Silver Nanoparticle-Assisted Electrochemically Exfoliated Graphene Inks Coated on PVA-Based Self-Healing Polymer Composites for Soft Electronics.

Smart sensors with self-healing capabilities have recently aroused increasing interest in applications in soft electronics. However, challenges remain in balancing the sensors' self-healing and compatibility between their sensing and substrate layers. This study evaluated several self-healing polymer substrates and graphene ink-based strain-sensing coatings. The optimum electrochemically exfoliated graphene (e-graphene)/silver nanoparticle-coated tannic acid (TA)/superabsorbent polymer/graphene oxide (GO) blended poly(vinyl alcohol) polymer composites exhibited improvements of 47.1 and 39.2%, respectively, for the healing efficiency in a substrate crack area and in the graphene-based sensing layer due to conductive layer adhesion. While TA was found to improve healing efficiency on the coating surface by forming hydrogen bonds between the sensing and polymer layers, GO healed the polymer surface due to its ability to form bonds in the polymer matrix. The superabsorbent polymer was found to absorb excess water in e-graphene dispersion due to its host-guest interaction, while also reducing the coating thickness.

Combined Structural and Plasmonic Enhancement of Nanometer-Thin Film Photocatalysis for Solar-Driven Wastewater Treatment.

Titanium dioxide (TiO2) thin films are commonly used as photocatalytic materials. Here, we enhance the photocatalytic activity of devices based on titanium dioxide (TiO2) by combining nanostructured glass substrates with metallic plasmonic nanostructures. We achieve a three-fold increase of the catalyst's surface area through nanoscale, three-dimensional patterning of periodic, conical grids, which creates a broadband optical absorber. The addition of aluminum and gold activates the structures plasmonically and increases the optical absorption in the TiO2 films to above 70% in the visible and NIR spectral range. We demonstrate the resulting enhancement of the photocatalytic activity with organic dye degradation tests under different light sources. Furthermore, the pharmaceutical drug Carbamazepine, a common water pollutant, is reduced in the aqueous solution by up to 48% in 360 min. Our approach is scalable and potentially enables future solar-driven wastewater treatment.

Effect of Particle Size on the Orientation and Order of Assemblies of Functionalized Microscale Cubes Formed at the Water/Air Interface.

The impact of the particle size and wettability on the orientation and order of assemblies obtained by self-organization of functionalized microscale polystyrene cubes at the water/air interface is reported. An increase in the hydrophobicity of 10- and 5-µm-sized self-assembled monolayer-functionalized polystyrene cubes, as assessed by independent water contact angle measurements, led to a change of the preferred orientation of the assembled cubes at the water/air interface from face-up to edge-up and further to vertex-up, irrespective of microcube size. This tendency is consistent with our previous studies with 30-µm-sized cubes. However, the transitions among these orientations and the capillary force-induced structures, which change from flat plate to tilted linear and further to close-packed hexagonal arrangements, were observed to shift to larger contact angles for smaller cube size. Likewise, the order of the formed aggregates decreased significantly with decreasing cube size, which is tentatively attributed to the small ratio of inertial force to capillary force for smaller cubes in disordered aggregates, which results in more difficulties to reorient in the stirring process. Experiments with small fractions of larger cubes added to the water/air interface increased the order of smaller homo-aggregates to values similar to neat 30 µm cube assemblies. Hence, collisions of larger cubes or aggregates are shown to play a decisive role in breaking metastable structures to approach a global energy minimum assembly.

Graphene Coating of Nafion Membranes for Enhanced Fuel Cell Performance.

Electrochemically exfoliated graphene (e-G) thin films on Nafion membranes exhibit a selective barrier effect against undesirable fuel crossover. This approach combines the high proton conductivity of state-of-the-art Nafion and the ability of e-G layers to effectively block the transport of methanol and hydrogen. Nafion membranes are coated with aqueous dispersions of e-G on the anode side, making use of a facile and scalable spray process. Scanning transmission electron microscopy and electron energy-loss spectroscopy confirm the formation of a dense percolated graphene flake network, which acts as a diffusion barrier. The maximum power density in direct methanol fuel cell (DMFC) operation with e-G-coated Nafion N115 is 3.9 times higher than that of the Nafion N115 reference (39 vs 10 mW cm-2@0.3 V) at a 5M methanol feed concentration. This suggests the application of e-G-coated Nafion membranes for portable DMFCs, where the use of highly concentrated methanol is desirable.

Evaluation of an on-site surface enhanced Raman scattering sensor for benzotriazole.

Benzotriazole (BTAH) has been used for decades as corrosion inhibitor and antifreeze. Since it is fairly soluble in water but very stable and can only be partly removed from wastewater treatment plants, it represents a threat to the environment and thus also to human health. Therefore, it is of uttermost importance to have a detection method capable of monitoring the concentration of BTAH at trace level on-site. Here, we demonstrate that a sensor based on surface-enhanced Raman spectroscopy is capable of detecting trace-level concentrations of BTAH. We carefully studied the concentration dependency and the time dependent coverage. Moreover, we could not only perform the measurements with clean solution but also with real samples from a wastewater treatment plant, ensuring that our method proposed works in a complex environment.

Role of Substrate Surface Morphology on the Performance of Graphene Inks for Flexible Electronics.

Two-dimensional (2D) materials, such as graphene, are seen as potential candidates for fabricating electronic devices and circuits on flexible substrates. Inks or dispersions of 2D materials can be deposited on flexible substrates by large-scale coating techniques, such as inkjet printing and spray coating. One of the main issues in coating processes is nonuniform deposition of inks, which may lead to large variations of properties across the substrates. Here, we investigate the role of surface morphology on the performance of graphene ink deposited on different paper substrates with specific top coatings. Substrates with good wetting properties result in reproducible thin films and electrical properties with low sheet resistance. The correct choice of surface morphology enables high-performance films without postdeposition annealing or treatment. Scanning terahertz time-domain spectroscopy (THz-TDS) is introduced to evaluate both the uniformity and the local conductivity of graphene inks on paper. A paper-based strain gauge is demonstrated and a variable resistor acts as an on-off switch for operating an LED. Customized surfaces can thus help in unleashing the full potential of ink-based 2D materials.

The fabrication of a flexible mold for high resolution soft ultraviolet nanoimprint lithography.

One key issue for all nanoimprint techniques is an appropriate method for the fabrication of desirable molds. We report on a novel flexible mold fabrication process-pressure-assisted molding (PAM)-for high resolution soft ultraviolet nanoimprint lithography (soft UV-NIL). In PAM, enhanced master filling is achieved by applying an external pressure during the mold fabrication process. Flexible molds, fabricated with PAM using different pressures in the range of 10-90 kPa, are compared to determine the role of pressures applied in the imprint performance.