Materials Today Chemistry
Colloidal chemistry as a guide to design intended dispersions of carbon nanomaterials☆
Graphical abstract
Introduction
Carbon-based inks are composed of carbon materials dispersed in liquid media, and they have been largely used for centuries in printing processes since Gutenberg's prints of the Bible in Mainz using carbon black inks [1] or in artistic works such as Roman frescos, Egyptian murals, and Japanese paintings, as shown in Fig. 1A [[2], [3], [4]], but their origin goes back to prehistoric times when the mankind was using charcoal inks (or only charcoal) to draw on caves walls [5,6]. Carbon inks composed of dispersed nanomaterials have gained importance since the reports by Coleman and collaborators using organic liquids to disperse carbon nanotubes [7] or to exfoliate pristine graphite [8] into monolayer and few-layer graphene and since the reports of graphene oxide (GO) dispersions in water [9] or polar solvents [10].
Carbon-based dispersions are composed of insoluble materials dispersed in a solvent or solution, and the majority are formed by sp2-hybridized carbon allotropes, such as graphite, graphene, carbon nanotubes, fullerenes, or carbon black that are electrical conductors [12,13] therefore, these dispersions can be classified as conductive inks. This type of inks can also be formed by other conductive materials such as conjugated polymers or metallic nanoparticles dispersed in liquids or solutions, and many works have investigated properties and applications of such systems aiming at flexible, organic, and/or printed electronic devices [[14], [15], [16], [17], [18], [19]].
There are a large number of works in the literature about different preparation routes and applications for dispersions of carbon nanomaterials [[20], [21], [22], [23], [24], [25], [26]]; however, recent reports have indicated that still there is room for new knowledge and applications for those systems, for example, (1) fabrication of field–effect transistor with an organic dispersion of semiconductor-enriched single-walled carbon nanotubes (SWCNT) showed in B [11], (2) graphite exfoliation using lithium and liquid ammonia into few-layer graphene that has properties not present in monolayer graphene or in bulk graphite [27], (3) fabrication of flexible electrodes based on conductive carbon inks derived from wood biomass [28], (4) inkjet printing of carbon-based dispersions electrostatically stabilized without insulating passivators [29], (5) preparation of turbostratic graphite from diverse sources by flash heating and dispersion of these materials in aqueous or organic media [30], and (6) fabrication of hydrovoltaic devices with carbon black inks [31,32], among many others recent reports that highlight the great potential of carbon-based colloidal systems.
Dispersions and inks are examples of colloids, and such systems have been investigated by almost two centuries since the pioneer work of Thomas Graham in the XIX century [33]. There are many theoretical and experimental studies in this field, including classical textbooks [[33], [34], [35], [36], [37]], and a deeper understanding of colloidal chemistry can guide the resolution of some issues in the field of carbon dispersions, such as the improvement of colloidal stability, mass concentration, exfoliation yield, and minimal use of additives, such as passivant agents or toxic solvents, as recent works have demonstrated [27,[38], [39], [40], [41], [42]].
There are many works on the literature concerning carbon-based conductive inks, presenting detailed preparation procedures, complex formulations with improved rheological properties, and different applications [[14], [15], [16], [17], [18],[43], [44], [45], [46]]; however, they will not be treated here. In this review, we focus on simple carbon colloids formed by one dispersed material (graphene or carbon nanotubes) and one dispersion medium (one solvent or solution) to highlight some basic colloidal properties relevant for carbon-based dispersions such as van der Waals (vdW) forces. We present some strategies used to overcome vdW interactions in carbon materials, but interpreting them in terms of colloidal interactions, and we show how the knowledge of some colloidal properties such as electrostatic and solvation interactions can guide the preparation and the improvement of carbon-based dispersions.
Section snippets
Carbon materials as vdW solids
Carbon allotropes are solids formed by carbon atoms bonded by covalent bonds. Among them, some allotropes formed by sp2-hybridized carbon atoms are special because they are formed by independent nanostructures, and these building units are stable as their respective bulk solids. This means that carbon nanostructures can exist independently from each other, in contrast to metallic nanoparticles for example, which are not stable without strong stabilizers attached on their surface, and without
Overcoming vdW forces in carbon materials
Eq. (1) is associated with vdW interactions for dispersed particles interacting across a liquid medium. However, it can be used to describe vdW interactions of carbon nanostructures in the solid state as a first approximation, keeping clearly that it is more complex and depends on the other factors (e.g. electronic properties for example, vdW interactions for metallic and semiconductor SWCNT can be different), but this same approach has been used for other authors to describe graphite
Outlooks
Although stabilization produced by adsorbed polymers and surfactants in general (such as amphiphilic molecules) or by functional groups are efficient in terms of maintaining particles dispersed for long periods and producing highly concentrated dispersions, due to the lyophilic nature of such colloids, especially with adsorbed polymers, these types of stabilization decrease the electrical contact between carbon materials after deposition because passivating molecules are insulators and
Concluding remarks
Preparation routes of carbon dispersions were interpreted in terms of colloidal interactions, which allowed to model dispersion and exfoliation procedures as means to overcome vdW interactions in carbon materials by decreasing their magnitude (e.g. intercalation procedures) and introducing opposite interactions to them (e.g. introducing electrostatic and solvation interactions). Procedures of liquid exfoliation applied for pristine carbon materials in organic media were highlighted because of
Authors’ contribution
J.P.V.D. has conceptualized the review in terms of subject and structure. Both authors have written and revised the paper.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge National Institute of Science & Technology in Bioanalytics (INCTBio, INCT-MCTI/CNPq/CAPES/FAPs nº 16/2014), Coordination for the Improvement of Higher Education Personnel (CAPES) (post-doc fellowship (J.P.V.D.) process: 88887.478317/2020–00), National Council for Scientific and Technological Development (CNPq) (Projects CNPq: 434303/2016-0 and MCTIC/CNPq/FNDCT/MS/SCTIE/Decit Nº 07/2020, and for the post-doc fellowship (J.P.V.D.) process: 301600/2021-0), and São Paulo
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This review is dedicated to the victims of the corona virus, and it is a protest against the destruction of Brazilian environment.