One-Dimensional π-d Conjugated Coordination Polymer Intercalated MXene Compound for High-Performance Supercapacitor Electrode.

MXenes possess the characteristics required for high-performance supercapacitors, such as high metallic conductivity and electrochemical activity, but their full potential remains unrealized owing to their tendency to self-restack when fabricated into an electrode. Designing an MXene interlayer with an effective intercalant has, therefore, become an important criterion to alleviate the restacking issue while also synergistically interact with MXene to further improve its electrochemical activity. This study reports the intercalation of 1D π-d conjugated coordination polymer (Ni-BTA) within the Ti3 C2 Tx nanosheet for the fabrication of a highly efficient supercapacitor electrode. Ni-BTA, which consists of a nickel center and 1,2,4,5-benzenetetramine (BTA) organic chain, is uniformly intercalated by direct synthesis on the abundant oxygen terminals on the Ti3 C2 Tx nanosheet surface. The intercalated Ni-BTA acts as an effective charge carrier transportation pathway through its 1D stretched delocalized π-d electrons while participating in pseudocapacitive activity with the Ni centers. As a result, the Ni-BTA/MXene film exhibits excellent rate performance and a gravimetric specific capacitance of 264.4 F g-1 at 5 mV s-1 . This magnitude is retained up to 94.6% after 10 000 cycles. The present study provides insights into the design of MXene interlayers for the fabrication of highly robust and stable supercapacitors.

Advances in high-voltage supercapacitors for energy storage systems: materials and electrolyte tailoring to implementation.

To achieve a zero-carbon-emission society, it is essential to increase the use of clean and renewable energy. Yet, renewable energy resources present constraints in terms of geographical locations and limited time intervals for energy generation. Therefore, there is a surging demand for developing high-performance energy storage systems (ESSs) to effectively store the energy during the peak time and use the energy during the trough period. To this end, supercapacitors hold great promise as short-term ESSs for rapid power recovery or frequency regulation to improve the quality and reliability of power supply. In particular, the electrical double layer capacitor (EDLC) which offers long and stable cycle retention, high power densities, and fast charge/discharge characteristics with a moderate operating voltage window, is a suitable candidate. Yet, for implementation of the EDLC in ESSs, further research effort is required in terms of increasing the operating voltage and energy densities while maintaining the long-term cycle stability and power densities which are desirable aspects for ESS operation. Here, we examine the advances in EDLC research to achieve a high operating voltage window along with high energy densities, covering from materials and electrolytes to long-term device perspectives for next-generation supercapacitor-based ESSs.

High-Capacity Ti3C2Tx MXene Electrodes Achieved by Eliminating Intercalated Water Molecules Using a Co-solvent System.

Synthesizing layered transition-metal carbides, MXenes, with a mesoporous structure remains challenging but is highly useful because it converts the laminated two-dimensional structures into versatile porous materials. Hydrogen bonds between intercalated H2O molecules and oxygen terminal groups on the surface are formed in aqueous solution processes, and this is a determining factor of surface area. We developed an extraction method to remove intercalated water molecules based on a simple intermolecular force attraction strategy in a co-solvent system using a combination of polar-protic/-aprotic and non-polar solvents. As a result, self-aggregated mesoporous Ti3C2Tx was realized without any additives. The dipole-dipole interaction between H2O and CHCl3 molecules under non-polar solvent conditions assists the extraction of intercalated H2O from the MXene suspension, which can form a self-aggregated morphology (not re-stacked horizontally). The process yields Ti3C2Tx with a layered structure of embedded mesopores and a specific surface area that is 13-fold higher than that of standard MXene. Electrodes made with the resulting MXene exhibited a larger specific capacitance of 224 F/g (1 A/g), with an improved cyclic retention of 96.4%@10,000 cycles. This intermolecular attraction-induced approach, involving the manipulation of morphology, is simple to mass-produce and can be used for MXene-based electrochemical applications.

Hierarchically Porous, Laser-Pyrolyzed Carbon Electrode from Black Photoresist for On-Chip Microsupercapacitors.

We report a laser-pyrolyzed carbon (LPC) electrode prepared from a black photoresist for an on-chip microsupercapacitor (MSC). An interdigitated LPC electrode was fabricated by direct laser writing using a high-power carbon dioxide (CO2) laser to simultaneously carbonize and pattern a spin-coated black SU-8 film. Due to the high absorption of carbon blacks in black SU-8, the laser-irradiated SU-8 surface was directly exfoliated and carbonized by a fast photo-thermal reaction. Facile laser pyrolysis of black SU-8 provides a hierarchically macroporous, graphitic carbon structure with fewer defects (ID/IG = 0.19). The experimental conditions of CO2 direct laser writing were optimized to fabricate high-quality LPCs for MSC electrodes with low sheet resistance and good porosity. A typical MSC based on an LPC electrode showed a large areal capacitance of 1.26 mF cm-2 at a scan rate of 5 mV/s, outperforming most MSCs based on thermally pyrolyzed carbon. In addition, the results revealed that the high-resolution electrode pattern in the same footprint as that of the LPC-MSCs significantly affected the rate performance of the MSCs. Consequently, the proposed laser pyrolysis technique using black SU-8 provided simple and facile fabrication of porous, graphitic carbon electrodes for high-performance on-chip MSCs without high-temperature thermal pyrolysis.

Chemical Patterning of Graphene via Metal-Assisted Highly Energetic Electron Irradiation for Graphene Homojunction-Based Gas Sensors.

To gain the target functionality of graphene for gas detection, nonfocused and large-scale compatible MeV electron beam irradiation on graphene with Ag patterns is innovatively adopted in air for chemical patterning of graphene. This strategy allows the metal-assisted site-specific oxidation of graphene to realize monolithically integrated graphene-chemically patterned graphene (CPG)-graphene homojunction-based gas sensors. The size-tunable CPG patterns can be mediated by regulating the size of Ag prepatterns. The impacts of highly energetic electron irradiation (HEEI) on graphene are summarized as follows: (i) the selective p-type doping and the defect generation of graphene by the HEEI-induced oxidation, (ii) the resistance of the homojunction devices manipulated by the HEEI dose, (iii) the band gap opening of graphene as well as the lowering of the Fermi level, (iv) the work function values for pristine graphene and CPG corresponding to 4.14 and 4.88 eV, respectively, and (v) graphene-CPG-graphene homojunction for NO2 gas, revealing an 839% enhanced gas response compared with that of the pristine graphene-based gas sensor.

Recyclable High-Performance Polymer Electrolyte Based on a Modified Methyl Cellulose-Lithium Trifluoromethanesulfonate Salt Composite for Sustainable Energy Systems.

Although energy-storage devices based on Li ions are considered as the most prominent candidates for immediate application in the near future, concerns with regard to their stability, safety, and environmental impact still remain. As a solution, the development of all-solid-state energy-storage devices with enhanced stability is proposed. A new eco-friendly polymer electrolyte has been synthesized by incorporating lithium trifluoromethanesulfonate into chemically modified methyl cellulose (LiTFS-LiSMC). The transparent and flexible electrolyte exhibits a good conductivity of near 1 mS cm-1 . An all-solid-state supercapacitor fabricated from 20 wt % LiTFS-LiSMC shows comparable specific capacitances to a standard liquid-electrolyte supercapacitor and an excellent stability even after 20 000 charge-discharge cycles. The electrolyte is also compatible with patterned carbon, which enables the simple fabrication of micro-supercapacitors. In addition, the LiTFS-LiSMC electrolyte can be recycled and reused more than 20 times with negligible change in its performance. Thus, it is a promising material for sustainable energy-storage devices.

Atomic-Level Customization of 4 in. Transition Metal Dichalcogenide Multilayer Alloys for Industrial Applications.

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large-scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS2(1- x ) Se2 x ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic-level substitution combined with a solution-based large-area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS1.62 Se0.38 , MoS1.37 Se0.63 , MoS1.15 Se0.85 , and MoS0.46 Se1.54 alloys, as well as MoS2 and MoSe2 . Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large-scale 2D TMD alloys are implemented. Se-stimulated strain effects and a detailed mechanism for the Se substitution in the MoS2 crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure-phase MoS2 and MoSe2 , are unambiguously achieved.

High energy electron beam stimulated nanowelding of silver nanowire networks encapsulated with graphene for flexible and transparent electrodes.

Low-dimensional nanostructures and their complementary hybridization techniques are in the vanguard of technological advances for applications in transparent and flexible nanoelectronics due to the intriguing electrical properties related to their atomic structure. In this study, we demonstrated that welding of Ag nanowires (NWs) encapsulated in graphene was stimulated by flux-optimized, high-energy electron beam irradiation (HEBI) under ambient conditions. This methodology can inhibit the oxidation of Ag NWs which is induced by the inevitably generated reactive ozone as well as improve of their electrical conductivity. We have systematically explored the effects of HEBI on Ag NWs and graphene. The optimized flux for HEBI welding of the Ag NWs with graphene was 150 kGy, which decreased the sheet resistance of the graphene/Ag NWs to 12 Ohm/sq. Following encapsulation with graphene, the initial chemical states of the Ag NWs were well-preserved after flux-tuned HEBI, whereas graphene underwent local HEBI-induced defect generation near the junction area. We further employed resonant Raman spectroscopy to follow the structural evolution of the sacrificial graphene in the hybrid film after HEBI. Notably, the sheet resistance of the welded Ag NWs encapsulated with graphene after HEBI was well-maintained even after 85 days.

A facile synthetic route to tungsten diselenide using a new precursor containing a long alkyl chain cation for multifunctional electronic and optoelectronic applications.

Single source precursors for coating and subsequent thermal decomposition processes enable a large-scale, low-cost synthesis of two-dimensional transition metal dichalcogenides (TMDs). However, practical applications based on two-dimensional TMDs have been limited by the lack of applicable single source precursors for the synthesis of p-type TMDs including layered tungsten diselenide (WSe2). We firstly demonstrate the simple and facile synthesis of WSe2 layers using a newly developed precursor that allows improved dispersibility and lower decomposition temperature. We study the thermal decomposition mechanism of three types of (Cat+)2[WSe4] precursors to assess the most suitable precursor for the synthesis of WSe2 layers. The resulting chemical and structural exploration of solution-processed WSe2 layers suggests that the (CTA)2[WSe4] may be a promising precursor because it resulted in the formation of high-crystalline WSe2. In addition, this study verifies the capability of WSe2 layers for multifunctional applications in optoelectronic and electronic devices. The photocurrent of WSe2-based photodetectors shows an abrupt switching behavior under periodic illumination of visible or IR light. The extracted photoresponsivity values for WSe2-based photodetectors recorded at 0.5 V correspond to 26.3 mA W-1 for visible light and 5.4 mA W-1 for IR light. The WSe2-based field effect transistors exhibit unipolar p-channel transistor behavior with a carrier mobility of 0.45 cm2 V-1 s-1 and an on-off ratio of ∼10.

Low temperature solution synthesis of reduced two dimensional Ti3C2 MXenes with paramagnetic behaviour.

MXenes - two dimensional, 2D, early transition metal, M, carbides and nitrides, X - are the latest addition to the 2D materials' world. Herein, we report on a facile low temperature solution chemical synthesis method to reduce Ti3C2Tx multilayered, ML, MXenes. Using X-ray photoelectron spectroscopy, electron spin resonance, magnetization measurements and other techniques, we concluded that immersing Ti3C2Tx MLs in the reducing agent Li-ethylenediamine (Li-EDA) - held at temperatures varying from room to 120 °C - reduces the 2D layers creating Ti3+ ions and oxygen vacancies. Above a temperature (T) of ≈10 K, the magnetic susceptibilities, χ, are temperature independent, implying that the resulting powders are Pauli paramagnetic. The loss of the magnetic signal upon intercalation of Li+ or EDA, together with a Curie-like increase in χ at T < 10 K, is consistent with that of a disordered metal that is close to a metallic to insulator transition and proves that the magnetism is associated with the 2D flakes. This result is the first evidence of any magnetism of any MXene.

Versatile porous graphene flakes derived from alkali metal carbonates using an ultrafast and sulfuric acid-free solid-state oxidation reaction.

Herein, we report on an unprecedented synthetic method for single-layered GO that takes just a few tens of minutes. This rationally designed solid-state oxidation based on alkali metal carbonates (Li2CO3, Na2CO3, K2CO3) involves a molten salt reaction, which enables the effective exfoliation and oxidation of graphene layers without using H2SO4 and KMnO4. The advantage of this approach is not only the ability to avoid the introduction of strong acid reactants in the reaction process, but this approach also leads to a 4.2 times larger specific surface area than conventional GO. For these reasons, we anticipate that this green, safe, fast and effective approach enables practical applications in graphene-based energy storage and light-absorbing black materials.

Roll-to-Roll Production of Layer-Controlled Molybdenum Disulfide: A Platform for 2D Semiconductor-Based Industrial Applications.

A facile methodology for the large-scale production of layer-controlled MoS2 layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll-to-roll-based thermal decomposition is developed. The resulting 50 cm long MoS2 layers synthesized on Ni foils possess excellent long-range uniformity and optimum stoichiometry. Moreover, this methodology is promising because it enables simple control of the number of MoS2 layers by simply adjusting the concentration of (NH4 )2 MoS4 . Additionally, the capability of the MoS2 for practical applications in electronic/optoelectronic devices and catalysts for hydrogen evolution reaction is verified. The MoS2 -based field effect transistors exhibit unipolar n-channel transistor behavior with electron mobility of 0.6 cm2 V-1 s-1 and an on-off ratio of ≈10³. The MoS2 -based visible-light photodetectors are fabricated in order to evaluate their photoelectrical properties, obtaining an 100% yield for active devices with significant photocurrents and extracted photoresponsivity of ≈22 mA W-1 . Moreover, the MoS2 layers on Ni foils exhibit applicable catalytic activity with observed overpotential of ≈165 mV and a Tafel slope of 133 mV dec-1 . Based on these results, it is envisaged that the cost-effective methodology will trigger actual industrial applications, as well as novel research related to 2D semiconductor-based multifaceted applications.

Chemically modulated graphene quantum dot for tuning the photoluminescence as novel sensory probe.

A band gap tuning of environmental-friendly graphene quantum dot (GQD) becomes a keen interest for novel applications such as photoluminescence (PL) sensor. Here, for tuning the band gap of GQD, a hexafluorohydroxypropanyl benzene (HFHPB) group acted as a receptor of a chemical warfare agent was chemically attached on the GQD via the diazonium coupling reaction of HFHPB diazonium salt, providing new HFHPB-GQD material. With a help of the electron withdrawing HFHPB group, the energy band gap of the HFHPB-GQD was widened and its PL decay life time decreased. As designed, after addition of dimethyl methyl phosphonate (DMMP), the PL intensity of HFHPB-GQD sensor sharply increased up to approximately 200% through a hydrogen bond with DMMP. The fast response and short recovery time was proven by quartz crystal microbalance (QCM) analysis. This HFHPB-GQD sensor shows highly sensitive to DMMP in comparison with GQD sensor without HFHPB and graphene. In addition, the HFHPB-GQD sensor showed high selectivity only to the phosphonate functional group among many other analytes and also stable enough for real device applications. Thus, the tuning of the band gap of the photoluminescent GQDs may open up new promising strategies for the molecular detection of target substrates.

Catalyst-free bottom-up growth of graphene nanofeatures along with molecular templates on dielectric substrates.

Synthesis of graphene nanostructures has been investigated to provide outstanding properties for various applications. Herein, we report molecular thin film-assisted growth of graphene into nanofeatures such as nanoribbons and nanoporous sheets along with a predetermined molecular orientation on dielectric substrates without metal catalysts. A Langmuir-Blodgett (LB) method was used for the formation of the molecularly patterned SiO2 substrates with ferric stearate layers, which acted as a template for the directional growth of the polypyrrole graphene precursor. The nanofeatures of the graphene were determined by the number of ferric stearate layers (e.g., nanoribbons from multiple layers and nanoporous sheets from a single layer). The graphene nanoribbons (GNRs) containing pyrrolic N enriched edges exhibited a p-type semiconducting behavior, whereas the nanoporous graphene sheets containing inhomogeneous pores and graphitic N enriched basal planes exhibited the typical electronic transport of nitrogen-doped graphene. Our approaches provide two central methods for graphene synthesis such as bottom-up and direct processes for the future development of graphene nanoelectronics.

Impermeable flexible liquid barrier film for encapsulation of DSSC metal electrodes.

Encapsulation of electronic devices such as dye-sensitized solar cells (DSSCs) is prone to degradation under normal atmospheric conditions, even with hermetic barriers on the metal electrodes. Overcoming this problem is crucial to increasing DSSC lifetimes and making them commercially viable. Herein, we report a new impermeable flexible liquid barrier film using polyvinyl alcohol (PVA) and partially reduced graphene oxide (PrGO), which dramatically enhances the lifetime of Ag metal electrodes (typically used in DSSCs) immersed in a highly acidic iodolyte solution. The Ag metal electrode encapsulated by the PVA/PrGO film survived for over 500 hrs, superior to existing barriers of glass frits, epoxy resins and polymers. The PVA/PrGO film strongly adheres to the Ag metal surface, and the resulting PVA/PrGO/Ag electrode is stable even on a curved substrate, with a sheet resistance nearly independent of curvature. These results give new insight for the design of high-performance and solution-processable flexible liquid barrier films for a wide range of applications, in particular for the encapsulation of electronic devices with liquid electrolytes.

Tunable Sub-nanopores of Graphene Flake Interlayers with Conductive Molecular Linkers for Supercapacitors.

Although there are numerous reports of high performance supercapacitors with porous graphene, there are few reports to control the interlayer gap between graphene sheets with conductive molecular linkers (or molecular pillars) through a π-conjugated chemical carbon-carbon bond that can maintain high conductivity, which can explain the enhanced capacitive effect of supercapacitor mechanism about accessibility of electrolyte ions. For this, we designed molecularly gap-controlled reduced graphene oxides (rGOs) via diazotization of three different phenyl, biphenyl, and para-terphenyl bis-diazonium salts (BD1-3). The graphene interlayer sub-nanopores of rGO-BD1-3 are 0.49, 0.7, and 0.96 nm, respectively. Surprisingly, the rGO-BD2 0.7 nm gap shows the highest capacitance in 1 M TEABF4 having 0.68 nm size of cation and 6 M KOH having 0.6 nm size of hydrated cation. The maximum energy density and power density of the rGO-BD2 were 129.67 W h kg(-1) and 30.3 kW kg(-1), respectively, demonstrating clearly that the optimized sub-nanopore of the rGO-BDs corresponding to the electrolyte ion size resulted in the best capacitive performance.

Solar-light photocatalytic disinfection using crystalline/amorphous low energy bandgap reduced TiO2.

A generation of reactive oxygen species (ROS) from TiO2 under solar light has been long sought since the ROS can disinfect organic pollutants. We found that newly developed crystalline/amorphous reduced TiO2 (rTiO2) that has low energy bandgap can effectively generate ROS under solar light and successfully remove a bloom of algae. The preparation of rTiO2 is a one-pot and mass productive solution-process reduction using lithium-ethylene diamine (Li-EDA) at room temperature. Interestingly only the rutile phase of TiO2 crystal was reduced, while the anatase phase even in case of both anatase/rutile phased TiO2 was not reduced. Only reduced TiO2 materials can generate ROS under solar light, which was confirmed by electron spin resonance. Among the three different types of Li-EDA treated TiO2 (anatase, rutile and both phased TiO2), the both phased rTiO2 showed the best performance to produce ROS. The generated ROS effectively removed the common green algae Chlamydomonas. This is the first report on algae degradation under solar light, proving the feasibility of commercially available products for disinfection.

Low-dimensional carbon and MXene-based electrochemical capacitor electrodes.

Due to their unique structure and outstanding intrinsic physical properties such as extraordinarily high electrical conductivity, large surface area, and various chemical functionalities, low-dimension-based materials exhibit great potential for application in electrochemical capacitors (ECs). The electrical properties of electrochemical capacitors are determined by the electrode materials. Because energy charge storage is a surface process, the surface properties of the electrode materials greatly influence the electrochemical performance of the cell. Recently, graphene, a single layer of sp(2)-bonded carbon atoms arrayed into two-dimensional carbon nanomaterial, has attracted wide interest as an electrode material for electrochemical capacitor applications due to its unique properties, including a high electrical conductivity and large surface area. Several low-dimensional materials with large surface areas and high conductivity such as onion-like carbons (OLCs), carbide-derived carbons (CDCs), carbon nanotubes (CNTs), graphene, metal hydroxide, transition metal dichalcogenides (TMDs), and most recently MXene, have been developed for electrochemical capacitors. Therefore, it is useful to understand the current issues of low-dimensional materials and their device applications.

Highly Stretchable and Conductive Silver Nanoparticle Embedded Graphene Flake Electrode Prepared by In situ Dual Reduction Reaction.

The emergence of stretchable devices that combine with conductive properties offers new exciting opportunities for wearable applications. Here, a novel, convenient and inexpensive solution process was demonstrated to prepare in situ silver (Ag) or platinum (Pt) nanoparticles (NPs)-embedded rGO hybrid materials using formic acid duality in the presence of AgNO3 or H2PtCl6 at low temperature. The reduction duality of the formic acid can convert graphene oxide (GO) to rGO and simultaneously deposit the positively charged metal ion to metal NP on rGO while the formic acid itself is converted to a CO2 evolving gas that is eco-friendly. The AgNP-embedded rGO hybrid electrode on an elastomeric substrate exhibited superior stretchable properties including a maximum conductivity of 3012 S cm(-1) (at 0 % strain) and 322.8 S cm(-1) (at 35 % strain). Its fabrication process using a printing method is scalable. Surprisingly, the electrode can survive even in continuous stretching cycles.

Cancer therapy using ultrahigh hydrophobic drug-loaded graphene derivatives.

This study aimed to demonstrate that curcumin (Cur)-containing graphene composites have high anticancer activity. Specifically, graphene-derivatives were used as nanovectors for the delivery of the hydrophobic anticancer drug Cur based on pH dependence. Different Cur-graphene composites were prepared based on polar interactions between Cur and the number of oxygen-containing functional groups of respective starting materials. The degree of drug-loading was found to be increased by increasing the number of oxygen-containing functional groups in graphene-derivatives. We demonstrated a synergistic effect of Cur-graphene composites on cancer cell death (HCT 116) both in vitro and in vivo. As-prepared graphene quantum dot (GQD)-Cur composites contained the highest amount of Cur nano-particles and exhibited the best anticancer activity compared to the other composites including Cur alone at the same dose. This is the first example of synergistic chemotherapy using GQD-Cur composites simultaneous with superficial bioprobes for tumor imaging.