Detection of ultralow concentrations of ammonia is very important in many applications such as fishing, poultry, agriculture, industry, biomedicine, and clinical diagnosis. However, detecting sub-ppm NH3 remains a challenge for chemiresistive-type gas sensors. Two-dimensional (2D) materials display tremendous potential for effective gas detectors that can be used in these applications. The as-developed MXene/SnS2 heterojunction-based chemiresistive-type sensor presents superior gas-sensing performance toward sub-ppm ammonia at room temperature. The sensor can detect NH3 concentrations down to 10 ppb at room temperature. It also displays excellent long-term stability, with a decline in the response at ∼3.4% for 20 days. The developed sensor also displays good selectivity toward NH3 relative to some potential interferents, such as HCHO, C2H5OH, CH3OH, C3H6O, benzene, and NO2. The measured in situ diffuse-reflectance infrared Fourier transform (DRIFT) spectra confirm that the products of nitric oxides during the chemical reactions occurred at the surface of MXene/SnS2. Density functional theory (DFT) based on the first principles was implemented to compute the adsorption ability of NH3 at the surface of the MXene/SnS2 heterostructure. This indicates that the enhancement in the sensing properties of the MXene/SnS2 heterostructure-based chemosensor could be ascribed to the stronger NH3 adsorption, better catalytical activity, and more effective charge transfer bestowed by the formed heterostructure and the electron-redistribution-assisted stronger extraction of electrons from the sensing material.

Hollow In 2 O 3 @TiO 2 double-layer nanospheres were prepared via a facile water bath method using the sacrifice template of carbon nanospheres. It is shown that the size of the In 2 O 3 /TiO 2 nanocomposites is 150−250 nm, the thickness of the In 2 O 3 shell is about 10 nm, and the thickness of the TiO 2 shell is about 15 nm. The sensing performances of the synthesized In 2 O 3 /TiO 2 nanocomposites-based chemiresistive-type sensor to formaldehyde (HCHO) gas under UV light activation at room temperature have been studied. Compared to the pure In 2 O 3 -and pure TiO 2based sensors, the In 2 O 3 /TiO 2 nanocomposite sensor exhibits much better sensing performances to formaldehyde. The response of the In 2 O 3 /TiO 2 nanocomposite-based sensor to 1 ppm formaldehyde is about 3.8, and the response time and recovery time are 28 and 50 s, respectively. The detectable formaldehyde concentration can reach as low as 0.06 ppm. The role of the formed In 2 O 3 /TiO 2 heterojunctions and the involved chemical reactions activated by UV light have been investigated by AC impedance spectroscopy and the in situ diffuse reflectance Fourier transform infrared spectroscopy. The improvement of the sensing properties of In 2 O 3 / TiO 2 nanocomposites could be attributed to the nanoheterojunctions between the two components and the "combined photocatalytic effects" of UV-light-emitting diode irradiation. Density functional theory calculations demonstrated that introducing heterojunctions could improve the adsorption energy and charge transfer between formaldehyde and sensing materials.

Semiconductor−metal contacts as one major challenge have severely hindered the further progress of two-dimensional (2D) electronics.Here, we present a simple and effective strategy to improve the contacts and electrical performances by fabricating van der Waals (vdW) heterostructures with 2D semiconductor MoS 2 and type-II Dirac semimetal PtTe 2 . The semiconductor MoS 2 and Dirac semimetal PtTe 2 nanoflakes are synthesized through CVD routes separately, followed by systematic material characterizations to confirm their structures. Furthermore, we constructed MoS 2 / PtTe 2 vdW heterostructures via a transfer technology with as-grown MoS 2 and PtTe 2 nanoflakes. The field-effect transistor based on MoS 2 /PtTe 2 heterostructures shows ohmic contact and improved electrical performances, such as two-terminal carrier mobility (∼38.2 cm 2 •V −1 •s −1 ) and ON/OFF ratio (∼10 4 ). We ascribe the improvement of contact and electrical performances to the utilization of ultrahigh-conductive layered PtTe 2 as an interlayer. The theoretical calculations demonstrate that the vdW contact can eliminate the Fermi level pinning effect; meanwhile, the ultrastrong covalent-like interlayer coupling guarantees the high-efficiency carrier injection across PtTe 2 and MoS 2 . The concept that synergizes 2D semiconductors as the channel and Dirac semimetal PtTe 2 as an interlayer will offer a promising approach toward the design of high-performance 2D electronics.

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