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Stimuli-Responsive Two-Dimensional Materials for Renewable Energy

Periodic Reporting for period 2 - 2DMAT4ENERGY (Stimuli-Responsive Two-Dimensional Materials for Renewable Energy)

Reporting period: 2019-09-18 to 2020-09-17

This research has been motivated by the ever-increasing need for reliable sources of renewable energy. Renewable energy sources provide clean and inexpensive electric energy and directly address the irreversible depletion of fossil fuels. However, their intermittent nature due to the day-night, tidal, and weather cycles, does not provide the desired, constant supply of electricity. Energy conversion and storage technologies, whose majority relies on electrochemical interfaces, balance this ‘demand vs. supply’ mismatch and prevent undesirable energy losses.

We aimed to engineer van der Waals (vdW) heterostructures of two-dimensional (2D) materials with tuneable electrochemical response for exploitation in renewable energy applications. These heterostructures, which are constructed by stacking 2D crystals on top of each other, have been attracting increasing attention in solid-state physics, optoelectronics, and photonics. Still, their full potential in electrochemical applications such as energy storage, conversion, and sensing remains completely unexploited. We will attempt to control their electrochemical response by external stimuli, including electric field, strain, and illumination. To achieve these objectives, we first focused on developing a solid understanding of the unexplored fundamental electrochemical properties of 2D materials and their dependence on these stimuli. Building on this knowledge, engineering of tuneable vdW heterostructures for electrochemical applications will follow.

Further general information about research on graphene and 2D materials at the University of Manchester can be found at the website accessible via the link below.
During my initial two-year secondment period outside the EU, we exploited our experience in 2D materials’ processing and characterisation and the world-class nanofabrication and metrology facilities and scientific expertise at Cornell University (USA). The main results achieved so far include:

1. Fabrication and characterisation of record-size transition metal dichalcogenides (TMDCs) on gold substrates. In this work, we exploited the large affinity between gold and chalcogenide elements, which leads to an unprecedented, nearly 100% exfoliation yield of MoS2 and other TMDCs. This strong, but vdW interaction leads to centimetre-sized MoS2 crystals, which maintained their chemical identity but “borrow” some of the electronic properties of the underlying Au substrate. These heterostructures possess a unique electrochemical behaviour, altering the MoS2 from semiconducting to metallic and in turn passivates the surface chemistry of the Au electrode. These published findings (Velický et al. ACS Nano 12, 2018, 10463-10472) provide essential guidance for the production of macroscopic-size TMDCs, and have important implications for many research areas, such as electrode modification, photovoltaics, and photocatalysis.

2. Development of suitable micro-/nano-fabrication of graphene electrodes for the potential-dependent measurement of the electron transfer kinetics. This objective was the largest stumbling block of this project so far. We have found that the photolithographic techniques, commonly employed in the semiconductor processing industry, are unsuitable for electrochemistry of 2D materials, due to the surface contamination left as a result of its contact with polymers. These polymers and/or their residues are hard to remove and adversely affect the electrochemical processes, which occur exclusively at the 2D surface and are therefore dominated by its chemistry and cleanliness. Our results indicate that these issues could be circumvented by employing electron-beam lithography as an alternative fabrication method.

3. Electron tunnelling across a graphite/hexagonal boron nitride(hBN)/liquid heterostructure. Here we exploited the precise control of the tunnelling distance using different layers of insulating hBN and electron-beam lithography to fabricate micro-sized electrodes of a defined geometry (Velický et al. ACS Nano 14, 2020, 993-1002). We found that the tunnelling current and corresponding electrochemical kinetics decay exhibits anomalies, which are neatly explained by a so-far unverified theoretical prediction from the Marcus-Hush theory of electron transfer. Such a unique match between theory and experiment is of significant implications for the scientific community across several fields, and prompts the application of hBN to explore electrochemical switching between different reaction mechanisms or long-range electron transfer.

During my final year period in the EU, at the University of Manchester (UK), I focused on the most promising research facets of the fellowship so far, which resulted in the two primary outcomes:

4. A spectroscopic study of the unique interaction between MoS2 and Au revealed strain and charge doping fingerprints in the Raman spectra of monolayer MoS2 on Au (Velický et al. J. Phys. Chem. Lett. 11, 2020, 6112-6118). We found that the strong interaction between MoS2 and Au is limited only to the MoS2 layer closest to Au – all the other MoS2 layers interact only weakly with the Au substrate. This behaviour, in turn, facilitates the preferential exfoliation of 1L MoS2 of macroscopic lateral dimensions. High-resolution tip-enhanced Raman spectroscopy confirmed that 1L MoS2 experiences both strain and charge doping induced by the Au, which are however spatially heterogeneous at the nanoscale, and reflect the nanoscale non-conformity between the two materials.

5. A study of MoS2 on a range of other metals, in which we found that Au outperforms all the other metals in their ability to exfoliate macroscale monolayer MoS2 with mm to cm lateral dimensions (Velický et al. Adv. Mater. Interfaces, 2020, 2001324, in press). This behaviour is due to the ability of Au to resist oxidation and the existence of the large strain-induced in MoS2, arising from the Au/MoS2 lattice mismatch.
All the scientific outputs, described above, progressed the 2D materials research beyond the current state-of-the-art. I believe that our results will have a significant impact on the field of 2D materials and electrochemistry. Our findings have already established an in-depth understanding of 2D materials’ electrochemistry and are paving the way toward tuneable electrochemical devices using 2D materials either individually or in vdW heterostructures. I envisage that a careful evaluation of these results by professionals in various industries can potentially lead to commercially viable applications, in areas such as energy storage, sensing, or electrocatalysis.
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