Large-Area 2D-MoS₂/black-Si Heterostructure for Next-Generation Energy Storage

A global shift to clean, low-carbon technologies requires development of low-cost, highly efficient energy storage. In particular, the electrode-electrolyte interface is key in development of stable, high-efficiency batteries [1], with high surface-area electrodes in the nano-dimensions being the pinnacle [2].<br/>The usage of two-dimensional (2D) materials in energy storage is continuously expanding [3] due to efficient ion transport between the single-atomic layers, and superior, atomically smooth, surface-areas leading to increased ion adsorption and surface reactions [4]. Further surface-area enhancement of the 2D materials, using scalable industrial compatible processes, could revolutionise the energy storage sector.<br/>In this work, ultra-high surface area “2D-MoS₂/black-Si” heterostructures were developed by merging two large-area scalable processes. Nanoscale grass-like black-Si acts as a scaffold for direct 2D growth, avoiding costly and complex transfer processes, significantly enhancing the surface-area of 2D-MoS₂. These hybrids are expected to out-perform current state-of-the-art, when used for applications such as battery anodes [5], but also lend themselves to gas sensing, water splitting and solar cells [6,7].<br/>Our two large-area processes are top-down vertical silicon nanowires (SiNWs) and a 2-step MoS₂ growth. The former uses AgNO₃ and hydrofluoric (HF) acid solution, with a cyclic process on the Si surface, caused by nucleation from Ag nanoparticles and etching from HF. This results in grass-like nanowires, whose height is controlled by etch time. The Si surfaces boast ultra-low broadband reflectivity (&lt;1%), and are typically used in photovoltaics [8]. The 2-step MoS₂ technique firstly deposits MoO₃ one atomic layer at a time via atomic layer deposition (ALD), followed by an anneal in hydrogen disulphide, converting the layers to MoS₂. This offers significant advantage over other MoS₂/SiNW heterostructure work published to date, which use electrodeposition, hydrothermal, or conventional chemical vapour depositions techniques [9-12]. Not only is our technique large-area compatible, but we are able to fine-tune the number of MoS₂ layers via ALD cycles, giving us greater control and quality, whilst also using the MoO₃ layer to protect the SiNW from the sulphur exposure in the anneal step. The result is a high-quality MoS₂, with layer number optimisation, conformally coating large-areas of black-Si.<br/>The MoS₂/black-Si stacks were characterised using Raman Spectroscopy and photoluminescence (PL) for measuring MoS₂ layer number and quality, scanning electron microscopy (SEM) for assessing potential nanowire damage and transmission electron microscopy (TEM) for conformality and in-depth MoS₂ analysis. The use of an alumina interfacial layer, via ALD, was also investigated.<br/>We successfully grew large-area monolayer and multilayer MoS₂ directly onto black-Si, with no nanowire degradation. By integrating an alumina interfacial layer, we further increased the MoS₂ quality, with fewer defects. This demonstrates the compatibility of our technique for fabricating scalable high-quality 2D-MoS₂/black-Si heterostructures in a tuneable and highly controllable way. Our next step is to directly assess the impact of our 2D material on silicon nanowire electrodes for next generation batteries, by using Electrochemical Impedance Spectroscopy.<br/>References:<br/>[1] 10.3389/fchem.2020.00821 [2] 10.1016/j.ssi.2016.11.028 [3] 10.1016/j.cclet.2019.10.028 [4] 10.1002/aenm.201600025 [5] 10.1002/adfm.200601186 [6] 10.1021/acsami.8b08114 [7] 10.1039/C4CS00455H [8] 10.1016/j.solmat.2016.10.044 [9] 10.1039/C7RA13484C [10] 10.1007/s12274-014-0673-y [11] 10.1007/s12633-018-0014-y [12] 10.1016/j.matdes.2016.07.098<br/>Acknowledgements: This work was funded by University of Southampton Zepler Institute Stimulus Fund 2020/21, EPSRC (EP/R005303/1) and (EP/N00762X/1).