INTRODUCTION

Gas sensors are essential for protecting human beings against hazardous gases such as flammable and toxic gases [1-4]. Traditional gas sensors based on semiconductor oxide materials display high sensitivity and operate at undesired high temperatures, i.e., 200-600 ˚C [5], so scientists have tried to produce one- and two-dimensional nano-materials gas sensors that operate at room temperature [5, 6].

In this regard, one-dimensional materials such as carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) due to their electronic and mechanical properties, high thermal conductivity [7], chemical stability [8, 9], high surface-to-volume rate, and hollow structure, have been different applications such as gas storage, gas sensors, etc. Also, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides, phosphorene, etc. have been used as suitable candidates for sensors in different applications due to their large surface area, thermal and electrical conductivity, high mechanical strength, and electron transfer rates [10-13]. Ref. 14 concluded that graphene functionalized with vanadium pentoxide (V₂O₅) using laser ablation technique is suitable for highly sensitive NH3 sensors [14]. Chen and coworkers have produced a Li+/CNT film that operates as an excellent sensor of the CH4 gas molecules [15].

Unfortunately, some 2D materials such as graphene and transition-metal dichalcogenides (TMDs) show low-temperature gas sensors [16, 17]. To overcome this restriction, a new family of 2D transition metal carbides and nitrides (named MXenes) have been studied for room-temperature (RT) gas-sensing applications [18-20]. MXenes are synthesized by selective etching of metal atoms from the structure of MAX phases, a large group of ternary carbides and nitrides with a hexagonal layered structure, with
the chemical composition: M_n+1AX_n, where n = 1, 2, or 3, “M” is an early transition metal, “A” is an A group (groups 13 and 14) element, an “X” is carbon, nitrogen, or both [21]. Previous studies showed that the surface of MXenes is randomly terminated by oxygen, hydroxyl, and/or fluorine where these ratios are mainly dependent on synthesis methods [22]. The unique chemistry of MXenes allows particular applications such as energy storage, water purification, gas sensors, catalysis, etc [23]. The first computational study on MXenes as gas sensors showed chemisorption of NH₃ on Ti₂CO₂ monolayer while other gas molecules (H₂, CH₄, CO, CO₂, N₂, NO₂, and O₂) showed much lower interaction [24]. Also, the first experimental report elaborated that, Ti₃C₂T_x, where T_x is a random combination of terminations present at the MXenes surfaces, showed p-type sensing behavior to methanol, ethanol, ammonia, and acetone gases [18]. Vanadium carbide (V₂CT_x) and molybdenum carbide (Mo₂C) also have been studied as gas sensors [25]. 

In addition, experimental and theoretical studies demonstrated that MXenes can be rolled as nanotubes [26]. Despite the potential applications of  MXenes as gas sensors, there are very limited studies in this area.  Interestingly, gas sensor MXene nanotubes have not been studied yet, so in this study, we investigate gas sensor properties of monolayer (Ti₂C-ML) and for the report, nanotube (Ti₂C-NT) of the Ti₂C for adsorption of CO₂ and CO gas molecules.

Simulation Method and System

In the present work, we investigated the adsorption of Co and CO₂ gas molecules on Ti₂C monolayer and nanotube by a reactive MD simulation method using the reactive force field (ReaxFF) developed by van Duin et al. [27], which is integrated with the “Large-scale Atomic/Molecular Massively Parallelized Simulator” (LAMMPs) code [28].

The ReaxFF force field used bond orders instead of the specific atomic connectivity, which allowed atoms to break and form bonds with other atoms during the simulation. The bond order is obtained from interatomic distances empirically.  The system’s total energy consists of various partial energy contributions, the bonded and non-bonded interactions (van der Waals, Coulomb). ReaxFF describes non-bonded interactions between all atoms, irrespective of connectivity. Unlike the fixed partial charges on atoms in classical MD, in ReaxFF MD, they are dynamically approximated in a charge equilibration procedure by minimizing the Coulomb energy using Electron Equilibration Method (EEM) when the atomic coordinates are updated at each time step [29].

Indeed, the Ti₂C nanotube is made of atomic coaxial cylinders whose walls include Ti@C@Ti coaxial cylinders. The simulation box had sides of 60×60×22 Å3 for monolayer and 40×40×50 Å3 for the nanotube systems that include Ti2C nanotube or monolayer in the box center and 172 CO or 109 CO₂ gas molecules, the amount of the gas molecule was chosen proportional to the volume of the boxes. The simulation ran through the canonical ensemble (NVT) calculation method with the Nosé Hoover thermostat temperature controller at T=298 K. the dynamic condition of the simulation was based on the popular Verlet algorithm. The time step and total time of the simulation were 0.01 fs and 0.3 ns, respectively. Approaching the steady states was checked by setting the total energy fluctuations to be less than 0.2%. Fig. 1 shows it against the simulation’s steps for the Ti₂C-ML@ CO as a sample.

Results and discussion

Fig. 2 and 3 show the interaction between Ti₂C monolayer and nanotube with CO and CO₂ gas molecules during simulation. As you can see in Figs. 2 and 3 at the final step, the interaction of gas molecules with Ti₂C-NTs distorts the tube cross-section. To better understand the behavior of sensing characteristics, binding energies, and charge transfer between Ti₂C-ML, Ti₂C-NT, and toxic gases, i.e., CO, and CO₂, have been calculated by the following relation and reported in the Table. 1,

Where E_ML/NT@gas, E_ML/NT, and Egas represent the total energy of the Ti₂C-ML or Ti₂C-NT with gas molecules, pristine Ti₂C-ML or Ti₂C-NT, and isolated gas molecules, respectively, divided by the whole number of the adsorbed gas molecules, Ngas.  According to Table. 1, negative values of the binding energies show that the adsorption process is energetically favorable and the gas molecules are strongly bonded to the Ti₂C-ML and Ti₂C-NT.After interactions, toxic gas molecules gain 0.518 and 0.554 e^-1 on the Ti₂C-ML and Ti₂C-NT, respectively. These charges are 0.469 and 0.551 e^-1 for CO₂ gas molecules on the Ti₂C-ML and Ti₂C-NT.

The weight percentage of the adsorbed gas molecules was calculated by Eq. 2 to find out the desired type of adsorption.

Where Ng: was the number of the adsorbed gas molecules, NTi and NC: were the number of the titanium and carbon atoms of the ML- Ti₂C or NT- Ti₂C, and W was the molecular or atomic weight. Table. 1, shows the simulation results for the chemical and physical adsorbed gas molecules by the structures. According to Table. 1, it seems that Ti₂C-ML has more sensitivity than Ti₂C- NT against CO and CO₂ toxic gas molecules. Also, physical adsorption is more probable than chemical ones. The most physical and chemical weight percentages are 27.74% and 13.11% for Ti₂C-ML against CO₂ gas molecules,  and both structures have more physical and chemical adsorption weight percentages for CO₂ gas molecules than CO.

Variation of distance densities (DDEN), which were the average number of equidistant adsorbed CO and CO2 gas molecules on two surfaces of the Ti₂C-ML and the walls of the Ti₂C-NT up to 10 Å, is shown in Fig. 4. This figure shows that dominant adsorption of CO and CO₂ gas molecules is the distance of 2 up to 6 Å from structure surfaces of ML and walls of the NT, so physical adsorption is the likely scenario as mentioned previously.  DDENs of the ML and NT for CO₂ gas molecules are comparable, and gas molecules have nearly the same distribution around these structures. Also, the adsorption of CO molecules on NT is more than twice, about 2.8 Å from the walls, and comparable with ML at other distances. The sharp peaks in Fig. 4 show the most accumulation around surfaces or walls of the structures.

Table. 2, compares this work with other works. For example, higher sensitivity to CO₂ gas molecules showed by the NbSeTe monolayer on the Te side, which operates like the Ti2C monolayer in our study [30]. In Ref. 31, the sensitivity of Ti₂CTx to CO₂ gas molecules is 0.1 %, which is very smaller than our result [31]. Khakbaz et.al reported that Ti₃C₂Tx MXene monolayer has more negative adsorption energy for CO gas molecules than CO₂ ones, in addition, physisorption is the dominant adsorption type, like our work [32]. in addition, these results have introduced about SnSe monolayer [33]. Also, SiC Bilayer adsorped CO and CO2 physically, as in our work, and they have nearly adsorption energy [34].

concluSion

In this study, the adsorption of the CO and CO₂ toxic gas molecules on Ti₂C-ML and NT has been presented, and we can conclude that two phases of the Ti₂C (ML and NT) have different behavior against these gases. The adsorption is energetically favorable. Physical adsorption is dominant, and the most physical and chemical adsorption is for CO₂ toxic gas molecules on Ti₂C-ML as well the least one is for CO toxic gas molecules on Ti₂C-NT. Results show that Ti₂C-ML and Ti₂C-NT can be used as CO and CO₂ toxic and polluted gas sensors.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.