Self-Assembled Graphene/Azo Polyelectrolyte Multilayer Film and Its Application in Electrochemical Energy Storage Device

Graphene/azo polyelectrolyte multilayer films were fabricated through electrostatic layer-by-layer (LbL) self-assembly, and their performance as electrochemical capacitor electrode was investigated. Cationic azo polyelectrolyte (QP4VP-co-PCN) was synthesized through radical polymerization, postpolymerization azo coupling reaction, and quaternization. Negatively charged graphene nanosheets were prepared by a chemically modified method. The LbL films were obtained by alternately dipping a piece of the pretreated substrates in the QP4VP-co-PCN and nanosheet solutions. The processes were repeated until the films with required numbers of bilayers were obtained. The self-assembly and multilayer surface morphology were characterized by UV−vis spectroscopy, AFM, SEM, and TEM. The performance of the LbL films as electrochemical capacitor electrode was estimated using cyclic voltammetry. Results show that the graphene nanosheets are densely packed in the multilayers and form random graphene network. The azo polyelectrolyte cohesively interacts with the nanosheets in the multilayer structure, which prevents agglomeration of graphene nanosheets. The sheet resistance of the LbL films decreases with the increase of the layer numbers and reaches the stationary value of 1.0 × 106 Ω/◻ for the film with 15 bilayers. At a scanning rate of 50 mV/s, the LbL film with 9 bilayers shows a gravimetric specific capacitance of 49 F/g in 1.0 M Na2SO4 solution. The LbL films developed in this work could be a promising type of the electrode materials for electric energy storage devices

Photoinduced Mass-Migration Behavior of Two Amphiphilic Side-Chain Azo Diblock Copolymers with Different Length Flexible Spacers

This article reports the synthesis and photoinduced mass-migration behavior of two amphiphilic side-chain azo diblock copolymers (PEG-b-P2CN, PEG-b-P6CN), which contain ethylene and hexamethylene as flexible spacers between main chains and pendent azobenzene functional groups. The copolymers were synthesized by atom transfer radical polymerization (ATRP) to obtain precursor block copolymers from a PEG-based macromolecular initiator and postpolymerization azo-coupling reaction of the precursors to introduce the azo chromophores. Spectroscopic analyses, GPC, and thermal analyses indicated that the diblock copolymers possessed well-defined structures and narrow molecular weight distributions. The photoinduced mass-migration behavior was studied through two different approaches, that is, investigating photoinduced deformation of colloidal spheres of the copolymers and surface relief grating (SRG) formation on the polymer thin films. The colloidal spheres used for the light irradiation experiments were prepared by solvent-induced phase separation and set on the substrates (such as TEM copper grids) as solid particles. Upon irradiation with a linearly polarized Ar+ laser beam (488 nm, 150 mW/cm2), the colloidal spheres of both copolymers were gradually elongated in the polarization direction of the laser beam, which evidenced the photoinduced mass migration on the micrometer scale. In the process, the PEG-b-P2CN colloids showed a significantly larger deformation rate and degree compared with the PEG-b-P6CN colloids. The mass-migration ability was also tested by inscribing surface relief gratings on PEG-b-P2CN and PEG-b-P6CN films with interfering Ar+ laser beams. The PEG-b-P2CN showed a significantly faster rate of grating formation and larger modulation depth in comparison with the PEG-b-P6CN. The observations suggest that the flexible spacer could play an important role in transferring the light-driving force from the azo chromophores to the polymer backbones, where the shorter spacers showed a more significant effect to cause the mass migration

Amphiphilic Diblock Copolymers Functionalized with Strong Push−Pull Azo Chromophores: Synthesis and Multi-Morphological Aggregation

This article reports the synthesis, characterization, and multimorphological aggregation of a series of amphiphilic diblock copolymers bearing strong push−pull azo chromophores. The diblock copolymers (PEGx-b-PCNy), which consist of poly(ethylene glycol) (PEG) and 2-(N-ethyl-N-(4-(4′-cyanophenylazo)phenyl)amino)ethyl methacrylate (PCN) blocks, were synthesized through atom transfer radical polymerization (ATRP) and postpolymerization azo-coupling reaction. PEGx-b-PCNy was prepared to have different hydrophilic/hydrophobic ratios (x = 122, y = 24, 62, 129, 224). Self-assembled aggregates were formed by the gradual addition of water to the solutions of the copolymers in THF. The formation process and morphology of the aggregates were characterized by DLS, SLS, SEM, and TEM. Results show that the block polymers start to form aggregates at the critical water content (CWC), which is related to the initial polymer concentration in THF and PCN block length. The morphology of the aggregates formed in the solutions is controlled by the PCN block length and preparation conditions. With the increase in the PCN length, the aggregates show different morphologies such as spherical micelles, rodlike aggregates, hollow nanotubes, and colloidal spheres. In the experimental range, a change in the polymer initial concentration in THF does not show an obvious effect on the aggregate morphology. The water-adding rate in the preparation process shows an important effect on the aggregate morphology. When the water-adding rate increases from 0.5 to 7.2 mL/h, the morphology of PEG122-b-PCN129 aggregates changes from nanotubes to a mixture of giant vesicles and colloidal spheres. Some well-organized aggregates developed from the photoresponsive copolymers could have potential applications in photocontrollable drug delivery and other uses

Flexible Disk Ultramicroelectrode for High-Resolution and Substrate-Tolerable Scanning Electrochemical Microscopy Imaging

A simple and universal strategy for fabricating flexible 25 μm platinum (Pt) disk ultramicroelectrodes (UMEs) was proposed, where a pulled borosilicate glass micropipette acted as a mold for shaping the flexible tip with flexible epoxy resin. The whole preparation procedure was highly efficient, enabling 10 or more probes to be manually fabricated within 10 h. Intriguingly, this technique permits an adjustable RG ratio, tip length, and stiffness, which could be tuned according to varying experimental demands. Besides, the electroactive area of the probe could be exposed and made renewable with a thin blade, allowing its reuse in multiple experiments. The flexibility characterization was then employed to optimize the resin/hardener mass ratio of epoxy resin and the tip position during HF etching in the fabrication process, suggesting that more hardener, a larger RG value, or a longer tip length obtained stronger deformation resistance. Subsequently, the as-prepared probe was examined by optical microscopy, cyclic voltammetry, and SECM approach curves. The results demonstrated the probe possessed good geometry with a small RG ratio of less than 3 and exceptional electrochemical properties, and its insulating sheath remained undeformed after blade cutting. Owing to the tip’s flexibility, it could be operated in contactless mode with an extremely low working distance and even in contact mode scanning to achieve high spatial resolution and high sensitivity while guaranteeing that the tip and samples would suffer minimal damage if the tip crashed. Finally, the flexible probe was successfully employed in three scanning scenarios where tilted and 3D structured PDMS microchips, a latent fingerprint deposited on the stiff copper sheet, and soft egg white were included. In all, the flexible probe encompasses the advantages of traditional disk UMEs and circumvents their principal drawbacks of tip crash and causing sample scratches, which is thus more compatible with large specimens of 3D structured, stiff, or even soft topography

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate’s hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate’s hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics

Origin of Capillary-Force-Induced Welding in Ag Nanowires and Ag Nanowire/Carbon Nanotube Conductive Networks

Capillary-force-induced welding can effectively reduce the contact resistance between two silver nanowires (AgNWs) by merging the NW–NW junctions. Herein, we report a model for quantifying the capillary force between two nano-objects. The model can be used to calculate the capillary force generated between AgNWs and carbon nanotubes (CNTs) during water evaporation. The results indicate that the radius of one-dimensional nano-objects is crucial for capillary-force-induced welding. AgNWs with larger radii can generate a greater capillary force (FAgNW‑AgNW) at NW–NW junctions. In addition, for AgNW/CNT hybrid films, the use of CNTs with a radius close to that of AgNWs can result in a larger capillary force (FAgNW‑CNT) at NW–CNT junctions. The reliability of the model is verified by measuring the change in sheet resistance before and after capillary-force-induced welding of a series of AgNW and AgNW/CNT conductive films with varying radii

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate’s hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate’s hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics

Customizable Stretchable Transparent Electrodes Based on AgNW/CNT Hybrids via Tailoring Sizes of Building Blocks

Stretchable transparent electrodes (STEs) based on silver nanowires (AgNWs) have received considerable attention for a variety of flexible and wearable electronic/optoelectronic devices. Up to now, most efforts have focused on optimizing the STEs composed by a single AgNW conductive network. On the contrary, the structure–performance correlations of STEs formed by a hybrid percolative network which comprises the AgNW and a second conductive nanomaterial have rarely been discussed. In this work, we fabricated hybrid-type STEs by selecting three kinds of carbon nanotubes (CNTs) with different diameters to pair with three types of AgNWs with variable length-to-diameter ratios. The size effect of building blocks of the nine combinations on the optical, electrical, and mechanical properties of resultant STEs was thoroughly investigated. The results reveal that AgNWs and CNTs with smaller diameters are beneficial to achieve hybrid electrodes with a high transmittance and low haze. AgNWs with larger length-to-diameter ratios contribute hybrid STEs with lower sheet resistance by adding a suitable amount of CNTs. Importantly, the smaller differences in diameters of AgNWs and CNTs lead to more effective capillary-force-induced welding, which boosts both the conductivity and stretchability of STEs. An optimized AgNW/CNT hybrid electrode demonstrated a transmittance of 66.4% and a haze of 11.0% at a sheet resistance of 8.70 Ω sq.–1 which could endure a uniaxial tensile strain as large as 490%, while its resistance increased only 46.9% after experiencing 1000 cycles of 50% tensile strain. Alternating current electroluminescent devices based on such AgNW/CNT hybrid STEs were also successfully developed, showing uniform and stable patterned luminescence