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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - 2DMAT4ENERGY (Stimuli-Responsive Two-Dimensional Materials for Renewable Energy)

Teaser

Summary

This research is 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 â€˜demand vs. supplyâ€™ mismatch and prevent undesirable energy losses.

We aim 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 but 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 key 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.

Work performed

During my initial 2-year secondment period outside the EU, we exploited our experience in 2D materialsâ€™ 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 my fortuitous discovery made while at Queenâ€™s University Belfast (UK) that the naturally large affinity between gold and chalcogenide elements leads to an unprecedented, nearly 100% exfoliation yield of MoS2 and other TMDCs. This strong, but non-chemical interaction leads to centimetre-sized MoS2 crystals, which maintained their chemical identity but â€œborrowâ€ some of the electronic properties of the underlying Au substrate. This leads to a unique electrochemical behaviour, altering the MoS2 from semiconducting to metallic and in turn passivates the surface chemistry of the Au electrode. These recently published findings (VelickÃ½ et al. ACS Nano 12, 2018, 10463-10472) provide important 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. We currently explore other routes to circumvent these issues, with the electron-beam lithography being the most promising, albeit the most expensive, method.

3. Electron tunnelling across a graphite/hexagonal boron nitride(hBN)/liquid heterostructure. Here we exploite 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. We found that the tunnelling current and corresponding electrochemical kinetics decay exponentially with the hBN thickness, as expected from other studies. However, the tunnelling behaviour exhibits anomalies, which are neatly explained by a previous theoretical prediction based on the Marcus-Hush theory of electron transfer, which has not yet been verified until now. Such a unique match between a theory and experiment is of significant implications for the scientific community across several fields, and prompts application of hBN to explore electrochemical switching between different reaction mechanisms or long-range electron transfer. Our manuscript is currently being revised and re-submitted to ACS Nano after an initial review. The manuscript preprint can be accessed from ChemRxiv (https://chemrxiv.org/articles/e/9275135).

Final results

The following objectives are currently under way, all of which are extending beyond the current state-of-the-art of this scientific discipline. We will continue to pursue these objectives for the remaining duration of the project.

1. Assessment of the electrochemical performance of TMDC/Au heterostructures as a function of TMDC identity and the number of layers. To boost the experimental capabilities in this project, we collaborate with Dr Andy Wain at NPL and Prof Kim McKelvey at Trinity College Dublin (Ireland), who are experts in the state-of-the-art scanning electrochemical probe microscopy techniques with a spatial resolution necessary to study the structure-activity relationships in 2D materials.

2. Exploiting the same materials, I have been working with Dr. Andrey Krayev at Horiba Scientific (USA), an expert in tip-enhanced Raman spectroscopy and photoluminescence measurement, allowing spatial resolution of tens of nanometres. We have recently observed a unique behaviour in the Raman spectra of MoS2 on Au, confirming the strong interaction between the two materials and leading to some unexpected spectral features. We believe that we are likely make substantial contribution toward fundamental understanding of the optical properties of 2D materials at the nanoscale.

3. Finally, I will continue working toward one of the most ambitious objectives of this project: the development of in-situ spectromechanoelectrochemical method for simultaneous application of mechanical strain and measurement of optical/electrochemical properties of 2D materials. This project is carried out in collaboration with Dr. Otakar Frank at the Czech Academy of Sciences (Czech Republic), a leading expert on Raman spectroscopy of 2D materials.

These efforts will have a significant impact on the field of 2D materials and electrochemistry. Our findings have already established in-depth understanding of 2D materialsâ€™ 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.