Theoretical study of the adsorption of NOx on TiO2/MoS2 nanocomposites: a comparison between undoped and N-doped nanocomposites

Titanium dioxide (TiO 2 , Titania) has aroused great attentions as an important semiconductor material due to its effectiveness and outstanding properties, such as non-toxicity, low cost, high catalytic efficiency, photoactivity [ 1 ], and stability. TiO 2 has been widely utilized in many fields, such as photo-catalysis, gas sensing, organic dye-sensitized solar cells, water splitting, and pollutant degradation [ 2 – 5 ]. Three important polymorphs were found for TiO 2, namely, anatase, rutile, and brookite [ 6 ], in which anatase and rutile forms are the most widely studied ones in different fields of science and technology. The photocatalytic applications of TiO 2 were restricted due to its wide bandgap (3.2 eV), which allows the absorption of the solar spectrum at the ultraviolet region by a lower percentage (3–5 % of the incoming solar light). The doping of TiO 2 anatase with some nonmetal elements, such as nitrogen, is a convenient solution, which would enhance the photo-efficiency of TiO 2 to the visible region and improve its photocatalytic activity [ 7 , 8 ]. Two-dimensional (2D) semiconductor materials, such as MoS 2 [ 9 ], and other dichalcogenides consisting of transition metals, such as MoSe 2 , WS 2, and so on, indicate the final scale for chalcogenide dimension around the vertical direction. MoS 2 , a layered structure consisting of Mo and S atoms arranged in hexagonal structure of atomic sheets of molybdenum and sulphur atoms, attracts numerous attentions due to the excellent electrical, mechanical, and optical properties, such as satisfied bandgap, thermal stability, carrier mobility, and so on [ 10 – 12 ]. MoS 2 has been broadly used in abundant applications, such as photocatalysts, nanotribology, lithium battery, dry lubrication, hydrodesulfurization catalyst, and photovoltaic cell because of its unique electronic, photosensitive, and catalytic properties [ 13 – 16 ]. Nanoelectronic devices fabricated on 2D materials, such as MoS 2, suggest also many efficiencies for these layered materials, which cause the further miniaturization of the integrated circuits beyond Moore’s law. Recently, numerous electronic devices were made using the few-layer MoS 2 as an important component, such as field-effect transistors [ 17 ], sensors [ 18 ], etc. However, several computational studies of N-doped TiO 2 anatase nanoparticles and few-layer MoS 2 structures have been separately published, describing some of the main electronic and physical properties of these materials. Particularly, the gas-sensing capabilities of MoS 2 -based field-effect transistors and sensing films for NO and NH 3 were experimentally revealed with an enhanced sensitivity in some other works [ 19 , 20 ]. TiO 2 /MoS 2 nanocomposites have been successfully synthesized for different purposes by some experimental methods [ 21 – 23 ]. There are no explanative computational studies on the adsorption behaviors of TiO 2 /MoS 2 nanocomposites. NO x molecules have been characterized as toxic gases which are mainly emitted from power plants and vehicle engines. For the general public, the most outstanding provenance of NO 2 is internal combustion engines that burn fossil fuels to work properly. In indoor places, NO 2 emission mainly stems from cigarette smoke, kerosene heaters, and stoves. Therefore, optimal removal of these harmful molecules is an important subject to human health and environmental protection [ 24 ]. In this study, the interaction of NO x molecules with TiO 2 /MoS 2 nanocomposites has been investigated by density functional theory (DFT) computations. We present here the results of calculations of complex systems consisting of NO x molecule positioned between the TiO 2 anatase nanoparticle and MoS 2 monolayer. The electronic structure of the adsorption systems has also been analyzed, including the projected density of states (PDOSs) and molecular orbitals (MOs). The main aim of this study is to supply an overall understanding on the adsorption behaviors of nano-TiO 2 /MoS 2 composites as highly sensitive NO x sensors.