Redox-Active Phenanthrenequinone Molecules and Nitrogen-Doped Reduced Graphene Oxide as Active Material Composites for Supercapacitor Applications.

Carbon-based supercapacitor electrodes are generally restricted in energy density, as they rely exclusively on electric double-layer capacitance (EDLC). The introduction of redox-active organic molecules to obtain pseudocapacitance is a promising route to develop electrode materials with improved energy densities. In this work, we develop a porous nitrogen-doped reduced graphene oxide and 9,10-phenanthrenequinone composite (N-HtrGO/PQ) via a facile one-step physical adsorption method. The electrochemical evaluation of N-HtrGO/PQ using cyclic voltammetry showed a high capacitance of 605 F g-1 in 1 M H2SO4 when the composite consisted of 30% 9,10-phenanthrenequinone and 70% N-HtrGO. The measured capacitance significantly exceeded pure N-HtrGO without the addition of redox-active molecules (257 F g-1). In addition to promising capacitance, the N-HtrGO/30PQ composite showed a capacitance retention of 94.9% following 20,000 charge/discharge cycles. Based on Fourier transform infrared spectroscopy, we postulate that the strong π-π interaction between PQ molecules and the N-HtrGO substrate enhances the specific capacitance of the composite by shortening pathways for electron transfer while improving structural stability.

Integration of Ultrathin Bubble Walls and Electrochemistry: Innovation in Microsensing for Forensic Nitrite Detection and Microscale Metallic Film Deposition.

We present a strategy for electrochemical measurements using a durable minute bubble wall with a thickness of 27 µm (D = 1.8 cm) as an innovative electrochemical medium. The composition, thickness, and volume of the tiny bubble film were investigated and estimated using the spectroscopic method and the Beer-Lambert law. A carbon microelectrode (D = 10 µm) was then employed as the working electrode, inserted through the bubble wall to function as the solution interface. First, the potential of this method for microelectrodeposition of metallic Ag and Pd films in a tiny bubble was investigated. Interestingly, microscopic images of the deposited film clearly demonstrated that the bubble thickness determines and confines the electrochemical deposition zone. In other words, innovative template-free microelectrodeposition was achieved. In the second phase of our work, microelectroanalysis of trace levels of nitrite ions was performed within the bubble wall and on a foam-covered hand, between the fingers directly, with a low limit of detection of 28 µM. This technique holds significance in criminal investigations, as the presence of NO2- ions on the hand indicates the potential presence of gunshot residue and aids in identifying suspects. In comparison to current methods, this approach is rapid, simple, cost-effective, and amenable to on-site applications, eliminating the need for sample treatment. Ultimately, the utilization of a bubble wall as a novel electrochemical microreactor can open new ways in microelectrochemical analysis, presenting novel opportunities and applications in the field of electrochemical sensors.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering.

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Defect Density-Dependent pH Response of Graphene Derivatives: Towards the Development of pH-Sensitive Graphene Oxide Devices.

In this study, we demonstrate that a highly pH-sensitive substrate could be fabricated by controlling the type and defect density of graphene derivatives. Nanomaterials from single-layer graphene resembling a defect-free structure to few-layer graphene and graphene oxide with high defect density were used to demonstrate the pH-sensing mechanisms of graphene. We show the presence of three competing mechanisms of pH sensitivity, including the availability of functional groups, the electrochemical double layer, and the ion trapping that determines the overall pH response. The graphene surface was selectively functionalized with hydroxyl, amine, and carboxyl groups to understand the role and density of the graphene pH-sensitive functional groups. Later, we establish the development of highly pH-sensitive graphene oxide by controlling its defect density. This research opens a new avenue for integrating micro-nano-sized pH sensors based on graphene derivatives into next-generation sensing platforms.

Detection of free chlorine in water using graphene-like carbon based chemiresistive sensors.

Free chlorine is the most commonly used water disinfectant. Measuring its concentration during and after water treatment is crucial to ensure its effectiveness. However, many of the existing methods do not allow for continuous on-line monitoring. Here we demonstrate a solid state chemiresistive sensor using graphene-like carbon (GLC) that overcomes that issue. GLC films that were either bare or non-covalently functionalized with the redox-active phenyl-capped aniline tetramer (PCAT) were successfully employed to quantify aqueous free chlorine, although functionalized devices showed better performance. The response of the sensors to increasing concentrations of free chlorine followed a Langmuir adsorption isotherm in the two tested ranges: 0.01-0.2 ppm and 0.2-1.4 ppm. The limit of detection was estimated to be 1 ppb, permitting the detection of breaches in chlorine filters. The devices respond to decreasing levels of free chlorine without the need for a reset, allowing for the continuous monitoring of fluctuations in the concentration. The maximum sensor response and saturation concentration were found to depend on the thickness of the GLC film. Hence, the sensitivity and dynamic range of the sensors can be tailored to different applications by adjusting the thickness of the films. Tap water samples from a residential area were tested using these sensors, which showed good agreement with standard colorimetric measurement methods. The devices did not suffer from interferences in the presence of ions commonly found in drinking water. Overall, these sensors are a cost-effective option for the continuous automated monitoring of free chlorine in drinking water.

Tuning the Chemical and Mechanical Properties of Conductive MoS2 Thin Films by Surface Modification with Aryl Diazonium Salts.

Molybdenum disulfide (MoS2) is a promising material for applications in sensors, energy storage, energy conversion devices, solar cells, and fuel cells. Because many of those applications require conductive materials, we recently developed a method for preparing a conductive form of MoS2 (c-MoS2) using dilute aqueous hydrogen peroxide in a simple and safe way. Here, we investigate modulating the chemical and mechanical surface properties of c-MoS2 thin films using diazonium chemistry. In addition to a direct passivation strategy of c-MoS2 with diazonium salts for electron-withdrawing groups, we also propose a novel in situ synthetic pathway for modification with electron-donating groups. The obtained results are examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The degree of surface passivation of pristine and functionalized c-MoS2 films was tested by exposing them to aqueous solutions of different metal cations (Fe2+, Zn2+, Cu2+, and Co2+) and detecting the chemiresistive response. While pristine films were found to interact with several of the cations, modified films did not. We propose that a surface charge transfer mechanism is responsible for the chemiresistive response of the pristine films, while both modification routes succeeded at complete surface passivation. Functionalization was also found to lower the coefficient of friction for semiconducting 2H-MoS2, while all conductive materials (modified or not) also had lower coefficients of friction. This opens up a pathway to a palette of dry lubricant materials with improved chemical stability and tunable conductivity. Thus, both in situ and direct diazonium chemistries are powerful tools for tuning chemical and mechanical properties of conductive MoS2 for new devices and lubricants based on conductive MoS2.

Defect Engineering of Graphene to Modulate pH Response of Graphene Devices.

Graphene-based pH sensors are a robust, durable, sensitive, and scalable approach for the sensitive detection of pH in various environments. However, the mechanisms through which graphene responds to pH variations are not well-understood yet. This study provides a new look into the surface science of graphene-based pH sensors to address the existing gaps and inconsistencies among the literature concerning sensing response, the role of defects, and surface/solution interactions. Herein, we demonstrate the dependence of the sensing response on the defect density level of graphene, measured by Raman spectroscopy. At the crossover point (ID/IG = 0.35), two countervailing mechanisms balance each other out, separating two regions where either a surface defect induced (negative slope) or a double layer induced (positive slope) response dominates. For ratios above 0.35, the pH-dependent induction of charges at surface functional groups (both pH-sensitive and nonsensitive groups) dominates the device response. Below a ratio of 0.35, the response is dominated by the modulation of charge carriers in the graphene due to the electric double layer formed from the interaction between the graphene surface and the electrolyte solution. Selective functionalization of the surface was utilized to uncover the dominant acid-base interactions of carboxyl and amine groups at low pH while hydroxyl groups control the high pH range sensitivity. The overall pH-sensing characteristics of the graphene will be determined by the balance of these two mechanisms.

Investigating the Microstructure and Mechanical Properties of Aluminum-Matrix Reinforced-Graphene Nanosheet Composites Fabricated by Mechanical Milling and Equal-Channel Angular Pressing.

Layered-graphene reinforced-metal matrix nanocomposites with excellent mechanical properties and low density are a new class of advanced materials for a broad range of applications. A facile three-step approach based on ultra-sonication for dispersion of graphene nanosheets (GNSs), ball milling for Al-powder mixing with different weight percentages of GNSs, and equal-channel angular pressing for powders' consolidation at 200 °C was applied for nanocomposite fabrication. The Raman analysis revealed that the GNSs in the sample with 0.25 wt.% GNSs were exfoliated by the creation of some defects and disordering. X-ray diffraction and microstructural analysis confirmed that the interaction of the GNSs and the matrix was almost mechanical, interfacial bonding. The density test demonstrated that all samples except the 1 wt.% GNSs were fully densified due to the formation of microvoids, which were observed in the scanning electron microscope analysis. Investigation of the mechanical properties showed that by using Al powders with commercial purity, the 0.25 wt.% GNS sample possessed the maximum hardness, ultimate shear strength, and uniform normal displacement in comparison with the other samples. The highest mechanical properties were observed in the 0.25 wt.% GNSs composite, resulting from the embedding of exfoliated GNSs between Al powders, excellent mechanical bonding, and grain refinement. In contrast, agglomerated GNSs and the existence of microvoids caused deterioration of the mechanical properties in the 1 wt.% GNSs sample.

Mechanochemical Green Synthesis of Exfoliated Edge-Functionalized Boron Nitride Quantum Dots: Application to Vitamin C Sensing through Hybridization with Gold Electrodes.

Two-dimensional boron nitride quantum dots (2D BNQDs) with excellent chemical stability, high photoluminescence efficiency, and low toxicity are a new class of advanced materials for biosensing and bioimaging applications. To overcome the current challenge about the lack of facile, scalable, and reproducible synthesis approach of BNQDs, we introduce a green and facile approach based on mechanochemical exfoliation of bulk h-BN particles in ethanol. Few-layered hydroxylated-functionalized QDs with a thickness of 1-2 nm and a lateral dimension of 2-6 nm have been prepared. The synthesized nanocrystals exhibit a strong fluorescence emission at 407 and 425 nm with a quantum efficiency of ∼6.2%. Spectroscopic analyses determine that interactions between oxygen groups of the solvent with boron sites occur, which along with the mechanical forces, lead to efficient exfoliation of the hexagonal structure and surface functionalization with -OH groups. We also demonstrate that the orbital interaction between BNQDs and the gold surface results in a profound electrochemical catalytic activity toward oxidation of vitamin C. It is shown that the BNQD-modified screen-printed gold electrode exhibits a decreased onset oxidation potential for about 0.37 V/AgCl. In addition to high catalytic activity, electrochemical studies also reveal that this electrode allows selective and sensitive detection of vitamin C with a good response over a wide range from 0.80 µM to 5.0 mM with a detection limit of 0.45 µM (S/N = 3) and a sensitivity of 1.3 µA µM-1 cm-2. Finally, the potential application of the hybrid sensor for detecting vitamin C in commercial drinks is demonstrated.