We have investigated the electronic and optical properties of monolayer SnP₃ adsorbed by small gas molecules by using first-principle calculations based on density functional theory. Six initial adsorption sites were selected to explore the most sensitive site to investigate the optimal adsorption positions of CH₄, H₂O, NH₃, NO, NO₂, and SO₂ adsorbed on the monolayer SnP₃. Aiming at the optimal adsorption configurations, the adsorption energy, adsorption distance, and charge densities are computed to investigate the adsorption types. Our results reveal that monolayer SnP₃ is sensitive to NO and SO₂ via strong physical adsorption, and NO₂ via chemical adsorption, thereby forming a new bond between O and P atoms. Analysis of the optical property shows that the adsorption of NO, NO₂, and SO₂ can remarkably influence the dielectric function and adsorption coefficient of monolayer SnP₃. Our theoretical studies indicate that monolayer SnP₃ is a good gas-sensitive material used for NO, NO₂, and SO₂ detection.

Ultraviolet (UV) disinfection is an early discovered technology that is currently and widely used for water treatment and food hygiene treatment. A newly emerging technology of UV disinfection, that is, UV light-emitting diodes (UV-LEDs), has aroused considerable research attention. UV-LEDs feature numerous advantages compared with traditional UV mercury vapor lamps and are expected to replace traditional UV lamps. Researchers currently perform studies to obtain data and develop methods for UV-LED water treatment systems. This article analyzes the latest research status and discusses the types of inactivation factors, such as the wavelength selectivity of UV light source, control of UV dose, effect of inactivation rate constant (K) (cm ² /mJ), working mode of water sample, external auxiliary system, and UV sensitivity of pathogenic bacteria in water. The wavelengths of approximately 260 and 280 nm normally feature strong inactivation characteristics. When compared with the approximately 260 nm wavelength chip, the around 280 nm wavelength chip proves to be a better choice as its higher wavelength light power can result in faster disinfection capacity of bacteria. UV dose can also be used as the reference value for disinfection of drinking water, whereas the inactivation rate constant (K) (cm ² /mJ) varies with different microorganism internal structures. Changing the working mode or adding an auxiliary system can also enhance the inactivation effect in water treatment system settings. In addition, we can compare the inactivation capacities of several pathogens as follows: ΦX174 > Escherichia coli > T type bacteriophage > Bacillus subtilis > MS2 or Qβ > human adenovirus. The in-depth investigation and discussion of inactivation factors and the mechanism of action in UV-LEDs water treatment systems will establish a more efficient UV-LED disinfection method in the future, provide a guiding direction, and promote the standardization and normalization of pathogen inactivation mechanism in UV-LED water treatment systems.

Using first-principles calculation, we have studied the adsorption effect of small gas molecules (H₂O, CO₂, CH₄, SO₂, H₂S, and NH₃) on GeP₃ monolayer. To determine the most stable adsorption site, five adsorption sites (center, Ge, P, bridge GeP, and bridge PP) were considered in the paper. Through calculations of adsorption energy, adsorption distance, and charge transfer, we preliminarily determined that H₂O, CO₂, and CH₄ were physically adsorbed on GeP₃ via weak van der Waals force. However, SO₂, H₂S, and NH₃ were chemically adsorbed on GeP₃ with new covalent bonds formed, as concluded by calculations of electron localization function and charge density difference. Gas molecule adsorption can cause significant changes in the band gap of single-layer GeP₃, indicating that pristine GeP₃ monolayer is sensitive to these gases. In addition, the adsorption energy of the H₂O, CO₂, and CH₄ adsorbed on GeP₃ can be tuned effectively by employing an external electric field. Our theoretical studies reveal that GeP₃ monolayer is a promising gas-sensitive material used in nanometer devices.