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Few-layer MoS2 with exceptional physical and chemical properties has attracted notable attention for next-generation gas sensors. The layer-dependent sensing performance for detecting NO2 based on MoS2 is not fully understood. Here, we report the direct synthesis of high-crystallinity uniform few-layer MoS2 via chemical vapor deposition. The influence of layer thickness on the NO2-sensing performance is evaluated. We show that, as compared to the monolayer and trilayer counterparts, the bilayer MoS2 presents the best sensitivity at room temperature. Further density functional theory calculations reveal that the bilayer MoS2 with an AA stacking sequence is more favorable for the physisorption of NO2 molecules due to the more negative adsorption energy, facilitating charge transfer during the NO2 adsorption and thus increasing the electrical response of the gas sensors. This work provides a basic understanding of layer-dependent gas-sensing performance and serves as a reference for designing room-temperature gas sensors for low power consumption and reliable responsivity. [ABSTRACT FROM AUTHOR]

Correlating the microscopic structural characteristics with the macroscopic electrochemical performance in electrode materials is critical for developing excellent-performance lithium-ion batteries, which however remains largely unexplored. Here, we show that the Zn 2 SnO 4 (ZTO) nanowires (NWs) with smaller diameters ( d < 5 nm) exhibit slower capacity fade rate and better cycling stability, as compared with the NWs with larger diameters ranging from tens to hundreds of nanometers. By applying in situ transmission electron microscopy (TEM), we discover a strong correlation of cracking behavior with the NW diameter. Upon the first lithiation, there exists a critical diameter of ∼80 nm, below which the NWs neither crack nor fracture, and above which the cracks could easily nucleate and propagate along the specific planes, resulting in the deteriorated cycling stability in larger sized electrodes. Further theoretical calculations based on the finite element model and the climbing image nudged elastic band method faithfully predict the size-dependent cracking behaviors, which may result from the synergistic effect of axial stress evolution as well as preferential Li-ion migration directions during the first lithiation. This work provides a real-time tracking of the tempo-spatial structural evolution of a single ZTO NW, which facilitates a fundamental understanding of how the sample size affects the electrochemical behavior and thus offers a reference for future battery design and application strategy.

Unveiling the effects of oxygen vacancies on the structural stability of layered α-MoO 3 is critical for optimizing its physical and chemical properties. Herein, we present experimental evidence regarding the phase stability of α-MoO 3 with ∼2% oxygen vacancy concentrations. Interestingly, we report a previously ignored oxygen-deficient orthorhombic MoO 3- x phase in space group Cmcm . Further density functional theory calculations reveal a detailed phase transition mechanism from α-MoO 3 to MoO 3- x . More importantly, we demonstrate that two-dimensional (2D) large polarons must exist to stabilize the MoO 3- x crystal structure. 2D large polarons are suspected to exist in numerous quasi-2D systems but have never been found in layered α-MoO 3 or other molybdenum oxides. Our work contributes to a basic understanding of the polaronic behavior in MoO 3- x and may broaden the application realm of molybdenum oxides.

Gallium oxide (Ga 2 O 3 ) has attracted extensive attention as a potential candidate for low-dimensional metal-oxide-semiconductor field-effect transistors (MOSFETs) due to its wide bandgap, controllable doping, and low cost. The structural stability of nanoscale Ga 2 O 3 is the key parameter for designing and constructing a MOSFET, which however remains unexplored. Using in situ transmission electron microscopy, we reveal the size-dependent phase transition of sub-2 nm Ga 2 O 3 nanowires. Based on theoretical calculations, the transformation pathways from the initial monoclinic β-phase to an intermediate cubic γ-phase and then back to the β-phase have been mapped and identified as a sequence of Ga cation migrations. Our results provide fundamental insights into the Ga 2 O 3 phase stability within the nanoscale, which is crucial for advancing the miniaturization, light weight, and integration of wide-bandgap semiconductor devices.

Framework structured tungsten bronzes serve as promising candidates for electrode materials in sodium-ion batteries (SIBs). However, the tungsten bronze framework structure changes drastically as mediated by the sodium ion concentration at high temperatures. While the three-dimensional ion channels facilitate fast ion storage and transport capabilities, the structural instability induced by Na + migration is a big concern regarding the battery performance and safety, which unfortunately remains elusive. Here, we show the real-time experimental evidence of the phase transitions in framework structured Na 0.36 WO 3.14 (triclinic phase) by applying different external voltages. The Na + -rich (Na 0.48 WO 3 , tetragonal phase) or -deficient (Na x WO 3 , x < 0.36, hexagonal phase) phase nucleates under the positive or negative bias, respectively. Combined with the theoretical calculations, the atomistic phase transition mechanisms associated with the Na + migration are directly uncovered. Our work sheds light on the phase instability in sodium tungsten bronzes and paves the way for designing advanced SIBs with high-stability.