Chemical degradation kinetics for two-dimensional materials in natural and biological environments – a data-driven review

Abstract

Two-dimensional (2D) materials are a large class of atomically thin, sheet-like solids, whose chemical stability is a critical issue for commercial devices. In addition, the chemical degradation processes that govern stability also determine 2D-material behaviour in biological systems and at the end of life, which have implications for environmental health and safety. There is a significant literature on the degradation of graphene-based materials, but the literature on other 2D materials is fragmentary, often semi-quantitative, and acquired under non-standardized conditions. The present critical review attempts to consolidate this fragmentary data on 2D materials “beyond graphene” in a rational form to allow quantitative meta-analyses and meaningful cross-material comparisons to identify trends and enable modelling. Data mined from 130 publications on 29 distinct chemical compositions were collected in a relational database and converted to 1st-order degradation rate constants. Under typical ambient conditions, either wet or dry, these rate constants vary over five orders of magnitude (1/k from 0.1 to 10 000 h). Through quantitative analysis of variance, 85% of this variation is associated with chemical composition, while other material variables do not show statistical significance in this dataset. The kinetic constants show a very poor correlation with free energy changes for the overall degradation reaction, but there are periodic table trends evident for the transition metal dichalcogenides (K_Te > K_Se > K_S). The atmospheric and aqueous-phase rate constants are cross correlated, allowing aqueous stability to be assessed from the more prevalent atmospheric data as an estimation tool. Introducing high reactivity species found in specialized environments (plasma, ozone, reactive oxygen species, reducing agents, enzymes), enhances degradation rates by 1–3 orders of magnitude, while other solutes or media components do not show large effects across the database. Finally, the results were used to classify 26 materials into three groups (biosoluble, biodegradable, persistent), which together with information on chemical degradation products can be used to accelerate safety assessment and to rationally select 2D materials for biomedical technologies.