Co-catalyst MoS2-nanosheets/TiO2 nanotubes for the enhancement of photocatalytic hydrogen production

The MoS2 nanosheets/TiO2 nanotubes heterojunctions are designed for enhancing photocatalytic hydrogen production. The results of XRD, SEM and TEM imply that the heterojunctions are successfully prepared via electrostatic spinning technology and hydrothermal co-deposition method. Besides, the results of UV and PL display that the MoS2 nanosheets enhance the absorption of visible light, drive the transfer of photo-electro and promote the separation of photo-generated carries. Further, compared with the pure TiO2 nanotubes, the heterojunctions exhibit excellent photocatalytic hydrogen production enhancement of about ∼340.739μmol/g.h, which is about 7 times of pure TiO2 nanotubes.


Introduction
Now, with the sharply increasing of energy demand, the traditional non-renew energy, such as fossil fuel, could no longer meet the requirement. Therefore, the exploration of new renew energy has drew lots of attentions from different fields [1] . Due to properties of renew, sustainable and clean, solar energy is regarded as an effective solution for energy shortage, especially the photocatalytic spilling water hydrogen production. Among these previous works, the TiO2, including rutile phase and anatase phase [2][3][4] , has been researched for over 40 years as a typical photocatalytic material [5][6] . Especially the various of morphologies, such as nanoparticle, nanosheet or nanofibers, are regarded as the unique advantages for the photocatalytic performances, and prepared via different methods, such as hydrothermal method, electrospinning technology [7][8][9] , etc. There, the researches demonstrate that different morphologies and preparations could influence the photocatalytic hydrogen production observably. There, the TiO2 nanotubes are prepared via the electrospinning technology for enhancing the specific surface area.
On the other hand, due to the well transmission of electron, stability and easy synthesis, the MoS2 is usually regarded as the co-catalyst in photocatalytic filed [10][11] . According to previous reports, the MoS2, including different physical dimensions of 0D, 1D, and 2D, are widely employed in photocatalytic production hydrogen [12][13][14] .
In this work, we prepared the TiO2 nanotubes via electrospinning technique for increasing the specific surface area, and the 2D MoS2 co-catalyst via the hydrothermal co-deposition method for promoting the photo-induced electron transmission and separation [15] . Further, the results manifest that the photocatalytic hydrogen production of the MoS2 nanosheets/TiO2 nanotubes heterojunctions exhibit a remarkable enhancement, and the mechanism of the photocatalytic enhancement is studied.
All the chemicals are analytical grade and purchased from Aladdin Industrial Corporation (shanghai, China).

The fabrication of TiO2 nanotube
In the work, we employed the electrostatic spinning technology for preparing TiO2 nanotube. Firstly, 1g TBOT, 0.35g PVP were dissolved in mixed solution, including 1.95g ethyl alcohol, 1.55g acetic acid, 1g oil and continuously string for composing precursor solution. And then the precursor solution was spined in 16.5 kv and rate of 0.05g/min. After spinning, the received sample was dried in 80 o C for 10 hours and annealed in 500 o C for 2 hours.

The fabrication of MoS2 nanosheets/TiO2 nanotubes heterojunctions (remarked as TM)
Firstly, different weight of (NH4)6Mo7O24H2O and CH4N2S were dissolved in 35 ml of distilled water and stirred for 1 hour to form a homogeneous solution. And then, 100 mg TiO2 nanotubes were added in pre-solution and continuously stirring for 1 hour. Following, the mix solution was transferred to a Teflon-lined stainless steel autoclave at 200 o C for 20 h. after cooling down to room temperature, the MoS2 nanosheets/TiO2 nanotubes heterojunctions (TM) was collected, washed with deionized water and ethanol, and dried at 80 C for 6 h. Here, we marked the samples with different ratios as TM-15%, TM-20%, TM-25% (the rate of MoS2/TiO2 are 15%, 20%, 25% respectively). The preparation of pure MoS2 nanosheet is same as above.

Characterization
The micromorphology of MoS2 nanosheets/TiO2 nanotubes heterojunctions were obtained by the scanning electron microscopy (FESEM Hitachi S-4800), and transmission electron microscopy (TEM JEM-2100). The phase composition of the heterojunctions were detected by the X-ray diffraction (XRD, Bruker D8 Discover) using Cu Kα (λ = 1.5406 Å) radiation at 40 kV and 40 mA. The UV-vis diffuse reflectance spectra were recorded by the UV-vis spectrophotometer (Hitachi-U3900). The PL spectra were collected by a Hitachi F-7000 spectrofluorimeter at the excitation wavelength of 330 nm.

Photocatalytic activity
Photocatalytic activity of the MoS2 nanosheets/TiO2 nanotubes heterojunctions were conducted by the photocatalytic hydrogen production using gas chromatograph (Techcomp GC-7900). A 300W Xe arc lamp was employed as light source. For the photocatalytic experiment, 50 mg samples were suspended in 100 mL solution (80 mL water and 20 mL triethanolamine as the sacrifice agented). Before the experiment, high purity Ar was transported 20 min for eliminating the dissolved oxygen from the reactor, and the suspension sample was ultrasonic bath for 20 min. In the whole process, the reaction was conducted at room temperature.

Result and Discussion
Figure1. The XRD spectra of the MoS2 nanosheets/TiO2 nanotubes heterojunctions with different ratio.    Accroding to the TEM of the sample (Figure. 3a), it could be observed that the MoS2 nanosheets grow on TiO2 nanotubes. Further, as revealed in the HRTEM of TM-20% ( Figure.     The Figure. 6 is the photocatalytic H2 production performance of the sample with different ratio. As revealed, the pure TiO2 obtains a photocatalytic H2 production performance of about ~46.29 μmol/g·h. Further, with the increasing of the MoS2, the photocatalytic HER performance of the samples are82.817, 340.739 and134.867μmol/g·h, respectively. It's obvious that the MoS2 without obvious photocatalytic HER performance and the TM-20% exhibits the optimal photocatalytic HER performance, which indicate that the MoS2 nanosheets could act as the co-catalyst in this system and the suitable MoS2 nanosheets could improve the photocatalytic HER performance efficiently.

Conclusion
In this work, the electrostatic spinning technology and hydrothermal co-deposition method are employed for the preparation of the MoS2 nanosheets/TiO2 nanotubes heterojunctions. Further, the photocatalytic HER performance of the samples are evaluated, there, the TM-20% exhibits remarkable photocatalytic HER performance of about ~340.739μmol/g·h, that is about ~7 folds than the pure TiO2, which is ascribed to the co-catalyst MoS2 nanosheets obtains well electro transmission could restrained the recombination of photo-induced electron-hole pairs, and the nanotubes and nanosheets could increase the specific surface area for increasing the photocatalytic HER performance. Thus, the MoS2 nanosheets/TiO2 nanotubes heterojunctions exhibits an excellent HER performance and provide a new path for designing the novel energy materials.