Enhancing the Sensing Performance of Zigzag Graphene Nanoribbon to Detect NO, NO2, and NH3 Gases

In this article, a zigzag graphene nanoribbon (ZGNR)-based sensor was built utilizing the Atomistic ToolKit Virtual NanoLab (ATK-VNL), and used to detect nitric oxide (NO), nitrogen dioxide (NO2), and ammonia (NH3). The successful adsorption of these gases on the surface of the ZGNR was investigated using adsorption energy (Eads), adsorption distance (D), charge transfer (∆Q), density of states (DOS), and band structure. Among the three gases, the ZGNR showed the highest adsorption energy for NO with −0.273 eV, the smallest adsorption distance with 2.88 Å, and the highest charge transfer with −0.104 e. Moreover, the DOS results reflected a significant increase of the density at the Fermi level due to the improvement of ZGNR conductivity as a result of gas adsorption. The surface of ZGNR was then modified with an epoxy group (-O-) once, then with a hydroxyl group (-OH), and finally with both (-O-) and (-OH) groups in order to improve the adsorption capacity of ZGNR. The adsorption parameters of ZGNR were improved significantly after the modification. The highest adsorption energy was found for the case of ZGNR-O-OH-NO2 with −0.953 eV, while the highest charge transfer was found for the case of ZGNR-OH-NO with −0.146 e. Consequently, ZGNR-OH and ZGNR-O-OH can be considered as promising gas sensors for NO and NO2, respectively.


Introduction
Graphene-based nanomaterials (G-NMs) have been subjected to intensive investigations in recent years due to their remarkable properties and promising application for electronics [1][2][3][4]. Nevertheless, it has been reported that graphene has some limitations in its sensing properties to some kinds of gas molecules such as CO, CO 2 , CH 4 , N 2 , NO 2 , NH 3 , and H 2 , affecting its usage for applications in the field of gas sensors [5][6][7][8]. These limitations can be solved either by functionalizing the surface of graphene or generating a G-NM which exhibits a tunable band gap that is referred to as a graphene nanoribbon (GNR) [9,10]. GNRs can be produced either by etching or patterning graphene along a specific direction [11][12][13]. In addition, practical challenges are still to be resolved in order to utilize graphene for practical sensors, such as its purity degree, trace metal content, application tests, and practical deposition of graphene-based materials on surfaces [14]. GNRs are considered a promising candidate for the field of gas sensors as compared with other C-NMs, thanks to their long and reactive edges that facilitate the adsorption of gas molecules [15][16][17][18][19][20]. Furthermore, GNRs have been reported in different forms of devices for utilization as a gas sensor, including field-effect transistors (FETs), chemiresistors, and capacitance sensors, etc. [21,22].
Although there have been great benefits to humans as a result of the continuous developments in technology and industry, there have been serious effects on human health due to the release of hazardous

Computational Method
Four different ZGNR systems (bare ZGNR, ZGNR-O, ZGNR-OH, and ZGNR-O-OH) have been investigated by DFT calculations based on the ATK-VNL package in this study. Herein, DFT was utilized to relax and to optimize all atoms and systems until the convergence of force was 0.01 eV/Å. The resulting systems were then used as gas sensors to detect NO, NO 2 , and NH 3 gases. The calculations were performed based on the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional [43,44]. The density mesh cutoff was taken to be 125 Hartree and the force tolerance to be 0.01 eV/Å. Monkhorst-Pack k point sampling of 4 × 2 × 1 was used during all calculations. The successful adsorption of NO, NO 2 , NH 3 gases on the surface of ZGNR systems was explored based on the adsorption energy (E ads ), adsorption distance (D), charge transfer (∆Q), band structure, and density of states (DOS). The adsorption energy of the gas molecules on ZGNR systems was calculated using the following formula [45][46][47]: where E ZGNR+gas is the total energy of the optimized structure of any gas molecules adsorbed on any of the different ZGNR systems. E ZGNR is the total energy of each of the ZGNR systems without gas adsorption, and E gas is the total energy of each of the optimized NO, NO 2 , and NH 3 gases. Increasing the value of E ads with a negative sign indicates that the adsorption of NO, NO 2 , and NH 3 gases on the surface of ZGNR systems is stronger [26,48]. Moreover, the charge transfer between the NO, NO 2 , and NH 3 gases and the different ZGNR systems was calculated using Mulliken population analysis [49,50].

ZGNR System
The bare ZGNR system was prepared based on ATK-VNL and investigated as a gas sensor to detect NO, NO 2 , and NH 3 gases. The optimized structures of ZGNR before and after adsorption of the three gases are given in Figure 1. The results show that the average C-C bond length of ZGNR is 1.425 Å. The adsorption of NO, NO 2 , and NH 3 gases on the surface of ZGNR was explored based on the adsorption energy, adsorption distance, charge transfer, band structure, and DOS. The calculations show that the adsorption energy of NO on ZGNR was −0.273 eV and the adsorption distance was 2.88 Å, as shown in Table 1. Moreover, the Mulliken charge analysis showed −0.104 e transfer between NO and ZGNR during the adsorption process. On the other hand, the adsorption energy, adsorption distance, and charge transfer values between NO 2 and ZGNR were −0.225 eV, 3.11 Å, and 0.040 e, respectively. The positive value of charge transfer in the case of NO 2 gas indicates that the electrons transfer from ZGNR to NO 2 . The results also reflect the capability of ZGNR to adsorb NH 3 with −0.091 eV adsorption energy and −0.018 e charge transfer.   The band structures of ZGNR before and after adsorption of NO, NO 2 , and NH 3 gases are shown in Figure 2a-d. The band structure results show that the band gap of ZGNR is 0 eV, reflecting its metallic nature, which has been reported [51,52]. Although no changes in the band gap were observed after the adsorption of NO, NO 2 , and NH 3 gases, considerable changes were detected in the DOS results in Figure 3.

ZGNR-O System
Figure 4a-d shows the optimized structure of the ZGNR-O system before and after adsorption of the gas molecules. The results show that the C-O bond length was 1.39 Å. The adsorption energy, adsorption distance, and charge transfer between the gas molecules and ZGNR-O are shown in Table  2. The adsorption distances between NO, NO2, and NH3 gases and ZGNR-O were 2.66, 3.15, and 3.15 Å, respectively. Although no considerable changes were observed in the adsorption parameters after the adsorption of NO2 gas, after the functionalization of ZGNR with -O-group, the adsorption energy increased to −0.318 and −0.124 eV for the cases of NO and NH3. Moreover, the charge transfer between NH3 gas and ZGNR-O system increased significantly to −0.128 e. Figure 5a-d shows the band structures of ZGNR-O before and after adsorption of NO, NO2, and NH3 gases. No significant changes were observed in the band gap after the adsorption of the gas molecules in the band structure results. Nevertheless, a remarkable increase in the DOS at the Fermi level was observed after the adsorption of the three gases on the surface of ZGNR-O, as shown in Figure 6. Figure 6a shows that The DOS results show that the DOS at the Fermi level increases significantly after adsorption of NO and NO 2 and slightly for the case of NH 3 . In addition to the increase of DOS at the Fermi level, a significant increase was also observed around −12.    Table 2. The adsorption distances between NO, NO 2 , and NH 3 gases and ZGNR-O were 2.66, 3.15, and 3.15 Å, respectively. Although no considerable changes were observed in the adsorption parameters after the adsorption of NO 2 gas, after the functionalization of ZGNR with -O-group, the adsorption energy increased to −0.318 and −0.124 eV for the cases of NO and NH 3 . Moreover, the charge transfer between NH 3 gas and ZGNR-O system increased significantly to −0.128 e. Figure 5a-d shows the band structures of ZGNR-O before and after adsorption of NO, NO 2 , and NH 3 gases. No significant changes were observed in the band gap after the adsorption of the gas molecules in the band structure results. Nevertheless, a remarkable increase in the DOS at the Fermi level was observed after the adsorption of the three gases on the surface of ZGNR-O, as shown in Figure 6.     10 . Figure 6. Density of states of the ZGNR-O system before and after the adsorption of (a) NO, (b) NO2, and (c) NH3.

ZGNR-OH System
In this part, the surface of ZGNR was functionalized with one -OH group and then used as a gas sensor to detect NO, NO2, and NH3 gases, as shown in Figure 7. The results show that the C-C bond length of ZGNR increased a little to 1.5 Å around the -OH group, while the C-O and O-H bonds were 1.49 Å and 0.98 Å, respectively. Table 3 shows the adsorption parameters of the

ZGNR-OH System
In this part, the surface of ZGNR was functionalized with one -OH group and then used as a gas sensor to detect NO, NO 2 , and NH 3 gases, as shown in Figure 7. The results show that the C-C bond length of ZGNR increased a little to 1.5 Å around the -OH group, while the C-O and O-H bonds were 1.49 Å and 0.98 Å, respectively. Table 3 shows the adsorption parameters of the optimized NO, NO 2 , and NH 3 gases adsorbed on ZGNR-OH. On one hand, the adsorption distances between NO, NO 2 , and NH 3 and ZGNR-OH decreased to 2.24, 1.74, and 2.18 Å, respectively. On the other hand, a significant improvement in the adsorption energy was observed with −0.641, −0.618, and −0.244 eV for the cases of NO, NO 2 , and NH 3 gases, respectively, as compared with the ZGNR and ZGNR-O systems. Moreover, the Mulliken charge analysis showed that 0.146 and 0.137 e transfer from NO and NH 3 to ZGNR-OH (negative ∆Q) during the adsorption process, while 0.074 e transfer from ZGNR-OH to NO 2 . Therefore, the sensing signal is expected to be negative for the former and positive of the latter.   The band structures of ZGNR-OH before and after adsorption of NO, NO 2 , and NH 3 gases are shown in Figure 8a-d, respectively. The results show no significant changes in the band gap after the adsorption of NO, NO 2 , and NH 3 gases; however, some changes were detected below and above the Fermi level as a result of the gas adsorption. The DOS results in Figure 9 show that the DOS at the Fermi level increases after the adsorption of NO and NH 3 , reflecting an improvement of ZGNR-OH conductivity upon gas adsorption. The results reveal that the DOS around −12.2, −8.9, −7.4, and 9.8 eV increased after the adsorption of NO gas, as shown in Figure 9a, while two new peaks were observed around 21.1 and 22.2 eV. In addition, the peak around 19.9 eV shifted to 19.0 eV. Although no significant change was detected at the Fermi level after the adsorption on NO 2 (Figure 9b), a considerable increase in the DOS around −8.9, −7.4, −2.1, 9.8, and 15.7 eV was detected. Moreover, the peak around −21.0 eV shifted to −21.8 eV, the peak around 19.9 eV shifted to 19.0 eV, and two new peaks around 21.1, and 22.2 eV were observed. Figure 9c shows that the DOS at Femi level increases slightly after the adsorption of NH 3 gas. A significant increase in the DOS around −5.2 and 6.5 eV was also observed. Figure 9. Density of states of the ZGNR-OH system before and after the adsorption of (a) NO, (b) NO2, and (c) NH3. Figure 9. Density of states of the ZGNR-OH system before and after the adsorption of (a) NO, (b) NO 2 , and (c) NH 3 .

ZGNR-O-OH System
To improve the adsorption capacity, the surface of ZGNR was functionalized with both -O-and -OH groups, as shown in Figure 10, and then used to detect NO, NO 2 , and NH 3 gases. The adsorption energies, adsorption distances, and charge transfer of the optimized NO, NO 2 , and NH 3 gases adsorbed on ZGNR-O-OH system are listed in Table 4. The results show that modifying the surface of ZGNR with -O-and -OH groups significantly enhanced its adsorption capacity. For instance, the adsorption distances decreased to 1.98, 1.68, and 2.45 Å, while the adsorption energies increased significantly to −0.625, −0.953, and −0.219 eV for the cases of the NO, NO 2 , and NH 3 gases, respectively. On the other hand, the Mulliken charge analysis demonstrated that 0.118 and 0.141 e transfer from NO and NH 3 to ZGNR-O-OH during the adsorption process, while 0.092 e transfer from ZGNR-O-OH to NO 2 gas.  The band structures of the ZGNR-O-OH system before and after the adsorption of NO, NO 2 , and NH 3 gases are shown in Figure 11a-d, respectively. The band structure results show that the band gap of ZGNR increased slightly to 0.043 eV after the functionalization with -O-and -OH groups. The band gap then decreased to 0 eV after the adsorption of the gas molecules, reflecting an improvement in its conductivity and the differentiation between the two cases before and after gas adsorption. Moreover, some other changes were observed below and above the Fermi level due to the adsorption of NO, NO 2 , and NH 3 gases. These changes are confirmed by the DOS results in Figure 12. The DOS results show that the DOS at Fermi level increased after the adsorption of NO and NH 3 . Figure 12a Figure 12c shows a significant increase in the DOS around −5.2, 6.5, 9.6, 18.2, 19.5, and 22.2 eV after the adsorption of NH 3 gas. In addition, a new peak around 23.7 eV was also observed.

Discussion
To improve the adsorption capacity of ZGNR, which is goal in this study, the surface of ZGNR was functionalized with -O-and -OH groups, creating three different systems (ZGNR-O, ZGNR-OH, and ZGNR-O-OH). Interestingly, the adsorption parameters of ZGNR reflect a significant improvement upon surface functionalization. This improvement is attributed mainly to the presence

Discussion
To improve the adsorption capacity of ZGNR, which is goal in this study, the surface of ZGNR was functionalized with -O-and -OH groups, creating three different systems (ZGNR-O, ZGNR-OH, and ZGNR-O-OH). Interestingly, the adsorption parameters of ZGNR reflect a significant improvement upon surface functionalization. This improvement is attributed mainly to the presence of the functional groups, which have been reported to play a significant role in facilitating the adsorption of the gas molecules during the sensing process [53][54][55].
The Mullekin charge analysis showed that the charge transfer is negative for the cases of NO and NH 3 gases, which is an indication that the charge transfers from the gas molecules to the ZGNR systems (negative sensing signal), which is in good agreement with the reported data stating that NO and NH 3 gases behave as donors [5,56]. On the other hand, for the case of NO 2 the charge transfer was positive, which means that the charge transfers from the ZGNR systems to the gas molecules (positive sensing signal), which have been reported to behave as an acceptor [5,56].
In addition to the adsorption energy, adsorption distance, and charge transfer, the adsorption of NO, NO 2 , and NH 3 gases on the surface of the ZGNR systems was also confirmed by the band structure and density of states results. The band structure results show some changes in the bands before and after the Fermi level as a result of the gas adsorption. These changes confirm the successful adsorption of the gas molecules on the surface of ZGNR systems. Moreover, the DOS results demonstrated a significant increase of the density at Fermi level and the appearance of some new peaks. The significant increase in the density of states is related to the improvement of the ZGNR system conductivity upon gas adsorption, which is in good agreement with the reported data that relate the change of adsorbent's conductivity after adsorption of gas molecules [57][58][59].
The sensing of nitrogen oxides can be assigned to their easy and rapid diffusion on the surface of ZGNR. This leads to formation of hydrogen bonds among the nitrogen oxides and ZGNR, which significantly contributes to increasing their binding energy. The interaction among nitrogen oxides with ZGNR with the epoxy group and hydroxyl groups can be attributed to the development of hydrogen bonds OH . . . O(N) between NO x and -OH and the formation of new weak covalent bonds (C . . . N as well as C . . . O), as well as H elimination to produce nitrous acid-and nitric acid-like moieties [60,61].

Conclusions
The main goal of this study was to enhance the sensing performance of zigzag graphene nanoribbon (ZGNR) towards the detection of NO, NO 2 , and NH 3 gases. First, primary calculations using density functional theory (DFT), based on Atomistic ToolKit Virtual NanoLab (ATK-VNL), were used to study the adsorption of the target gases on the surface of the ZGNR. The best sensing performance was found for the case of NO gas, with −0.273 eV adsorption energy and −0.104 e charge transfer during the adsorption process. In order to improve the sensing performance of ZGNR, three different functionalized systems were built: ZGNR-O, ZGNR-OH, and ZGNR-O-OH. The sensing performance towards the three gases was improved significantly after the functionalization, especially for the cases of the ZGNR-OH and ZGNR-O-OH systems. Nevertheless, the best sensing performance from the four systems based on the adsorption energy, adsorption distance, charge transfer, and conductivity improvement was identified for the case of NO gas. Among the four systems, ZGNR-OH with adsorption energy of −0.641 eV and −0.146 e charge transfer was the best. Thus, it is the most selective to NO gas, reflecting that ZGNR-OH can be considered as a promising gas sensor to detect NO gas.