photocatalytic activity is very low. To address this issue, the Bi2MoO6 was usually combined with other semiconductor photocatalysts to obtain an type of heterostructure to achieve high efficient separation of photo-generated charge carriers, which finally lead to a substantial improvement in photocatalytic activity [40e43], e.g., CdS/Bi2MoO6, g-C3N4/Bi2MoO6, Ag2WO4/Ag/Bi2MoO6, Bi2MoO6/ZnSnO3 and CeO2/Bi2MoO6
composites [44e49]. Up to now, however, there is no any report of Bi2MoO6/MnxCd1-xS composite photocatalysts, although MnxCd1-xS solid solution is one of the most important photocatalysts with visible light response. Herein, we have reported the synthesis of Bi2MoO6/MnxCd1-xS composite and investigated its photocatalytic activity for hydrogen evolution from water with visible light response in more detail. A series of MnxCd1-xS composites were synthesized by a co-precipitation method firstly. Then the photocatalytic activity of the MnxCd1-xS composites was optimized by adjusting the relative ratio of Mn2þ to Cd2þ. The highest photocatalytic activity of MnxCd1-xS composites was determined to be Mn0.2Cd0.8S in composition. Further combining Mn0.2Cd0.8S composite with Bi2MoO6 nanocrystalline through a hydrothermal process yielded hierarchical heterostructure Bi2MoO6/Mn0.2Cd0.8S composite. Our investigation shows that the obtained Bi2MoO6/Mn0.2Cd0.8S composites exhibited a significantly improved photocatalytic activity for hydrogen evolution from water with visible light response when comparing with pure Bi2MoO6 and Mn0.2Cd0.8S composites. Furthermore, the Bi2MoO6/Mn0.2Cd0.8S composites’ photocatalytic activity can be further optimized by adjusting the relative ratio of Bi2MoO6 to Mn0.2Cd0.8S to a certain degree. A possible photocatalytic mechanism for Bi2MoO6/Mn0.2Cd0.8S composite has been proposed. Due to its good stability and excellent photocatalytic activity, the obtained Bi2MoO6/ Mn0.2Cd0.8S composite has potential application in photocatalytic water-splitting for hydrogen evolution by using solar power.
Experimental Materials Manganese acetate tetrahydrate (Mn(CH3COO)2$4H2O), cadmium acetate dehydrate (Cd(CH3COO)2$2H2O), bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), sodium molybdate dehydrate (Na2MoO4$2H2O), sodium sulfite (Na2SO3) and sodium sulfide nonahydrate (Na2S$9H2O) were obtained from Aladdin Reagent Co., Ltd. All chemicals used in the experiments are of analytical grade and were used without further purified. Deionized water was utilized in whole experiment.
The synthesis of MnxCd1-xS solid solution nanocrytines A suitable amount of Mn(CH3COO)2$4H2O and Cd(CH3COO)2$2H2O in different ratio according to the stoichiometric ratio of MnxCd1-xS (x ¼ 0e1) solid solution were dissolved in 75 mL deionized water to obtain a homogeneous mixture solution.
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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After that, an appropriate amount of 1.0 M Na2S solution was added into above solution drop by drop with fast stirring to obtain yellow sediments suspended in above mixture solution. The obtained yellow suspension solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, and then the autoclave was maintained at 200 C for 24 h to get MnxCd1xS solid solution nanocrytines. After that, the autoclave was cooled to room temperature naturally. The obtained final MnxCd1-xS solid solution nanocrytines were collected by centrifugation, and dried at 60 C in vacuum for 24 h.
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microscopy (HAADF-STEM) and elemental mapping analysis were conducted by using a JEM-ARM 200F scanning transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measurement was carried out in a PHI 5000C ESCA system. Diffuse reflectance spectra were obtained by using a Lambda 750 s UVevisible diffuse reflectance spectrometer with a wavelength range of 200e800 nm and pure BaSO4 power as a standard sample. Photoluminescence (PL) spectra were measured by using an F-7000 fluorescence spectrophotometer. All the measurements were performed at room temperature.
The synthesis of Bi2MoO6 microspheres Photoelectrochemical measurements Bi2MoO6 microspheres were synthesized via a solvothermal method. Typically, 1.9402 g Bi(NO3)3$5H2O and 0.4839 g Na2MoO4$2H2O with molar ratio (Bi/Mo) of 2:1 were dissolved in the mixture solvent composed of 20 mL ethylene glycol and 40 mL ethanol. After fast stirring for 1 h at room temperature, the mixture solution was transferred into a 100 mL Teflonlined stainless steel autoclave, and then the autoclave was maintained at 160 C for 5 h to get Bi2MoO6 microspheres. The final yellow Bi2MoO6 precipitate was collected by centrifugation, then it was washed several times and dried at 60 C in vacuum for 24 h.
The synthesis of Bi2MoO6/Mn0.2Cd0.8S composites The hierarchical Bi2MoO6/Mn0.2Cd0.8S heterostructure composites were synthesized by a co-precipitation method followed by a hydrothermal process. For the preparation of Mn0.2Cd0.8S precursor, typically, 0.4902 g Mn(CH3COO)2$4H2O, 2.132 g Cd(CH3COO)2$2H2O were dissolved in 50 mL deionized water to form a solution. After that, 2.4018 g Na2S$9H2O was added into above solution drop by drop. Finally, the yellow precipitate of Mn0.2Cd0.8S precursor was obtained. Then, an appropriate amount of as-prepared Bi2MoO6 power was added into the above obtained Mn0.2Cd0.8S precursor solution. After fast stirring at room temperature for 1 h, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, and then the autoclave was maintained at 200 C for 24 h. After that it was naturally cooled to room temperature. The prepared Bi2MoO6/Mn0.2Cd0.8S composite sample was collected by centrifugation, and it was wished by deionized water and absolute alcohol alternately for three times and dried at 60 C in vacuum for 24 h. The Bi2MoO6/Mn0.2Cd0.8S composite with 1 wt %, 3 wt %, 5 wt %, 7 wt %, 10 wt % Bi2MoO6 of Mn0.2Cd0.8S was labeled as 1-BMO/MCS, 3-BMO/MCS, 5BMO/MCS, 7-BMO/MCS, 10-BMO/MCS, respectively.
The photocurrent measurements were measured by a conventional three Electrode CHIe660C electrochemical workstation in a conventional three electrodes configuration using Pt foil as counter electrode and an Ag/AgCl (saturate KCl) as reference electrode. 0.5 M Na2SO4 aqueous solution was used as electrolyte. The working bias voltage is 0.5 V, a 300 W Xenon lamp was used as visible light source. The Preparation of working electrode: 0.01 g of the sample was dispersed in 1 mL of deionized water. After sonication for 15 min, the slurry was uniformly cast onto an indium tin oxide (ITO) glass film in size of 4 cm 1 cm, and then it was dried at 60 C for 6 h to obtain final electrode.
Photocatalytic activity test The photocatalytic hydrogen production was conducted in a quartz glass reactor with a top exposure and a closed circulation system. Light source was provided by the 300 W Xenon lamp (PLS-SXE300C, Beijing Perfectlight Co. Ltd., China) coupled with a UV cut-off filter (l > 420 nm). Before the start of the photocatalytic reaction, the air in system was vacuumized several times to eliminate the air completely in whole circulation system. The argon gas was filled into the whole circulation system during the intervals of vacuuming process. In a typically experiment, 50 mg photocatalyst was dispersed in 100 mL aqueous solution containing 0.25 M Na2SO3 and 0.35 M Na2S as sacrificial reagents. The temperature of photocatalytic reaction was stabilized at 6 C by a constant temperature bath. The obtained H2 was analyzed by a linked gas chromatograph (Agilent GC 7890b) equipped with high purity argon as carrier gas.
Results and discussion
Sample characterization
Crystal phase composition of samples
X-ray diffraction (XRD) patterns of the prepared samples were characterized by a Bruker D8ADVANCE X-ray diffractometer using Cu Ka (l ¼ 0.154 nm) radiation. The morphologies and microstructures of samples were analyzed by a field-emission scanning electron microscopy (Nava Nano SEM450) equipped with an energy-dispersive X-ray spectrometry (EDS), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM, JEM-2100HR). The high angle annular dark field scanning transmission electron
Fig. 1 exhibits the XRD patterns of the MnxCd1-xS (x ¼ 0e1) solid solutions, pure Bi2MoO6, and Bi2MoO6/Mn0.2Cd0.8S composites. The standard diffraction patterns data of CdS, g-MnS and Bi2MoO6 are also shown in Fig. 1 for references. It can be seen clearly from Fig. 1 that the diffraction patterns of CdS sample are consistent with the standard data of the hexagonal phase CdS (JCPDS No. 41e1049). There is no other diffraction peak in the patterns of the CdS sample, indicating the pure hexagonal phase CdS was obtained. Also, it can be seen from
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 1 e The representative XRD patterns of MnxCd1-xS (x ¼ 0, 0.1, 0.2, 0.5, 0.8, 0.9, MnS, Bi2MoO6) and 10-BMO/ MCS composite; the standard data for CdS (JCPDS No.41e1049), MnS (JCPDS No. 40e1289) and Bi2MoO6(JCPDS No. 21e0102) were shown as references.
Fig. 1 that the diffraction peaks of the MnxCd1-xS composites shifted to higher diffraction angles slowly until its phase changes from a hexagonal phase CdS to a hexagonal g-MnS (JCPDS No. 40e1289) when increasing the x value from 0.1 to 1, which indicates that the MnxCd1-xS was not just the simple mixture of CdS and MnS, but a new kind of solid solution. For example, the XRD peaks of the Mn0.2Cd0.8S present an obvious left-shift compared to the standard data of hexagonal g-MnS (JCPDS No. 40e1298), indicating the formation of Mn0.2Cd0.8S A) is solid solution successfully [50]. The radii of Cd2þ ion (0.97 A), therefore, when Cd2þ larger than that of the Mn2þ ion (0.46 ion is partly replaced by Mn2þ ion in MnxCd1-xS solid solution, it will lead to the shift of XRDs to some extent [50]. The characteristic peaks of pure Bi2MoO6 match well with those of the standard data of orthorhombic Bi2MoO6 (JCPDS No. 21e0102) [49]. Due to the low content of Bi2MoO6 and high dispersion in Mn0.2Cd0.8S solid solution, the diffraction peaks of Bi2MoO6 is not seen obvious in 10-BMO/MCS composite. Although, the presence of Bi2MoO6 phase in 10-BMO/MCS composite can be further demonstrated by the following characterizations of TEM, HRTEM, HAADF-STEM and XPS measurements.
Morphologies of samples Fig. 2aec displays the FE-SEM images of the pristine Bi2MoO6 (Fig. 2a), Mn0.2Cd0.8S (Fig. 2b), and 5-BMO/MCS (Fig. 2c) composite, respectively. As shown in Fig. 2a, it can be seen clearly that the pure Bi2MoO6 material presents a flower-like microsphere shape consisting of many small nanosheets. The Mn0.2Cd0.8S (Fig. 2b) composite is made up of some irregular nanoparticles. After combining Mn0.2Cd0.8S composite with Bi2MoO6, it can be seen clearly from Fig. 2c that the 5-BMO/ MCS shows an irregular shape consisting of some microcrystallines of Mn0.2Cd0.8S and Bi2MoO6. Further analysis indicates that the Mn0.2Cd0.8S composite presents an irregular shape and the Bi2MoO6 disperses homogeneously on the surface of
Mn0.2Cd0.8S composite with close contact in a nanosheet shape. The EDS spectrum of 5-BMO/MCS composite shown in Fig. 2d, which demonstrates the presence of Mn, Cd, S, Bi, Mo, O elements in 5-BMO/MCS. Fig. 2e shows the TEM image of 5BMO/MCS composite. It can be seen clearly from Fig. 2e that the Bi2MoO6 nanosheets were dispersed homogenously on the surface of Mn0.2Cd0.8S composite, which is consistent with the measurement results of SEM shown in Fig. 2c. The HRTEM image of 5-BMO/MCS composite shown in Fig. 2f clearly shows two distinctly different lattice fringes. The two different lattice spacing are estimated to be around 0.314 nm and 0.336 nm, corresponding to (131) crystal plane of Bi2MoO6 and (002) crystal plane of Mn0.2Cd0.8S, respectively [49,51]. The EDS elements mapping of the HAADF-STEM of the 5BMO/MCS composite is shown in Fig. 3. As shown in Fig. 3b, the EDS elements mapping from Fig. 3a clearly shows the existence of Mn, Cd, S, Bi, Mo, O in 5-BMO/MCS composite, and all these elements are distributed homogeneously in 5BMOMCS composite. The EDS elements mapping results are basically in line with those of FE-SEM and STEM micrographs. All these measurement results further indicate that Bi2MoO6 has been dispersed homogeneously on the surface of Mn0.2Cd0.8S composite. X-ray photoelectron spectroscopy (XPS) was carried out to analyze the surface chemical composition of 5-BMO/MCS composite. The typical XPS spectra of the 5-BMO/MCS are shown in Fig. 4. The survey spectrum (Fig. 4a) indicates that the composite’s surface is composed of Mn, Cd, S, Bi, Mo and O elements, which is in line with the results of EDS measurement shown in Figs. 2d and 3. The high resolution XPS spectra of Mn 2p, Cd 3d, S 2p, Bi 4f, Mo 3d and O 1s are shown in Fig. 4beg, respectively. For Mn0.2Cd0.8S composite, the energy peaks of Mn, Cd, S ions located at 652.8 eV (Mn2þ 2p1/2), 641.4 eV (Mn2þ 2p3/2), 411.8 eV (Cd2þ 3d3/2), 405.1 eV (Cd2þ 3d5/ 2 2p1/2) and 161.5 eV (S2 2p3/2), respectively, can 2), 162.6 eV (S be observed, which is basically consistent with literature date with very small variations [50]. For the section of Bi2MoO6, the main binding energy peaks located at 159.1eV, 164.3 eV and 232.4 eV, 235.5 eV corresponding to Bi 4f7/2, Bi 4f5/2 of Bi3þ and Mo 3d5/2, Mo 3d3/2 of Mo6þ, respectively can be also found in Fig. 4e,d, respectively, which mainly agree with the results in previous reports, except for a slight shift toward higher binding energy [49,50]. For O 1s, three main binding energy peaks located at 529.3 eV, 530.6 eV and 532.2 eV are observed, which can be assigned to the BieO, MoeO and surface hydroxyl groups (OeH), respectively [49]. The XPS results of 5BMO/MCS further confirmed its composition, and it basically agrees with the results of FE-SEM and EDS mapping micrographs.
Optical properties Fig. 5a shows the UVevisible diffuse reflectance spectra of MnxCd1-xS (x ¼ 0e1) solid solutions. As shown in Fig. 5a, for single phase of g-MnS, it shows wide band absorption from 200 to 650 nm with a peak at around 350 nm and some shoulder peaks from 350 to 500 nm. The strong absorption band around 350 nm is ascribed to the interband transition, which basically agrees with its band gap of 3.54 eV, and the absorption band beyond 350 nm with shoulders in the visible
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 2 e The FE-SEM images of Bi2MoO6 (a), Mn0.2Cd0.8S (b), and 5-BMO/MCS composite (c), and the EDS spectrum (d), TEM (e) and HRTEM (f) of 5-BMO/MCS composite.
light region is associated with internal transitions in the partly occupied 3d states of Mn2þ and the bulk defects in crystals [52e54]. For pure phase of CdS, it shows a strong absorption from 200 to 500 nm, and its absorption intensity decreases sharp beyond 500 nm with the absorption edge of around 600 nm. For MnxCd1-xS (x ¼ 0e1) solid solution, its absorption band changes with the different ratio of Mn2þ to Cd2þ. Basically, it absorption band edges vary from 350 to 500 nm. At the low content of Cd2þ (x < 0.2 in MnxCd1-xS), although some shoulder peaks in its absorption spectra can be observed but its intensity is lower than that of pure g-MnS, indicting bulk defects in g-MnS crystals have been suppressed by introducing Cd2þ. At high content of Cd2þ (x 0.2 in MnxCd1-xS), shoulder peak disappear and the MnxCd1-xS solid solutions
exhibit intense absorption bands with gradually steep edges in the visible light region, indicating that the MnxCd1-xS solid solutions is not just the simple mixture of MnS and CdS, but a true meaning of solid solution with its own valence band and conduction band [50]. With increasing the content of Cd2þ, the absorption edges of MnxCd1-xS (x 0.2) solid solutions shows a gradual red shift until it reach up to that of pure CdS phase at around 500 nm. Fig. 5b shows the UVevisible diffuse reflectance spectra of Bi2MoO6, Mn0.2Cd0.8S, and BMO/MCS composites. As shown in Fig. 5b, pure Bi2MoO6 shows strong wide band absorption from 200 to 470 nm peaking at 320 nm, the corresponding band gap is estimated to be around 2.64 eV. For Mn0.2Cd0.8S solid solution, it shows strong wide band absorption from 200 to
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 3 e HADDF-STEM image of 5-BMO/MCS composite (a, b), the corresponding elemental mapping images for Mn, Cd, S, Bi, Mo, and O are presented in (c), (d), (e), (f), (g) and (h), respectively.
570 nm, the corresponding band gap is estimated to be about 2.15 eV. For BMO/MCS composite (shown in Fig. 5b), due to the low content of Bi2MoO6, its absorption spectrum shows similar shape to that of pure Mn0.2Cd0.8S solid solution.
Photocatalytic properties Photocatalytic activities for hydrogen production from water of MnxCd1-xS, Bi2MoO6 and Bi2MoO6/Mn0.2Cd0.8S composites with different mass ratio of Bi2MoO6 to Mn0.2Cd0.8S contents were evaluated with visible-light (l > 420 nm) irradiation using Na2SO3 and using Na2S as sacrifice reagents. From Fig. 6a, it can be seen clearly that there is no H2 evolution from blank MnS under the irradiation of visible light. For pure CdS, it shows a reasonable photocatalytic activity for hydrogen evolution under the excitation of visible light. However, for MnxCd1-xS solid solution, it shows a novel improved photocatalytic activity for hydrogen evolution under the irradiation of visible light. All MnxCd1-xS solid solution sample shows higher photocatalytic activity for hydrogen evolution with visible light
response than that of pure CdS, and its photocatalytic activity changes with changing the relative ration of Mn2þ to Cd2þ to some extent, which can be seen more clearly in inset in Fig. 6a. For MnxCd1-xS solid solution sample, its photocatalytic activity for H2 evolution increases firstly with increasing the content of Mn2þ, until it content reach the 20% mole ratio of the MnxCd1-xS composites (x ¼ 0.2 in MnxCd1-xS), then it starts to decrease when further increasing the content of Mn2þ from x ¼ 0.2 to 1.0 in MnxCd1-xS composite. Therefore, the optimized mole ratio of Mn2þ to Cd2þ was estimated to be x ¼ 0.2 in MnxCd1-xS composites, and the Mn0.2Cd0.8S sample’s photocatalytic activity for hydrogen evolution of is around 3 times higher than that of pure CdS. Fig. 6b shows the photocatalytic activity for hydrogen evolution of Bi2MoO6/Mn0.2Cd0.8S composite in different mass ratio of Bi2MoO6 to Mn0.2Cd0.8S. For the purpose of comparision, the photocatalytic activities of pure Bi2MoO6 and Mn0.2Cd0.8S for hydrogen evolution are also shown in Fig. 6b. Due to the fast recombination of photo-generated electronsholes pairs, the pure Bi2MoO6’s photocatalytic activity for
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 4 e XPS survey spectra of 5-BMO/MCS (a), high resolution XPS of Mn 2p (b), Cd 3d (c), S 2p (d), Bi 4f (e), Mo 3d (f) and O 1s (g) in 5-BMO/MCS. Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 5 e UVevisible diffuse reflectance spectra of MnxCd1-xS (x ¼ 0e1, a) and Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/ Mn0.2Cd0.8S composites (b).
hydrogen evolution with visible light response is very low (almost zero). Although, single Mn0.2Cd0.8S composite shows reasonable photocatalytic activity for hydrogen evolution with visible light response, its practical application is still a challenge. However, after coupling with Bi2MoO6, the Bi2MoO6/Mn0.2Cd0.8S composite’s photocatalytic activity increased significantly. As shown in Fig. 6b, the Bi2MoO6/ Mn0.2Cd0.8S composite’s photocatalytic activity for hydrogen evolution is much higher than that of single Mn0.2Cd0.8S composite. We have optimized the relative contents of Bi2MoO6 to Mn0.2Cd0.8S in Bi2MoO6/Mn0.2Cd0.8S composite. As shown in inset in Fig. 6b, the Bi2MoO6/Mn0.2Cd0.8S composite’s photocatalytic activity for hydrogen evolution increase slowly when Bi2MoO6’s relative content increase from 0 to 5 wt % of Mn0.2Cd0.8S composite, after reaching a maximum at 5 wt %, its photocatalytic activity starts to decrease when further increasing its content from 5 wt % to higher content. Therefore, the optimized the relative contents of Bi2MoO6 is determined to be around 5 wt % of Mn0.2Cd0.8S composite in Bi2MoO6/Mn0.2Cd0.8S composite, and the 5-BMO/MCS composite’s photocatalytic activity for hydrogen evolution is around 3 times higher than that of single Mn0.2Cd0.8S solid solution
Fig. 6 e Plots of photocatalytic hydrogen evolution amount vs. irradiation time for MnxCd1-xS (a) and Bi2MoO6/ Mn0.2Cd0.8S (b) composites. The inset in Figure 6a shows hydrogen evolution rate of MnxCd1-xS composite vs. the molar ratio of Mn2þ to Cd2þ, and the inset in Figure 6b shows hydrogen evolution rate of Bi2MoO6/Mn0.2Cd0.8S composite vs. the molar ratio of Bi2MoO6 to Mn0.2Cd0.8S.
nanoparticles and around 10 times of pristine CdS. Here, the Bi2MoO6 acts like a cocatalyst facilitating the separation and transportation of carriers (including electrons and holes) in Bi2MoO6/Mn0.2Cd0.8S composite. Usually, excessive Bi2MoO6 will act as recombination centers for the electrons and holes [49e51]. Also, some excessive of them might occupy the active sites on its surface, which will lead to the decrease of photocatalytic activity of Bi2MoO6/Mn0.2Cd0.8S composite accordingly [49e51]. Photocatalytic stability is also an important parameter in evaluating the properties of semiconductor photocatalyst materials. The photocatalytic stability of Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites has been evaluated by 4 times cycling hydrogen production experiment. As shown in Fig. 7, for pure Mn0.2Cd0.8S, its photocatalytic activity for hydrogen evolution under visible light response decreased significantly (only keeps 55.8% of original activity) in 2.5 h cycled for four times. However, for Bi2MoO6/Mn0.2Cd0.8S composite, there is no obvious decrease of its photocatalytic
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 7 e Recycling tests of Mn0.2Cd0.8S and 5-BMO/MCS composites for photocatalytic hydrogen evolution with visible light irradiation.
activity under same experimental conditions (still keeps 90.5% of original activity). The experiment results indicate that Bi2MoO6/Mn0.2Cd0.8S composite has excellent stability during photocatalytic hydrogen production process.
Photocatalytic mechanism As it is well known, some factors, e.g., the efficiency of light absorption, charge transportation and charge separation, etc., are the key elements of influencing the photocatalytic activities of semiconductor photocatalysts. From Fig. 5b, the light
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absorption efficiency of Bi2MoO6/Mn0.2Cd0.8S composites does not show much difference from that of Mn0.2Cd0.8S. Basically, their absorption spectra have similar shapes and intensities. Therefore, for Bi2MoO6/Mn0.2Cd0.8S composite, light absorption is not the key element that determines its photocatalytic activities for hydrogen evolution. The efficiency of charge transportation and charge separation in semiconductor photocatalyst can be estimated by its photoluminescence efficiency to some extent. For semiconductor photocatalyst, the higher of photoluminescence efficiency, the lower of the charge transportation and charge separation’s efficiency. The PL spectra of Mn0.2Cd0.8S and a series of Bi2MoO6/Mn0.2Cd0.8S composites with excitation of 350 nm UV light are shown in Fig. 8. With excitation of 350 nm UV light, the Mn0.2Cd0.8S solid solution shows an orange luminescence and the corresponding emission spectra consists of a board band from 500 to 700 nm with a maximum at 550 nm [38,49]. For a series of Bi2MoO6/Mn0.2Cd0.8S composites, with excitation of 350 nm UV light, all of them present an orange luminescence, and their corresponding emission spectra have similar shapes to that of Mn0.2Cd0.8S solid solution. From Fig. 8, it is seen clearly that luminescence intensity of Mn0.2Cd0.8S solution is much higher than that of Bi2MoO6/ Mn0.2Cd0.8S composites. It means that the luminescence of Mn0.2Cd0.8S solution has been quenched by incorporating Bi2MoO6. By introducing Bi2MoO6 into Mn0.2Cd0.8S, the recombination of photo-generated electrons and holes is inhibited greatly, therefore, the luminescence intensity of Bi2MoO6/ Mn0.2Cd0.8S composite is much lower than that of Mn0.2Cd0.8S solid solution (as shown in Fig. 8). The luminescence intensity decreases in the following order: Mn0.2Cd0.8S > 1-BMO/MCS >
Fig. 8 e The photoluminescence spectra of Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites with excitation of 350 nm UV light. Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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3-BMO/MCS >10-BMO/MCS > 7-BMO/MCS > 5-BMO/MCS, which is consistent with the variation trends of the photocatalytic activities of Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites (shown in Fig. 6b). The charge separation efficiency in semiconductor photocatalyst can be also illustrated by photoelectrochemical techniques. The transient photocurrent-time curves (I-t) of Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites electrodes with visible light excitation for several of on and off cycles are shown in Fig. 9. As shown in Fig. 9, the pure Bi2MoO6 show the lowest photocurrent density in current experimental condition, indicating that photo-generated electrons and holes in Bi2MoO6 disappeared with rapid recombination. The photocurrent density of Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites electrodes decrease in the following order: 5-BMO/MCS > 7-BMO/MCS >10-BMO/MCS > 3BMO/MCS > 1-BMO/MCS >Mn0.2Cd0.8S > Bi2MoO6, which is basically consistent with their experimental photocatalytic activities for hydrogen evolution with visible light response (as shown in Fig. 6b). The valence band (VB) potential and conduction band (CB) potential of Bi2MoO6 and Mn0.2Cd0.8S can be estimated by the following equations [49]: EVB ¼ X Ee þ0:5Eg ECB ¼ EVB Eg In above equations, EVB is the valence band (VB) of semiconductor, ECB is the conduction band (CB) of semiconductor, X is the absolute electronegativity of semiconductor. The “X” values for Bi2MoO6 and Mn0.2Cd0.8S are calculated to be 5.54 and 4.88 eV, respectively. Ee is the energy of free electrons on hydrogen (around 4.5 eV) and Eg is the band gap energy of semiconductor. For Bi2MoO6 and Mn0.2Cd0.8S, their Eg are estimated to be 2.64 eV and 2.15 eV, respectively. As a result, according to above equations, the EVB of Bi2MoO6 and Mn0.2Cd0.8S can be calculated to be around 2.36 and 1.45 eV, respectively. Therefore, the ECB of Bi2MoO6 and Mn0.2Cd0.8S is
Fig. 10 e Proposed photocatalytic mechanism diagram of Bi2MoO6/Mn0.2Cd0.8S composite.
estimated to be 0.28 and 0.7 eV, respectively. The results demonstrated that the band gap structure, and energy band positions of Bi2MoO6 and Mn0.2Cd0.8S is beneficial to the separation of photo-generated electron-hole pairs in Bi2MoO6/ Mn0.2Cd0.8S composite. In order to further understand the photocatalytic mechanism of Bi2MoO6/Mn0.2Cd0.8S composites, we proposed a possible mechanism for charges separation and charges transformation in Bi2MoO6/Mn0.2Cd0.8S composite [55]. As shown in Fig. 10, with excitation of visible light, both Bi2MoO6 and Mn0.2Cd0.8S in the composites can be excited and engender photo-generated electron-hole pairs. Since the CB (0.7 eV) and VB (þ1.4 eV) of Mn0.2Cd0.8S lies below the CB (0.28 eV) and VB (þ2.36 eV) of Bi2MoO6, the photo-generated electrons can be transferred from the CB of Mn0.2Cd0.8S to CB of Bi2MoO6, while the photo-generated holes can be transferred from the VB of Bi2MoO6 to VB of Mn0.2Cd0.8S. The photogenerated electrons at the CB of Bi2MoO6 can reduce Hþ and evolve H2, the photo-generated holes at the VB of Mn0.2Cd0.8S can be consumed by the sacrificial reagents (Na2S and Na2SO3). Therefore, the photo-generated electron-hole pair in Bi2MoO6/Mn0.2Cd0.8S composite can be effectively separated, resulting in its signally improvement of photocatalytic activity for H2 evolution from water.
Conclusions
Fig. 9 e The transient photocurrent e Time (I-t) curves of Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites.
Due to the well match of the energy band position of Bi2MoO6 and Mn0.2Cd0.8S, and the existence of heterostructure in Bi2MoO6/Mn0.2Cd0.8S composite, the obtained hierarchical Bi2MoO6/Mn0.2Cd0.8S heterostructure nanocomposite shows significantly improved photocatalytic activity for hydrogen evolution from water with visible light response comparing with pure Bi2MoO6 and Mn0.2Cd0.8S solid solution. Considering their high photocatalytic activities for hydrogen evolution with visible light response and good stabilities, the obtained Bi2MoO6/Mn0.2Cd0.8S composites have potential application in photocatalytic water-splitting for hydrogen evolution by using solar power. The current research may provide a new insight
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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into the design and fabrication of novel composites photocatalysts with high effect.
Acknowledgements This project is financially supported by the National Natural Science Foundation of China (NSFC 51762035, 21161015, 51750110511, 51568049, 51932009), the Natural Science Foundation of the Jiangxi Province of China (20164BCD40098, 20152ACB20011, 2009GZH0082), the Natural Science Foundation of the Jiangxi Higher Education Institutions of China (GJJ09180, GJJ14513), Jiangmen Innovative Research Team Program (2017) and Major Program of Basic Research and Applied Research of Guangdong Province (2017KZDXM083).
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Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visible-light hydrogen production Xiaoming Liu a,b, Binquan Wu a, Xueya Chen a, Liushui Yan a, Huiqin Guo a, Kexin Li a,*, Luping Xu a, Jun Lin b,c,** a
School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang, 330063, PR China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China c School of Applied Physics and Materials, Wuyi University, Jiangmen, Guangdong, 529020, PR China b
highlights
graphical abstract
The Mn0.2Cd0.8S shows an excellent photocatalytic activity for H2 evolution. The Bi2MoO6/Mn0.2Cd0.8S exhibits a significantly high photocatalytic activity. A possible photocatalytic mechanism for Bi2MoO6/Mn0.2Cd0.8S has been proposed.
article info
abstract
Article history:
In this work, a series of MnxCd1-xS solid solutions as efficient photocatalysts for hydrogen
Received 16 August 2019
evolution with visible light response were synthesized via a co-precipitation method firstly.
Received in revised form
Then, the hierarchical Bi2MoO6/Mn0.2Cd0.8S heterostructured composite was prepared by
21 October 2019
combining Mn0.2Cd0.8S composite with Bi2MoO6 nanocrystalline through a hydrothermal
Accepted 26 November 2019
process. The optimized Mn0.2Cd0.8S composite’s photocatalytic activity is more than 3
Available online xxx
times of pristine CdS and the prepared Bi2MoO6/Mn0.2Cd0.8S nanocomposites exhibited a significantly improved photocatalytic activity for hydrogen evolution from water with
Keywords:
visible light response comparing with single Mn0.2Cd0.8S composite. The optimized pho-
Bi2MoO6/Mn0.2Cd0.8S composites
tocatalytic activity of Bi2MoO6/Mn0.2Cd0.8S composite is around 10 times of pristine CdS.
Heterostructure
The excellent photocatalytic activity of Bi2MoO6/Mn0.2Cd0.8S composite might be ascribed
Hydrogen production
to the well-matched energy band structures of the Bi2MoO6 and Mn0.2Cd0.8S. Furthermore,
* Corresponding author. ** Corresponding author. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China E-mail addresses: [email protected] (K. Li), [email protected] (J. Lin). https://doi.org/10.1016/j.ijhydene.2019.11.197 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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the heterojunctions in Bi2MoO6/Mn0.2Cd0.8S composite might also do some contributions to improve its photocatalytic activities to some extent. A possible photocatalytic mechanism was proposed. Due to its excellent photocatalytic activity and good stability for hydrogen evolution from water, the obtained hierarchical Bi2MoO6/Mn0.2Cd0.8S composite has potential application in photocatalytic hydrogen evolution from water by using solar power. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the 21st century, the energy crisis and environmental issues have driven human society to explore new clean, effect and renewable resources [1e4]. Hydrogen energy, due to its clean, efficiency and environmental friendliness, has been considered as a promising energy resources in the future [5]. Since TiO2 semiconductor was first utilized for photoelectronchemical water splitting with ultraviolet light excitation by Fujishima and Honda in 1972，hydrogen evolution via a photocatalytic reaction using semiconductor has been widely regarded as a promising technique to convert solar energy into chemical energy stored in hydrogen [6e8]. However, most of semiconductors photocatalysts only absorb ultraviolet light, which accounts for 3e5% of solar spectrum. On the contrary, nearly half of solar energy is in visible light region [9e16]. Therefore, achieving highly efficient and stable solar hydrogen generation from water is still a challenging and promising research topic [17e20]. Recently, the solid solution of multi-component sulfides, e.g., CoxCd1-xS, Zn1-xCdxS, (Zn0.95Cu0.05)1-xCdxS, MnxCd1-xS, etc., have attracted much more attention owing to their remarkable optical performances in visible light region, adjustable band structures and potential application in solar hydrogen generation from water [21e34]. The multicomponent sulfides can be synthesized by different methods. Usually, different synthesis methods will lead to different photocatalytic properties [31]. It is reported that the MnxCd1-xS solid solution can be modified as a type of efficient photocatalyst for hydrogen production from water with visible-light response even in the absence of cocatalyst [35e37]. However, due to its poor stability, limited surface active sites and low efficiency of charges separation, the MnxCd1-xS solid solution’s photocatalytic activity for hydrogen production from water is still low [36,37]. Therefore, the MnxCd1-xS solid solution’s photocatalytic activity need to be further improved for photochemical solar H2 production with visible light response. Bismuth molybdate (Bi2MoO6) has been regarded as a promising semiconductor photocatalyst due to its relatively narrow band gap, favorable thermal stability and excellent photocatalytic performance [38,39]. However, ascribing to the fast recombination of photo-generated charge carriers, its photocatalytic activity is very low. To address this issue, the Bi2MoO6 was usually combined with other semiconductor photocatalysts to obtain an type of heterostructure to achieve high efficient separation of photo-generated charge carriers, which finally lead to a substantial improvement in photocatalytic activity [40e43], e.g., CdS/Bi2MoO6, g-C3N4/Bi2MoO6, Ag2WO4/Ag/Bi2MoO6, Bi2MoO6/ZnSnO3 and CeO2/Bi2MoO6
composites [44e49]. Up to now, however, there is no any report of Bi2MoO6/MnxCd1-xS composite photocatalysts, although MnxCd1-xS solid solution is one of the most important photocatalysts with visible light response. Herein, we have reported the synthesis of Bi2MoO6/MnxCd1-xS composite and investigated its photocatalytic activity for hydrogen evolution from water with visible light response in more detail. A series of MnxCd1-xS composites were synthesized by a co-precipitation method firstly. Then the photocatalytic activity of the MnxCd1-xS composites was optimized by adjusting the relative ratio of Mn2þ to Cd2þ. The highest photocatalytic activity of MnxCd1-xS composites was determined to be Mn0.2Cd0.8S in composition. Further combining Mn0.2Cd0.8S composite with Bi2MoO6 nanocrystalline through a hydrothermal process yielded hierarchical heterostructure Bi2MoO6/Mn0.2Cd0.8S composite. Our investigation shows that the obtained Bi2MoO6/Mn0.2Cd0.8S composites exhibited a significantly improved photocatalytic activity for hydrogen evolution from water with visible light response when comparing with pure Bi2MoO6 and Mn0.2Cd0.8S composites. Furthermore, the Bi2MoO6/Mn0.2Cd0.8S composites’ photocatalytic activity can be further optimized by adjusting the relative ratio of Bi2MoO6 to Mn0.2Cd0.8S to a certain degree. A possible photocatalytic mechanism for Bi2MoO6/Mn0.2Cd0.8S composite has been proposed. Due to its good stability and excellent photocatalytic activity, the obtained Bi2MoO6/ Mn0.2Cd0.8S composite has potential application in photocatalytic water-splitting for hydrogen evolution by using solar power.
Experimental Materials Manganese acetate tetrahydrate (Mn(CH3COO)2$4H2O), cadmium acetate dehydrate (Cd(CH3COO)2$2H2O), bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), sodium molybdate dehydrate (Na2MoO4$2H2O), sodium sulfite (Na2SO3) and sodium sulfide nonahydrate (Na2S$9H2O) were obtained from Aladdin Reagent Co., Ltd. All chemicals used in the experiments are of analytical grade and were used without further purified. Deionized water was utilized in whole experiment.
The synthesis of MnxCd1-xS solid solution nanocrytines A suitable amount of Mn(CH3COO)2$4H2O and Cd(CH3COO)2$2H2O in different ratio according to the stoichiometric ratio of MnxCd1-xS (x ¼ 0e1) solid solution were dissolved in 75 mL deionized water to obtain a homogeneous mixture solution.
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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After that, an appropriate amount of 1.0 M Na2S solution was added into above solution drop by drop with fast stirring to obtain yellow sediments suspended in above mixture solution. The obtained yellow suspension solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, and then the autoclave was maintained at 200 C for 24 h to get MnxCd1xS solid solution nanocrytines. After that, the autoclave was cooled to room temperature naturally. The obtained final MnxCd1-xS solid solution nanocrytines were collected by centrifugation, and dried at 60 C in vacuum for 24 h.
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microscopy (HAADF-STEM) and elemental mapping analysis were conducted by using a JEM-ARM 200F scanning transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measurement was carried out in a PHI 5000C ESCA system. Diffuse reflectance spectra were obtained by using a Lambda 750 s UVevisible diffuse reflectance spectrometer with a wavelength range of 200e800 nm and pure BaSO4 power as a standard sample. Photoluminescence (PL) spectra were measured by using an F-7000 fluorescence spectrophotometer. All the measurements were performed at room temperature.
The synthesis of Bi2MoO6 microspheres Photoelectrochemical measurements Bi2MoO6 microspheres were synthesized via a solvothermal method. Typically, 1.9402 g Bi(NO3)3$5H2O and 0.4839 g Na2MoO4$2H2O with molar ratio (Bi/Mo) of 2:1 were dissolved in the mixture solvent composed of 20 mL ethylene glycol and 40 mL ethanol. After fast stirring for 1 h at room temperature, the mixture solution was transferred into a 100 mL Teflonlined stainless steel autoclave, and then the autoclave was maintained at 160 C for 5 h to get Bi2MoO6 microspheres. The final yellow Bi2MoO6 precipitate was collected by centrifugation, then it was washed several times and dried at 60 C in vacuum for 24 h.
The synthesis of Bi2MoO6/Mn0.2Cd0.8S composites The hierarchical Bi2MoO6/Mn0.2Cd0.8S heterostructure composites were synthesized by a co-precipitation method followed by a hydrothermal process. For the preparation of Mn0.2Cd0.8S precursor, typically, 0.4902 g Mn(CH3COO)2$4H2O, 2.132 g Cd(CH3COO)2$2H2O were dissolved in 50 mL deionized water to form a solution. After that, 2.4018 g Na2S$9H2O was added into above solution drop by drop. Finally, the yellow precipitate of Mn0.2Cd0.8S precursor was obtained. Then, an appropriate amount of as-prepared Bi2MoO6 power was added into the above obtained Mn0.2Cd0.8S precursor solution. After fast stirring at room temperature for 1 h, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, and then the autoclave was maintained at 200 C for 24 h. After that it was naturally cooled to room temperature. The prepared Bi2MoO6/Mn0.2Cd0.8S composite sample was collected by centrifugation, and it was wished by deionized water and absolute alcohol alternately for three times and dried at 60 C in vacuum for 24 h. The Bi2MoO6/Mn0.2Cd0.8S composite with 1 wt %, 3 wt %, 5 wt %, 7 wt %, 10 wt % Bi2MoO6 of Mn0.2Cd0.8S was labeled as 1-BMO/MCS, 3-BMO/MCS, 5BMO/MCS, 7-BMO/MCS, 10-BMO/MCS, respectively.
The photocurrent measurements were measured by a conventional three Electrode CHIe660C electrochemical workstation in a conventional three electrodes configuration using Pt foil as counter electrode and an Ag/AgCl (saturate KCl) as reference electrode. 0.5 M Na2SO4 aqueous solution was used as electrolyte. The working bias voltage is 0.5 V, a 300 W Xenon lamp was used as visible light source. The Preparation of working electrode: 0.01 g of the sample was dispersed in 1 mL of deionized water. After sonication for 15 min, the slurry was uniformly cast onto an indium tin oxide (ITO) glass film in size of 4 cm 1 cm, and then it was dried at 60 C for 6 h to obtain final electrode.
Photocatalytic activity test The photocatalytic hydrogen production was conducted in a quartz glass reactor with a top exposure and a closed circulation system. Light source was provided by the 300 W Xenon lamp (PLS-SXE300C, Beijing Perfectlight Co. Ltd., China) coupled with a UV cut-off filter (l > 420 nm). Before the start of the photocatalytic reaction, the air in system was vacuumized several times to eliminate the air completely in whole circulation system. The argon gas was filled into the whole circulation system during the intervals of vacuuming process. In a typically experiment, 50 mg photocatalyst was dispersed in 100 mL aqueous solution containing 0.25 M Na2SO3 and 0.35 M Na2S as sacrificial reagents. The temperature of photocatalytic reaction was stabilized at 6 C by a constant temperature bath. The obtained H2 was analyzed by a linked gas chromatograph (Agilent GC 7890b) equipped with high purity argon as carrier gas.
Results and discussion
Sample characterization
Crystal phase composition of samples
X-ray diffraction (XRD) patterns of the prepared samples were characterized by a Bruker D8ADVANCE X-ray diffractometer using Cu Ka (l ¼ 0.154 nm) radiation. The morphologies and microstructures of samples were analyzed by a field-emission scanning electron microscopy (Nava Nano SEM450) equipped with an energy-dispersive X-ray spectrometry (EDS), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM, JEM-2100HR). The high angle annular dark field scanning transmission electron
Fig. 1 exhibits the XRD patterns of the MnxCd1-xS (x ¼ 0e1) solid solutions, pure Bi2MoO6, and Bi2MoO6/Mn0.2Cd0.8S composites. The standard diffraction patterns data of CdS, g-MnS and Bi2MoO6 are also shown in Fig. 1 for references. It can be seen clearly from Fig. 1 that the diffraction patterns of CdS sample are consistent with the standard data of the hexagonal phase CdS (JCPDS No. 41e1049). There is no other diffraction peak in the patterns of the CdS sample, indicating the pure hexagonal phase CdS was obtained. Also, it can be seen from
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 1 e The representative XRD patterns of MnxCd1-xS (x ¼ 0, 0.1, 0.2, 0.5, 0.8, 0.9, MnS, Bi2MoO6) and 10-BMO/ MCS composite; the standard data for CdS (JCPDS No.41e1049), MnS (JCPDS No. 40e1289) and Bi2MoO6(JCPDS No. 21e0102) were shown as references.
Fig. 1 that the diffraction peaks of the MnxCd1-xS composites shifted to higher diffraction angles slowly until its phase changes from a hexagonal phase CdS to a hexagonal g-MnS (JCPDS No. 40e1289) when increasing the x value from 0.1 to 1, which indicates that the MnxCd1-xS was not just the simple mixture of CdS and MnS, but a new kind of solid solution. For example, the XRD peaks of the Mn0.2Cd0.8S present an obvious left-shift compared to the standard data of hexagonal g-MnS (JCPDS No. 40e1298), indicating the formation of Mn0.2Cd0.8S A) is solid solution successfully [50]. The radii of Cd2þ ion (0.97 A), therefore, when Cd2þ larger than that of the Mn2þ ion (0.46 ion is partly replaced by Mn2þ ion in MnxCd1-xS solid solution, it will lead to the shift of XRDs to some extent [50]. The characteristic peaks of pure Bi2MoO6 match well with those of the standard data of orthorhombic Bi2MoO6 (JCPDS No. 21e0102) [49]. Due to the low content of Bi2MoO6 and high dispersion in Mn0.2Cd0.8S solid solution, the diffraction peaks of Bi2MoO6 is not seen obvious in 10-BMO/MCS composite. Although, the presence of Bi2MoO6 phase in 10-BMO/MCS composite can be further demonstrated by the following characterizations of TEM, HRTEM, HAADF-STEM and XPS measurements.
Morphologies of samples Fig. 2aec displays the FE-SEM images of the pristine Bi2MoO6 (Fig. 2a), Mn0.2Cd0.8S (Fig. 2b), and 5-BMO/MCS (Fig. 2c) composite, respectively. As shown in Fig. 2a, it can be seen clearly that the pure Bi2MoO6 material presents a flower-like microsphere shape consisting of many small nanosheets. The Mn0.2Cd0.8S (Fig. 2b) composite is made up of some irregular nanoparticles. After combining Mn0.2Cd0.8S composite with Bi2MoO6, it can be seen clearly from Fig. 2c that the 5-BMO/ MCS shows an irregular shape consisting of some microcrystallines of Mn0.2Cd0.8S and Bi2MoO6. Further analysis indicates that the Mn0.2Cd0.8S composite presents an irregular shape and the Bi2MoO6 disperses homogeneously on the surface of
Mn0.2Cd0.8S composite with close contact in a nanosheet shape. The EDS spectrum of 5-BMO/MCS composite shown in Fig. 2d, which demonstrates the presence of Mn, Cd, S, Bi, Mo, O elements in 5-BMO/MCS. Fig. 2e shows the TEM image of 5BMO/MCS composite. It can be seen clearly from Fig. 2e that the Bi2MoO6 nanosheets were dispersed homogenously on the surface of Mn0.2Cd0.8S composite, which is consistent with the measurement results of SEM shown in Fig. 2c. The HRTEM image of 5-BMO/MCS composite shown in Fig. 2f clearly shows two distinctly different lattice fringes. The two different lattice spacing are estimated to be around 0.314 nm and 0.336 nm, corresponding to (131) crystal plane of Bi2MoO6 and (002) crystal plane of Mn0.2Cd0.8S, respectively [49,51]. The EDS elements mapping of the HAADF-STEM of the 5BMO/MCS composite is shown in Fig. 3. As shown in Fig. 3b, the EDS elements mapping from Fig. 3a clearly shows the existence of Mn, Cd, S, Bi, Mo, O in 5-BMO/MCS composite, and all these elements are distributed homogeneously in 5BMOMCS composite. The EDS elements mapping results are basically in line with those of FE-SEM and STEM micrographs. All these measurement results further indicate that Bi2MoO6 has been dispersed homogeneously on the surface of Mn0.2Cd0.8S composite. X-ray photoelectron spectroscopy (XPS) was carried out to analyze the surface chemical composition of 5-BMO/MCS composite. The typical XPS spectra of the 5-BMO/MCS are shown in Fig. 4. The survey spectrum (Fig. 4a) indicates that the composite’s surface is composed of Mn, Cd, S, Bi, Mo and O elements, which is in line with the results of EDS measurement shown in Figs. 2d and 3. The high resolution XPS spectra of Mn 2p, Cd 3d, S 2p, Bi 4f, Mo 3d and O 1s are shown in Fig. 4beg, respectively. For Mn0.2Cd0.8S composite, the energy peaks of Mn, Cd, S ions located at 652.8 eV (Mn2þ 2p1/2), 641.4 eV (Mn2þ 2p3/2), 411.8 eV (Cd2þ 3d3/2), 405.1 eV (Cd2þ 3d5/ 2 2p1/2) and 161.5 eV (S2 2p3/2), respectively, can 2), 162.6 eV (S be observed, which is basically consistent with literature date with very small variations [50]. For the section of Bi2MoO6, the main binding energy peaks located at 159.1eV, 164.3 eV and 232.4 eV, 235.5 eV corresponding to Bi 4f7/2, Bi 4f5/2 of Bi3þ and Mo 3d5/2, Mo 3d3/2 of Mo6þ, respectively can be also found in Fig. 4e,d, respectively, which mainly agree with the results in previous reports, except for a slight shift toward higher binding energy [49,50]. For O 1s, three main binding energy peaks located at 529.3 eV, 530.6 eV and 532.2 eV are observed, which can be assigned to the BieO, MoeO and surface hydroxyl groups (OeH), respectively [49]. The XPS results of 5BMO/MCS further confirmed its composition, and it basically agrees with the results of FE-SEM and EDS mapping micrographs.
Optical properties Fig. 5a shows the UVevisible diffuse reflectance spectra of MnxCd1-xS (x ¼ 0e1) solid solutions. As shown in Fig. 5a, for single phase of g-MnS, it shows wide band absorption from 200 to 650 nm with a peak at around 350 nm and some shoulder peaks from 350 to 500 nm. The strong absorption band around 350 nm is ascribed to the interband transition, which basically agrees with its band gap of 3.54 eV, and the absorption band beyond 350 nm with shoulders in the visible
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 2 e The FE-SEM images of Bi2MoO6 (a), Mn0.2Cd0.8S (b), and 5-BMO/MCS composite (c), and the EDS spectrum (d), TEM (e) and HRTEM (f) of 5-BMO/MCS composite.
light region is associated with internal transitions in the partly occupied 3d states of Mn2þ and the bulk defects in crystals [52e54]. For pure phase of CdS, it shows a strong absorption from 200 to 500 nm, and its absorption intensity decreases sharp beyond 500 nm with the absorption edge of around 600 nm. For MnxCd1-xS (x ¼ 0e1) solid solution, its absorption band changes with the different ratio of Mn2þ to Cd2þ. Basically, it absorption band edges vary from 350 to 500 nm. At the low content of Cd2þ (x < 0.2 in MnxCd1-xS), although some shoulder peaks in its absorption spectra can be observed but its intensity is lower than that of pure g-MnS, indicting bulk defects in g-MnS crystals have been suppressed by introducing Cd2þ. At high content of Cd2þ (x 0.2 in MnxCd1-xS), shoulder peak disappear and the MnxCd1-xS solid solutions
exhibit intense absorption bands with gradually steep edges in the visible light region, indicating that the MnxCd1-xS solid solutions is not just the simple mixture of MnS and CdS, but a true meaning of solid solution with its own valence band and conduction band [50]. With increasing the content of Cd2þ, the absorption edges of MnxCd1-xS (x 0.2) solid solutions shows a gradual red shift until it reach up to that of pure CdS phase at around 500 nm. Fig. 5b shows the UVevisible diffuse reflectance spectra of Bi2MoO6, Mn0.2Cd0.8S, and BMO/MCS composites. As shown in Fig. 5b, pure Bi2MoO6 shows strong wide band absorption from 200 to 470 nm peaking at 320 nm, the corresponding band gap is estimated to be around 2.64 eV. For Mn0.2Cd0.8S solid solution, it shows strong wide band absorption from 200 to
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 3 e HADDF-STEM image of 5-BMO/MCS composite (a, b), the corresponding elemental mapping images for Mn, Cd, S, Bi, Mo, and O are presented in (c), (d), (e), (f), (g) and (h), respectively.
570 nm, the corresponding band gap is estimated to be about 2.15 eV. For BMO/MCS composite (shown in Fig. 5b), due to the low content of Bi2MoO6, its absorption spectrum shows similar shape to that of pure Mn0.2Cd0.8S solid solution.
Photocatalytic properties Photocatalytic activities for hydrogen production from water of MnxCd1-xS, Bi2MoO6 and Bi2MoO6/Mn0.2Cd0.8S composites with different mass ratio of Bi2MoO6 to Mn0.2Cd0.8S contents were evaluated with visible-light (l > 420 nm) irradiation using Na2SO3 and using Na2S as sacrifice reagents. From Fig. 6a, it can be seen clearly that there is no H2 evolution from blank MnS under the irradiation of visible light. For pure CdS, it shows a reasonable photocatalytic activity for hydrogen evolution under the excitation of visible light. However, for MnxCd1-xS solid solution, it shows a novel improved photocatalytic activity for hydrogen evolution under the irradiation of visible light. All MnxCd1-xS solid solution sample shows higher photocatalytic activity for hydrogen evolution with visible light
response than that of pure CdS, and its photocatalytic activity changes with changing the relative ration of Mn2þ to Cd2þ to some extent, which can be seen more clearly in inset in Fig. 6a. For MnxCd1-xS solid solution sample, its photocatalytic activity for H2 evolution increases firstly with increasing the content of Mn2þ, until it content reach the 20% mole ratio of the MnxCd1-xS composites (x ¼ 0.2 in MnxCd1-xS), then it starts to decrease when further increasing the content of Mn2þ from x ¼ 0.2 to 1.0 in MnxCd1-xS composite. Therefore, the optimized mole ratio of Mn2þ to Cd2þ was estimated to be x ¼ 0.2 in MnxCd1-xS composites, and the Mn0.2Cd0.8S sample’s photocatalytic activity for hydrogen evolution of is around 3 times higher than that of pure CdS. Fig. 6b shows the photocatalytic activity for hydrogen evolution of Bi2MoO6/Mn0.2Cd0.8S composite in different mass ratio of Bi2MoO6 to Mn0.2Cd0.8S. For the purpose of comparision, the photocatalytic activities of pure Bi2MoO6 and Mn0.2Cd0.8S for hydrogen evolution are also shown in Fig. 6b. Due to the fast recombination of photo-generated electronsholes pairs, the pure Bi2MoO6’s photocatalytic activity for
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 4 e XPS survey spectra of 5-BMO/MCS (a), high resolution XPS of Mn 2p (b), Cd 3d (c), S 2p (d), Bi 4f (e), Mo 3d (f) and O 1s (g) in 5-BMO/MCS. Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 5 e UVevisible diffuse reflectance spectra of MnxCd1-xS (x ¼ 0e1, a) and Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/ Mn0.2Cd0.8S composites (b).
hydrogen evolution with visible light response is very low (almost zero). Although, single Mn0.2Cd0.8S composite shows reasonable photocatalytic activity for hydrogen evolution with visible light response, its practical application is still a challenge. However, after coupling with Bi2MoO6, the Bi2MoO6/Mn0.2Cd0.8S composite’s photocatalytic activity increased significantly. As shown in Fig. 6b, the Bi2MoO6/ Mn0.2Cd0.8S composite’s photocatalytic activity for hydrogen evolution is much higher than that of single Mn0.2Cd0.8S composite. We have optimized the relative contents of Bi2MoO6 to Mn0.2Cd0.8S in Bi2MoO6/Mn0.2Cd0.8S composite. As shown in inset in Fig. 6b, the Bi2MoO6/Mn0.2Cd0.8S composite’s photocatalytic activity for hydrogen evolution increase slowly when Bi2MoO6’s relative content increase from 0 to 5 wt % of Mn0.2Cd0.8S composite, after reaching a maximum at 5 wt %, its photocatalytic activity starts to decrease when further increasing its content from 5 wt % to higher content. Therefore, the optimized the relative contents of Bi2MoO6 is determined to be around 5 wt % of Mn0.2Cd0.8S composite in Bi2MoO6/Mn0.2Cd0.8S composite, and the 5-BMO/MCS composite’s photocatalytic activity for hydrogen evolution is around 3 times higher than that of single Mn0.2Cd0.8S solid solution
Fig. 6 e Plots of photocatalytic hydrogen evolution amount vs. irradiation time for MnxCd1-xS (a) and Bi2MoO6/ Mn0.2Cd0.8S (b) composites. The inset in Figure 6a shows hydrogen evolution rate of MnxCd1-xS composite vs. the molar ratio of Mn2þ to Cd2þ, and the inset in Figure 6b shows hydrogen evolution rate of Bi2MoO6/Mn0.2Cd0.8S composite vs. the molar ratio of Bi2MoO6 to Mn0.2Cd0.8S.
nanoparticles and around 10 times of pristine CdS. Here, the Bi2MoO6 acts like a cocatalyst facilitating the separation and transportation of carriers (including electrons and holes) in Bi2MoO6/Mn0.2Cd0.8S composite. Usually, excessive Bi2MoO6 will act as recombination centers for the electrons and holes [49e51]. Also, some excessive of them might occupy the active sites on its surface, which will lead to the decrease of photocatalytic activity of Bi2MoO6/Mn0.2Cd0.8S composite accordingly [49e51]. Photocatalytic stability is also an important parameter in evaluating the properties of semiconductor photocatalyst materials. The photocatalytic stability of Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites has been evaluated by 4 times cycling hydrogen production experiment. As shown in Fig. 7, for pure Mn0.2Cd0.8S, its photocatalytic activity for hydrogen evolution under visible light response decreased significantly (only keeps 55.8% of original activity) in 2.5 h cycled for four times. However, for Bi2MoO6/Mn0.2Cd0.8S composite, there is no obvious decrease of its photocatalytic
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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Fig. 7 e Recycling tests of Mn0.2Cd0.8S and 5-BMO/MCS composites for photocatalytic hydrogen evolution with visible light irradiation.
activity under same experimental conditions (still keeps 90.5% of original activity). The experiment results indicate that Bi2MoO6/Mn0.2Cd0.8S composite has excellent stability during photocatalytic hydrogen production process.
Photocatalytic mechanism As it is well known, some factors, e.g., the efficiency of light absorption, charge transportation and charge separation, etc., are the key elements of influencing the photocatalytic activities of semiconductor photocatalysts. From Fig. 5b, the light
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absorption efficiency of Bi2MoO6/Mn0.2Cd0.8S composites does not show much difference from that of Mn0.2Cd0.8S. Basically, their absorption spectra have similar shapes and intensities. Therefore, for Bi2MoO6/Mn0.2Cd0.8S composite, light absorption is not the key element that determines its photocatalytic activities for hydrogen evolution. The efficiency of charge transportation and charge separation in semiconductor photocatalyst can be estimated by its photoluminescence efficiency to some extent. For semiconductor photocatalyst, the higher of photoluminescence efficiency, the lower of the charge transportation and charge separation’s efficiency. The PL spectra of Mn0.2Cd0.8S and a series of Bi2MoO6/Mn0.2Cd0.8S composites with excitation of 350 nm UV light are shown in Fig. 8. With excitation of 350 nm UV light, the Mn0.2Cd0.8S solid solution shows an orange luminescence and the corresponding emission spectra consists of a board band from 500 to 700 nm with a maximum at 550 nm [38,49]. For a series of Bi2MoO6/Mn0.2Cd0.8S composites, with excitation of 350 nm UV light, all of them present an orange luminescence, and their corresponding emission spectra have similar shapes to that of Mn0.2Cd0.8S solid solution. From Fig. 8, it is seen clearly that luminescence intensity of Mn0.2Cd0.8S solution is much higher than that of Bi2MoO6/ Mn0.2Cd0.8S composites. It means that the luminescence of Mn0.2Cd0.8S solution has been quenched by incorporating Bi2MoO6. By introducing Bi2MoO6 into Mn0.2Cd0.8S, the recombination of photo-generated electrons and holes is inhibited greatly, therefore, the luminescence intensity of Bi2MoO6/ Mn0.2Cd0.8S composite is much lower than that of Mn0.2Cd0.8S solid solution (as shown in Fig. 8). The luminescence intensity decreases in the following order: Mn0.2Cd0.8S > 1-BMO/MCS >
Fig. 8 e The photoluminescence spectra of Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites with excitation of 350 nm UV light. Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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3-BMO/MCS >10-BMO/MCS > 7-BMO/MCS > 5-BMO/MCS, which is consistent with the variation trends of the photocatalytic activities of Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites (shown in Fig. 6b). The charge separation efficiency in semiconductor photocatalyst can be also illustrated by photoelectrochemical techniques. The transient photocurrent-time curves (I-t) of Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites electrodes with visible light excitation for several of on and off cycles are shown in Fig. 9. As shown in Fig. 9, the pure Bi2MoO6 show the lowest photocurrent density in current experimental condition, indicating that photo-generated electrons and holes in Bi2MoO6 disappeared with rapid recombination. The photocurrent density of Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites electrodes decrease in the following order: 5-BMO/MCS > 7-BMO/MCS >10-BMO/MCS > 3BMO/MCS > 1-BMO/MCS >Mn0.2Cd0.8S > Bi2MoO6, which is basically consistent with their experimental photocatalytic activities for hydrogen evolution with visible light response (as shown in Fig. 6b). The valence band (VB) potential and conduction band (CB) potential of Bi2MoO6 and Mn0.2Cd0.8S can be estimated by the following equations [49]: EVB ¼ X Ee þ0:5Eg ECB ¼ EVB Eg In above equations, EVB is the valence band (VB) of semiconductor, ECB is the conduction band (CB) of semiconductor, X is the absolute electronegativity of semiconductor. The “X” values for Bi2MoO6 and Mn0.2Cd0.8S are calculated to be 5.54 and 4.88 eV, respectively. Ee is the energy of free electrons on hydrogen (around 4.5 eV) and Eg is the band gap energy of semiconductor. For Bi2MoO6 and Mn0.2Cd0.8S, their Eg are estimated to be 2.64 eV and 2.15 eV, respectively. As a result, according to above equations, the EVB of Bi2MoO6 and Mn0.2Cd0.8S can be calculated to be around 2.36 and 1.45 eV, respectively. Therefore, the ECB of Bi2MoO6 and Mn0.2Cd0.8S is
Fig. 10 e Proposed photocatalytic mechanism diagram of Bi2MoO6/Mn0.2Cd0.8S composite.
estimated to be 0.28 and 0.7 eV, respectively. The results demonstrated that the band gap structure, and energy band positions of Bi2MoO6 and Mn0.2Cd0.8S is beneficial to the separation of photo-generated electron-hole pairs in Bi2MoO6/ Mn0.2Cd0.8S composite. In order to further understand the photocatalytic mechanism of Bi2MoO6/Mn0.2Cd0.8S composites, we proposed a possible mechanism for charges separation and charges transformation in Bi2MoO6/Mn0.2Cd0.8S composite [55]. As shown in Fig. 10, with excitation of visible light, both Bi2MoO6 and Mn0.2Cd0.8S in the composites can be excited and engender photo-generated electron-hole pairs. Since the CB (0.7 eV) and VB (þ1.4 eV) of Mn0.2Cd0.8S lies below the CB (0.28 eV) and VB (þ2.36 eV) of Bi2MoO6, the photo-generated electrons can be transferred from the CB of Mn0.2Cd0.8S to CB of Bi2MoO6, while the photo-generated holes can be transferred from the VB of Bi2MoO6 to VB of Mn0.2Cd0.8S. The photogenerated electrons at the CB of Bi2MoO6 can reduce Hþ and evolve H2, the photo-generated holes at the VB of Mn0.2Cd0.8S can be consumed by the sacrificial reagents (Na2S and Na2SO3). Therefore, the photo-generated electron-hole pair in Bi2MoO6/Mn0.2Cd0.8S composite can be effectively separated, resulting in its signally improvement of photocatalytic activity for H2 evolution from water.
Conclusions
Fig. 9 e The transient photocurrent e Time (I-t) curves of Bi2MoO6, Mn0.2Cd0.8S and Bi2MoO6/Mn0.2Cd0.8S composites.
Due to the well match of the energy band position of Bi2MoO6 and Mn0.2Cd0.8S, and the existence of heterostructure in Bi2MoO6/Mn0.2Cd0.8S composite, the obtained hierarchical Bi2MoO6/Mn0.2Cd0.8S heterostructure nanocomposite shows significantly improved photocatalytic activity for hydrogen evolution from water with visible light response comparing with pure Bi2MoO6 and Mn0.2Cd0.8S solid solution. Considering their high photocatalytic activities for hydrogen evolution with visible light response and good stabilities, the obtained Bi2MoO6/Mn0.2Cd0.8S composites have potential application in photocatalytic water-splitting for hydrogen evolution by using solar power. The current research may provide a new insight
Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197
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into the design and fabrication of novel composites photocatalysts with high effect.
Acknowledgements This project is financially supported by the National Natural Science Foundation of China (NSFC 51762035, 21161015, 51750110511, 51568049, 51932009), the Natural Science Foundation of the Jiangxi Province of China (20164BCD40098, 20152ACB20011, 2009GZH0082), the Natural Science Foundation of the Jiangxi Higher Education Institutions of China (GJJ09180, GJJ14513), Jiangmen Innovative Research Team Program (2017) and Major Program of Basic Research and Applied Research of Guangdong Province (2017KZDXM083).
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Please cite this article as: Liu X et al., A novel hierarchical Bi2MoO6/Mn0.2Cd0.8S Heterostructured Nanocomposite for Efficient Visiblelight hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.197