1D heterojunction enhancement on photocatalytic activity through assembling MoS2 nanosheets onto super-long TiO2 nanofibers

Journal Pre-proofs Full Length Article Direct evidence of 2D/1D heterojunction enhancement on photocatalytic activity through assembling MoS2 nanosheets onto super-long TiO2 nanofibers Yukun Li, Peng Zhang, Dongyang Wan, Chao Xue, Jiangtao Zhao, Guosheng Shao PII: DOI: Reference:

S0169-4332(19)33177-0 https://doi.org/10.1016/j.apsusc.2019.144361 APSUSC 144361

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

23 April 2019 20 September 2019 9 October 2019

Please cite this article as: Y. Li, P. Zhang, D. Wan, C. Xue, J. Zhao, G. Shao, Direct evidence of 2D/1D heterojunction enhancement on photocatalytic activity through assembling MoS2 nanosheets onto super-long TiO2 nanofibers, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144361

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Direct evidence of 2D/1D heterojunction enhancement on photocatalytic activity through assembling MoS2 nanosheets onto super-long TiO2 nanofibers Yukun Lia,b, Peng Zhanga,b,c*, Dongyang Wana,b, Chao Xue*a,b, Jiangtao Zhaoa,b and Guosheng Shao*a,b,c a School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 45001, China. b State Centre for International Cooperation on Designer Low-Carbon & Environmental Materials (CDLCEM), Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, China. c Zhengzhou Materials Genome Institute, Building 2, Zhongyuanzhigu, Xingyang 40100, China. * School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Street, Zhengzhou 450001, People’s Republic of China. Corresponding [email protected]

author:

[email protected];

[email protected];

Abstract Nano-heterostructures (2D/2D, 2D/1D, 2D/0D etc.) have received special attention due to their remarkable performances beyond those of their single-component semiconductor. Direct measurements of the band structure and band bending at the interface of the semiconductors illustrate an important method to understand the fundamental catalytic mechanism and explore promising nanomaterials with improved catalytic property. In this work, the 2D/1D heterojunction with assembling MoS2 nanosheets onto the super-long TiO2 nanofibers is successfully prepared via the electrospinning and hydrothermal method. More importantly, the band bending and electrons transfer of the TiO2/MoS2 heterostructures are directly evidenced by ultraviolet photoelectron spectra (UPS) and in situ irradiated X-Ray photoelectron spectroscopy (ISI-XPS). We find that the as-prepared composite heterostructure exhibited superior photocatalytic hydrogen production activity. Especially, the optimized 2D/1D TiO2/MoS2 heterojunction containing 60 wt.% MoS2 showed the highest hydrogen production activity of 171.24 μmoL‧ g−1‧ h−1, which was about 24 times higher than that of the pure TiO2. To justify the corresponding mechanism of enhanced performance of hydrogen production, photocurrent analysis, electrochemical impedance spectroscopy, ISI-XPS and UPS are also employed to investigate the separation of the photo-generated electron-hole pairs. Keywords: 2D/1D; Hydrogen production; Electrons transfer; ISI-XPS; UPS

1. Introduction In order to alleviate the energy crisis, nano-heterostructures have drawn special attention recently for the potential alternatives in the field of photocatalytic split water to produce hydrogen [1, 2]. And it is found that modifying the structure of the existing materials can dramatically improve their chemical and physical properties, especially for nanomaterials [3]. So far, nanomaterials have been decorated mainly by constructing heterojunctions, such as 2D/2D, 2D/1D, 2D/0D etc. [4, 5]. As a typical semiconductor nanomaterial, TiO2 has an excellent performance in the field of photocatalysis, due to its non-toxic, low cost and strong chemical and thermal stability [6-10]. Unfortunately, similar to other photocatalysts, the rapid recombination of photogenerated electron-hole pairs is a decisive factor that affect the practical application of TiO2. To modify TiO2 [11] and suppress the recombination of charge carrier, many measures have been carried out, such as loading noble metals [12, 13] or transition metal ions [14]. In our previous work [15], we have researched the task about TiO2 decorated by noble metal of Pt and Au. Nevertheless, due to short longevity of photogenerated electron-hole pairs and high cost, the photocatalysts modified with noble metal is still not satisfactory [15, 16]. Therefore, combining TiO2 with a new type of semiconductor nanomaterial with a narrow band gap is an effective strategy, which can not only replace noble metals, but also improves the response range of visible light [17, 18]. The intrinsic n-type semiconductor MoS2 [19, 20] is a latent material for combining with TiO2. The resultant heterojunction possesses the comfortable energy level to produce hydrogen [21]. Furthermore, the unique structure of large surface area and rich edge structure of MoS2 nanosheets is beneficial to improving the photocatalytic property [22]. In addition, the 2D MoS2 nanosheet has the comfortable band-edge positions and narrow band gap, [23] which made it being a promising co-catalysts to replace noble metal for photocatalytic proton reduction reaction. Among various morphologies, 1D nanofiber and 2D nanosheet have caused great concern for their evident improvement in particle performance in photocatalysis and other applications [24-27]. It is also greatly enhanced the utilization of visible light through the unique structure of high specific surface area, which is consistent with the investigation in our

previous study [28]. Theoretically, the photocatalytic activity is mainly related to the phase structure, adsorption ability and the separation efficiency of photo-generated electron-hole pairs. Although the phase structure [29] and adsorption efficiency [30] can be easily characterized and clarified by existing techniques, there is no direct evidence to clarify the process about the band bending contributing to the transfer of photo-induced electrons in the heterostructure. Besides, previous studies have shown that the 2D/2D, 2D/1D, 2D/0D composites can exhibit a Fermi level shift [31, 32] and enhance the separation efficiency of photo-induced electrons and holes in heterojunction. The aforementioned results evidently showed that the enhancement of photocatalytic activity is related to the formation of heterostructure. However, there is no further investigation about the behavior of charge separation and transfer in heterostructure, which is important for demonstrating the mechanism of the enhanced photocatalytic performance in the hybrid photocatalytic system. Particularly, the direct evidence of the electrons and holes migration across the interface between the component semiconductors is lacking. Moreover, there is no related study that directly connects band bending with electrons transfer. Here, the 2D/1D heterostructure with assembling MoS2 nanosheets onto super-long TiO2 nanofibers was fabricated through the combination of electrospinning and hydrothermal method [33]. The ultraviolet photoelectron spectroscopy (UPS) was carried out to investigate the band bending. The work functions of two single components were also directly calculated by UPS. Besides, the binding energies under and without UV-light irradiation were obtained via in-situ irradiated X-Ray photoelectron spectroscopy (ISI-XPS) measurement [34]. The binding energy under different condition can be obtained through XPS measurement. Finally, we conclude that the enhanced photocatalytic hydrogen production performance of TiO2/MoS2 heterojunction system under solar-light irradiation mainly attributes to the effective separation of photo-generated electron-hole pairs in the heterostructure.

2. Experiment 2.1. Preparation of TiO2 nanofibers Firstly, 10 mL absolute ethanol, 6 mL acetic acid and 1.1 g polyvinyl pyrrolidone powder (PVP, Mw = 1300000) was dispersed into a beaker. To generate a homogeneous solution, the beaker with mixed solution was magnetic stirred for 1 h. Then, 2.0 mL tetrabutyl titanate (TBOT) was added into the above mixture under magnetic stirring for 12 h. Then, the obtained precursor solution was sucked into a syringe. The distance between the needle tip and the collector plate was about 15 cm. The high voltage power of positive terminal connected was set at 13 kV. At last, the as-prepared precursor was calcined at 520 °C in air atmosphere for 30 min with a rate of 4 °C‧ min-1 to harvest the anatase TiO2 nanofibers. 2.2. Fabrication of TiO2/MoS2 composite The TiO2/MoS2 heterostructure was prepared by the hydrothermal method. It was fabricated as follows: 270 mg thioacetamide (C2H5NS), 135 mg sodium molybdate (Na2MoO4 · 2H2O) and 60 mg calcined TiO2 nanofibers were dissolved in 60 mL deionized water and magnetically stirred for 30 min. Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 240 °C for 24 h. After the reaction, the whole system was cooled to room temperature naturally. The solid production was collected by multiple centrifugation and dried. The concentrations of MoS2 in TiO2/MoS2 of the composition was adjusted by changing the amount of Mo source and S source via the same method. 2.3. Characterization The field emission scanning electron microscopy (FESEM, JSM-7500F) was used to investigate the morphologies of the prepared samples. And the morphologies and structure were also studied through transmission electron microscope (TEM, FEI Tecnai G2 F20). The BET data was obtained by MicroActive for ASAP 2460 2.01. XRD (Rigaku Ultima IV) was applied to study the crystallinity under a scanning rate of 4°‧ min-1 from 10° to 80°. The X-Ray photoelectron spectroscopy (AXIS UltraDLD,

Kratos Analytical Inc) was used to investigate the photo-generated electrons migration pathway across the interface of TiO2/MoS2. A UV-Vis spectrophotometer (Shi madzu, model UV 3600) was took to characterization all samples. 2.4. Electrochemical measurements Photoelectrochemical performance was characterized through an electrochemical workstation (AMETEK, PARSTAT 4000, America) in a cubic quartz reactor. The 0.2 M Na2SO4 aqueous solution as electrolyte. The Pt wire served as a counter electrode and the Ag/AgCl electrode acted a reference electrode. The synthetic photoanode acted a working electrode. The mixture including water (0.35 mL), polyethylene glycol (0.05 g) and the sample (0.1 g) was deposited on the conducting glass of FTO with an area of 1.5×1.5 cm2. A 300 W Xe lamp (Beijing Perfectlight Co. Ltd, PLS-SXE-300) was served as the light source. The electrochemical impedance spectra (EIS) and photocurrent response spectroscopy were carried out at a working photoanode with constant potential open-circuit voltage of +0.9 V. 2.5. Photocatalytic hydrogen generation Photocatalytic splitting water reaction was measured by gas chromatography (GC7860) connecting online analysis system (LabSolar-III AG, Beijing Perfect Light Co. Ltd.). The photocatalytic hydrogen evolution reaction was carried out in a quartz reactor. And 300 W Xe lamp (PLS-SXE 300, Beijing Perfect Light Co. Ltd) was selected as a UV-vis light source. Generally, the 50 mg samples, 40 mL deionized water and 25 mL methyl alcohol (anhydrous, Sinopharm Chemical Regent, 99.5%) was added into the quartz reactor with a stirring. The 300 W Xe lamp was used to illuminate the quartz reactor after vacuumizing. The gas chromatography (GC-7860) was equipped with a thermal conductivity detector (TCD), which was used to analysis the gas product composition from quartz reactor via liquid suspension reaction. 3. Results and discussions

To evaluate the phase structure of TiO2/MoS2 samples, XRD analyses were carried out. As shown in Fig. 1, seven diffraction peaks appeared at 2θ = 25.5°, 37.9°, 48.0°, 53.8°, 62.8°, 68.7°and 75.0° could be readily indexed to (101), (004), (200), (105), (204), (116) and (215) planes of the anatase TiO2 (JCPDF # 65-5714), respectively. It indicates that the nanofibers have a highly crystalline character due to the sharp and intense diffraction peaks of the TiO2 nanofibers. Meanwhile, the MoS2 pattern is the blue line in Fig. 1. As can be seen from the XRD diffraction pattern of TiO2/MoS2 heterostructures, the characteristic diffraction peaks located at 14.5°, 33.0°, 41.1° and 58.3° can be well matched with the typical (003), (101), (015) and (110) planes of 2HMoS2 (JCPDF #65-3656). It further certificates that the crystal phase structure of TiO2 did not change when the MoS2 nanosheets in-situ anchored on the surface of the TiO2 nanofibers under the hydrothermal treatment. It can be obviously observed in Fig. 1, the intensity of the (003) plane of MoS2 in the 2D/1D TiO2/MoS2 heterojunction was significantly decreased compared with the pure MoS2, which can be attributed to the fact that the c-axis stacking of MoS2 sheets was effectively inhibited during the formation process of 2D/1D TiO2/MoS2 heterojunction. Besides, characteristic values of both (101) and (110) planes were slightly shifted to more positive positions. According to the Bragg's Law, the narrow of interlayer spacing will lead to the enlarge of diffraction angle. The XRD analysis results demonstrated the formation of a new hierarchical layered structure of MoS2/TiO2 composite. The morphologies of the as-prepared samples were characterized by SEM and TEM measurement. As can be seen from the SEM images of the pure TiO2 nanofibers and TiO2/MoS2 heterojunction in Fig. 2, the diameter of pure TiO2 nanofibers is about 300 nm with smooth surface (Fig. 2A). The SEM image shown in Fig. 2B clearly revealed that the ultrathin MoS2 nanosheets with a 100 nm horizontal size are and vertically anchored on the surface of the super-long TiO2 nanofibers. The diameter of 2D/1D nanofiber heterojunction is obviously enlarged compared to that of pure TiO2 nanofiber. As demonstrated in Fig. 3A, the surface of TiO2 nanofibers were wrapped by the ultrathin MoS2 nanosheets, indicating the formation of the TiO2/MoS2 nanofiber heterojunction. In addition, the constructed hierarchical TiO2/MoS2 nanofiber

heterojunction will not only increase the specific surface areas but also favor the harvesting of incident light due to the multiple scattering within the hierarchical structure. As a result, these structural advantages are beneficial to exposing more catalytic active sites for the adsorption of reactants on the surface of MoS2 co-catalyst. Moreover, HR-TEM analysis (Fig. 3B) showed that the MoS2 nanosheets are vertically aligned to the surface of TiO2 nanofibers to form the intimate contact interfacial junction. The interactive heterojunction could provide charge transfer channels for achieving the spatial charge separation rapid migration, thus dramatically enhancing the photocatalytic activities. The lattice spacing measurement of 0.35 and 0.62 nm can be well assigned to the (101) diffraction plane of anatase TiO2 nanocrystal and (002) diffraction plane of 2H-MoS2, respectively. As shown in Fig. 3C, the SAED pattern contains two sets of diffraction signals, indicating the polycrystalline structure of the 2D/1D TiO2/MoS2 heterostructures. And the R1 and R2 are assigned to (003) plane of MoS2 and (101) plane of TiO2, respectively. Further confirming the co-existence of TiO2 nanofibers and MoS2 nanosheets in the hybrid heterojunction. The photocatalytic hydrogen production performance of the pure TiO2 nanofibers and 2D/1D heterojunctions with different MoS2 concentrations from 40 wt.% to 70 wt.% were investigated. The gas chromatogram of hydrogen is shown in Fig. S1. As shown in Fig. 4A, the pure TiO2 nanofibers exhibited apparently low H2 production rate due to the highly-efficient charge carrier recombination. By contrast, the as-prepared composites exhibit superior photocatalytic hydrogen production activities after anchoring the vertical 2D few-layer MoS2 nanosheets onto the surface of 1D TiO2 nanofibers. The constructed 2D/1D TiO2/MoS2 heterostructure containing 60 wt.% MoS2 showed the highest hydrogen production rates of 171.24 μmoL‧ g−1‧ h−1, which was about 24 times higher than that of the pure TiO2 under UV-vis light irradiation. The results indicate that the introduce of the appropriate MoS2 nanosheet co-catalyst can significantly boost the photocatalytic hydrogen production activity. As shown in Fig. 4B, the stability of the 2D/1D TiO2/MoS2 heterostructure containing 60 wt.% MoS2 was tested for five runs. The performance of photocatalytic hydrogen production displayed no noticeable decrease in the cycling tests, indicating

that the 2D/1D TiO2/MoS2 heterostructure exhibits very good stability in photocatalytic hydrogen production from water splitting under UV-vis light irradiation. Meanwhile, the specific surface area and porosity of the as-prepared 2D/1D heterostructure before and after light irradiation were elucidated by N2 adsorption-desorption analysis. It can be seen from Fig. 5A and C that the N2 adsorption-desorption isotherms exhibited typical type IV isotherm patterns both before and after the photocatalytic reaction, indicating the presence of slit-shaped mesopores in these nanostructures. It is worth noting that the pores size was decreased after the reaction. Before the H 2 evolution reaction, there are part of the pores size distribution between 10 and 100 nm (in Fig. 5B). However, almost all the pores size is under 30 nm after the reaction, as shown in Fig. 5D. Besides, the absorbed gas volume after reaction (Fig. 5C) was increased compared with the fresh sample (Fig. 5A), indicating the smaller pores size. The hierarchical MoS2 nanosheets anchored onto the surface of TiO2 nanofibers were sabotaged by mechanical force and the thermal effect to some extent during the photocatalytic reaction. This structural reconstruction behavior results in the formation of more slit-stacking pores, smaller pore size and higher specific surface area, which increases the BET surface area (in Table 1). It is well known that a large surface area is beneficial to exposing more catalytic active sites for the adsorption of reactants on the surface of photocatalyst, thus improving the photocatalytic activity for hydrogen evolution. To further characterize the stability of the 2D/1D heterojunction, the inductively coupled plasma (ICP) emission spectrometer measurement was carried out. Before the photocatalytic reaction, the 50 mg MoS2/TiO2 was dispersed into the mixed solution containing 60 mL deionized water and 25 mL methyl alcohol. The samples are taken out from quartz reactor every 30 minutes during the photocatalytic reduction reaction and is diluted 10 times before the test. As demonstrated in Fig. 6A, the dissolution amount of Mo4+ increases with reaction continues, and after UV-vis light illumination for 120 minutes, the concentration of Mo4+ reaches to 6.18 μg‧ mL-1, which means that only 0.12 wt.% of photocatalyst decomposed into the solution during the entire reaction. Meanwhile, the Ti4+ is barely not dissolution even after UV-vis light illumination for

120 minutes. And the changes of concentration of Mo4+ and Ti4+ ions can also be obviously observed in Fig. 6B and C, respectively. The ICP analysis results are consistent with those of recyclability of hydrogen evolution tests. Furthermore, the transient photocurrent responses and electrochemical impedance spectroscopy (EIS) of various photocatalysts were measured to investigate the separation and transfer efficiency of photo-generated carriers. As shown in Fig. 7A, the pure MoS2 barely has no intensity of photocurrent, while the composites showed significantly enhanced photocurrent intensity. The photocurrent density reaches an average of 27μA for the 2D/1D TiO2/MoS2 heterojunction containing 60 wt.% MoS2, which is 4 times higher than that of the pure TiO2. It demonstrates that the 2D/1D heterojunction may provide an effective separation of photo-generated electron hole pairs. According to the nonlinear regression fitting results of the EIS for as-prepared electrodes (Fig. 7B), the fitted results measured by the traditional Randle circuit program [35] were shown as following: TiO2/MoS2
In addition, work function (Wf) measurement via UPS technique is also valuable to investigate the photo-generated electron transfer behavior of the 2D/1D TiO2/MoS2 heterojunction. For purpose of confirming the suitable valence band of the heterostructure, UPS was carried out shown in Fig 8B. As can be seen from UPS spectra of TiO2/MoS2 composite, it was calculated that the Wf of the 2D/1D TiO2/MoS2 heterojunction containing 60 wt.% MoS2 located at 4.7 eV. As displayed in Fig. 9, the band structures of pure TiO2 and MoS2 were also determined by the UPS spectra. And the position of the band structure versus vacuum energy level and normal hydrogen electrode were calculated. For single component of TiO2 and MoS2, the work functions are calculated to be 4.5 eV and 4.3 eV, respectively. Then the valence band (VB) edges for pure TiO2 and MoS2 are calculated to be 7.3 eV and 6.3 eV, respectively, and the corresponding conductive band (CB) are calculated to be 4.1 eV and 4.3 eV, respectively. The detailed parameters of band level of the single phase TiO2 and MoS2 are illustrated in Fig. 10. When a TiO2/MoS2 heterostructure forming, there will be a shift of Femi levels in the semiconductor interface due to the different work function. And the Femi level shift will lead to a band bending [36-40]. Photo-generated charge transfer behavior can be affected under the band bending. It's worth noting that the intimate contact interfacial junction leads to a better spatial separation of photogenerated charge carriers due to the presence of built-in electric field induced by the difference work functions of various semiconductors. The corresponding energy band bending structure and photogenerated charge transfer direction of TiO2/MoS2 heterojunction were shown in Scheme 1. According to the band theory and results of UPS measurement, the Wf of pure TiO2 is calculated to be 4.5 eV, which is higher than that of MoS 2 (4.3 eV). Therefore, the band bending of MoS2 is upward, while the band bending of TiO2 is downward. The electrons in VB of TiO2 are excited to the CB, leaving the holes on the VB under UV-vis light illumination. Then, the photo-generated electrons will immediately shift to the CB of MoS2 under the effect of built-in electric field. Consequently, the photogenerated electron-hole pairs of TiO2 could be efficiently separated. And a large

number of photo-induced electrons rapidly migrate to the edge active sites of MoS2 nanosheet for participating in the proton reduce reaction, thus significantly enhance of the photocatalytic hydrogen activity of the 2D/1D TiO2/MoS2 heterojunction. To further investigate the interfacial photo-generated charge transfer between TiO2 and MoS2, the in-situ irradiated XPS (ISI-XPS) was performed. As shown in Fig. 11A, without light irradiation the TiO2/MoS2 exhibited two peaks at 464.4 eV and 458.7 eV, which are related with Ti 2p3/2 and Ti 2p1/2 of TiO2. Under light irradiation, there was a slight positive shift in the Ti 2p binding energy, suggesting the decrease of electron density of TiO2 under light irradiation. Meanwhile, as shown in Fig. 11B, two characteristic peaks located at 232.7 eV and 229.5 eV can be well assigned to Mo 3d5/2 and Mo 3d3/2, respectively, without light irradiation, while the corresponding two characteristic peaks for Mo 3d underwent a negative shift upon light irradiation, suggesting an increase electron density on MoS2. The shift of binding energy provides a direct evidence of the charge transmission across the interface between TiO2 and MoS2 under the same light irradiation condition during the photocatalytic reaction. In detail, the photo-generated electrons migrate from TiO2 to MoS2, which is in good agreement with the band structure in Scheme 1. The direct evidence of electrons transfer is given in this work, which is not provided by the previous relative papers (Table S1). Compared with traditional XPS measurement, the ISI-XPS is also carried out under UV light irradiation. As demonstrated in Fig. 12A, the photo-emitted electrons excited by X-Ray can escape from the surfaces of TiO2 and MoS2, respectively. And these photo-emitted electrons can be used to characterize different elements by the difference of binding energy. Once the TiO2/MoS2 compound is illuminated by both the UV light and X-Ray, both the TiO2 and MoS2 in the hybrid system will be excited by UV light (Fig. 12B). As mentioned above, the band bending occurs with the strong interfacial interaction between TiO2 and MoS2 because of the different Wf of TiO2 and MoS2. The band bending in the heterojunction was favorable for the photo-generated electrons in CB of TiO2 injecting to the CB of MoS2 nanosheet co-catalyst (Fig. 12C), thus greatly prolonging the lifetime of electrons. In addition, the binding energy also can be affected

by surface shielding effect. As demonstrated in Fig. 12D, the valence electrons at the outermost layer of Ti could transfer to Mo and increase the amounts of outermost electrons of Mo, thus leading to the decrease and increase of surface shielding effect of Ti and Mo, respectively. Increased shielding effect leads to a decrease in binding energy, which can be apparently observed in Fig. 9A and B. Such a transferring of electrons can effectively enhance the electron-hole separation efficiency. Conclusions In summary, the direct evidence based on the UPS and ISI-XPS characterization confirms the band bending and photo-generated electrons transfer between the interface of TiO2 nanofibers and MoS2 nanosheets. The formation of heterojunction can greatly enhance the separation efficiency of the photo-generated electrons and holes. As a result, the photocatalytic hydrogen reduction performance of the 2D/1D TiO2/MoS2 heterojunction is significantly improved compared with those of the pure TiO2 nanofibers and MoS2 nanosheets. This work comprehensively confirms the spatical separation and rapidly transfer the photo-generated electrons between the intimate contact interfacial junction in 2D/1D heterojunction through experimental and theoretical analyses, providing important inspiration for future development of semiconductor composites in efficient solar energy utilization.

Acknowledgments The work was supported by the National Natural Science Foundation of China (Nos. 51972287, 51502269, 51001091, 111174256, 91233101), Natural Science Foundation of Henan Province (No. 182300410187) and Outstanding Young Talent Research Fund of Zhengzhou University (No. 1521320023).

References [1] M. Batzill, Energ. Environ. Sci. 4 (2011) 3275-3286. [2] M. R. Hoffmann, S. T. Martin, W. Choi, D. W. Bahnemann, Chem. Rev. 95 (1995) 69-96. [3] H. Park, Y. Park, W. Kim, W. Choi, J. Photochem. Photobio. 15 (2013): 1-20. [4] W. Zhou, C. Cheng, J. Liu, Y. Y. Tay, J. Jiang, X. Jia, H. J. Fan, Adv. Funct. Mater. 21 (2011), 2439-2445. [5] L. Huang, F. Peng, H. J. Wang, H. Yu, Z. Li, Catal. Commun. 10 (2009), 1839-1843. [6] G. Liu, L. Z. Wang, H. G. Yang, H. M. Cheng, G. Q. Lu, J. Mater. Chem. 20 (2010) 831-843. [7] P. Ruban, K. Sellappa, Energy 73 (2014) 926-932. [8] J. Schneider, M. Matsuoka, M. Takeuchi, Chem. Rev. 114 (2014) 9919-9986. [9] Z. G. Zou, J. H. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625-627. [10] C. Sattler, L. de Oliveira, M. Tzschirner, A. E. H. Machado, Energy 29 (2004) 835843. [11] S. G. Kumar, K. S. R. K. Rao, Appl. Surf. Sci. 391 (2017) 124-148. [12] M. Anpo, M. Takeuchi J. Catal. 216 (2003) 505-516. [13] D. Li, J. L. Jia, T. Zheng, X. W. Cheng, X. J. Yu, Appl. Catal. B: Environ. 188 (2016) 259-271. [14] J. Arana, O. G. Diaz, M. M. Saracho, J. M. D. Rodriguez, J. A. H. Melian, J. P. Pena, Appl. Catal. B: Environ. 36 (2002), 113-124. [15] H. Q. Gao, P. Zhang, J. H. Hu, J. M. Pan, J. J. Fan, G. S. Shao, Appl. Surf. Sci. 391 (2017) 211-217. [16] H. Q. Gao, P. Zhang, J. T. Zhao, Y. S. Zhang, J. H. Hu, G. S. Shao, Appl. Catal. B: Environ. 210 (2017) 297-305. [17] Y. Lu, H. T. Yu, S. Chen, X. Quan, H. M. Zhao, Environ. Sci. Technol. 46 (2012) 1724-1730. [18] W. Y. Cheng, T. H. Yu, K. J. Chao, S. Y. Lu, Int. J. Hydrogen Energ. 38 (2013) 9665-9672. [19] R. Ganatra, Q. Zhang, ACS nano 8 (2014) 4074-4099.

[20] W. Teng, Y. Wang, Q. Lin, J. Nanosci Nanotechno. 19 (2019) 3519-3527. [21] T. S. Rahman, D. A. Chen, J. Phys. Chem. C 12 (2019) 7185-7201 [22] E. Parzinger, B. Miller, B. Blaschke, J. A. Garrido, J. W. Ager, A. Holleitner, U. Wurstbauer, ACS nano 9 (2015) 11302-11309. [23] P. Zhang, D. Wan, Z. Zhang, G. Wang, J. Hu, G. Shao, Environ. Sci.: Nano 5(2018) 1813-1820. [24] S. Sahu, L. Cao, M. J. Meziani, C. E. Bunker, K. A. S. Fernando, P. Wang, Y. P. Sun, Chem. Phys. Lett. 634 (2015) 122-128. [25] J. Hu, L. Wang, P. Zhang, C. Liang, G. Shao, J. Power Sources 328(2016) 28-36. [26] P. Zhang, L. J. Wang, X. Zhang, J. H. Hu, G. S. Shao, Appl. Catal. B: Environ. 166-167(2015) 193–201. [27] P. Zhang, L. J. Wang, X. Zhang, J. H. Hu, G. S. Shao, Nano-Micro Lett. 7 (2015) 86-95. [28] Y. Ma, X. L. Wang, C. Li, Chinese J. Catal. 2015 36 (2015) 1519-1527. [29] S. Kanda, T. Akita, M. Fujishima, H. Tada, J. Colloid Interf. Sci. 354 (2011) 607610. [30] V. Subramanian, E. E. Wolf , P. V. Kamat, J. Am. Chem. Soc. 126 (2004), 49434950. [31] M. Jakob, H. Levanon, P. V. Kamat, Nano Lett. 3 (2003), 353-358. [32] C. B. Liu, L. L. Wang, Y. H. Tang, S. L. Luo, Y. T. Liu, S. Q. Zhang, Y. X. Zeng, Y. Z. Xu, Appl. Catal. B: Environ. 164 (2015) 1-9. [33] H. Zhang, X. J. Lv, Y. M. Li, Y. M. Wang, J. H. Li, ACS nano 4 (2009), 380-386. [34] J. X. Low, B. Z. Dai, T. Tong, C. J. Jiang, J. G. Yu, Adv. Mater. 5 (2018), 1802981 (1-5) [35] B. Ershler, Faraday Soc.1 (1947) 269-277. [36] T. F. Jiang, T. F. Xie, L. P. Chen, Z. W. Fu, D. J. Wang, Nanoscale 5 (2013), 29382944. [37] J. Zhao, P. Zhang, J. Fan, J. Hu, G. Shao, Appl. Surf. Sci. 430(2018) 466-474. [38] J. Zhao, P. Zhang, Z. Wang, S. Zhang, H. Gao, J. Hu, G. Shao, Scientific Reports 7(2017) 16116.

[39] S. Zhang, W. Xiao, Y. Zhang, K. Liu, X. Zhang, J. Zhao, Z. Wang, P. Zhang, G. Shao, J. Mater. Chem. A 6(2018) 22555-22565. [40] P. Zhang , X. Li, C. Shao, Y. Liu, J. Mater. Chem. A 3(2015) 3281-3284.

Fig. 1. XRD patterns of pure TiO2 nanofibers, MoS2 nanosheets and the TiO2/MoS2 composite.

Fig. 2. (A) SEM images of pure TiO2 nanofibers and (B) TiO2/MoS2 composite nanofibers.

Fig. 3. (A) TEM image, (B) HRTEM image and (C) SAED pattern of TiO2/MoS2 heterostructures

Fig. 4. (A) Photocatalytic hydrogen production activity of different samples and (B) the recycling test of the 60% wt. sample.

Fig. 5. The N2 adsorption−desorption isotherms and corresponding pore-size distribution curves of the samples (A, B) before and (C, D) after reaction.

Table 1. BET data of the materials Status of MoS2/TiO2

BET Surface Area (m2/g)

before reaction after reaction

55.79 85.61

Fig.6. (A) The changes of ion concentration of Mo4+ and Ti4+ after a period of time reaction, the measurement curves of (B) Mo4+ and (C) Ti4+.

Fig. 7. (A) Photocurrents and (B) EIS of the as-prepared samples.

Fig.8. (A) UV-vis absorption spectroscopy of the as-prepared samples; (B) UPS spectra of pure TiO2 nanofibers and TiO2/MoS2 heterostructure with 60% MoS2.

Fig.9. UPS spectra of pure TiO2 and MoS2.

Fig.10. The band structures of pure TiO2 and MoS2.

Scheme 1. Mechanism graph of electronic transfer in TiO2/MoS2 heterostructure.

Fig. 11. High-resolution XPS for Ti 2p (A) and Mo 3d (B) of TiO2/MoS2 in the dark or under Ultraviolet light LED irradiation(UV-TiO2/MoS2).

Fig.12. The principle of XPS (A) and ISI-XPS (B); electrons transfer through the interface of TiO2/MoS2 (C and D).

Graphical abstract We describe a general route to arrive at direct evidence based on the UPS and ISI-XPS characterization to confirm the band bending and photogenerated electrons transfer between the TiO2/MoS2.

Highlights 

Clean fuel production using solar energy.



One dimensional TiO2/MoS2 heterostructure are prepared.

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The mechanism of charge-carries separation and transfer is proved by UPS and ISIXPS.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: