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. TiCT 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.

In this work, we demonstrate that gas adsorption is significantly higher in edge sites of vertically aligned MoS2 compared to that of the conventional basal plane exposed MoS2 films. To compare the effect of the alignment of MoS2 on the gas adsorption properties, we synthesized three distinct MoS2 films with different alignment directions ((1) horizontally aligned MoS2 (basal plane exposed), (2) mixture of horizontally aligned MoS2 and vertically aligned layers (basal and edge exposed), and (3) vertically aligned MoS2 (edge exposed)) by using rapid sulfurization method of CVD process. Vertically aligned MoS2 film shows about 5-fold enhanced sensitivity to NO2 gas molecules compared to horizontally aligned MoS2 film. Vertically aligned MoS2 has superior resistance variation compared to horizontally aligned MoS2 even with same surface area exposed to identical concentration of gas molecules. We found that electrical response to target gas molecules correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. Density functional theory (DFT) calculations corroborate the experimental results as stronger NO2 binding energies are computed for multiple configurations near the edge sites of MoS2, which verifies that electrical response to target gas molecules (NO2) correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. We believe that this observation extends to other 2D TMD materials as well as MoS2 and can be applied to significantly enhance the gas sensor performance in these materials.

Miniaturization of electronics demands electromagnetic interference (EMI) shielding of nanoscale dimension. The authors report a systematic exploration of EMI shielding behavior of 2D Ti3C2Tx MXene assembled films over a broad range of film thicknesses, monolayer by monolayer. Theoretical models are used to explain the shielding mechanism below skin depth, where multiple reflection becomes significant, along with the surface reflection and bulk absorption of electromagnetic radiation. While a monolayer assembled film offers ≈20% shielding of electromagnetic waves, a 24‐layer film of ≈55 nm thickness demonstrates 99% shielding (20 dB), revealing an extraordinarily large absolute shielding effectiveness (3.89 × 106 dB cm2 g−1). This remarkable performance of nanometer‐thin solution processable MXene proposes a paradigm shift in shielding of lightweight, portable, and compact next‐generation electronic devices.

We discuss the dominant factors influencing the rate of oxidation of Ti3C2TxMXene flakes, and present guidelines for their storage with the aim of maintaining the intrinsic properties of the as-prepared material.

Ti3C2T x MXene is an attractive two-dimensional (2D) material for a wide variety of applications; however, measured properties vary widely from study to study. A potential factor to the property differences relates to variability in the MAX phase precursors. To illustrate this, Ti3AlC2, the precursor for Ti3C2T x MXene, was synthesized using three carbon sources (graphite, carbon lampblack, and titanium carbide (TiC)) at 1650 °C for 2 h. Thermal analysis was utilized to examine the reaction mechanism, indicating that the three carbon sources experience different reaction pathways. The Ti3AlC2 MAX powders were then converted into Ti3C2T x MXene and delaminated. The products revealed differences with respect to the lateral flake size, chemical composition, chemical stability in deionized water, and electrical conductivity. Graphite-produced Ti3C2T x showed the highest conductivity (∼4400 S/cm) and is the most stable (time constant of 10.1 days), while TiC-produced MXene has comparable conductivity (∼3480 S/cm), but the lowest colloidal stability (4.8 days), and carbon lampblack has the lowest conductivity (∼1020 S/cm) and low chemical stability (5.1 days). Furthermore, gas sensors were fabricated from all three MXenes to probe differences in their performance. The TiC-produced Ti3C2T x showed the highest response, followed by graphite-produced, and last Ti3C2T x produced from carbon lampblack. This illustrates that synthesis of the MAX precursor material leads to significant difference within the resultant MXene and provides another pathway for further control over their properties.

We have demonstrated a highly stable electric heater made of oxidation-resistant MXene film, which was capable of stable operation in air under highly oxidizing conditions (70 °C, 100% RH).

Engineering electrode nanostructures is critical in developing high-capacity, fast rate-response, and safe Li-ion batteries. This study demonstrates the synthesis of orthorhombic Nb 2 O 5 @Nb 4 C 3 T x (or @Nb 2 CT x ) hierarchical composites via a one-step oxidation -in fl owing CO 2 at 850 °C -of 2D Nb 4 C 3 T x (or Nb 2 CT x ) MXene. The composites possess a layered architecture with orthorhombic Nb 2 O 5 nanoparticles decorated uniformly on the surface of the MXene fl akes and interconnected by disordered carbon. The composites have a capacity of 208 mAh g −1 at a rate of 50 mA g −1 (0.25 C) in 1-3 V versus Li + /Li, and retain 94% of the specifi c capacity with 100% Coulombic effi ciency after 400 cycles. The good electrochemical performances could be attributed to three synergistic effects: (1) the high conductivity of the interior, unoxidized Nb 4 C 3 T x layers, (2) the fast rate response and high capacity of the external Nb 2 O 5 nanoparticles, and (3) the electron "bridge" effects of the disordered carbon. This oxidation method was successfully extended to Ti 3 C 2 T x and Nb 2 CT x MXenes to prepare corresponding composites with similar hierarchical structures. Since this is an early report on producing this structure, there is much room to push the boundaries further and achieve better electrochemical performance.

Orthorhombic niobium pentoxide (T-Nb2O5) offers high capacitance and fast charging–discharging rate capabilities when used as an electrode material for Li-ion capacitors. A homogeneous distribution of T-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 x , a member of the MXenes family of 2D transition metal carbides, which leads to a hierarchical hybrid material with T-Nb2O5 nanoparticles uniformly supported on the surface of Nb2CT x sheets with disordered carbon. The oxidation temperature, duration, and CO2 flow rate determine the T-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 T-Nb2O5 coupled with the fast charge transfer pathways provided by the conductive 2D Nb2CT x sheets and the as-formed disordered carbon.