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Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation
international journal of hydrogen energy xxx (xxxx) xxx
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MoS2/Ti3C2 heterostructure for efficient visiblelight photocatalytic hydrogen generation Juhui Zhang 1, Chao Xing 1, Feng Shi* School of Material Science & Engineering, Shandong University of Science and Technology, Qingdao, 266590, PR China
highlights
graphical abstract
MoS2/Ti3C2 photocatalyst was prepared by hydrothermal method. MoS2 with 30% Ti3C2 loading exhibits the H2 production rate of 6144.7 mmol g1 h1. MoS2/Ti3C2
photocatalyst
can
regulate the electron transport pathway.
article info
abstract
Article history:
The MoS2/Ti3C2 catalyst with a unique sphere/sheet structure were prepared by hydro-
Received 13 October 2019
thermal method. The MoS2/Ti3C2 heterostructure loading 30% Ti3C2 has a maximum
Received in revised form
hydrogen production rate of 6144.7 mmol g1 h1, which are 2.3 times higher than those of
3 December 2019
the pure MoS2. The heterostructure maintains a high catalytic activity within 4 cycles. The
Accepted 16 December 2019
heterostructure not only effectively reduce the recombination of photogenerated electrons
Available online xxx
and holes, but also provide more activation sites, which promotes the photocatalytic hydrogen evolution reaction (HER). These works can provide reference for the development
Keywords: Photocatalytic hydrogen evolution
of efficient catalysts in photocatalytic hydrogen evolution. © 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
MoS2 spheres Ti3C2 MXene Heterostructure Photogenerated electrons Hydrothermal methods
* Corresponding author. E-mail address: [email protected] (F. Shi). 1 These authors contributed equally to this work and should be considered co-first authors. https://doi.org/10.1016/j.ijhydene.2019.12.109 0360-3199/© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Introduction In order to solve the energy crisis and environment pollution, photocatalysts that converts light energy into fuel is becoming a promising strategy [1e3]. Thence, the development of efficient photo catalyst for H2 generation was regarded as an urgent item in this field [4,5]. Designing and preparing semiconductor photocatalysts with high effectiveness is crucial for the commercialization of photocatalytic H2 generation [6e9]. Molybdenum disulfide (MoS2), a graphene like layered semiconductor [10], is widely using as chemically active photocatalysts for hydrodesulfurization and hydrogen evolution [11,12]. Bulk MoS2 has low catalytic activity due to insufficient oxidation potential. However, nanoscale MoS2 will shift the valence band enough to have a certain catalytic activity. This characteristic can be ascribed to the quantum confinement of nanomaterials [13,14]. Since the catalytic application of MoS2 for the hydrogen evolution reaction (HER) was first reported by Nørskov et al. [15], researchers had discovered various MoS2 nanostructure with HER characteristic. Both experimental and computational results showed that nanoscale MoS2 has a large specific surface area and a rich catalytic active site, making it become a potential material for photocatalysis [16e18]. Transition metal carbides (MXene) have been proved to have excellent performance in the field of catalytic hydrogen evolution [19e21]. Ti3C2 MXene can be used as a co-catalyst to regulate the transfer direction of photogenerated electrons because of the high electrical conductivity [22e24], thereby improving the separation efficiency of photo-generated electrons and holes [25,26]. Wang et al. showed that the hydrogen evolution rate of TiO2/Ti3C2 was 2.3 times higher than that of pure rutile TiO2 [27]. Ran et al. demonstrated that the combination of Ti3C2 MXene and metal sulfide can enhance visible light photocatalytic hydrogen production [28]. As the cocatalyst, Ti3C2 MXene exhibits metallic characteristics with substantial electronic states crossing the Fermi level, implying its exceptional capability to transport electrons. However, there is a lack of discussion in present literature that highlights Ti3C2 as the main co-catalyst. Here we show a reasonable structural design between Ti3C2 and MoS2 by changing the load of Ti3C2. Such heterostructure can improve the interfacial conductivity and regulate the electron transportation pathway along the basal planes. Therefore, the photogenerated electrons are more effectively separated from MoS2, enhancing the photocatalytic activity dramatically [29,30]. In this work, the novel MoS2/Ti3C2 heterostructure photocatalyst were obtained. This heterostructure can reform the structure of the material and regulate the electron transport pathway to achieve efficient separation of photogenerated electrons and holes. Through a rational design and morphology tuning of Ti3C2, Ti3C2 interacts closely with MoS2 to achieve a high photocatalytic H2 production activity under visible light irradiation.
Experimental section All reagents were commercially available from China National Pharmaceutical Group Corporation Ltd. All reagents were
analytical grade and used as received without further purification. Preparation of Ti3C2 MXene sheets: Ti3AlC2 power (1 g) was mixed with 50% HF solution (40 ml), then the reaction mixture was stirred 24 h at room temperature for selectively etching the Al layers from the Ti3AlC2. After that, the mixture was centrifuged at 3500 rpm and rinsed by deionized water until the PH of supernatant reached 7. Finally, the Ti3C2 MXene were dried at 60 C for 12 h under vacuum. Synthesis of MoS2/Ti3C2 heterostructures: Preparation of MoS2/Ti3C2 heterostructures is conventional hydrothermal methods. First 1 mmol Na2MoO4$2H2O, 5 mmol CH4N2S, and a certain amount of Ti3C2 (the load of Ti3C2 accounts for 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt% of the total mass, the obtained samples were marked as T0, T10, T20, T30, T40 and T50, respectively) were added in 60 ml of distilled water. Then, the suspension was placed in a 100 ml Teflon-lined autoclave and heated at 200 C for 24 h after ultrasonic treatment 20 min. After reaction, the production was centrifuged and washed with distilled water several times, the left black precipitate was dried in a vacuum oven at 80 C for 12 h. Characterization: Rigaku D/max-2000 X-ray diffractometer with Cu-Ka incident source was used to characterize the synthesized phases. The morphology of the MoS2/Ti3C2 were collected though SEM (model as Nova Nano SEM45) and TEM (FEI Tecnai G20). XPS was obtained on a VG scientific ESCALAB 250 spectrometer equipped with 300 W Mg Ka radiation to characterize the chemical states of the MoS2/Ti3C2. Optical performance was obtained with UV-3101PC. Quadrasorb EVO was used for N2 adsorption test to obtain specific surface area and pore size. Fluorescence spectrophotometer (F-4700, Hitachi, Japan) was used for the PL testing. The electrochemical impedance performance and the transient photocurrent response was tested by electrochemical Workstation. Photoactivity Measurements: Photocatalytic hydrogen evolution experiments were carried out using the CEL-SPH2ND9 photocatalytic reaction system. A Xe lamp (300 W) was used as the light source which is equipped with an optical filter (l > 420 nm). 30 mg of the sample was dispersed in 50 ml of methanol solution (methanol: 30%, deionized water: 70%) after sonication 30 min, and then the reaction flask containing the dispersion was placed in a multi-channel photocatalytic reactor. Through 5 h of LED light irradiation, the generated gas was monitored online using a gas chromatograph. The detector was a TCD thermal conductivity detector. The carrier gas was made of high purity nitrogen or argon.
Results and discussion The synthesis process of MoS2/Ti3C2 heterostructures is explained in Fig. 1. Through the hydrofluoric acid (HF) selectively etching, the aluminium (Al) atoms in Ti3C2 MXene phases were removed to form Ti3C2. Due to the metal electron donors of the Ti3C2, molybdate and thiourea can uniformly absorbed on the layer surface of Ti3C2 under the ultrasonication [31]. Under the subsequent hydrothermal treatment, a part of MoS2 spheres were growing on the Ti3C2 layers
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 1 e Schematic illustration of synthesis process of MoS2/Ti3C2 heterostructure.
and forming heterostructure, and the Ti3C2 layers provided fast charge transport channel for high photocatalytic activity (see Fig. 2). The crystalline structure of MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts were investigated by Xray power diffraction (XRD). With increase of Ti3C2 loading, some Ti3C2 peaks intensity gradually increased in the Fig.2, such as the (111), (200), (220), (311) and (222). In addition, the peaks at 25.03 marked with heart correspond to the peaks of TiO2, which revealed that Ti3C2 was gradually oxidized in the hydrothermal process. The diffraction peaks at 14.39 and 58.76 corresponded to the (002) and (110) planes of hexagonal 2HeMoS2 (JCPDS card file No. 75-1539). Displayed from the pattern, the (002) and (110) peaks of 2HeMoS2 shift to a high angle, indicating that the lattice parameter of MoS2 changed small due to the doping of O2 ions with a smaller radius during the reaction, which confirmed by the Mo 3d spectrum of XPS. Scanning electron microscopy (SEM) images presented the morphology of the samples after Ti3C2 intercalation. MoS2
(Fig. 3 (a)) possessed large surface area, which can provide plenty of active sites for photocatalytic hydrogen generation, and the size of the initial MoS2 spheres was approximately 1 mm. The obtained Ti3C2 (Fig. 3(g)) exhibited a clear multilayer structure, similar to exfoliated layered graphite. As the hydrothermal time prolonged, the MoS2 spheres and the Ti3C2 layer were combined under thermodynamic driving. With the Ti3C2 loading increases, a part of MoS2 spheres began to combined on the surface of the Ti3C2 layer until it completely covered the surface of the sheet (from Fig.3 (b) to Fig.3 (f)). Elemental mapping images further show the presence and homogeneous distribution of Mo, S, Ti and C in entire MoS2/ Ti3C2 heterostructure (Fig. 3(h)). The morphology and microstructure of MoS2/Ti3C2 heterostructure was futher analyzed by high resolution transmission electron microscope (HRTEM). The general images of MoS2/Ti3C2 (T30) heterostructure can be observed in Fig. 4 (a)e(b). As shown in Fig. 4 (c), Ti3C2 sheets and MoS2 spheres were cross-connected to each other and corrugations were clearly visible. Fig. 4 (d) shows the lattice fringes
Fig. 2 e XRD patterns of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 3 e SEM images of MoS2/Ti3C2 heterostructure synthesized at different Ti3C2 content: (a) T0, (b) T10, (c) T20, (d) T30, (e) T40, (f) T50, and (g) Ti3C2 MXene. (h) Elemental mapping images of MoS2/Ti3C2 heterostructure (T30).
Fig. 4 e HRTEM images of MoS2/Ti3C2 (T30) heterostructure. Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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corresponding to different phases, fringes with lattice spacing of 0.62 nm correspond to the (002) plane of hexagonal MoS2. The lattice spacing of 0.98 nm are agreed to the (002) plane of Ti3C2, which extracted from the fast Fourier transform (FFT) patterns of selected area (marked by blue frame) shown in the inset [32]. The HRTEM images indicated that the MoS2 and Ti3C2 sheets were in intimate contact, ensuring favorable electrical transfer between the two materials and will improve the photocatalytic performance of the MoS2/ Ti3C2 heterostructure. In order to analyze the chemical state and elemental composition of MoS2/Ti3C2, X-ray photoelectron spectroscopy (XPS) measurements of T30 heterostructure were carried out.
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Fig. 5 (a) depicted the XPS full spectrum of T30 contained Mo, S, Ti, C and O elements, where the O element was derived from TiO2 crystals and surface-terminated functional groups of Ti3C2. In Fig. 5 (b), Mo 3d spectrum contains diffraction two peaks, the peak at 228.73 eV corresponds to the Mo 3d5/2 binding energy in MoS2, and the other peak at 231.91 eV corresponds to the Mo 3d3/2 binding energy in MoO3, which indicates the oxygen is incorporated in MoS2 during the reaction [33,34]. The peak of Mo6þ-O appears at a binding energy of 236.1 eV due to partial oxidation of MoS2 in the T30 heterostructure [35]. From the S 2p spectrum of Fig. 5 (c), the binding energies of the S 2p3/2 and S 2p1/2 peaks are detected at 161.54 eV and 162.72 eV, respectively [36]. Four peaks were
Fig. 5 e XPS spectra of T30 heterostructure (a) Full spectrum of T30 heterostructure, (b) Mo 3d spectrum, (c) S 2p spectrum, (d) Ti 2p spectrum, (e) C 2s spectrum, (f) O 1s spectrum.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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observed in Ti 2p spectroscopy (Fig. 5 (d)), the peak at 455.21 eV and 461.98 eV are related to the Ti 2p3/2 and Ti 2p1/2 binding energy in Ti3C2. The two prominent peaks appeared at the binding energy of 458.73 eV and 464.52 eV, further proving that the formation of TieO bonds is due to oxygen terminated surfaces etched by HF [37]. Fig. 5 (e) exhibits two strong peaks, the main peak at 283.78 eV corresponds to the CeTi in Ti3C2, and the peak at 284.84 eV is attributed to sp [2] hybridization. Two peaks in the O 1s spectrum (Fig. 5 (f)) with binding energy of 530.17 eV and 531.7 eV are related to TieO bonds and hydroxyl groups adsorbed on the surface of the sample (-OH) [38,39]. Light absorption plays a vital role as a precondition for H2 production. Fig. 6 shows the UVevis diffuse reflectance absorption spectra of Ti3C2 and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. The Ti3C2 sheets exhibit poor absorbance intensity. However, the semiconducting property of MoS2/Ti3C2 contributes to the absorption enhancement and has higher and wider absorption regions. Therefore, the incident sunlight will be utilized at the maximum for the following photon excitation. The surface areas and pore size distributions of MoS2/Ti3C2 heterostructure were obtained by N2 adsorption-desorption measurement. Fig. 7 (a) shows the N2 adsorption-desorption isotherms of MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. The isotherms in the higher nips increased rapidly, and the hysteresis loops occurred during desorption. When the relative pressure was close to 1, the curve was closed. The MoS2/Ti3C2 heterostructure exhibits reversible type IV isotherm, which is one of the main characteristics of mesoporous materials. Furthermore, it was found that the N2 adsorption amount of T30 was the largest, indicating that the specific surface area of the sample was the largest. The change in specific surface area of MoS2/Ti3C2 heterostructure
in Fig. 7 (b) confirmed this. When the loading of Ti3C2 reached 40%, the specific surface area of MoS2/Ti3C2 heterostructure began to decrease, indicating that the MoS2 spheres combined on the surface of the Ti3C2 layers, which was consistent with the SEM image. As shown in Fig. 7 (b), the pore size of the MoS2/Ti3C2 heterostructure is close to 19 nm. This kind of pore size acts as a channel for electron transport, which facilitates the full contact of the catalyst with the reactants in the photocatalytic process and improves the catalytic efficiency. Fig. 8 (a) depicts photocatalytic H2 production of different Ti3C2 contented MoS2/Ti3C2 heterostructure under visible light irradiation for 5 h. As the irradiation time is extended, the amount of H2 produced increases linearly. As shown in Fig. 8 (b), photocatalytic hydrogen evolution rate of pure MoS2 is 2626 mmol g1 h1, indicating a poor photocatalytic activity, and Ti3C2 also demonstrates little photocatalytic activity in hydrogen evolution. However, the combination of MoS2 and Ti3C2 greatly enhances catalytic activity, T30 can improve the photocatalytic hydrogen evolution rate to 6144.7 mmol g1 h1, which are 2.3 times higher than those of the pure MoS2. This enhancement can be attributed to the presence of Ti3C2 to promote efficient transfer of photogenerated electrons. With the loading of Ti3C2 increases, the morphology of MoS2 is from spheres to fully attached to the Ti3C2 layer, which reduces the specific surface area and weakens catalytic hydrogen evolution performance. Among the samples, T30 shows the best HER activity, indicating that the direction of charge transfer can be effectively controlled by a reasonable specific structural design. In addition, a cycle stability experiment was carried out for T30, the sample still maintained a high H2 production after four cycles (Fig. 8 (c)), which suggested the cyclic stability of T30.
Fig. 6 e UVevis absorption spectra of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 7 e (a) N2 adsorption-desorption isotherms of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. (b) The surface area and pore size distribution curves of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts.
According to PL attributes, the PL spectrum can directly reflect the separation situation of photogenerated electrons carriers. For PL signals, the lower the PL intensity, the higher the separation rate of photogenerated electrons and, possibly, the higher the photocatalytic activity [40]. Therefore, the photocatalytic activity of samples can be quickly evaluated by means of PL measurements under a certain condition. As shown in Fig. 9 (a), the PL intensity of MoS2/ Ti3C2 heterostructure (T30) was lower than pure MoS2,
because the internal electric field formed by T30 provided an electron transport channel, the photogenerated electrons transferred from MoS2 to Ti3C2 easily, which effectively inhibited the recombination of photogenerated electronsholes. From the fluorescence lifetime decay curve (Fig. 9 (b)), the carrier lifetimes of pure MoS2 and T30 under light were 2.69 ns and 3.44 ns, indicating that the recombination of photogenerated electrons and holes of T30 was effectively suppressed. Therefore, T30 can effectively delay the
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 8 e (a) Photocatalytic H2 production of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. (b) Average photocatalytic H2 evolution rate of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. (c) Cyclicity test of hydrogen production performance for T30 heterostructure.
recombination of charges and prolong the life of electrons, thereby having excellent photocatalytic activity. This conclusion is further confirmed by transient photocurrent response testing. In Fig. 10, the photocurrent intensity of the T30 sample rose immediately after the light was turned on, and immediately decreased after the lamp was turned off, which indicated that most of the photogenerated electrons migrate to the surface of the sample at the moment of turning on the light, and then transferred to the working electrode to generate photocurrent. The photocurrent density of T30 is significantly higher than pure MoS2. On the one hand, Ti3C2 as a co-catalyst to promote electron transport, on the other hand, the addition of Ti3C2 inhibits the recombination of photogenerated electrons and holes. These results indicate the MoS2/Ti3C2 heterostructure is beneficial to photocatalytic activity. Moreover, Charge transfer and recombination process of photogenerated electrons and holes were investigated by electrochemical impedance spectroscopy (EIS). As shown in Fig. 11 (a), the impedance spectrum of the samples is not a
perfect semicircle, which means that there is a charge transfer resistance (Rct) between the electrolyte and the catalyst. The smaller the arc radius of the electrochemical impedance, the smaller the Rct, indicating that the efficiency separation of carrier is higher [41]. The arc radius of the electrochemical impedance under illumination was significantly smaller than that in the dark state, which demonstrated that illumination promotes photogenerated electron transport. In Fig. 11 (b), the T30 exhibited much smaller semicircular arcs than pure MoS2, it is proved that the combination of Ti3C2 improved the separation efficiency of electron-hole pairs and enhanced the activity of the catalyst. The photocatalytic reaction involves three main processes: light absorption, charge separation and transfer, and surface redox reactions. The quantum size effects that nanoscale MoS2 will shift the valence band enough to have a certain catalytic activity. As a co-catalyst compounded to MoS2, Ti3C2 has low catalytic activity and has the following effects on photocatalytic H2 generation performances: (1) 2D-layered Ti3C2 materials can
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 9 e (a) Photoluminescence (PL) spectra of MoS2 and MoS2/Ti3C2 heterostructure (T30). (b) Time-resolved fluorescence decay spectra of MoS2 and MoS2/Ti3C2 heterostructure (T30).
Fig. 10 e Photocurrent densityetime curves of MoS2 and MoS2/Ti3C2 heterostructure (T30).
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Fig. 11 e (a) Eletrochemical impedance spectroscopy (EIS) of MoS2 and MoS2/Ti3C2 heterostructure (T30), (b) spectra in light conditions.
quickly transfer the electrons and reduce the recombination due to high electrical conductivity. (2) The layered structure can provide the passageway for the diffusion of produced H2 [42]. Fig. 12 depicts the photocatalytic hydrogen generation mechanism of MoS2/Ti3C2. When illuminated, electrons of MoS2 were transferred from the valence band (VB) to the conduction band (CB), and left holes in the valence band. Methanol as a hole sacrificial agent in the reaction consumed photogenerated holes, which increased the quantum yield. Part of the photogenerated electrons at the MoS2 conduction band were transferred to Ti3C2, which resulted in efficient separation of photogenerated electron-hole pairs and improved photocatalytic hydrogen evolution efficiency. MoS2 and TiO2 produced by the oxidation process have a certain catalytic activity, which also promote the reduction of Hþ into H2 by photogenerated electrons. This semiconductor catalyst effectively promotes the rapid separation of photo-generated electrons and holes, which improves the photocatalytic performance [43,44].
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 12 e Schematic photocatalytic mechanism for MoS2/Ti3C2 heterostructure.
Conclusion The novel MoS2/Ti3C2 composite photocatalyst was prepared by hydrothermal method. According to a series of structural characterizations, the MoS2 spheres and the Ti3C2 sheets were effectively combined to achieve charge separation and transfer direction adjustment. The performance experiments showed that the addition of Ti3C2 can effectively improve the photocatalytic activity, and T30 exhibited the highest H2 production rate (6144.7 mmol g1 h1) and still maintains a high H2 production after four cycles. Simulation analysis of the reaction process shows that the electron channel formed between MoS2 and Ti3C2 accelerates charge transport and inhibits charge recombination, which is beneficial to the photocatalytic hydrogen evolution reaction. This study provides a new way to establish an effective composite photocatalytic system.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant 11874240), the Opening Project of Key Laboratory of Inorganic Functional Materials and Devices, Chinese Academy of Sciences (Grant No. KLIFMD201803), the Natural Science Foundation of Shandong Province, China (No. ZR2016EMM21), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (No. 2016RCJJ002).
references
[1] Takashi H, Jun K, Kazunari D. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. ChemInform 2014;43(22):7520e35. [2] Chhowalla M, Shin HS, Eda G, Li LJ, Loh KP, Hua Z. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat Chem 2013;5(4):263e75.
[3] Juan L, Yang L, Naiyun L, Yuzhi H, Xing Z, Hui H, Yeshayahu L, Shuit-Tong L, Jun Z, Zhenhui K. Water splitting. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015;46(23):970e4. [4] Mckone JR, Marinescu SC, Brunschwig BS, Winkler JR, Gray HB. Earth-abundant hydrogen evolution electrocatalysts. Chem Sci 2014;5(3):865e78. [5] Ying Y, Han J, Zhang X, Zhang Y, Zhou J, Muir D, Sutarto R, Zhang Z, Liu S, Bo S. Facile synthesis of few-layer-thick carbon nitride nanosheets by liquid ammonia-assisted lithiation method and their photocatalytic redox properties. RSC Adv 2014;4(62):32690e7. [6] Xu Z, Hongjian Y, Guopeng W, Guijun M, Fuyu W, Lu W, Can L. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as Cocatalyst under visible light irradiation. J Am Chem Soc 2008;130(23):7176e7. [7] Tu W, Yong Z, Qi L, Yan S, Bao S, Wang X, Min X, Zou Z. An in situ simultaneous reduction-hydrolysis technique for fabrication of TiO2-graphene 2D sandwich-like hybrid nanosheets: graphene-promoted selectivity of photocatalytic-driven hydrogenation and coupling of CO2 into methane and ethane. Adv Funct Mater 2013;23(14):1743e9. [8] Wang Yawen, Zhang Lizhi, Deng Kejian, Chen Xinyi, Zou Zhigang. Low temperature synthesis and photocatalytic activity of rutile TiO2 nanorod superstructures. J Phys Chem C 2007;111(6):2709e14. [9] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238(5358):37e8. [10] Jia MS, Loh KP. Electrochemical double-layer capacitance of MoS2 nanowall films. Electrochem Solid State Lett 2007;10(11):A250. [11] Claudia A, Maria S, Paolo C, Adolfo S, Vincenzo P. New ’chimie douce approach to the synthesis of hybrid nanosheets of MoS2 on CNT and their anti-friction and antiwear properties. Nanotechnology 2013;24(12):125601. [12] Dasgupta NP, Yang P. Semiconductor nanowires for photovoltaic and photoelectrochemical energy conversion. Front Phys 2014;9(3):289e302. [13] Nguyen TP, Van Le Q, Choi KS, Oh JH, Kim YG, Lee SM, Chang ST, Cho YH, Choi S, Kim TY. MoS2 nanosheets exfoliated by sonication and their application in organic photovoltaic cells. Sci Adv Mater 2015;7(4):700e5 (6). [14] Hinnemann B, Moses PG, Bonde J, Jørgensen KP, Nielsen JH, Horch S, Chorkendorff I, Nørskov JK. Biomimetic hydrogen
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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[15]
[16]
[17]
[18]
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. J Am Chem Soc 2005;127(15):5308e9. Karunadasa HI, Chang CJ. A molecular MoS2 edge site mimic for catalytic hydrogen generation. Science 2012;335(6069):698e702. Chen WF, Wang CH, Sasaki K, Marinkovic N, Xu W, Muckerman JT, Zhu Y, Adzic RR. Highly active and durable nanostructured molybdenum carbide electrocatalysts for hydrogen production. Energy Environ Sci 2013;6(3):943e51. Ghassemi H, Harlow W, Mashtalir O, Beidaghi M, Lukatskaya MR, Gogotsi Y, Taheri ML. In situ environmental transmission electron microscopy study of oxidation of twodimensional Ti3C2 and formation of carbon-supported TiO2. J Mater Chem 2014;2(35):14339e43. Splendiani A, Sun L, Zhang Y, Li T, Kim J, Chim CY, Galli G, Wang F. Emerging photoluminescence in monolayer MoS2. Nano Lett 2010;10(4):1271e5. Lukatskaya MR, Mashtalir O, Ren CE, Dall’Agnese Y, Rozier P, Taberna PL, Naguib M, Simon P, Barsoum MW, Gogotsi Y. Cation intercalation and high volumetric capacitance of twodimensional titanium carbide. Science 2013;341(6153):1502e5. Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Catal Surv Asia 2003;7(1):31e8. Qiuming P, Jianxin G, Qingrui Z, Jianyong X, Baozhong L, Aiguo Z, Riping L, Yongjun T. Unique lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide. J Am Chem Soc 2014;136(11):4113e6. Yidong H, Abrams BL, Vesborg PCK, BjRketun MRE, Konrad H, Lone B, Setti AM, Damsgaard CD, Thomas P, Ole H. Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nat Mater 2011;10(6):434e8. Khazaei M, Arai M, Sasaki T, Chung CY, Kawazoe Y. Novel electronic and magnetic properties of two-dimensional transition metal carbides and nitrides. Adv Funct Mater 2013;23(17):2185e92. Minoli G. Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide. Science 2013;341(6153):1502e5. Peng C, Yang X, Li Y, Yu H, Wang H, Peng F. Hybrids of twodimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity. ACS Appl Mater Interfaces 2016;8(9). acsami.5611973. Wang H, Peng R, Hood ZD, Naguib M, Adhikari SP, Wu Z. Titania composites with 2 D transition metal carbides as photocatalysts for hydrogen production under visible-light irradiation. Chemsuschem 2016;9(12):1490e7. Ran J, Gao G, Li FT, Ma TY, Du A, Qiao SZ. Ti3C2 MXene cocatalyst on metal sulfide photo-absorbers for enhanced visible-light photocatalytic hydrogen production. Nat Commun 2017;8:13907. Xiaobo C, Shaohua S, Liejin G, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110(11):6503e70. Osterloh FE. Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting. Chem Soc Rev 2013;42(6):2294e320. Luo J, Tao X, Zhang J, Xia Y, Huang H, Zhang L, Gan Y, Liang C, Zhang W. Sn4þ ions decorated highly conductive
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
11
Ti3C2 MXene: promising lithium-ion anodes with enhanced volumetric capacity and cyclic performance. ACS Nano 2016;10(2):2491. Gao G, O’Mullane AP, Du A. 2D MXenes: a new family of promising catalysts for the hydrogen evolution reaction. ACS Catal 2016;7(1):494e500. 7(1). Ma TY, Cao JL, Jaroniec M, Qiao SZ. Interacting carbon nitride and titanium carbide nanosheets for high-performance oxygen evolution. Angew Chem 2016;128(3):1150e4. Liu J, Tang S, Lu Y, Cai G, Liang S, Wang W, Chen X. Synthesis of Mo2N nanolayer coated MoO2 hollow nanostructures as high-performance anode materials for lithium-ion batteries. Energy Environ Sci 2013;6(9):2691e7. Sanfeng W, Chunming H, Grant A, Ross JS, Cobden DH, Xiaodong X. Vapor-solid growth of high optical quality MoS2 monolayers with near-unity valley polarization. ACS Nano 2013;7(3):2768e72. Li X, Zhang L, Zang X, Li X, Zhu H. Photo-promoted platinum nanoparticles decorated MoS2@Graphene woven fabric catalyst for efficient hydrogen generation. ACS Appl Mater Interfaces 2016;8(17):10866e73. Ren W, Zhou W, Zhang H, Cheng C. ALD TiO2-coated flowerlike MoS2 nanosheets on carbon cloth as sodium ion battery anode with enhanced cycling stability and rate capability. ACS Appl Mater Interfaces 2016;9(1):487e95. Xuefeng W, Xi S, Yurui G, Zhaoxiang W, Richeng Y, Liquan C. Atomic-scale recognition of surface structure and intercalation mechanism of Ti3C2X. J Am Chem Soc 2015;137(7):2715e27. Yoon Y, Lee M, Kim SK, Bae G, An KS. A strategy for synthesis of carbon nitride induced chemically doped 2D MXene for high-performance supercapacitor electrodes. Adv Energy Mater 2018;8(15):1703173. Rakhi RB, Ahmed B, Hedhili MN, Anjum DH, Alshareef HN. Effect of postetch annealing gas composition on the structural and electrochemical properties of Ti2CTx MXene electrodes for supercapacitor applications. Chem Mater 2015;27(15):5314e23. Liu Q, Ai L, Jing J. MXene-derived TiO2@C/g-C3N4 heterojunctions for highly efficient nitrogen photofixation. J Mater Chem 2018;6(9):4102e10. Zhang J, Yu J, Jaroniec M, Gong JR. Noble metal-free reduced graphene oxide-ZnxCd₁-xS nanocomposite with enhanced solar photocatalytic H₂-production performance. Nano Lett 2012;12(9):4584e9. Li L, Yu L, Lin Z, Yang GW. Reduced TiO2-graphene oxide heterostructure as broad spectrum-driven efficient water splitting photocatalysts. ACS Appl Mater Interfaces 2016;8(13):8536e45. Yuan W, Cheng L, Zhang Y, Wu H, Lv S, Chai L, Guo X, Zheng L. 2D-Layered carbon/TiO2 hybrids derived from Ti3C2MXenes for photocatalytic hydrogen evolution under visible light irradiation. Adv Mater Interfaces 2017;4(20):1700577. Nakata K, Ochiai T, Murakami T, Fujishima A. Photoenergy conversion with TiO2 photocatalysis: new materials and recent applications. Electrochim Acta 2012;84(12):103e11.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
MoS2/Ti3C2 heterostructure for efficient visiblelight photocatalytic hydrogen generation Juhui Zhang 1, Chao Xing 1, Feng Shi* School of Material Science & Engineering, Shandong University of Science and Technology, Qingdao, 266590, PR China
highlights
graphical abstract
MoS2/Ti3C2 photocatalyst was prepared by hydrothermal method. MoS2 with 30% Ti3C2 loading exhibits the H2 production rate of 6144.7 mmol g1 h1. MoS2/Ti3C2
photocatalyst
can
regulate the electron transport pathway.
article info
abstract
Article history:
The MoS2/Ti3C2 catalyst with a unique sphere/sheet structure were prepared by hydro-
Received 13 October 2019
thermal method. The MoS2/Ti3C2 heterostructure loading 30% Ti3C2 has a maximum
Received in revised form
hydrogen production rate of 6144.7 mmol g1 h1, which are 2.3 times higher than those of
3 December 2019
the pure MoS2. The heterostructure maintains a high catalytic activity within 4 cycles. The
Accepted 16 December 2019
heterostructure not only effectively reduce the recombination of photogenerated electrons
Available online xxx
and holes, but also provide more activation sites, which promotes the photocatalytic hydrogen evolution reaction (HER). These works can provide reference for the development
Keywords: Photocatalytic hydrogen evolution
of efficient catalysts in photocatalytic hydrogen evolution. © 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
MoS2 spheres Ti3C2 MXene Heterostructure Photogenerated electrons Hydrothermal methods
* Corresponding author. E-mail address: [email protected] (F. Shi). 1 These authors contributed equally to this work and should be considered co-first authors. https://doi.org/10.1016/j.ijhydene.2019.12.109 0360-3199/© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Introduction In order to solve the energy crisis and environment pollution, photocatalysts that converts light energy into fuel is becoming a promising strategy [1e3]. Thence, the development of efficient photo catalyst for H2 generation was regarded as an urgent item in this field [4,5]. Designing and preparing semiconductor photocatalysts with high effectiveness is crucial for the commercialization of photocatalytic H2 generation [6e9]. Molybdenum disulfide (MoS2), a graphene like layered semiconductor [10], is widely using as chemically active photocatalysts for hydrodesulfurization and hydrogen evolution [11,12]. Bulk MoS2 has low catalytic activity due to insufficient oxidation potential. However, nanoscale MoS2 will shift the valence band enough to have a certain catalytic activity. This characteristic can be ascribed to the quantum confinement of nanomaterials [13,14]. Since the catalytic application of MoS2 for the hydrogen evolution reaction (HER) was first reported by Nørskov et al. [15], researchers had discovered various MoS2 nanostructure with HER characteristic. Both experimental and computational results showed that nanoscale MoS2 has a large specific surface area and a rich catalytic active site, making it become a potential material for photocatalysis [16e18]. Transition metal carbides (MXene) have been proved to have excellent performance in the field of catalytic hydrogen evolution [19e21]. Ti3C2 MXene can be used as a co-catalyst to regulate the transfer direction of photogenerated electrons because of the high electrical conductivity [22e24], thereby improving the separation efficiency of photo-generated electrons and holes [25,26]. Wang et al. showed that the hydrogen evolution rate of TiO2/Ti3C2 was 2.3 times higher than that of pure rutile TiO2 [27]. Ran et al. demonstrated that the combination of Ti3C2 MXene and metal sulfide can enhance visible light photocatalytic hydrogen production [28]. As the cocatalyst, Ti3C2 MXene exhibits metallic characteristics with substantial electronic states crossing the Fermi level, implying its exceptional capability to transport electrons. However, there is a lack of discussion in present literature that highlights Ti3C2 as the main co-catalyst. Here we show a reasonable structural design between Ti3C2 and MoS2 by changing the load of Ti3C2. Such heterostructure can improve the interfacial conductivity and regulate the electron transportation pathway along the basal planes. Therefore, the photogenerated electrons are more effectively separated from MoS2, enhancing the photocatalytic activity dramatically [29,30]. In this work, the novel MoS2/Ti3C2 heterostructure photocatalyst were obtained. This heterostructure can reform the structure of the material and regulate the electron transport pathway to achieve efficient separation of photogenerated electrons and holes. Through a rational design and morphology tuning of Ti3C2, Ti3C2 interacts closely with MoS2 to achieve a high photocatalytic H2 production activity under visible light irradiation.
Experimental section All reagents were commercially available from China National Pharmaceutical Group Corporation Ltd. All reagents were
analytical grade and used as received without further purification. Preparation of Ti3C2 MXene sheets: Ti3AlC2 power (1 g) was mixed with 50% HF solution (40 ml), then the reaction mixture was stirred 24 h at room temperature for selectively etching the Al layers from the Ti3AlC2. After that, the mixture was centrifuged at 3500 rpm and rinsed by deionized water until the PH of supernatant reached 7. Finally, the Ti3C2 MXene were dried at 60 C for 12 h under vacuum. Synthesis of MoS2/Ti3C2 heterostructures: Preparation of MoS2/Ti3C2 heterostructures is conventional hydrothermal methods. First 1 mmol Na2MoO4$2H2O, 5 mmol CH4N2S, and a certain amount of Ti3C2 (the load of Ti3C2 accounts for 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt% of the total mass, the obtained samples were marked as T0, T10, T20, T30, T40 and T50, respectively) were added in 60 ml of distilled water. Then, the suspension was placed in a 100 ml Teflon-lined autoclave and heated at 200 C for 24 h after ultrasonic treatment 20 min. After reaction, the production was centrifuged and washed with distilled water several times, the left black precipitate was dried in a vacuum oven at 80 C for 12 h. Characterization: Rigaku D/max-2000 X-ray diffractometer with Cu-Ka incident source was used to characterize the synthesized phases. The morphology of the MoS2/Ti3C2 were collected though SEM (model as Nova Nano SEM45) and TEM (FEI Tecnai G20). XPS was obtained on a VG scientific ESCALAB 250 spectrometer equipped with 300 W Mg Ka radiation to characterize the chemical states of the MoS2/Ti3C2. Optical performance was obtained with UV-3101PC. Quadrasorb EVO was used for N2 adsorption test to obtain specific surface area and pore size. Fluorescence spectrophotometer (F-4700, Hitachi, Japan) was used for the PL testing. The electrochemical impedance performance and the transient photocurrent response was tested by electrochemical Workstation. Photoactivity Measurements: Photocatalytic hydrogen evolution experiments were carried out using the CEL-SPH2ND9 photocatalytic reaction system. A Xe lamp (300 W) was used as the light source which is equipped with an optical filter (l > 420 nm). 30 mg of the sample was dispersed in 50 ml of methanol solution (methanol: 30%, deionized water: 70%) after sonication 30 min, and then the reaction flask containing the dispersion was placed in a multi-channel photocatalytic reactor. Through 5 h of LED light irradiation, the generated gas was monitored online using a gas chromatograph. The detector was a TCD thermal conductivity detector. The carrier gas was made of high purity nitrogen or argon.
Results and discussion The synthesis process of MoS2/Ti3C2 heterostructures is explained in Fig. 1. Through the hydrofluoric acid (HF) selectively etching, the aluminium (Al) atoms in Ti3C2 MXene phases were removed to form Ti3C2. Due to the metal electron donors of the Ti3C2, molybdate and thiourea can uniformly absorbed on the layer surface of Ti3C2 under the ultrasonication [31]. Under the subsequent hydrothermal treatment, a part of MoS2 spheres were growing on the Ti3C2 layers
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 1 e Schematic illustration of synthesis process of MoS2/Ti3C2 heterostructure.
and forming heterostructure, and the Ti3C2 layers provided fast charge transport channel for high photocatalytic activity (see Fig. 2). The crystalline structure of MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts were investigated by Xray power diffraction (XRD). With increase of Ti3C2 loading, some Ti3C2 peaks intensity gradually increased in the Fig.2, such as the (111), (200), (220), (311) and (222). In addition, the peaks at 25.03 marked with heart correspond to the peaks of TiO2, which revealed that Ti3C2 was gradually oxidized in the hydrothermal process. The diffraction peaks at 14.39 and 58.76 corresponded to the (002) and (110) planes of hexagonal 2HeMoS2 (JCPDS card file No. 75-1539). Displayed from the pattern, the (002) and (110) peaks of 2HeMoS2 shift to a high angle, indicating that the lattice parameter of MoS2 changed small due to the doping of O2 ions with a smaller radius during the reaction, which confirmed by the Mo 3d spectrum of XPS. Scanning electron microscopy (SEM) images presented the morphology of the samples after Ti3C2 intercalation. MoS2
(Fig. 3 (a)) possessed large surface area, which can provide plenty of active sites for photocatalytic hydrogen generation, and the size of the initial MoS2 spheres was approximately 1 mm. The obtained Ti3C2 (Fig. 3(g)) exhibited a clear multilayer structure, similar to exfoliated layered graphite. As the hydrothermal time prolonged, the MoS2 spheres and the Ti3C2 layer were combined under thermodynamic driving. With the Ti3C2 loading increases, a part of MoS2 spheres began to combined on the surface of the Ti3C2 layer until it completely covered the surface of the sheet (from Fig.3 (b) to Fig.3 (f)). Elemental mapping images further show the presence and homogeneous distribution of Mo, S, Ti and C in entire MoS2/ Ti3C2 heterostructure (Fig. 3(h)). The morphology and microstructure of MoS2/Ti3C2 heterostructure was futher analyzed by high resolution transmission electron microscope (HRTEM). The general images of MoS2/Ti3C2 (T30) heterostructure can be observed in Fig. 4 (a)e(b). As shown in Fig. 4 (c), Ti3C2 sheets and MoS2 spheres were cross-connected to each other and corrugations were clearly visible. Fig. 4 (d) shows the lattice fringes
Fig. 2 e XRD patterns of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 3 e SEM images of MoS2/Ti3C2 heterostructure synthesized at different Ti3C2 content: (a) T0, (b) T10, (c) T20, (d) T30, (e) T40, (f) T50, and (g) Ti3C2 MXene. (h) Elemental mapping images of MoS2/Ti3C2 heterostructure (T30).
Fig. 4 e HRTEM images of MoS2/Ti3C2 (T30) heterostructure. Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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corresponding to different phases, fringes with lattice spacing of 0.62 nm correspond to the (002) plane of hexagonal MoS2. The lattice spacing of 0.98 nm are agreed to the (002) plane of Ti3C2, which extracted from the fast Fourier transform (FFT) patterns of selected area (marked by blue frame) shown in the inset [32]. The HRTEM images indicated that the MoS2 and Ti3C2 sheets were in intimate contact, ensuring favorable electrical transfer between the two materials and will improve the photocatalytic performance of the MoS2/ Ti3C2 heterostructure. In order to analyze the chemical state and elemental composition of MoS2/Ti3C2, X-ray photoelectron spectroscopy (XPS) measurements of T30 heterostructure were carried out.
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Fig. 5 (a) depicted the XPS full spectrum of T30 contained Mo, S, Ti, C and O elements, where the O element was derived from TiO2 crystals and surface-terminated functional groups of Ti3C2. In Fig. 5 (b), Mo 3d spectrum contains diffraction two peaks, the peak at 228.73 eV corresponds to the Mo 3d5/2 binding energy in MoS2, and the other peak at 231.91 eV corresponds to the Mo 3d3/2 binding energy in MoO3, which indicates the oxygen is incorporated in MoS2 during the reaction [33,34]. The peak of Mo6þ-O appears at a binding energy of 236.1 eV due to partial oxidation of MoS2 in the T30 heterostructure [35]. From the S 2p spectrum of Fig. 5 (c), the binding energies of the S 2p3/2 and S 2p1/2 peaks are detected at 161.54 eV and 162.72 eV, respectively [36]. Four peaks were
Fig. 5 e XPS spectra of T30 heterostructure (a) Full spectrum of T30 heterostructure, (b) Mo 3d spectrum, (c) S 2p spectrum, (d) Ti 2p spectrum, (e) C 2s spectrum, (f) O 1s spectrum.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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observed in Ti 2p spectroscopy (Fig. 5 (d)), the peak at 455.21 eV and 461.98 eV are related to the Ti 2p3/2 and Ti 2p1/2 binding energy in Ti3C2. The two prominent peaks appeared at the binding energy of 458.73 eV and 464.52 eV, further proving that the formation of TieO bonds is due to oxygen terminated surfaces etched by HF [37]. Fig. 5 (e) exhibits two strong peaks, the main peak at 283.78 eV corresponds to the CeTi in Ti3C2, and the peak at 284.84 eV is attributed to sp [2] hybridization. Two peaks in the O 1s spectrum (Fig. 5 (f)) with binding energy of 530.17 eV and 531.7 eV are related to TieO bonds and hydroxyl groups adsorbed on the surface of the sample (-OH) [38,39]. Light absorption plays a vital role as a precondition for H2 production. Fig. 6 shows the UVevis diffuse reflectance absorption spectra of Ti3C2 and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. The Ti3C2 sheets exhibit poor absorbance intensity. However, the semiconducting property of MoS2/Ti3C2 contributes to the absorption enhancement and has higher and wider absorption regions. Therefore, the incident sunlight will be utilized at the maximum for the following photon excitation. The surface areas and pore size distributions of MoS2/Ti3C2 heterostructure were obtained by N2 adsorption-desorption measurement. Fig. 7 (a) shows the N2 adsorption-desorption isotherms of MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. The isotherms in the higher nips increased rapidly, and the hysteresis loops occurred during desorption. When the relative pressure was close to 1, the curve was closed. The MoS2/Ti3C2 heterostructure exhibits reversible type IV isotherm, which is one of the main characteristics of mesoporous materials. Furthermore, it was found that the N2 adsorption amount of T30 was the largest, indicating that the specific surface area of the sample was the largest. The change in specific surface area of MoS2/Ti3C2 heterostructure
in Fig. 7 (b) confirmed this. When the loading of Ti3C2 reached 40%, the specific surface area of MoS2/Ti3C2 heterostructure began to decrease, indicating that the MoS2 spheres combined on the surface of the Ti3C2 layers, which was consistent with the SEM image. As shown in Fig. 7 (b), the pore size of the MoS2/Ti3C2 heterostructure is close to 19 nm. This kind of pore size acts as a channel for electron transport, which facilitates the full contact of the catalyst with the reactants in the photocatalytic process and improves the catalytic efficiency. Fig. 8 (a) depicts photocatalytic H2 production of different Ti3C2 contented MoS2/Ti3C2 heterostructure under visible light irradiation for 5 h. As the irradiation time is extended, the amount of H2 produced increases linearly. As shown in Fig. 8 (b), photocatalytic hydrogen evolution rate of pure MoS2 is 2626 mmol g1 h1, indicating a poor photocatalytic activity, and Ti3C2 also demonstrates little photocatalytic activity in hydrogen evolution. However, the combination of MoS2 and Ti3C2 greatly enhances catalytic activity, T30 can improve the photocatalytic hydrogen evolution rate to 6144.7 mmol g1 h1, which are 2.3 times higher than those of the pure MoS2. This enhancement can be attributed to the presence of Ti3C2 to promote efficient transfer of photogenerated electrons. With the loading of Ti3C2 increases, the morphology of MoS2 is from spheres to fully attached to the Ti3C2 layer, which reduces the specific surface area and weakens catalytic hydrogen evolution performance. Among the samples, T30 shows the best HER activity, indicating that the direction of charge transfer can be effectively controlled by a reasonable specific structural design. In addition, a cycle stability experiment was carried out for T30, the sample still maintained a high H2 production after four cycles (Fig. 8 (c)), which suggested the cyclic stability of T30.
Fig. 6 e UVevis absorption spectra of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 7 e (a) N2 adsorption-desorption isotherms of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. (b) The surface area and pore size distribution curves of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts.
According to PL attributes, the PL spectrum can directly reflect the separation situation of photogenerated electrons carriers. For PL signals, the lower the PL intensity, the higher the separation rate of photogenerated electrons and, possibly, the higher the photocatalytic activity [40]. Therefore, the photocatalytic activity of samples can be quickly evaluated by means of PL measurements under a certain condition. As shown in Fig. 9 (a), the PL intensity of MoS2/ Ti3C2 heterostructure (T30) was lower than pure MoS2,
because the internal electric field formed by T30 provided an electron transport channel, the photogenerated electrons transferred from MoS2 to Ti3C2 easily, which effectively inhibited the recombination of photogenerated electronsholes. From the fluorescence lifetime decay curve (Fig. 9 (b)), the carrier lifetimes of pure MoS2 and T30 under light were 2.69 ns and 3.44 ns, indicating that the recombination of photogenerated electrons and holes of T30 was effectively suppressed. Therefore, T30 can effectively delay the
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 8 e (a) Photocatalytic H2 production of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. (b) Average photocatalytic H2 evolution rate of Ti3C2 MXene and MoS2/Ti3C2 heterostructure with different Ti3C2 loading amounts. (c) Cyclicity test of hydrogen production performance for T30 heterostructure.
recombination of charges and prolong the life of electrons, thereby having excellent photocatalytic activity. This conclusion is further confirmed by transient photocurrent response testing. In Fig. 10, the photocurrent intensity of the T30 sample rose immediately after the light was turned on, and immediately decreased after the lamp was turned off, which indicated that most of the photogenerated electrons migrate to the surface of the sample at the moment of turning on the light, and then transferred to the working electrode to generate photocurrent. The photocurrent density of T30 is significantly higher than pure MoS2. On the one hand, Ti3C2 as a co-catalyst to promote electron transport, on the other hand, the addition of Ti3C2 inhibits the recombination of photogenerated electrons and holes. These results indicate the MoS2/Ti3C2 heterostructure is beneficial to photocatalytic activity. Moreover, Charge transfer and recombination process of photogenerated electrons and holes were investigated by electrochemical impedance spectroscopy (EIS). As shown in Fig. 11 (a), the impedance spectrum of the samples is not a
perfect semicircle, which means that there is a charge transfer resistance (Rct) between the electrolyte and the catalyst. The smaller the arc radius of the electrochemical impedance, the smaller the Rct, indicating that the efficiency separation of carrier is higher [41]. The arc radius of the electrochemical impedance under illumination was significantly smaller than that in the dark state, which demonstrated that illumination promotes photogenerated electron transport. In Fig. 11 (b), the T30 exhibited much smaller semicircular arcs than pure MoS2, it is proved that the combination of Ti3C2 improved the separation efficiency of electron-hole pairs and enhanced the activity of the catalyst. The photocatalytic reaction involves three main processes: light absorption, charge separation and transfer, and surface redox reactions. The quantum size effects that nanoscale MoS2 will shift the valence band enough to have a certain catalytic activity. As a co-catalyst compounded to MoS2, Ti3C2 has low catalytic activity and has the following effects on photocatalytic H2 generation performances: (1) 2D-layered Ti3C2 materials can
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 9 e (a) Photoluminescence (PL) spectra of MoS2 and MoS2/Ti3C2 heterostructure (T30). (b) Time-resolved fluorescence decay spectra of MoS2 and MoS2/Ti3C2 heterostructure (T30).
Fig. 10 e Photocurrent densityetime curves of MoS2 and MoS2/Ti3C2 heterostructure (T30).
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Fig. 11 e (a) Eletrochemical impedance spectroscopy (EIS) of MoS2 and MoS2/Ti3C2 heterostructure (T30), (b) spectra in light conditions.
quickly transfer the electrons and reduce the recombination due to high electrical conductivity. (2) The layered structure can provide the passageway for the diffusion of produced H2 [42]. Fig. 12 depicts the photocatalytic hydrogen generation mechanism of MoS2/Ti3C2. When illuminated, electrons of MoS2 were transferred from the valence band (VB) to the conduction band (CB), and left holes in the valence band. Methanol as a hole sacrificial agent in the reaction consumed photogenerated holes, which increased the quantum yield. Part of the photogenerated electrons at the MoS2 conduction band were transferred to Ti3C2, which resulted in efficient separation of photogenerated electron-hole pairs and improved photocatalytic hydrogen evolution efficiency. MoS2 and TiO2 produced by the oxidation process have a certain catalytic activity, which also promote the reduction of Hþ into H2 by photogenerated electrons. This semiconductor catalyst effectively promotes the rapid separation of photo-generated electrons and holes, which improves the photocatalytic performance [43,44].
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
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Fig. 12 e Schematic photocatalytic mechanism for MoS2/Ti3C2 heterostructure.
Conclusion The novel MoS2/Ti3C2 composite photocatalyst was prepared by hydrothermal method. According to a series of structural characterizations, the MoS2 spheres and the Ti3C2 sheets were effectively combined to achieve charge separation and transfer direction adjustment. The performance experiments showed that the addition of Ti3C2 can effectively improve the photocatalytic activity, and T30 exhibited the highest H2 production rate (6144.7 mmol g1 h1) and still maintains a high H2 production after four cycles. Simulation analysis of the reaction process shows that the electron channel formed between MoS2 and Ti3C2 accelerates charge transport and inhibits charge recombination, which is beneficial to the photocatalytic hydrogen evolution reaction. This study provides a new way to establish an effective composite photocatalytic system.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant 11874240), the Opening Project of Key Laboratory of Inorganic Functional Materials and Devices, Chinese Academy of Sciences (Grant No. KLIFMD201803), the Natural Science Foundation of Shandong Province, China (No. ZR2016EMM21), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (No. 2016RCJJ002).
references
[1] Takashi H, Jun K, Kazunari D. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. ChemInform 2014;43(22):7520e35. [2] Chhowalla M, Shin HS, Eda G, Li LJ, Loh KP, Hua Z. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat Chem 2013;5(4):263e75.
[3] Juan L, Yang L, Naiyun L, Yuzhi H, Xing Z, Hui H, Yeshayahu L, Shuit-Tong L, Jun Z, Zhenhui K. Water splitting. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015;46(23):970e4. [4] Mckone JR, Marinescu SC, Brunschwig BS, Winkler JR, Gray HB. Earth-abundant hydrogen evolution electrocatalysts. Chem Sci 2014;5(3):865e78. [5] Ying Y, Han J, Zhang X, Zhang Y, Zhou J, Muir D, Sutarto R, Zhang Z, Liu S, Bo S. Facile synthesis of few-layer-thick carbon nitride nanosheets by liquid ammonia-assisted lithiation method and their photocatalytic redox properties. RSC Adv 2014;4(62):32690e7. [6] Xu Z, Hongjian Y, Guopeng W, Guijun M, Fuyu W, Lu W, Can L. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as Cocatalyst under visible light irradiation. J Am Chem Soc 2008;130(23):7176e7. [7] Tu W, Yong Z, Qi L, Yan S, Bao S, Wang X, Min X, Zou Z. An in situ simultaneous reduction-hydrolysis technique for fabrication of TiO2-graphene 2D sandwich-like hybrid nanosheets: graphene-promoted selectivity of photocatalytic-driven hydrogenation and coupling of CO2 into methane and ethane. Adv Funct Mater 2013;23(14):1743e9. [8] Wang Yawen, Zhang Lizhi, Deng Kejian, Chen Xinyi, Zou Zhigang. Low temperature synthesis and photocatalytic activity of rutile TiO2 nanorod superstructures. J Phys Chem C 2007;111(6):2709e14. [9] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238(5358):37e8. [10] Jia MS, Loh KP. Electrochemical double-layer capacitance of MoS2 nanowall films. Electrochem Solid State Lett 2007;10(11):A250. [11] Claudia A, Maria S, Paolo C, Adolfo S, Vincenzo P. New ’chimie douce approach to the synthesis of hybrid nanosheets of MoS2 on CNT and their anti-friction and antiwear properties. Nanotechnology 2013;24(12):125601. [12] Dasgupta NP, Yang P. Semiconductor nanowires for photovoltaic and photoelectrochemical energy conversion. Front Phys 2014;9(3):289e302. [13] Nguyen TP, Van Le Q, Choi KS, Oh JH, Kim YG, Lee SM, Chang ST, Cho YH, Choi S, Kim TY. MoS2 nanosheets exfoliated by sonication and their application in organic photovoltaic cells. Sci Adv Mater 2015;7(4):700e5 (6). [14] Hinnemann B, Moses PG, Bonde J, Jørgensen KP, Nielsen JH, Horch S, Chorkendorff I, Nørskov JK. Biomimetic hydrogen
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109
international journal of hydrogen energy xxx (xxxx) xxx
[15]
[16]
[17]
[18]
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. J Am Chem Soc 2005;127(15):5308e9. Karunadasa HI, Chang CJ. A molecular MoS2 edge site mimic for catalytic hydrogen generation. Science 2012;335(6069):698e702. Chen WF, Wang CH, Sasaki K, Marinkovic N, Xu W, Muckerman JT, Zhu Y, Adzic RR. Highly active and durable nanostructured molybdenum carbide electrocatalysts for hydrogen production. Energy Environ Sci 2013;6(3):943e51. Ghassemi H, Harlow W, Mashtalir O, Beidaghi M, Lukatskaya MR, Gogotsi Y, Taheri ML. In situ environmental transmission electron microscopy study of oxidation of twodimensional Ti3C2 and formation of carbon-supported TiO2. J Mater Chem 2014;2(35):14339e43. Splendiani A, Sun L, Zhang Y, Li T, Kim J, Chim CY, Galli G, Wang F. Emerging photoluminescence in monolayer MoS2. Nano Lett 2010;10(4):1271e5. Lukatskaya MR, Mashtalir O, Ren CE, Dall’Agnese Y, Rozier P, Taberna PL, Naguib M, Simon P, Barsoum MW, Gogotsi Y. Cation intercalation and high volumetric capacitance of twodimensional titanium carbide. Science 2013;341(6153):1502e5. Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Catal Surv Asia 2003;7(1):31e8. Qiuming P, Jianxin G, Qingrui Z, Jianyong X, Baozhong L, Aiguo Z, Riping L, Yongjun T. Unique lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide. J Am Chem Soc 2014;136(11):4113e6. Yidong H, Abrams BL, Vesborg PCK, BjRketun MRE, Konrad H, Lone B, Setti AM, Damsgaard CD, Thomas P, Ole H. Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nat Mater 2011;10(6):434e8. Khazaei M, Arai M, Sasaki T, Chung CY, Kawazoe Y. Novel electronic and magnetic properties of two-dimensional transition metal carbides and nitrides. Adv Funct Mater 2013;23(17):2185e92. Minoli G. Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide. Science 2013;341(6153):1502e5. Peng C, Yang X, Li Y, Yu H, Wang H, Peng F. Hybrids of twodimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity. ACS Appl Mater Interfaces 2016;8(9). acsami.5611973. Wang H, Peng R, Hood ZD, Naguib M, Adhikari SP, Wu Z. Titania composites with 2 D transition metal carbides as photocatalysts for hydrogen production under visible-light irradiation. Chemsuschem 2016;9(12):1490e7. Ran J, Gao G, Li FT, Ma TY, Du A, Qiao SZ. Ti3C2 MXene cocatalyst on metal sulfide photo-absorbers for enhanced visible-light photocatalytic hydrogen production. Nat Commun 2017;8:13907. Xiaobo C, Shaohua S, Liejin G, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110(11):6503e70. Osterloh FE. Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting. Chem Soc Rev 2013;42(6):2294e320. Luo J, Tao X, Zhang J, Xia Y, Huang H, Zhang L, Gan Y, Liang C, Zhang W. Sn4þ ions decorated highly conductive
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
11
Ti3C2 MXene: promising lithium-ion anodes with enhanced volumetric capacity and cyclic performance. ACS Nano 2016;10(2):2491. Gao G, O’Mullane AP, Du A. 2D MXenes: a new family of promising catalysts for the hydrogen evolution reaction. ACS Catal 2016;7(1):494e500. 7(1). Ma TY, Cao JL, Jaroniec M, Qiao SZ. Interacting carbon nitride and titanium carbide nanosheets for high-performance oxygen evolution. Angew Chem 2016;128(3):1150e4. Liu J, Tang S, Lu Y, Cai G, Liang S, Wang W, Chen X. Synthesis of Mo2N nanolayer coated MoO2 hollow nanostructures as high-performance anode materials for lithium-ion batteries. Energy Environ Sci 2013;6(9):2691e7. Sanfeng W, Chunming H, Grant A, Ross JS, Cobden DH, Xiaodong X. Vapor-solid growth of high optical quality MoS2 monolayers with near-unity valley polarization. ACS Nano 2013;7(3):2768e72. Li X, Zhang L, Zang X, Li X, Zhu H. Photo-promoted platinum nanoparticles decorated MoS2@Graphene woven fabric catalyst for efficient hydrogen generation. ACS Appl Mater Interfaces 2016;8(17):10866e73. Ren W, Zhou W, Zhang H, Cheng C. ALD TiO2-coated flowerlike MoS2 nanosheets on carbon cloth as sodium ion battery anode with enhanced cycling stability and rate capability. ACS Appl Mater Interfaces 2016;9(1):487e95. Xuefeng W, Xi S, Yurui G, Zhaoxiang W, Richeng Y, Liquan C. Atomic-scale recognition of surface structure and intercalation mechanism of Ti3C2X. J Am Chem Soc 2015;137(7):2715e27. Yoon Y, Lee M, Kim SK, Bae G, An KS. A strategy for synthesis of carbon nitride induced chemically doped 2D MXene for high-performance supercapacitor electrodes. Adv Energy Mater 2018;8(15):1703173. Rakhi RB, Ahmed B, Hedhili MN, Anjum DH, Alshareef HN. Effect of postetch annealing gas composition on the structural and electrochemical properties of Ti2CTx MXene electrodes for supercapacitor applications. Chem Mater 2015;27(15):5314e23. Liu Q, Ai L, Jing J. MXene-derived TiO2@C/g-C3N4 heterojunctions for highly efficient nitrogen photofixation. J Mater Chem 2018;6(9):4102e10. Zhang J, Yu J, Jaroniec M, Gong JR. Noble metal-free reduced graphene oxide-ZnxCd₁-xS nanocomposite with enhanced solar photocatalytic H₂-production performance. Nano Lett 2012;12(9):4584e9. Li L, Yu L, Lin Z, Yang GW. Reduced TiO2-graphene oxide heterostructure as broad spectrum-driven efficient water splitting photocatalysts. ACS Appl Mater Interfaces 2016;8(13):8536e45. Yuan W, Cheng L, Zhang Y, Wu H, Lv S, Chai L, Guo X, Zheng L. 2D-Layered carbon/TiO2 hybrids derived from Ti3C2MXenes for photocatalytic hydrogen evolution under visible light irradiation. Adv Mater Interfaces 2017;4(20):1700577. Nakata K, Ochiai T, Murakami T, Fujishima A. Photoenergy conversion with TiO2 photocatalysis: new materials and recent applications. Electrochim Acta 2012;84(12):103e11.
Please cite this article as: Zhang J et al., MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.109