Analytische Chemie
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The implementation of new 2-D materials based technologies in production processes requires the development of quality management tools. These have to be underpinned by appropriate measurements. Consequently there is a need for the development of the metrology for measurement methods, the development of certified reference materials (CRM) and finally standardization. This chain represents the ideal way to practically useful standards. The presentation will give an overview on the main players in the field and summarize the recent status of activities. At the highest level the metrology of chemical characterization of 2D materials is in the scope of the International Meter Convention, specifically the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology (CCQM). Pre-standardization is an activity field of the Versailles Project on Advanced Materials and Standards (VAMAS). Standardization is mainly addressed by addressed ISO Technical Committees. A summary on available CRMs relevant to the characterization of nano materials has been prepared by BAM. Examples from the work of BAM’s Division 6.1 “Surface Analysis and Interfacial Chemistry” showcasing the characterization of chemically modified graphene surfaces are given and specific needs for the development of metrology are addressed.
In the discussion the audience is invited to define specific needs which will be streamlined to the respective bodies!
Graphene is a two-dimensional carbon network with unique properties, including high mechanical stiffness, strength, and elasticity, outstanding electrical and thermal conductivity, and many others. Despite these advantages, its low solubility, poor reactivity and the limited accessibility of a well-defined basal plane are major challenges for applications. An ideal method to overcome these problems is the covalent attachment of functional molecules to its surface which enables further reactive modifications for specific applications. There is a number of different technologies for surface functionalization of graphene and related CNT materials. However, to get control on the functionalization process and to optimize the performance of the modified surfaces analytical tools for surface chemical characterization are required. X-ray absorption (NEXAFS) and photoelectron spectroscopy (XPS) have been identified to be rather powerful here. Specifically NEXAFS spectroscopy underpinned by quantum chemical spectrum simulations is unique in a way to address changes of aromaticity and defect formation at the graphene surface during functionalization.
For relevant surface modification technologies, we present examples on how NEXAFS and XPS can perform well. All presented modifications aim on the production of platforms for defined functional 2D nanomaterials, as for example multifunctional hybrid architectures. In detail, we investigated:
• Graphene and carbon nanotube functionalized by a Vacuum-Ultraviolet (VUV) induced photochemical process in NH3 or O2 atmospheres in order to introduce amino or hydroxy functionalities, respectively.
• Br bonding on r.f. cw low pressure plasma brominated graphite surfaces by using Br2 and bromoform as plasma gases.
• A wet chemical method for covalent functionalization of graphene sheets by a one-pot nitrene [2+1] cycloaddition reaction under mild conditions. Here a reaction between 2,4,6-trichloro-1,3,5-triazine and sodium azide with thermally reduced graphene oxide (TRGO) results in defined dichlorotriazine-functionalized graphene sheets.
Graphene is a two-dimensional carbon network with unique properties. However, its low solubility, poor reactivity and the limited accessibility of a well-defined basal plane are major challenges for applications. An ideal method to overcome these problems is the covalent attachment of functional molecules to its surface which enable further reactive modifications for specific applications. There are several technologies for surface functionalization of graphene and related CNT materials. To get control on the functionalization process and to optimize the performance of the modified surfaces analytical tools for surface chemical characterization are required. X-ray absorption (NEXAFS) and photoelectron spectroscopy (XPS) have been identified to be rather powerful here [1-3]. Specifically, NEXAFS spectroscopy underpinned by quantum chemical spectrum simulations [4] is unique in a way to address changes of aromaticity and defect formation at the graphene surface during functionalization.
For relevant surface modification technologies, we present examples on how NEXAFS and XPS can do a good job. All presented modifications aim on the production of platforms for defined functional 2D nanomaterials, as for example multifunctional hybrid architectures. In detail, we investigated:
• A wet chemical method for covalent functionalization of graphene sheets by a one-pot nitrene [2+1] cycloaddition reaction under mild conditions. Here a reaction between 2,4,6-trichloro-1,3,5-triazine and sodium azide with thermally reduced graphene oxide (TRGO) results in defined dichlorotriazine-functionalized graphene sheets.
• Graphene and carbon nanotube functionalized by Vacuum-Ultraviolet (VUV) induced photochemical or r.f. cw low pressure plasma processes to introduce amino, hydroxy or brominated functionalities.
To underpin finger-print information delivered by C K-edge NEXAFS we studied the effects of selected point and line defects as well as chemical modifications for a single graphene layer model by density functional theory based spectrum simulations.
References
[1] P.-L. Girard-Lauriault et al., Appl. Surf. Sci., 258 2012 8448-8454, DOI: 10.1016/j.apsusc.2012.03.012
[2] A. Lippitz et al., Surf. Sci., 611 2013 L1-L7, DOI: 10.1016/j.susc.2013.01.020
[3] A. Faghani et al., Angew. Chemie (International ed.), 56 2017 2675-2679, DOI:10.1002/anie.201612422
[4] C. Ehlert, et al., Phys.Chem.Chem.Phys., 16 2014 14083-14095, DOI: 10.1039/c4cp01106f
Since its discovery, graphene has got growing attention in the industrial and application research due to its unique properties . However, graphene has not been yet implemented into the industrial market, in particularly due to the difficulty of properly characterizing this challenging material. As most of other nanomaterials, graphene’s properties are closely linked to its chemical and structural properties, such as number of layers, flake thickness, degree of functionalisation and C/O ratio. For the commercialization, suitable procedures for the measurement and characterization of the ultrathin flakes, of lateral dimensions in the range from µm to tens of µm, are essential.Surface chemical methods, especially XPS, have an outstanding role of providing chemical information on the composition. Thereby, one well-known problem for surface analytical methods is the influence of contamination on the composition as in the case of adventitious carbon. The differentiation between carbon originated from the contamination or from the graphene sample itself is often not obvious, which can lead to altered results in the determination of the composition. To overcome this problem, Hard Energy X-ray Photoelectron Spectroscopy (HAXPES) offers new possibilities due to its higher information depth. Therefore, XPS measurement obtained with Al Kα radiation (E = 1486. 6 eV) were compared with analyses performed with a Cr Kα (E = 5414. 8 eV) excitation on functionalized graphene samples. Differences are discussed in terms of potential carbon contamination, but also of oxygen on the composition of the samples. Measurements are performed on O-, N- and F-functionalized graphene. Different preparation procedures (powder, pellet, drop cast from liquid suspension) will be also discussed, correlation of the results with the flakes morphology as well as their validation with other independent methods are in progress.