Large-Area Buckled MoS₂ Films on the Graphene Substrate

In this study, a novel buckled structure of edge-oriented MoS₂ films is fabricated for the first time by employing monolayer graphene as the substrate for MoS₂ film growth. Compared to typical buckling methods, our technique has several advantages: (1) external forces such as heat and mechanical strain are not applied; (2) uniform and controllable buckling over a large area is possible; and (3) films are able to be transferred to a desired substrate. Dual MoS₂ orientation was observed in the buckled film where horizontally aligned MoS₂ layers of 7 nm thickness were present near the bottom graphene surface and vertically aligned layers dominated the film toward the outer surface, in which the alignment structure was uniform across the entire film. The catalytic ability of the buckled MoS₂ films, measured by performing water-splitting tests in acidic environments, shows a reduced onset potential of −0.2 V versus reversible hydrogen electrode (RHE) compared to −0.32 V versus RHE for pristine MoS₂, indicating that the rough surface provided a higher catalytic activity. Our work presents a new method to generate a buckled MoS₂ structure, which may be extended to the formation of buckled structures in various 2D materials for future applications

Direct Observation of Molybdenum Disulfide, MoS₂, Domains by Using a Liquid Crystalline Texture Method

Because the properties of molybdenum disulfide (MoS₂) are strongly influenced by the sizes and boundaries of its domains, the direct visualization of large-area MoS₂ domains is one of the most important challenges in MoS₂ research. In the current study, we developed a simple and rapid method to observe and determine the boundaries of MoS₂ domains. The technique, which depends on observations of nematic liquid crystal textures on the MoS₂ surface, does not damage the sample and is not limited by domain size. Thus, this approach should significantly aid not only efforts aimed at gaining an understanding of the relationships between grain boundaries and properties of MoS₂ but also those focusing on how domain sizes are controlled during large-area synthesi

The Role of Layer-Controlled Graphene for Tunable Microwave Heating and Its Applications to the Synthesis of Inorganic Thin Films

In this paper, we present the first method for precisely controlling the heat generated by microwave heating by tuning the number of graphene layers grown by chemical vapor deposition. The conductivity of the graphene increases linearly with the number of graphene layers, indicating that Joule heating plays a primary role in the temperature control of the graphene layer. In this method, we successfully synthesize TiO₂ and MoS₂ thin films, which do not interact well with microwaves, on a layer-controlled graphene substrate for a very short time (3 min) through microwave heating

Rational Design of Aminopolymer for Selective Discrimination of Acidic Air Pollutants

Strong acidic gases such as CO₂, SO₂, and NO₂ are harsh air pollutants with major human health threatening factors, and as such, developing new tools to monitor and to quickly sense these gases is critically required. However, it is difficult to selectively detect the acidic air pollutants with single channel material due to the similar chemistry shared by acidic molecules. In this work, three acidic gases (i.e., CO₂, SO₂, and NO₂) are selectively discriminated using single channel material with precise moiety design. By changing the composition ratio of primary (1°), secondary (2°), and tertiary (3°) amines of polyethylenimine (PEI) on CNT channels, unprecedented high selectivity between CO₂ and SO₂ is achieved. Using in situ FT-IR characterizations, the distinct adsorption phenomenon of acidic gases on each amine moiety is precisely demonstrated. Our approach is the first attempt at controlling gas adsorption selectivity of solid-state sensor via modulating chemical moiety level within the single channel material. In addition, discrimination of CO₂, SO₂, and NO₂ with the single channel material solid-state sensor is first reported. We believe that this approach can greatly enhance air pollution tracking systems for strong acidic pollutants and thus aid future studies on selective solid-state gas sensors

Selective Molecular Separation on Ti₃C₂T<i>_x</i>–Graphene Oxide Membranes during Pressure-Driven Filtration: Comparison with Graphene Oxide and MXenes

In this work, we prepared 90 nm thick Ti₃C₂T<i>_x</i>–graphene oxide (GO) membranes laminated on a porous support by mixing GO with Ti₃C₂T<i>_x</i>. This process was chosen to prevent the penetration of target molecules through inter-edge defects or voids with poor packing. The lattice period of the prepared membrane was 14.28 Å, as being swelled with water, resulting in an effective interlayer spacing of around 5 Å, which corresponds to two layers of water molecules. The composite membranes effectively rejected dye molecules with hydrated radii above 5 Å, as well as positively charged dye molecules, during pressure-driven filtration at 5 bar. Rejection rates were 68% for methyl red, 99.5% for methylene blue, 93.5% for rose Bengal, and 100% for brilliant blue (hydrated radii of 4.87, 5.04, 5.88, and 7.98 Å, respectively). Additionally, the rejections of composite membrane were compared with GO membrane and Ti₃C₂T<i>_x</i> membrane

Metallic Ti₃C₂T_<i>x</i> MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio

Achieving high sensitivity in solid-state gas sensors can allow the precise detection of chemical agents. In particular, detection of volatile organic compounds (VOCs) at the parts per billion (ppb) level is critical for the early diagnosis of diseases. To obtain high sensitivity, two requirements need to be simultaneously satisfied: (i) low electrical noise and (ii) strong signal, which existing sensor materials cannot meet. Here, we demonstrate that 2D metal carbide MXenes, which possess high metallic conductivity for low noise and a fully functionalized surface for a strong signal, greatly outperform the sensitivity of conventional semiconductor channel materials. Ti₃C₂T_<i>x</i> MXene gas sensors exhibited a very low limit of detection of 50–100 ppb for VOC gases at room temperature. Also, the extremely low noise led to a signal-to-noise ratio 2 orders of magnitude higher than that of other 2D materials, surpassing the best sensors known. Our results provide insight in utilizing highly functionalized metallic sensing channels for developing highly sensitive sensors

Metallic Ti₃C₂T_<i>x</i> MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio

Achieving high sensitivity in solid-state gas sensors can allow the precise detection of chemical agents. In particular, detection of volatile organic compounds (VOCs) at the parts per billion (ppb) level is critical for the early diagnosis of diseases. To obtain high sensitivity, two requirements need to be simultaneously satisfied: (i) low electrical noise and (ii) strong signal, which existing sensor materials cannot meet. Here, we demonstrate that 2D metal carbide MXenes, which possess high metallic conductivity for low noise and a fully functionalized surface for a strong signal, greatly outperform the sensitivity of conventional semiconductor channel materials. Ti₃C₂T_<i>x</i> MXene gas sensors exhibited a very low limit of detection of 50–100 ppb for VOC gases at room temperature. Also, the extremely low noise led to a signal-to-noise ratio 2 orders of magnitude higher than that of other 2D materials, surpassing the best sensors known. Our results provide insight in utilizing highly functionalized metallic sensing channels for developing highly sensitive sensors

Synthesis and Charge Storage Properties of Hierarchical Niobium Pentoxide/Carbon/Niobium Carbide (MXene) Hybrid Materials

Orthorhombic niobium pentoxide (<i>T</i>-Nb₂O₅) offers high capacitance and fast charging–discharging rate capabilities when used as an electrode material for Li-ion capacitors. A homogeneous distribution of <i>T</i>-Nb₂O₅ nanoparticles in a highly conductive matrix represents a promising approach to maximize its energy and power densities. Here we report a one-step CO₂ oxidation of two-dimensional (2D) Nb₂CT_<i>x</i>, a member of the MXenes family of 2D transition metal carbides, which leads to a hierarchical hybrid material with <i>T</i>-Nb₂O₅ nanoparticles uniformly supported on the surface of Nb₂CT_<i>x</i> sheets with disordered carbon. The oxidation temperature, duration, and CO₂ flow rate determine the <i>T</i>-Nb₂O₅ crystallite size as well as the structure, composition, and the charge storage properties of the hybrid material. Fifty micrometer thick electrodes of the hybrid material exhibit high capacitance (330 C g^–1 and 660 mF cm^–2 at a charge–discharge time of 4 min) and good cycling performance in a nonaqueous lithium electrolyte. The charge storage kinetics are dominated by a surface-controlled process. The observed electrochemical performance is attributed to the intrinsic pseudocapacitive response and excellent energy storage capability of <i>T</i>-Nb₂O₅ coupled with the fast charge transfer pathways provided by the conductive 2D Nb₂CT_<i>x</i> sheets and the as-formed disordered carbon