Rolling up graphene oxide sheets through solvent-induced self-assembly in dispersions

Photomanipulatable colloidal clusters from the aggregation of azo molecular glass spheres

Colloidal clusters with well-controlled shapes have attracted extensive interest in the fields of materials, chemistry, physics, and biology. This communication reports the controllable fabrication of photoresponsive colloidal clusters with a wide range of adjustable sizes and complex architectures through an approach of microsphere formation and fusion. The clusters of colloidal spheres were obtained via adding ethanol dropwise into a tetrahydrofuran solution of an isosorbide-based azo compound (IAC-4). In the process, the colloidal spheres with soft and sticky shells were first formed in the dispersion. After stirring at an appropriate rate and time, clusters composed of controlled numbers of colloidal spheres were obtained. With increasing stirring time, the colloidal spheres in the clusters underwent fusion transforming into a range of structures with particular architectures. The structure formation, evolution and control were investigated by scanning electron microscopy (SEM) and dynamic light scattering (DLS). Under linearly polarized light irradiation, colloidal spheres in the clusters in the solid state were observed to be stretched along the direction of electric-field oscillation and these clusters were thus transformed into complex particles with unique morphologies. This exploration can lead to a new methodology to effectively fabricate colloidal clusters with complex architectures and shed new light on colloidal packing and organization under the driving forces of extrinsic energy input.

Shear annealing of a self-interacting sheet

2D materials such as graphene, graphene oxide, transition metal dichalcogenides, and 2D polymers have unique properties which allow them to be used in many applications from electronics to energy to biotechnology. Producing and applying these materials often involves solution processing. Previous computational studies have observed 2D sheets in shear and extensional flows, but have focused on steady flows, even though the dynamics of these materials might exhibit hysteresis. In this work, we study 2D sheets with short-ranged attractive interactions under time-varying shear. We show that, even with relatively simple protocols, the properties of sheet suspensions can be tuned.

Dynamics of a self-interacting sheet in shear flow

Solution processing of 2D materials such as graphene is important for applications thereof, yet a complete fundamental understanding of how 2D materials behave dynamically in solution is lacking. Here, we extend previous work by Silmore et al., Soft Matter, 2021, 17(18), 4707-4718 by adding short-ranged Lennard-Jones interactions to 2D sheets in shear flow. We find that the addition of these interactions allows for a rich landscape of conformations which depend on the balance between shear strength, bending rigidity, and interaction strength as well as the initial configuration of the sheet. We explore this conformational space and classify sheets as flat, tumbling, 1D folded, or 2D folded based on their conformational properties. We use kinetic and energetic arguments to explain why sheets adopt certain conformations within the folded regime. Finally, we calculate the stresslet and find that, even in the absence of thermal fluctuations and multiple sheet interactions, shear-thinning followed by shear-thickening behavior can appear.

Real-Time Monitoring of Adsorption-Induced Scrolling of Colloidal Inorganic Nanosheets

Inorganic nanotubes have attracted much attention due to their unique physicochemical properties. Nanotubes can be prepared by scrolling exfoliated nanosheets under ambient conditions. However, how the nanosheet scrolled in its colloidal state has not been experimentally visualized. In this paper, we directly observed the scrolling process of nanosheets upon adsorption of organic cations. Exfoliated flat nanosheets of niobate and clay in aqueous colloids were found to scroll by adding organic cations, such as exfoliation reagents, to the colloids. Employment of cationic stilbazolium dye enabled in situ observation of the dye adsorption and scrolling by optical microscopy based on changes in color and morphology of the nanosheets. The scrolling was promoted for nanosheets adsorbed with a stilbazolium dye with a longer alkyl chain, suggesting that the interaction between the hydrophobic parts of the dye cations is the driving force of the scrolling. This finding should encourage research on the formation of nanotubes from nanosheets and also provides important guidelines for the selection of appropriate exfoliation reagents when exfoliating nanosheets from layered crystals.

Porous Flow Field for Next-Generation Proton Exchange Membrane Fuel Cells: Materials, Characterization, Design, and Challenges

Porous flow fields distribute fuel and oxygen for the electrochemical reactions of proton exchange membrane (PEM) fuel cells through their pore network instead of conventional flow channels. This type of flow fields has showed great promises in enhancing reactant supply, heat removal, and electrical conduction, reducing the concentration performance loss and improving operational stability for fuel cells. This review presents the research and development progress of porous flow fields with insights for next-generation PEM fuel cells of high power density (e.g., ∼9.0 kW L^-1). Materials, fabrication methods, fundamentals, and fuel cell performance associated with porous flow fields are discussed in depth. Major challenges are described and explained, along with several future directions, including separated gas/liquid flow configurations, integrated porous structure, full morphology modeling, data-driven methods, and artificial intelligence-assisted design/optimization.

Water Quality Carbon Nanotube-Based Sensors Technological Barriers and Late Research Trends: A Bibliometric Analysis

Water is the key element that defines and individualizes our planet. Relative to body weight, water represents 70% or more for the majority of all species on Earth. Taking care of water as a whole is equivalent with taking care of the entire biodiversity or the whole of humanity itself. Water quality is becoming an increasingly important component of terrestrial life, hence intensive work is being conducted to develop sensors for detecting contaminants and assessing water quality and characteristics. Our bibliometric analysis is focused on water quality sensors based on carbon nanotubes and highlights the most important objectives and achievements of researchers in recent years. Due to important measurement characteristics such as sensitivity and selectivity, or low detection limit and linearity, up to the ability to measure water properties, including detection of heavy metal content or the presence of persistent organic compounds, carbon nanotube (CNT) sensors, taking advantage of available nanotechnologies, are becoming increasingly attractive. The conducted bibliometric analysis creates a visual, more efficient keystones mapping. CNT sensors can be integrated into an inexpensive real-time monitoring data acquisition system as an alternative for classical expensive and time-consuming offline water quality monitoring. The conducted bibliometric analysis reveals all connections and maps all the results in this water quality CNT sensors research field and gives a perspective on the approached methods on this specific type of sensor. Finally, challenges related to integration of other trends that have been used and proven to be valuable in the field of other sensor types and capable to contribute to the development (and outlook) for future new configurations that will undoubtedly emerge are presented.

Emergence of carbon nanoscrolls from single walled carbon nanotubes: an oxidative route

Carbon nanoscrolls (CNS), a one dimensional (1D) helical form of carbon, have received enormous attention recently due to their unique structure, superior properties and potential applications. In this work, radial merging of HiPCO single walled nanotube (SWNT) bundles and emergence of CNS are reported following a reflux action involving wet oxidation, HCl washing and annealing at 900 °C. We observe macroscopic quantities of graphene sheets (GS) in the post-treated sample and beautiful manifestation of curling and folding of the GS into CNS. Here, a simple solution based oxidative route for successful merging and exfoliation of SWNT bundles and subsequent formation of CNS are demonstrated and discussed in view of Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies. Direct evidence of emergence of CNS from SWNTs via synthesis of GS through a simple oxidative method is reported for the first time.

Low reflectance of carbon nanotube and nanoscroll-based thin film coatings: a case study

Research on carbon material-based thin films with low light reflectance has received significant attention for the development of high absorber coatings for stray light control applications. Herein, we report a method for the successful fabrication of stable thin films comprised of carbon nanotubes (CNTs) and nanoscrolls (CNS) on an aluminium (Al) substrate, which exhibited low reflectance of the order of 2-3% in the visible and near-infrared (NIR) spectral bands. Changes in the structural and chemical composition of pristine single-walled carbon nanotube (SWCNT) samples were analyzed after each processing step. Spectroscopy, microscopy and microstructural studies demonstrated emergence of CNS and multi-walled carbon nanotubes (MWCNTs) due to the sequential chemical processing of the sample. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies revealed the formation of CNS via curling and folding of graphene sheets. Microstructural investigations including SEM and atomic force microscopy (AFM) confirmed the presence of microcavities and pores on the surface of the film. These cavities and pores significantly contribute to the observed low reflectance value of CNTs, CNS compound films by trapping the incident light. Fundamental space environmental simulation tests (SEST) were performed on the coated films, that showed promising results with reflectance values almost unaltered in the visible and NIR spectral bands, demonstrating the durability of these films as potential candidates to be used in extreme space environmental conditions. This study describes the preparation, characterization, and testing of blended CNT and CNS coatings for low-light scattering applications.

Fabrication of ultrasound-mediated tunable graphene oxide nanoscrolls

In this work, a cost-effective and facile method was adopted for the fabrication of graphene oxide nanoscrolls (GONS) by low frequency (20 kHz) ultrasonication with tunable dimensions. The graphene oxide (GO) was synthesized by modified Hummer's method using synthetic graphite as a base material. Later, GO suspension (0.05 g L^-1) were made using methanol as solvent and subjected to different ultrasonication conditions. It was found that GO sheets curls themselves into nanoscrolls by overcoming the energy barrier for scrolling with the help of bubble cavitation energy provided by ultrasonication. Also, the effect of ultrasonication power (100-150 W) for irradiation time (0.5-3 h) over the GONS dimensions were investigated. The spiral wounded GONS structures were shown using electron microscopy. Raman Spectroscopy, Thin-film X-Ray Diffraction, Energy Dispersive X-Ray, FT Infrared Spectroscopic analysis were also done to endorse GONS formation. Factors affecting GONS formation such as sonication power and solvent selection were studied as scrolling of GO sheets are strongly dependent on sonication parameters and solvent characteristics. It was found that GONS length varies inversely with irradiation time for identical power density. Also, a solvent with relatively large Hansen solubility parameter, lower dipole movement and less negative value of zeta potential support GONS formation of longer length. Raman analysis overlays the rapid oxygen-defect site cleavage mechanism. The obtained GONS unlocks further developments in various engineering applications like adsorption, drug delivery and filtration membrane.

Vortex fluidic mediated transformation of graphite into highly conducting graphene scrolls

Two-dimensional graphene has remarkable properties that are revolutionary in many applications. Scrolling monolayer graphene with precise tunability would create further potential for niche applications but this has proved challenging. We have now established the ability to fabricate monolayer graphene scrolls in high yield directly from graphite flakes under non-equilibrium conditions at room temperature in dynamic thin films of liquid. Using conductive atomic force microscopy we demonstrate that the graphene scrolls form highly conducting electrical contacts to highly oriented pyrolytic graphite (HOPG). These highly conducting graphite-graphene contacts are attractive for the fabrication of interconnects in microcircuits and align with the increasing interest in building all sp²-carbon circuits. Above a temperature of 450 °C the scrolls unravel into buckled graphene sheets, and this process is understood on a theoretical basis. These findings augur well for new applications, in particular for incorporating the scrolls into miniaturized electronic devices.

Tube-rolling and formation of mechanically robust micro-tubes in graphene oxide aqueous dispersions during shear flow

We showed that GO domains at low pH are under a tube-rolling motion with a vorticity alignment at low shear rates. Mechanically robust micro-tubes were formed during tube-rolling. The micro-tubes were highly bendable and exhibited excellent elastic recovery. There was no restacking of GO sheets to graphitic structures for the GO micro-tube wall in a wet state.

A facile synthesis of perforated reduced graphene oxide for high performance electrochemical sensors

Highly active perforated reduced graphene oxide (P-rGO) was synthesized by a facile methodology based on co-deposition of graphene oxide with sacrificial Prussian blue. Electrode surface properties were characterized by SEM and EDS. The GC/P-rGO electrode exhibited a larger specific surface area than that of GCE. These findings highlighted that the signal was enhanced for both dopamine detection and selenium detection by using P-rGO as a relevant supporting substrate. The result indicated that the large number of perforated structures formed numerous electrically conductive channels in the structure, improving the electrocatalytic properties.