Drastic Gas Sensing Selectivity in 2-Dimensional MoS2 Nanoflakes by Noble Metal Decoration

New Insights into Gas-Solid Interactions of NO2/MoS2 Monolayers: a Comparative Study with MoSe2 and MoTe2 Monolayers

Understanding the microscopic electronic structure determines the macroscopic properties of the materials. Sufficient sampling has the same foundational importance in understanding the interactions. The NO2/MoS2 interaction is well known, but there are still many inconsistencies in the basic data, and the source of the NO2 direct dissociation activity has not been revealed. Based on a large-scale sampling density functional theory (DFT) study, the optimal adsorption of the NO2/MoS2 monolayer system is determined. The impurity state on the top of the valence band of the S-vacancy monolayer (MoS2-VS) was determined by cross-analysis of the band structure and density of states, which has been neglected for a long time. This provides a reasonable explanation for the direct dissociation of NO2 on the MoX2 monolayers. Further atomic structure analysis reveals that the impurity state originates from the not-fully occupied valence orbitals. This also corroborates the fact that the Mo material has dissociation activity, while the W material does not. There is no impurity state on the top of the valence band of the X-vacancy WS2 and WSe2 monolayers. Interestingly, NO2 dissociation did not occur in the MoTe2-VTe monolayer. This may be related to the 6s inert electron pair effect of the Te atom. The double-oriented adsorption behavior of NO2is also revealed. In contrast to the MoSe2 and MoTe2 monolayers, NO2-oriented adsorption on the MoS2 perfect monolayer deviates obviously, which is speculated to be related to space limitation and larger electronegativity of the S atom. The oriented adsorption ability of the MoX2 monolayers followed the order MoTe2 (64.4%) > MoSe2 (44.8%) > MoS2 (42.7%), according to the directed proportion. Renewed insights into the adsorption basic data and the understanding of the electronic structure of NO2/MoX2 (X = S, Se, Te) monolayer systems provide a basic understanding of the gas-surface interactions and various future surface-related advanced applications.

Mechanism of Interfacial Molecular Interactions Reveals the Intrinsic Factors for the Highly Enhanced Sensing Performance of Ag-Loaded Co3O4

The noble metal-loaded strategy can effectively improve the gas-sensing performances of metal oxide sensors. However, the gas-solid interfacial interactions between noble metal-loaded sensing materials and gaseous species remain unclear, posing a significant challenge in correlating the physical and chemical processes during gas sensing. In this study, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in situ Raman spectroscopy were conducted to collaboratively investigate the interfacial interactions involved in the ethanol gas-sensing processes over Co3O4 and Ag-loaded Co3O4 sensors. In situ DRIFTS revealed differences in the compositions and quantities of sensing reaction products, as well as in the adsorption-desorption interactions of surface species, among Co3O4 and Ag-loaded Co3O4 materials. In parallel, in situ Raman spectroscopy demonstrated that the ethanol atmosphere can modulate the electron scattering of Ag-loaded Co3O4 materials but not of raw Co3O4. In situ experimental results revealed the intrinsic reason for the highly enhanced sensing performances of the Ag-loaded Co3O4 sensors toward ethanol gas, including a decreased optimal working temperature (from 250 to 150 °C), an improved gas response level (from 24 to 257), and accelerated gas recovery dynamics. This work provides an effective platform to investigate the interfacial interactions of sensing processes at the molecular level and further advances the development of high-performance gas sensors.

Substantially Accelerated Response and Recovery in Pd-Decorated WO3 Nanorods Gasochromic Hydrogen Sensor

The development of hydrogen (H2) gas sensors is essential for the safe and efficient adoption of H2 gas as a clean, renewable energy source in the challenges against climate change, given its flammability and associated safety risks. Among various H2 sensors, gasochromic sensors have attracted great interest due to their highly intuitive and low power operation, but slow kinetics, especially slow recovery rate limited its further practical application. This study introduces Pd-decorated amorphous WO3 nanorods (Pd-WO3 NRs) as an innovative gasochromic H2 sensor, demonstrating rapid and highly reversible color changes for H2 detection. In specific, the amorphous nanostructure exhibits notable porosity, enabling rapid detection and recovery by facilitating effective H2 gas interaction and efficient diffusion of hydrogen ions (H+) dissociated from the Pd nanoparticles (Pd NPs). The optimized Pd-WO3 NRs sensor achieves an impressive response time of 14 s and a recovery time of 1 s to 5% H2. The impressively fast recovery time of 1 s is observed under a wide range of H2 concentrations (0.2-5%), making this study a fundamental solution to the challenged slow recovery of gasochromic H2 sensors.

Flicker Noise in Resistive Gas Sensors-Measurement Setups and Applications for Enhanced Gas Sensing

We discuss the implementation challenges of gas sensing systems based on low-frequency noise measurements on chemoresistive sensors. Resistance fluctuations in various gas sensing materials, in a frequency range typically up to a few kHz, can enhance gas sensing by considering its intensity and the slope of power spectral density. The issues of low-frequency noise measurements in resistive gas sensors, specifically in two-dimensional materials exhibiting gas-sensing properties, are considered. We present measurement setups and noise-processing methods for gas detection. The chemoresistive sensors show various DC resistances requiring different flicker noise measurement approaches. Separate noise measurement setups are used for resistances up to a few hundred kΩ and for resistances with much higher values. Noise measurements in highly resistive materials (e.g., MoS2, WS2, and ZrS3) are prone to external interferences but can be modulated using temperature or light irradiation for enhanced sensing. Therefore, such materials are of considerable interest for gas sensing.

Contactless photoelectrochemical biosensors based on hierarchical MXene/Bi2S3 nanosheets with the branched hybridization chain reaction

Ethyl carbamate, a substance frequently occurring in fermented foods, seriously affects people's health; however, poor sensitivity constrains the development of ethyl carbamate sensors. In this work, hierarchical Bi2S3/MXene nanosheets were synthesized using a hydrothermal method, and experimentally their coupled UV light is an efficient NH3 sensing material. Meanwhile, the density functional theory (DFT) confirms that the MXene/Bi2S3 nanosheet interface has an excellent ability to adsorb NH3, resulting in a change of photocurrent. As a proof-of-concept, a highly sensitive ethyl carbamate photoelectrochemical (PEC) biosensor was constructed based on the ammonia generation strategy of glutamate dehydrogenase coupled to the branched hybridization chain reaction (bHCR). Specifically, the target-triggered bHCR enriches a large number of enzyme-encapsulated liposomes, while the enzymatic NH3-generation reaction will cause a change in the Bi2S3/MXene photocurrent, which completes the target detection process. Under optimal conditions, the constructed PEC biosensors exhibited superior analytical performance toward ethyl carbamate in the range of 0.01 μg/mL to 1 μg/mL and limit of detection (LOD) down to 0.001 μg/mL. In addition, it offers an effective method for food safety monitoring due to its excellent stability, fast response, and maneuverability on real samples (red wine, yellow wine, and brandy).

Porous carbon nanosheets derived from two-dimensional Fe-MOF for simultaneous voltammetric sensing of dopamine and uric acid

It is of great significance for electrochemical sensors to simultaneously detect dopamine (DA) and uric acid (UA) related to biological metabolism. In this work, two-dimensional (2D) porous carbon nanosheets (CNS) was prepared as electrocatalysts to improve the sensitivity, the selectivity, and the detection limit of the simultaneous detection. First, 2D amorphous iron-metal organic frameworks (Fe-MOF) was synthesized with Fe3+and terephthalic acid via a facile wet chemistry method at room temperature. And then, CNS was prepared by pyrolysis and pickling of Fe-MOF. CNS had large specific surface area, good electrical conductivity and lots of carbon defects. The response currents of the CNS modified electrode was larger than those of the control electrodes in the simultaneous determination. The simultaneous determination was measured via differential pulse voltammetry to reduce the effect of capacitive currents on quantitative analysis. The CNS modified electrodes showed high sensitivity and low detection limit for the simultaneous detection of DA and UA. The modified electrodes have been successfully used to detect DA and UA in normal human serum.

Reconfigurable Manipulation of Oxygen Content on Metal Oxide Surfaces and Applications to Gas Sensing

Oxygen vacancies and adsorbed oxygen species on metal oxide surfaces play important roles in various fields. However, existing methods for manipulating surface oxygen require severe settings and are ineffective for repetitive manipulation. We present a method to manipulate the amount of surface oxygen by modifying the oxygen adsorption energy by electrically controlling the electron concentration of the metal oxide. The surface oxygen control ability of the method is verified using first-principles calculations based on density functional theory (DFT), X-ray photoelectron spectroscopy (XPS), and electrical resistance analysis. The presented method is implemented by fabricating oxide thin film transistors with embedded microheaters. The method can reconfigure the oxygen vacancies on the In2O3, SnO2, and IGZO surfaces so that specific chemisorption dominates. The method can selectively increase oxidizing (e.g., NO and NO) and reducing gas (e.g., H2S, NH3, and CO) reactions by electrically controlling the metal oxide surface to be oxygen vacancy-rich or adsorbed oxygen species-rich. The proposed method is applied to gas sensors and overcomes their existing limitations. The method makes the sensor insensitive to one gas (e.g., H2S) in mixed-gas environments (e.g., NO2+H2S) and provides a linear response (R2 = 0.998) to the target gas (e.g., NO2) concentration within 3 s. We believe that the proposed method is applicable to applications utilizing metal oxide surfaces.

Molecular modeling and simulation of transition metal-doped molybdenum disulfide biomarkers in exhaled gases for early detection of lung cancer

BACKGROUND

The presence of volatile organic compounds (VOCs) in the exhaled breath of lung cancer patients is the only available source for detecting the disease at its initial stage. Exhaled breath analysis depends purely on the performance of the biosensors. The interaction between VOCs and pristine MoS₂ is repulsive in nature. Therefore, modifying MoS₂ via surficial adsorption of the transition metal nickel is of prime importance. The surficial interaction of six VOCs with Ni-doped MoS₂ led to substantial variations in the structural and optoelectronic properties compared to those of the pristine monolayer. The remarkable improvement in the conductivity, thermostability, good sensing response, and recovery time of the sensor exposed to six VOCs revealed that a Ni-doped MoS₂ exhibits impressive properties for the detection of exhaled gases. Different temperatures have a significant impact on the recovery time. Humidity has no effect on the detection of exhaled gases upon exposure to VOCs. The obtained results may encourage the use of exhaled breath sensors by experimentalists and oncologists to enable potential advancements in lung cancer detection.

METHODS

The surface adsorption of transition metal and its interaction with volatile organic compounds on a MoS₂ surface was studied by using Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA). The pseudopotentials used in the SIESTA calculations are norm-conserving in their fully nonlocal forms. The atomic orbitals with finite support were used as a basis set, allowing unlimited multiple-zeta and angular momenta, polarization, and off-site orbitals. These basis sets are the key for calculating the Hamiltonian and overlap matrices in O(N) operations. The present hybrid density functional theory (DFT) is a combination of PW92 and RPBE methods. Additionally, the DFT+U approach was employed to accurately ascertain the coulombic repulsion in the transition elements.

The Oxygen Evolution Reaction at MoS2 Edge Sites: The Role of a Solvent Environment in DFT-Based Molecular Simulations

Density functional theory (DFT) calculations are employed to study the oxygen evolution reaction (OER) on the edges of stripes of monolayer molybdenum disulfide. Experimentally, this material has been shown to evolve oxygen, albeit with low efficiency. Previous DFT studies have traced this low catalytic performance to the unfavourable adsorption energies of some reaction intermediates on the MoS₂ edge sites. In this work, we study the effects of the aqueous liquid surrounding the active sites. A computational approach is used, where the solvent is modeled as a continuous medium providing a dielectric embedding of the catalyst and the reaction intermediates. A description at this level of theory can have a profound impact on the studied reactions: the calculated overpotential for the OER is lowered from 1.15 eV to 0.77 eV. It is shown that such variations in the reaction energetics are linked to the polar nature of the adsorbed intermediates, which leads to changes in the calculated electronic charge density when surrounded by water. These results underline the necessity to computationally account for solvation effects, especially in aqueous environments and when highly polar intermediates are present.

Highly Selective NH3 Sensor Based on MoS2/WS2 Heterojunction

In this paper, the heterostructure of MoS₂/WS₂ was prepared by a hydrothermal method; the n-n heterostructure was demonstrated using TEM combined with Mott-Schottky analysis. The valence and conduction band positions were further identified by the XPS valence band spectra. The NH₃-sensing properties were assessed at room temperature by changing the mass ratio of the MoS₂ and WS₂ components. The 50 wt%-MoS₂/WS₂ sample exhibited the best performance, with a peak response of 23643% to NH₃ at a concentration of 500 ppm, a minimum detection limit of 20 ppm, and a fast recovery time of 2.6 s. Furthermore, the composites-based sensors demonstrated an excellent humidity immune property with less than one order of magnitude in the humidity range of 11-95% RH, revealing the practical application value of these sensors. These results suggest that the MoS₂/WS₂ heterojunction is an intriguing candidate for fabricating NH₃ sensors.