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Plasmon-Enhanced Monolayer MoS2 for Efficient and Sustainable Photocatalytic Water Splitting Platform

  • Author(s): Ye, Peiyi
  • Advisor(s): Xie, Ya-Hong
  • et al.

H2 has long been known to be one of the highest energy density fuels. Fuel cell technology has been actively pursued as an environmentally friendly power source for automobiles. The promise of the technology is hindered by the lacking of a sustainable way of producing H2. Solar water-splitting via photo-electrochemical cells (PEC) is the most promising approach which converts the sustainable solar energy to the chemical energy store inside the H2 bonding. The materials that can be used in a PEC cell must fulfil a variety of thermodynamic and kinetic requirements to ensure good efficiency and durability. Single-layer MoS2 possess the corrected energy bandgap of 1.9 eV which allows for sufficient over-potential while still being capable of absorbing the majority of the solar spectrum. However, the limited optical absorbance from single-layer MoS2 prevent it from widely used. This shortcoming of the single-layer thickness of MoS2 is overcome by superimposing the MoS2 with plasmonic surface that serves to amplify the enhanced electromagnetic field where the MoS2 is located, allowing single-layer MoS2 to efficiently absorb Sun light thus producing H2.

In the first part of dissertation, a novel two-step chemical vapor deposition method is developed to consistently grow high coverage and exclusive single-layer MoS2. Up to 90% surface coverage and single-layer MoS2 is successfully fabricated. This is a crucial step to conduct the follow-up experiments.

In the second part of dissertation, A facile one-pot synthetic approach for synthesis hollow Au nanoframes structure is reported for the first time. A growth mechanism has been revealed that involves a synergistic function of Ag and Br ions. The presence of Ag+ lead to observed self-limiting of Au film thickness whereas Au {111} facets are preferentially attacked by the presence of Br- in the reaction ambient. Combined simulation and experimental studies show strong plasmonic effect that the hybrid platform made of graphene/Au nanoframes is capable of detecting analytes at concentration levels down to 10-9 M by using the surface-enhanced Raman spectroscopy (SERS) technique.

In the last part of the dissertation, the plasmonic effect generated from nano-structured metal surfaces is introduced to offset the small thickness from single-layer MoS2 and improve the overall absorption. It is demonstrated for the first time that using single-layer MoS2 as a well-defined nanospacer between Au-nanoparticles and Au-film (gap plasmon system). The field enhancement is known to be inversely proportional to this gap thickness. Hence reducing the gap thickness is important to achieve the highest possible field enhancement. �In this work, it is demonstrated for the first time that using single-layer MoS2 as a well-defined nanospacer between Au-nanoparticles (AuNPs) and Au-film, which could offer an extremely high localized electric field enhancement within the gap. The MoS2 Raman intensity with the Surface Enhanced Raman Scattering (SERS) enhancement factor (EF) up to 5x106 is obtained from the MoS2-Au gap plasmon system. A 5-fold increase in the photocurrent is obtained from the MoS�2-Au gap plasmon as the working electrode compared to that from bare MoS2 prepared under the same condition. Compared with individual metal nanoparticles commonly used to enhance thin-film photocatalytic process, gap-plasmon could theoretically produce 8 orders of magnitude higher SERS EF and precise control the hot spot location to superimpose where ultrathin materials locate thus using the higher incident energy available.

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