Spectroscopic and Microscopic Characterisation of Liquid Phase Exfoliated 2D Materials
Citation:
Griffin, Aideen, Spectroscopic and Microscopic Characterisation of Liquid Phase Exfoliated 2D Materials, Trinity College Dublin.School of Physics, 2021Download Item:
Abstract:
Since the experimental isolation of graphene nearly two decades ago, the research into two-dimensional materials has accelerated at a tremendous rate across a vast range of scientific and engineering fields. Liquid phase exfoliation, a common production method, has progressed in recent years with studies reported on a variety of materials including graphene, transition metal dichalcogenides and layered double hydroxides. While the collection of liquid exfoliated crystals is substantial and growing year on year, the spectroscopy of hexagonal-boron nitride (h-BN) nanosheets, a widely used insulator, and the effect of stabilizers on the exfoliation process have yet to be explored in detail.
Spectroscopy of two dimensional h-BN, unlike many other nanomaterials, contains
relatively little information due to the fact that both the absorption and Raman spectra consist of only one feature each. By size-selecting liquid exfoliated h-BN nanosheets the dependence of h-BN optical spectra on nanosheet dimension is analysed. After decoupling the light to separate the absorbance from the scattering, the size-dependence
is removed apart from a well-defined variation in the energy of peak absorbance with
nanosheet thickness. Furthermore, while the position of the h-BN Raman G band remains
constant with nanosheet dimension, the linewidth appears to vary weakly with nanosheet thickness. Through the analysis of solvatochromatic effects in the liquid environment, the behaviour of the h-BN Raman G band width with nanosheet thickness is modelled. The size-dependent spectroscopic properties from both spectroscopic techniques can be used as metrics to estimate nanosheet thickness.
During exfoliation, surfactants are commonly used to stabilize nanosheets against
reaggregation. Using WS2 as a model system, the effect of varying surfactant type and
concentration on the yield and dimensions of exfoliated nanosheets is explored. This
study shows that for ionic surfactants, the mass of nanosheets decreases sharply past ~10 mM, regardless of surfactant. Very similar dependences were observed for both nanosheet length and thickness. In contrast to previous studies, this data implies that the optimum surfactant concentration is not linked to the critical micelle concentration. In addition, surfactant concentrations as low as 0.07 mM yielded stable nanosheet dispersions with zeta potentials above 40 mV. By decoupling the exfoliation and stabilization effects of the surfactant, it is the (de)stabilization process, rather than the exfoliation process, that is shown to link nanosheet concentration, size and thickness to surfactant concentration.
In addition to frequently used 2D materials such as h-BN and WS2, novel materials
have been predicted in recent databases. The need to improve the performance of lithium ion batteries is ever-increasing due to the current climate crisis and these previously unexplored materials might hold the key. In this work, liquid phase exfoliation was used to produce nanosheets of SnP3 and SiP, two novel materials with extremely high theoretical capacity of 1670 mAh/g and 3060 mAh/g respectively for Li storage. Nanosheet-nanotube composite thin films were formed for use as lithium storing anodes. Active mass-normalised capacities of 1657 mAh/g for SnP3 and 2654 mAh/g for SiP were measured, close to the theoretical values and state-of-the-art for 2D-based electrodes.
Apart from new materials, scalable techniques are important in the production of nanomaterials for applications. Bubble collapse, the mechanism behind the exfoliation of material in a sonicator, is replicated through boiling graphite and surfactant in an everyday kitchen kettle. Two techniques were demonstrated, the first a standard kitchen kettle producing very low yields of graphene after multiple cycles and the second a modification of the kettle, where a cooling coil maintains a rolling boil for long periods of time, increasing the number of bubbles. The increased heating time in the optimised setup resulted in a 10-fold increase in nanosheet concentration. EDX, XPS and Raman measurements revealed characteristic graphene signatures but also oxidation peaks and a high D band (Raman) indicative of a defective final product. Two regimes were proposed to exist in the kettle exfoliation. In the first regime, only partial exfoliation of graphite occurred resulting in thick graphitic platelets, mostly observed in a standard kettle. In the second regime large, thin nanosheets were exfoliated through the cleavage of neighbouring basal-plane defective sheets as a result of the increased energy input from the optimised set up.
Sponsor
Grant Number
European Commission
Graphene Flagship
Description:
APPROVED
Author: Griffin, Aideen
Advisor:
Coleman, JonathanPublisher:
Trinity College Dublin. School of Physics. Discipline of PhysicsType of material:
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Full text availableKeywords:
Liquid phase exfoliation, 2D Materials, Spectroscopy, Microscopy, Nanomaterials, h-BN, SnP3, surfactant, grapheneMetadata
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