Narrow Results

The gas response of metal oxide-based sensors depends strongly on its adsorption properties. To explore the potential sensing capability of pristine and nitrogen modified TiO₂/graphene oxide (GO) heterostructures, the adsorption of NO₂ molecule on the N-doped nanocomposites was investigated using density functional theory (DFT) calculations. Six possible configurations were simulated based on the estimated adsorption energies. The binding sites were located over the oxygen, doped nitrogen and five-fold coordinated titanium atoms of TiO₂. The electronic properties including atomic Mulliken population, projected density of states and molecular orbitals were investigated in detail. The N–O bonds of the NO₂ molecule were significantly increased after the adsorption process. The adsorption of NO₂ molecule on the N-doped nanocomposite is more energetically favorable than the adsorption on the undoped one. The results suggest that NO₂ chemisorbs on the considered nanocomposites. Mulliken population analysis reveals a noticeable charge transfer from the nanocomposite to the molecule, which indicate that NO₂ acts as a charge acceptor. Molecular orbital calculations show that the highest occupied molecular orbitals (HOMOs) of the studied systems were mainly localized on the adsorbed NO₂ molecule. The significant overlaps in the projected density of states (PDOS) spectra of the interacting atoms confirm the formation of chemical bonds between them. There is a direct relationship between the results of charge transfer and sensing responses. N-doped nanocomposites have better sensing response than the undoped ones. The results highlight the possibility to develop innovative highly efficient NO₂ sensors based on novel TiO₂/GO nanocomposites.

Regarding two-dimensional (2D) nanomaterials as gas sensors, we have studied the adsorption of gas molecules (NH₃, NO₂, NO) on the graphitic GaN sheet (PL-GaN) using density functional theory calculations. “Langmuir” adsorption of gas molecules of NH₃, NO₂, and NO on the graphitic GaN sheet has been accomplished using density functional theory. The changes in charge density have shown a more important charge transfer on the hexagonal honeycomb nanosheet of gallium nitride (GaN) which acts as the electron acceptor while gas molecules act as the stronger electron donors through adsorption on the graphitic-like GaN surface. The adsorption of NH₃, NO${}_{2,}$ and NO, respectively, on GaN nanosheet has more contribution with high expansion curves of hydrogen, nitrogen and oxygen. The results extracted from PDOS curves after optimization show the electron transferring from nitrogen and oxygen of gas molecules (adsorbate/donor) towards gallium in the active site of GaN surface (adsorbent/acceptor). GaN nanosheet represents enough capability for adsorbing gases of NH₃, NO₂, and NO through charge transfer from nitrogen and oxygen atoms to the gallium atom regarding the intra-atomic and interatomic interactions. Concerning the change of physicochemical properties of the PL-GaN sheet before and after molecule adsorption, the PL-GaN nanosheet can be applied as an appropriate selective gas sensor for NH₃, NO₂ and NO detection.

TeO₂ nanorods functionalized with Pd were prepared by a three-step process comprising thermal evaporation of Te powders, Pd deposition by photo-reduction, and annealing. Sensors were fabricated by using the Pd-functionalized TeO₂ nanorods. Scanning electron microscopy images exhibited that the nanorods with diameters in a range of 50–100 nm and lengths of a few micrometers were covered with the Pd nanoparticles with an average diameter of ~ 15 nm. Transmission electron microscopy and X-ray diffraction analysis revealed that the nanorods were monocrystalline simple tetragonal TeO₂. On the other hand, the nanoparticles on them were confirmed to be nanocrystalline face-centered cubic Pd. The multiple-networked TeO₂ nanorod sensors exhibited a sensitivity of 3.13% at 100 ppm NO₂ at 300°C, whereas the Pd-functionalized TeO₂ nanorod sensors exhibited a sensitivity of 11.97% under the same condition. The recovery time of TeO₂ nanorods was decreased considerably at every NO₂ concentration by the Pd-functionalization even if the response time decreased or increased slightly depending upon the NO₂ concentration. In addition, the origin of the enhancement of the sensing properties of the TeO₂ nanorods by functionalization with Pd is discussed.

To enhance the poor sensing capabilities of traditional p-type metal oxide semiconductor sensors, a novel gas sensor was designed with Co₃O₄/CuO heterostructure modified by glutathione (GSH)-reduced gold (Au), which was characterized by different analyses. The experiments showed that the 0.5wt.% Au–Co₃O₄/CuO gas sensor exhibited excellent sensing performance with a response value of up to 28.3 for 3ppm NO₂ at a relatively low operating temperature ($100^{\circ }$C). Besides, it demonstrated a linear relationship between detection concentration and response value, displayed good selectivity for NO₂, showcased stability and exhibited repeatability. Moreover, the sensor revealed the ability to resist humidity at the optimal temperature. Finally, we proposed a gas sensing mechanism to explain the enhanced NO₂ detection performance of the sensor, attributed to the heterojunction structure between Co₃O₄/CuO and the catalytic properties of Au. This led to an increase in adsorbed oxygen, as confirmed by XPS characterization. Therefore, the 0.5wt.% Au–Co₃O₄/CuO ternary nanocomposite gas sensor is deemed to be a highly effective sensing material that can be utilized to detect NO₂ gas in real-life applications.

Titanium dioxide (TiO₂) nanoparticles were prepared by resistive heating Ti filament under an oxygen atmosphere. The obtained nanoparticles were confirmed to be a tetragonal crystal TiO₂ and the particle size increased with increasing the oxygen pressure. The NO₂ gas sensing properties of the TiO₂ nanoparticles were investigated. All the sensors made of TiO₂ nanoparticles with different particle sizes exhibited the maximum sensitivities to 1 ppm NO₂ at a relative low operating temperature of 150°C. Comparing with the large particle size of TiO₂ nanoparticles, the ones with the smallest particle size exhibited the highest sensitivity and the best response and recovery characteristic to various NO₂ gas concentrations.

A detailed understanding of the mechanism of nitrogen oxides (NOx) absorption by ionic liquids (ILs) is of great significance for the application of ILs in the gas capture and separation process. The interaction energies between the cation ([Emim] +) or the anion ([OTf] -) of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ILs and NO₂ (or N₂O₄) are estimated at the M06-2X/6-31+G^** level with basis set superposition error (BSSE) correction. Two interaction models in the complexes of cation with NO₂ (or N₂O₄) have been considered. The π_cation-π_NO2 mode is favorable than the hydrogen-bonding model while there is a preference for the hydrogen-bonding model to the π_cation-π_N2O4 in the cation- N₂O₄ system. The calculated results demonstrate that the interaction between the anion and NO₂ (or N₂O₄) is the dominant factor in determining the absorption process of NO₂ by the ILs and the expected π_cation-π stacking contribution to the efficient absorption of NO₂ is not reproduced. This may indicate that the role of the π_cation-π_N2O4 stacking interaction may have been overestimated. The NBO and AIM analysis further provide evidence for the interaction model and strength between the ion and the gas molecules.