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Investigation of Photoelectrochemical Properties of Porous W\WO3 Nanoplates Sensitized with Transition Metal Dichalcogenides Nanolayers for Water Splitting

Mojaddami, Majdoddin | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53608 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Simchi, Abdolreza
  7. Abstract:
  8. Nowadays, storing solar renewable energy in hydrogen (as a clean fuel) is an attractive method for substitution of environmentally pollutant energy resources (fossil fuels) which is applicable through water splitting with the use of semiconductors. Among metal oxide photoanodes, WO3 as a moderate band gap semiconductor with high photo-stability, environmental compatibility, and low cost has attracted significant attention. However, photoelectron conversion efficiency of WO3 is still limited due to poor oxygen evolution reaction kinetics and charge carrier recombination. Herein, two strategies of nanostructuring and compositing with cocatalysts were applied to envisage with these limitations. The electrodes were prepared by electrochemical etching of Cu networks followed by hydrothermal growth of WO3 nanoflakes. The optimized photoelectrode exhibited an impressive current density of 4.36 mA.cm-2 comprising a remarkable photocurrent of 1.71 mA.cm-2 at 1.23 V vs. RHE under 100 mW.cm-2 simulated sunlight. This achievement is amongst the highest reported photocurrents for WO3 photoelectrodes with tungsten substrate reported so far. Impedance and Mott-Schottky analyses evidenced fast charge transfer, low recombination rate, and accelerated O2 detachment provided by optimum 3D porous WO3/W electrode. To improve the performance of this electrode transition metal dichalcogenides (TMDs) of WS2 and MoS2 were utilized as cocatalysts. Exfoliated WS2 and MoS2 nanosheets were deposited on the surface of WO3 nanoflakes by the electrophoretic technique to fabricate a type II heterojunction. It is the first report of this triple semiconductors composite. Moreover, synergistic effects of WS2 and MoS2 due to improved electrical conduction and the stream of charge carriers between these sulfides are demonstrated. The photoanode exhibits an impressive current density of 14.9 mA.cm-2 at 1.23 VRHE under 100 mW.cm-2 simulated sunlight. Deposition of MoS2 and WS2 sheets on WO3 nanoflakes improved the current density of porous WO3/W electrodes by about 340%, while only about 36% improvement is achieved by deposition of single TMD (MoS2/WO3 or WS2/WO3). The high stability of the photoanode in highly acidic media is shown. The synergetic effects of TMDs on active sites, charge transfer resistance, and charge transport properties are elaborated by electrochemical impedance spectroscopy and Mott-Schottky analyses. A mechanism proposed for improved activity of heterojunction electrode. The composite electrode of WO3 and boron carbonitride is also examined. Current density increased due to catalytic properties of boron carbonitride. However, its wide band gap decreased the photocurrent. Taking all the results together revealed that light absoption and buble detachment should be improved to enhance the electrode performance. Therefore, 3D printing was utilized to develop ordered structures. For the first time, arrangement, shape and angle of printed elements were examined. Concluding the results of this part and combining them with our previous electrodes, it is expected that 3D printing W-Cu, etching Cu and adding cocatalysts could provide a robust photoelectrode. Moreover, these findings can pave a new route to improve the performance of other renowned water splitting electrodes
  9. Keywords:
  10. Water Splitting ; Three Dimentional Printing ; Tungsten Oxide ; Transmission Metal Dichalcogenides (TMDs) ; Photoelectrode ; Boron Carbonitride ; Photoelectrochemical Water Splitting

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