Systematic, high-throughput characterisation of carbon nanomaterials
Carbon nanomaterials are a very large family of carbon allotropes – carbon in different physical forms with varying physical, chemical, thermal, electrical and magnetic properties. These materials include carbon nanotubes (CNTs), graphene, fullerenes, carbon quantum dots and nanodiamonds. Even within the sub-class of CNTs, there are single-walled, multi-walled, and single-double multi-walled CNTs. The walls themselves (formed by ‘rolling’ a graphene sheet) can be zigzag, chiral or armchair – names describing the relationship of the carbon atoms across multiple hexagonal rings. Accelerating application requires a better understanding of the physiochemical properties of specific carbon nanomaterials and how these are related to performance. With the support of the Marie Skłodowska-Curie Actions (MSCA) programme, the TACOMA project made a substantial contribution to filling this knowledge gap by developing and applying in situ soft X-ray absorption spectroscopy (XAS) and transition-edge sensor (TES) technology.
Oxygen functional groups and metal residues
Electron orbitals define the most likely locations of electrons around an atom’s nucleus. Hybrid orbitals result when two or more orbitals are ‘combined’ and carbon nanomaterials are characterised by sp2 or sp3 hybridisation. According to MSCA fellow Sami Sainio of the University of Oulu: “The properties and applicability of carbon nanomaterials are directly linked to their hybridisation and orbital orientation, and possibly to other parameters. Along with hybridisation and orbital orientation, we investigated surface oxygen functionalisation and metals.” TACOMA sought to identify the major oxygen-containing functional groups on carbon nanomaterials’ surfaces and to find out which metals were present in these materials. Furthermore, the project aimed to determine the contribution to the materials’ observed electrochemical performance.
TES technology operated with soft XAS
Sainio harnessed two cutting-edge technologies – XAS and TES. Soft XAS uses synchrotron radiation in a vacuum to excite electrons and obtain element-specific atomic information regarding local chemical and electronic structures. TES are thermometers operating at near-absolute-zero Kelvin that enable detection of single photons released during de-excitation. Together with colleagues at the Stanford Synchrotron Radiation Lightsource (SSRL) in the USA, Sainio employed industrial automation to better serve the user community given the remote requirements of the COVID-19 pandemic. This enhanced the throughput of SSRL beamlines 8-2 and 10-1 about 10 fold compared to manual operation, enabling Sainio and other users to systematically characterise the surface chemistry of a plethora of different materials.
Rigorous scientific method: a guaranteed path to success
“We now know that nearly all carbon nanomaterials are contaminated by some metal impurities and that the 13 different carbon nanomaterials studied show remarkably similar surface oxygen functionalisation. This suggests that the materials’ behaviour is a result of a combination of properties, and that oxygen functionalities might play a less important role than previously expected,” explains Sainio. Sainio underscored the necessity and value of rigorous scientific method. Much research is focused on surface properties and surface functionalisation given that surfaces form the interface with other materials. TACOMA’s rigorous, high-throughput approach has shown that surface functionalisation may not be the ‘Holy Grail’ when it comes to designing carbon nanomaterials with exotic properties for applications. Such strategic studies will enable scientists to characterise carbon nanomaterials in a standardised way, informing models to accurately predict the properties of yet uncharacterised materials. “Those interested can now look at our outcomes and make informed decisions on which of these materials, if any, is a potential candidate for their applications,” concludes Sainio.
Keywords
TACOMA, carbon nanomaterials, TES, XAS, CNTs, surface oxygen functionalisation, SSRL, synchrotron, soft X-ray absorption spectroscopy, transition-edge sensor
