Enhanced Visible Light Absorption of MoS(2-X)/TiO2/Ti Multilayered Heterojunction Photoelectrode
Authors: Cheng Chaoqun , Du Kang , Li Gang , Wang Kaiying 
Published: February 2020

Design of efficient heterojunction photoelectrode for solar driven photoelectrochemical water splitting is one of the foremost challenges in the development of a solar hydrogen production system [1]. Representatively, n-type titanium dioxide (TiO2) has been extensively investigated as a promising photoelectrode material for photoexcited hydrogen from water. However, most attempts on modifying this material for improving the utilization of light in the visible region of the solar spectrum have met with limited performance [2]. As Molybdenum Disulfide (MoS2) has been examined potentially as efficient hydrogen evolution catalyst containing no precious metals [3]. Herein, we fabricated a novel multilayered heterojunction photoelectrode MoS(2-x)/TiO2/Ti by effortlessly spraying liquid nanoflake MoS2 on the surface of TiO2substrate after thermal oxidation of Ti foil, following thermally textualizing process, and then investigated the visible light characteristics.
The whole fabrication process of the samples, as noted respectively a, b, c and d in the fig.1(1), consists of three steps, thermal oxidation of clean Ti foils in high temperature tube furnace at 500 ℃ for 3 hours creating TiO2 film layer on the surface, then spraying liquid nanoflake MoS2 (commercially available) on the surface of TiO2 (top and back side) developing layers of MoS2 film as top covering the TiO2 after 24 hours drying in a drying oven, and subsequently thermal annealing in the tube at 500 ℃ for 1 hour in forming gas environment (N2/H2 = 95%/5%). The elemental and quantity analysis of elemental dispersive X-ray spectrum (EDS, fig.1(2)) on surficial area shows expected TiO2 and MoS2-x existing. The scanning electron microscope (SEM) images for as-annealed sample d are shown in the fig.1 (3) and (4) with different resolutions. After annealing in the forming gas atmosphere, the surface shows signs of dissociation thereby increasing the defects density, including edge sites. This is because during the thermal annealing in hydrogen environment, partly sulfur atoms in MoS2 would be removed as forming H2S gas resulting into the creation of vacancies and active edges, while the remained excess Mo atoms form metal clusters on the film [3]. Finally, for comparison, the absorption spectrum of four samples is measured as displayed in the fig.2 in detail. The processed samples all show obvious absorption enhancement in the wavelength from 400 to 800 nm. Specially, the annealed sample d maintains good feature of visible light adsorption feature, indicating the formation of multilayered heterojunction without destroying the composite function of TiO2 as absorbing light energy material and MoS2as catalyst excessively.
In conclusion, the MoS(2-x)/TiO2/Ti multilayered heterojunction photoelectrode with enhanced visible light absorption are fabricated through a facile method, which the multilayered heterojunction present stable structure and composite function. What’s more, this novel low-cost heterojunction photoelectrode posses potential application in the photoelectrochemical water splitting system.

Acknowledgments: The author gratefully acknowledges the financial support from Norwegian Research Council FRINATEK programme (231416/F20), and China Scholarship Council (CSC, Grant No.201506930002). The Research Council of Norway is acknowledged for the support to the NorFab, project number 245963/F50.

References

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Figures

Figure 1