A Theoretical Investigation on Nitrogen Dioxide Adsorption on Self-Assembled Monolayer-Functionalized ZnO Monolayer

This study employs Density Functional Theory (DFT) calculations to elucidate the adsorption mechanism in the context of a ZnO monolayer system integrated with a Self-Assembled Monolayer (SAM) for NO2 detection. Through rigorous computational analysis, we delve into the intricate interplay of geometric transformations, the dissociation of NO2 bonds, and shifts in electronic properties ensuing from the introduction of the SAM. The observed modifications underscore the pronounced influence exerted by the SAM on the system’s behaviour. This investigation not only sheds light on the underlying mechanisms but also paves the way for potential experimental applications involving the functionalization of ZnO with SAM for enhanced gas sensing performance. The findings hold significant promise for the advancement of gas sensor technologies with improved sensitivity and selectivity.


Introduction
The development of advanced materials with tailored surface properties has become a paramount pursuit in the realm of materials science and nanotechnology.Among these materials, zinc oxide (ZnO) has emerged as a versatile platform due to its remarkable electronic, optical, and catalytic properties [1][2][3].In recent years, the functionalization of ZnO surfaces through self-assembled monolayers (SAMs) has garnered significant attention for its potential to engineer surface reactivity, enhance selectivity, and confer tunable characteristics for various applications ranging from gas sensing to catalysis [4][5][6].One particularly intriguing avenue is the controlled adsorption of gas molecules onto these functionalized surfaces.
Among the numerous gases of interest, nitrogen dioxide (NO2) stands out due to its substantial environmental implications.As a major component of air pollution and a precursor to various secondary pollutants, NO2's impact on human health and ecosystem integrity is well-documented [7].Therefore, gaining insights into the interaction between NO2 molecules and engineered surfaces is of great significance, offering avenues for designing effective gas sensors, environmental remediation strategies, and catalysts.
This study endeavours to delve into the intriguing interplay between NO2 molecules and ZnO monolayers functionalized with self-assembled monolayers (SAMs).By employing DFT calculations, we aim to unravel the underlying mechanisms governing the adsorption of NO2 onto these modified surfaces.Our investigation seeks to shed light on the role of SAM functionalization in tailoring the adsorption behaviour, electronic structure modifications, and potential application of the ZnO monolayer.

Computational Details
Utilizing density functional theory (DFT) calculations, we embarked on an in-depth exploration of the ZnO surface and its intricate interaction with nitrogen dioxide (NO 2 ) molecules.To elucidate the exchange-correlation interaction during the relaxation process, we employed the well-regarded generalized gradient approximation (GGA) coupled with the Perdew-Burke-Ernzerhof (PBE) functional, employing the Vienna Ab initio Simulation Package (VASP) technique [8,9].
Our pursuit of accuracy extended to the realm of ionic relaxations, with stringent convergence criteria demanding a maximal force threshold of 0.01 eV/Å and an exacting tolerance of 10 5 [10].Notably, a plane-wave basis set was meticulously tailored, featuring a cutoff energy parameter set at 490 eV to ensure reliable outcomes.The computational models we crafted consisted of a 4 x 4 ZnO monolayer supercell, housing a precise arrangement of 32 atoms.The inclusion of a generous 15 Å vacuum buffer between layers strategically prevented any undue interlayer interactions, preserving the fidelity of our investigations.For our comprehensive analyses, we strategically configured K-point setups of 5 x 5 x 1 and 20 x 20 x 1, for geometry optimization and electronic structure calculations, respectively.These refinements in our methodology underscore our commitment to unveiling a nuanced understanding of the intriguing interplay between ZnO surfaces and sulfur dioxide molecules.
The adsorption energy calculation of the nitrogen dioxide on the ZnO monolayer surface was achieved using Eq.1 Where E (NO2+ZnO) , E (ZnO) , and E (NO2) are the total energies of the adsorption system, ZnO monolayer surface, and an isolated NO2 molecule, respectively.

Result and Discussion
The monolayer configuration of ZnO (referred to as ZnOML) was established through the cleavage of the bulk wurtzite ZnO crystal along the [001] axis.Subsequently, this configuration was subjected to optimization within a [4x4] supercell arrangement, where a vacuum region of 15 Å was incorporated to mitigate interlayer interactions.This process resulted in the formation of a honeycomb-like lattice structure.The analysis of Zn-O bond distances revealed a computed value of 1.87 Å, consistent with findings reported in the previous research [11,12].Furthermore, we functionalized the ZnO ML with a self-assembled monolayer (SAM) of thiol ((3-Mercaptopropyl) trimethoxy silane).After geometry optimization and self-consistent calculation, we placed the NO2 molecule on the bridge site which is the most optimum site based on previous calculations [12].After conducting adsorption simulations, an analysis was performed on the geometric structure of ZnOML-SAM-NO 2 , which is observable in Figure 1.The initially flat ZnOML surface transformed into a rippled configuration, concurrently distorted by both NO2 molecules and the SAM.It is also evident that the constituent atoms of the NO2 molecules disassociated, dispersing across the ZnO and SAM surface.The oxygen atoms of the former NO2 molecules also engaged in interactions with the ZnO surface and with the bonding structure of the SAM.Meanwhile, one of the nitrogen atoms facilitated the bonding of one hydrogen atom (from the SAM structure), forming an N-H bond and diffusing onto the ZnO surface.The Si-C bonds within the SAM structure were likewise observed to break, transitioning from their initial bond length of 1.98 Å to 4.17 Å.Through the preliminary analysis of this geometric arrangement, it is apparent that interactions transpired among the ZnO surface, the SAM, and the NO2 molecules.The angle and bond length data of atoms within this adsorption system can be found in Table 1.Compared to the bond length between the pure ZnO surface and NO 2 molecules as presented in Table 1, it becomes evident that the presence of the SAM within the system leads to a decreased separation distance between the gas molecules and the ZnO surface.This decrease in distance indicates a stronger occurrence of adsorption.In this simulation, the adsorption energy of the system is calculated to be -5.26eV.Contrasting with the adsorption of ZnO onto NO2 without the SAM, the adsorption energy of this system is more negative (more optimal), signifying that the SAM has the capability to enhance the adsorption performance of ZnO towards NO2 [12].From this observation, it's apparent that during the adsorption process, the SAM structure plays a role that contributes to the increased adsorption energy exhibited by this system.Hybridization also occurs between the N atom (from NO2) and the H atom (from the SAM), both of which are products of the dissociation of their original molecules, as depicted in Figure 2d, as previously discussed.We can also examine the band structure of ZnO-SAM/NO2, as indicated by the curve in Figure 3. It's evident that there is asymmetry in the spin-up and spin-down curves, suggesting the emergence of magnetization within this adsorption system.When compared to the ZnO/NO2 structure (without SAM), there are no longer bands appearing near the Fermi level as before, causing the material to revert to its semiconductor properties.In this system, there's also a larger bandgap change compared to the ZnO/NO2.The energy bandgap of the system for spin-up and spin-down is 0.79 eV and 0.71 eV, respectively, which are relatively broader.Consequently, it can be concluded that the SAM can alter the electronic properties of the ZnO/NO2 adsorption system.

Conclusion
In this study, we conducted DFT calculations to investigate the adsorption mechanism following the introduction of a Self-Assembled Monolayer (SAM) onto the ZnO monolayer system in the presence of NO2.The alterations observed in geometric structure, the dissociation of NO2 bonds, and the shifts in electronic properties collectively signify the substantial impact of the SAM on the system.This research unveils the potential of experimentally exploring the functionalization of ZnO with SAM for gas sensing applications, highlighting its performance as a gas sensor.

Figure 2 .
Figure 2. PDOS of the ZnO/SAM adsorption system towards NO2 Further analysis was carried out on the PDOS curve of the ZnO/SAM adsorption system in comparison to ZnO without the SAM, as depicted in Figure 2. As you can see in the PDOS graph (Figure 2a), hybridization is observed around -1.2 eV, indicating the presence of interactions between the Zn atoms (ZnO surface) and the O atoms (NO2 molecules).The captured O atom originates from the dissociated NO2 molecule during the adsorption process, resulting in the ZnO surface acquiring an additional O atom.The dissociated N atom also bonds to the ZnO surface, as illustrated in Figure 2b.The presence of hybridization around -7.2 eV also indicates the adsorption of the N atom onto the ZnO surface.Another dissociated O atom (from NO2) becomes captured by the SAM, as shown in Figure 2c.From this observation, it's apparent that during the adsorption process, the SAM structure plays a role that contributes to the increased adsorption energy exhibited by this system.Hybridization also occurs between the N atom (from NO2) and the H atom (from the SAM), both of which are products of the dissociation of their original molecules, as depicted in Figure2d, as previously discussed.

Figure 3 .
Figure 3. Band Structure of ZnO/SAM adsorption system towards NO2