Introduction

Agricultural activities result 80–90% of the total global ammonia (NH3) emission [1]. Its emission results mainly through volatilization of livestock manure as well as a synthetic mineral for nitrogen based fertilizers [2, 3]. NH3 emissions alongside with other greenhouse gases that result mainly from agricultural activities have negative effects on air quality [4]. It is associated with two main environmental hazards: acidification and eutrophication [5]. When ammonia interacts with water, it increases the acidification of both soil and water [6]. Furthermore, the deposition of ammonia in soil and water can increase their nitrogen level that may increase the eutrophication of aquatic ecosystems [5]. Human exposure to ammonia concentrations of 25 ppm or higher may negatively influence the lung functions and respiratory track, and it may result injuries and risking burns for high exposure concentrations [7]. Accordingly, precise environmental monitoring of ammonia level and control of its emission (especially at the industrial production plants such as production plants of plastics, fabrics, and explosives) are essential [8].

Conductometric chemical sensors are electronic devices that detect the presence of selected chemicals and their concentrations at their surroundings through the change in the electrical conductance [9,10,11,12,13]. The chemical sensors are important for domestic safety in addition to environmental assessment because they identify hazardous gas concentrations, and send their values to safety as well as control systems.

Graphene is known as a two dimensional material of a mono layer of carbon atoms in a lattice structure of honeycomb [14, 15]. Graphene exhibits two main structures following the arrangement of its atoms: zigzag (ZGNR) and armchair (AGNR). The two structures have different energy band structures where ZGNR is categorized as a conductor whereas AGNR can be either conductor or semiconductor [16,17,18]. Graphene is considered a material with extraordinary characteristics that include excellent thermal conduction, optical transparency, alongside its high density that inhibits its penetration by gases [19]. Experimentally, the band gap of AGNR can be controlled through decreasing its dimensions to one or quasi one-dimensional [20]. AGNR may be classified into three categories depending on the quantity of dimers through a nanoribbon line (\({N}_{d}\)), with \({N}_{d}=3n, 3n+1, or 3n+2\), with \(n\) is an integer. The first two categories are semiconductor, whereas the last one is metallic [21]. The semiconductor AGNR is more attractive for device applications such as optical devices, biodevices, and chemical sensors [12, 22, 23]. Additionally, it is investigated heavily for gas sensor applications due to its ability to adsorb various gases [18, 24,25,26]. The sensitivity as well as the selectivity may be promoted by modification of AGNR surface, i.e., by (i) do** that involves replacement of selected carbon atom(s) by dopant atom(s) and (ii) decoration that involves precise addition of atom(s) on its surface [23, 27, 28].

The effect of modifying graphene nanoribbon has been investigated intensively recently for gas sensing utilization [29], since its impact on enhancing AGNR adsorption features was illustrated experimentally [12, 19]. Miao et al. investigated the decomposition of ammonia on graphene modified by metal clusters of Ni6, Co6, and Fe6 using DFT computations [30]. They found that graphene modified with Ni6 is more active for ammonia adsorption and dissociation. Barkov examined modifying graphene nanoribbons with carboxylic acid (COOH) group and its effect on ammonia adsorption using DFT calculations [31]. They found that wet nanoribbons interact further with ammonia. The carboxyl group attracts water molecule that is favorable in terms of energy that in turn promotes ammonia adsorption where COOH and water represent adsorption centers for NH3. VE C. Padilla et al. examined ammonia adsorption on graphene doped with P and Si atoms by means of DFT computations [32]. Phosphorus-modified graphene exhibits metallic behavior, while the silicon-modified graphene is a semiconductor (band gap ~ 0.25 eV). The phosphorus-modified structure was found to be more favorable for ammonia adsorption than the silicon-modified one.

Herein, the effect of modification of AGNR (\({N}_{a}=3p\)) by ZrOx (\(x=0, 1, \mathrm{or }2\)) on its adsorption for ammonia gas is investigated using first principles and DFT computations [33]. The objective of this work is to explore the influence of AGNR do** and surface decoration by ZrOx; thus, AGNR structures are modified by ZrOx species through either substitution of carbon atom(s) by ZrOx (do**), or addition of ZrOx on the central position of AGNR surface (decoration). The adsorption of ammonia on pristine and modified graphene structures is explored by detailed investigation of band structure, adsorption energy (\({E}_{\mathrm{d}}\)), adsorption length (\(D\)), charge transferred (\(\Delta Q\)), and density of states (DOS).

Simulation methods

Kohn–Sham (KS) DFT calculations were employed to explore the influence of ZrOx modification of AGNR structures on their adsorption of NH3 [34]. The different structures were labeled as presented in Table 1. The computations utilized a framework that consists of a linear combination of atomic orbitals (LCAO) along with pseudopotential perturbation. The electrons are modeled as a system of non-interacting gas that exhibits a density (\(n\)) within an effective potential (\({V}^{\mathrm{eff}}\left(n\right)\)) according to the equation [35]:

Table 1 AGNR structures and their descriptions alongside with the band gap energy (Eg) prior to NH3 adsorption
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$${V}^{\mathrm{eff}}\left(n\right)={V}^{H}\left(n\right)+{V}^{xc}\left(n\right)+{V}^{\mathrm{ext}}\left(n\right)$$
(1)

The electrostatic interaction amongst electrons is represented by the Hartree potential (\({V}^{H}\)). The exchange–correlation (XC) potential is represented by (\({V}^{xc}\)), whereas the electrostatic potential energy of electrons is indicated by (\({V}^{\mathrm{ext}}\)).

The Hamiltonian of KS (\({\widehat{H}}^{\mathrm{KS}}\)) may be presented as [35]

$${\widehat{H}}^{\mathrm{KS}}=-\frac{{\hslash }^{2}}{2m}{\nabla }^{2}+{V}^{\mathrm{eff}}$$
(2)

where \(m\) is the mass of electron and \(\mathrm{\hslash }=\frac{h}{2\pi }\) (\(h\) is Planck’s constant). The KS equation is represented by a DFT-LCAO process through numerical values to permit effective implementation of KS-DFT computations for all different structures. Two types of approximations were implemented within the KS-DFT formalism: local density (LDA) alongside with generalized gradient (GGA). Their implementation was according to Perdew–Burke–Ernzerhof (PBE) scheme combined with Grimme approximations (DFT-D2) that account for Van der Waals force [36, 37]. The above estimates were selected because of their reasonable consideration of both accuracy and efficiency of DFT computations as presented by various reports [38].

A simulation package from Synopsys: Quantum ATK associated with a virtual nanolab (VNL) was utilized to perform the DFT calculations. Single sheets of AGNR with the edge atoms terminated by hydrogen were used in this investigation [39]. Zr, ZrO, and ZrO2 (referred as ZrOx) were used to modify the structures either by do** or by decoration. Prior to any computations, an LBFGS optimizer was employed to optimize the structures [40], and they were relaxed where the residual force per atom and stress tolerance were under 0.05 eV/Å and 0.1 GPa, correspondingly. The \(k\)-point sampling mesh was 4 × 2 × 1, with a mesh density of 100 Hartree. This \(k\)-point sampling mesh was selected since it provided accurate results with reasonable computation time. The binding energy of ammonia can be evaluated by calculation of adsorption energy for a structure was evaluated using [17, 18, 41]:

$${E}_{d}={E}_{\mathrm{structure}+{\mathrm{NH}}_{3}}-({E}_{\mathrm{structure}}+{E}_{{\mathrm{NH}}_{3}})$$
(3)

Herein, \({E}_{\mathrm{structure}+{\mathrm{NH}}_{3}}\) is the total energy of AGNR structure with adsorbed NH3, \({E}_{\mathrm{structure}}\) is the total energy of an unexposed AGNR structure, and \({E}_{{\mathrm{NH}}_{3}}\) is the total energy for NH3 molecule. The value of \({E}_{d}\) is an indication of the suitability of a structure to adsorb NH3: the further the negative \({E}_{d}\), the more appropriate the structure for NH3 adsorption. The charge relocation among NH3 and a structure is another indicator of preference for its adsorption, and it was evaluated by the Mulliken method as [20, 23]

$$\Delta Q={q}_{f}-{q}_{0}$$
(4)

where \({q}_{f}\) and \({q}_{0}\) denote the final and initial Mulliken charges of the gas, respectively. If electros were moved from NH3 to AGNR, \(\Delta Q\) will be negative.

Results and discussion

The effect on the adsorption capacity for ammonia gas of AGNR structures modified by ZrOx is examined. The modification is established either by do** or decoration for AGNR structure. The detailed description of the different examined nanostructures is presented in Table 1. The table also reveals the band gap energy (\({E}_{g}\)) of the different structures. The maximum value of \({E}_{g}\) is for the pristine structure, while the modified structures exhibit lower values of \({E}_{g}\). AGNR structures doped with ZrO and ZrO2 demonstrate the lowest \({E}_{g}\) values. The optimized doped AGNR structures are presented in Fig. 1a–d. The carbon atoms are rearranged as a result of energy optimization near the do** site. The edge atoms of AGNR structures are passivated using hydrogen atoms to avoid reconstruction of the structures as a result of satisfying dangling bonds for the edge atoms [39]. The modification cites of ZrOx are selected at the center of the structures to minimize edge effect, where many cites reveal similar results because of the high symmetry of nanoribbons. The C–C bond length of pristine AGNR is 1.42–1.43 Å. The doped structures exhibit the following bond lengths: (i) Zr + AGNR structure: the Zr-C bond length is 1.89–1.90 Å; (ii) ZrO + AGNR structure: the Zr-C bond length is 1.86–1.87 Å, the Zr-O bond length is 1.90 Å, and the O-C bond length is 1.42–1.44 Å; (iii) ZrO2 + AGNR structure: the Zr-C bond length is 1.83 Å, the Zr-O bond length is 1.89 Å, and the O-C bond length is 1.42–1.44 Å. The bond lengths are as follows for the decorated structures: (i) Zr on AGNR structure, the Zr-C bond length is 2.30 Å; (ii) ZrO on AGNR structure, the Zr-C and Zr-O bond lengths are 2.41–2.44 Å and 1.78 Å, respectively; and (iii) ZrO2 on AGNR structure, the Zr-C and Zr-O bond lengths are 2.75–2.95 Å and 1.82 Å, correspondingly. Do** an AGNR structure with ZrOx by substitution of carbon atoms produces a stress on the structure that results modifications of the bond lengths for C-Zr, Zr-O, and C-O to relief the stress [42]. The optimized decorated structures are presented in Fig. 2a–c. Both Zr and ZrO are bonded to interstitial locations with the structures that minimize their overall energy. Nevertheless, ZrO2 remains on top of the structure unbounded due to its high stability as compared with both Zr and ZrO [43].

Fig. 1
figure 1

Optimized AGNR structures (top and side views) prior (a–d) and post (e–h) to NH3 adsorption: (a, e) AGNR, (b, f) doped Zr + AGNR, (c, g) doped ZrO + AGNR, and (d, h) doped ZrO2 + AGNR

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Fig. 2
figure 2

Optimized AGNR structures (top and side views) prior (a–c) and post (d–f) to NH3 adsorption: (a, d) decorated Zr on AGNR, (b, e) decorated ZrO on AGNR, and (c, f) decorated ZrO2 on AGNR

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The effect of AGNR structure modification on the band structure is presented in Fig. 3b–d for doped structures and Fig. 4b–d for decorated structures. The band structure of the pristine AGNR is presented in Figs. 3a and 4a. The figures demonstrate that al the band structures have a parabolic form near the Γ point. The evaluated density of states within the band structures within both valance and conduction bands increases as a result of modification (for both modified structures, doped and decorated) as compared with the unmodified structure. The modification leads to the introduction of new bands near the Fermi level, and shift of the valance band close to the Fermi level in agreement with the results presented in Table 1.

Fig. 3
figure 3

Band structures for the optimized AGNR structures prior (a–d) and post (e–h) to NH3 adsorption: (a, e) AGNR, (b, f) doped Zr + AGNR, (c, g) doped ZrO + AGNR, and (d, h) doped ZrO2 + AGNR

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Fig. 4
figure 4

Band structures for the optimized AGNR structures prior (a–c) and post (d–f) to NH3 adsorption: (a, d) decorated Zr on AGNR, (b, e) decorated ZrO on AGNR, and (c, f) decorated ZrO2 on AGNR

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Adsorption energy

Figure 1e–h reveals the doped AGNR structures post to adsorption of NH3 gas. The figures illustrate that NH3 molecule is unbonded to the pristine structure; however, it is bonded to all other ZrOx-doped AGNR structures. Upon introduction of an NH3 gas molecule to an AGNR structures, where the molecule orientation is set during the optimization stage to exhibit the minimum energy leading to the maximum stable configuration, NH3 gas is adsorbed on AGNR through physisorption and chemisorption. Physisorption involves weak attraction force among the gas and AGNR, while chemisorption involves strong chemical bonding among the gas and AGNR [44, 45]. In case of Pristine-Gr substrate, NO and NO2 molecules are physisorbed on its surface while in case of BC2P-Gr and BCP2-Gr, both NO and NO2 molecules are chemisorbed on substrate.

The chemisorption of NH3 gas on the ZrOx-doped AGNR structures indicates that they are favorable for NH3 adsorption unlike the pristine structure. The decorated AGNR structures post to adsorption of NH3 gas are presented in Fig. 2d–f. The figures reveal that NH3 molecule is not chemisorbed on any decorated AGNR structure, i.e. no bond can be observed between NH3 gas and the structures.

Band structure

The effect of ammonia adsorption on AGNR structures on the band structure is demonstrated in Figs. 3a and 4a for pristine structure, Fig. 3e–h for doped structures, and Fig. 4e–h for decorated structures. Further, bands appear as a result of NH3 gas adsorption and, in general, the band gap increases after adsorption as presented in Table 2. The modifications in the band structure post to NH3 adsorption and the introduced bands are indicators of the generation of extra electronic states as a result of adsorption [32]. The developments appear within the band structure specify that the ZrOx modification of AGNR is an effective approach for adsorption of NH3 molecule.

Table 2 Band gap energy, adsorption energy, ammonia adsorption length, and charge transferred among the gas and the structures post to adsorption of NH3
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Density of states

The effect of AGNR do** on band structure is examined using DOS as presented in Fig. 5a. The figure illustrates that DOS decreases generally due to do**, and new bands appear within both conduction and valance bands. For example, the following bands appear as a result of do** at: 1.06, 3.52, − 0.65, and − 4.00 eV for the Zr-doped AGNR; and 0.726, 3.48, and − 1.43 eV for the ZrO-doped AGNR; and 0.726, 2.85, 3.33, − 3.63, and − 4.26 eV for the ZrO2 -doped AGNR. Decoration of AGNR modifies the DOS as illustrated in Fig. 5b. Although the intensity of DOS is lower for the decorated structures as compared with the pristine structure, it is higher than the equivalent for the case of do**. New bands appear within both conduction and valance bands of decorated AGNR such as at 0.67, 0.93, 1.37, 1.85, and − 0.19 eV for the Zr-doped AGNR; 0.36, 0.61, 1.96, and − 0.24 eV for the ZrO-doped AGNR; and 0.22, 1.69, and − 1.52 eV for the ZrO2-doped AGNR. Figure 5 reveals that the DOS of both doped and decorated structures increases near the Fermi level, in agreement with the results of Figs. 3 and 4. In addition, the conduction band exhibits lower DOS than the valence band for all AGNR structures. Figure 6 shows the effect of NH3 adsorption on DOS for the different ZrOx-modified AGNR sheets. The DOS preserves its overall features prior to adsorption of NH3 gas. The DOS near the Fermi level is higher for the decorated structures as compared with the doped structures. Adsorption of NH3 decreases the intensity of DOS of many bands for the doped AGNR structures, such as at 1.28, − 1.34, − 2.69, and − 4.42 eV. Nevertheless, lower decrease in the DOS is observed for the decorated AGNR structures. The results of DOS designate that many electronic states are available within the AGNR structures post to NH3 adsorption [46]. Accordingly, the ZrOx-modified AGNR structures can be considered as effective systems for NH3 gas adsorption.

Fig. 5
figure 5

Electronic density of states of both (a) doped and (b) decorated AGNR structures before adsorption of NH3 gas

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Fig. 6
figure 6

Electronic density of states of both (a) doped and (b) decorated AGNR structures after adsorption of NH3 gas

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Discussion

The adsorption capacity for NH3 of AGNR modified by ZrOx is explored using adsorption energy, charge transferred to the NH3 molecule, along with the adsorption length as shown in Table 2. The table demonstrates that the modified AGNR structures have greater adsorption energies in comparison with the unmodified AGNR, indicating that ZrOx modification of AGNR structures enhances their NH3 adsorption. The adsorption energy of the doped structures is higher than its equivalent of the decorated structures. In fact, all doped structure are suitable for NH3 adsorption. Herein, ammonia molecule bonds to the ZrOx dopant which is already bonded to AGNR structure. Nevertheless, the highest adsorption energy for NH3 is for the ZrO2 + AGNR-doped structure, where it is 22.2 times greater than that of the unmodified AGNR structure. Furthermore, the adsorption length results are in agreement with those of the adsorption energy. Here, all doped structures exhibit low adsorption length of ~ 2.4 Å indicating strong bonds among NH3 and the structures. The adsorption length is higher for the decorated structures, in comparison with the doped structures, which is in agreement with the observation (Fig. 2), i.e., no chemisorption is established for those structures. The charge transferred from/to ammonia molecule for the doped structures is ~ -0.2e which is almost similar for the three structures, and it is higher than that of decorated structures (except that for the Zr decoration case). The negative sign indicates that the charge is transferred from NH3 to AGNR structures. The considerable amount of transferred charge from NH3 molecule to the doped structures specifies the formation of chemical bonds between them. The charge transfer observation indicates that adsorption of NH3 molecule on doped AGNR structures modify their electronic features notably (in agreement with DOS in Fig. 6) to generate strong interactions among NH3 and the AGNR structures resulting high energy of adsorption [47]. The charge transferred among NH3 and the ZrOx-modified AGNR structures are extracted within AGNR [12]. On the other hand, the low magnitude of charge transferred among NH3 and the decorated AGNR structures is allocated to the fact that NH3 is not chemisorbed on either of them.

The adsorption energy towards ammonia of the doped AGNR structure is boosted due to ZrO2 modification, for instance, it is ~ 41 times greater than that presented for zigzag graphene nanoribbon modified with OH [20]. The illustrated results indicate that the ZrOx-doped structures are more favorable for NH3 adsorption than the decorated ones [48,49,50]. Accordingly, one can conclude that although the small differences in the adsorption energy for the doped structures, their values are close, signifying that do** AGNR with Zr metal and its oxides is a suitable approach for adsorption of NH3. Additionally, if decoration is considered for AGNR adsorption of NH3, the ZrO decorated structure is the suitable one due to its relatively high adsorption energy. The improved NH3 adsorption on doped AGNR structures can be allocated to the high affinity of ZrOx to the gas upon do** [51]. Here, the reaction of both Zr1+ and Zr.2+ with nitrogen and oxygen atoms is favorable [52]. Do** AGNR structures with ZrOx improves their reactivity by supplying additional negative charges for the delocalized π bond due to the low electronegativity of ZrOx in comparison with carbon atoms [42]. The low ammonia adsorption energy of decorated AGNR structures in comparison with the doped ones may be allocated to the weak hybridization among their overlapped orbitals: 3p for nitrogen, and 3d and 4 s for zirconium [53]. On the other hand, the 3p orbital of nitrogen, 3d orbital of zirconium, and 2p orbital of oxygen exhibit strong hybridization for the do** case [53]. The influence of zirconium and its oxides on enhancing ammonia adsorption is examined and presented experimentally [54]. This is assigned to trap** of electrons within the conduction band that generates electron depletion region. NH3 molecule reacts with the adsorbed ions of oxygen, causing the release of the trapped electrons and narrowing the space charge [54]. The adsorption of NH3 is enhanced by the transfer of charge due to the presence of oxygen ions on the AGNR surface which may be presented as [55]

$${4\mathrm{NH}}_{3}+3{\mathrm{O}}_{2}^{-}\left(\mathrm{ads}\right)\to {2\mathrm{N}}_{2}+6{\mathrm{H}}_{2}\mathrm{O}+6{\mathrm{e}}^{-}$$

The equation reveals that NH3 adsorption generates additional charges as demonstrated by the charge transferred (Table 2). In all doped structures, NH3 is bonded to the Zr atom. The reaction of NH3 with zirconium can be presented as below [56]:

$${\mathrm{NH}}_{3}+\mathrm{Zr}\to {\mathrm{ZrNH}}_{3}$$

However, for decorated structures, the bond of Zr atom are established with carbon rather than the ammonia. The present value of adsorption energy for ZrO2 + ZGNR structure towards NH3 is compared with latest results for NH3 adsorption of graphene-based structures, as presented in Table 3. The table demonstrates the major improvement of the present work for ammonia adsorption due to ZrO2 do**.

Table 3 Recent reported values for the adsorption energy of NH3 on graphene-based structures
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Conclusion

First-principle computations of density functional theory (DFT) were employed to explore the effect of modification of nanoribbon with armchair-graphene structure (AGNR) by ZrOx (\(x=0, 1,\mathrm{ or} 2\)) on its adsorption of ammonia (NH3) gas. The modification of AGNR structures was established either by do** (i.e., substitution of carbon atom(s)), and decoration on the surface. Ammonia adsorption was examined by thorough investigation of the adsorption energy (\({E}_{d}\)), band structure, charge transferred (\(\Delta Q\)), adsorption length (\(D\)), and density of states (DOS). The findings reveal that NH3 adsorption was greatly enhanced upon modification with ZrOx; nevertheless, decoration was less effective than do**. The highest adsorption energy of NH3 was found for the structure doped by ZrO2, followed by that doped by Zr. Adsorption of NH3 on the ZrO2 doped AGNR exhibits a favorable adsorption of \({E}_{d}\sim -10.05 eV\), \(D\sim 2.4 \dot{\mathrm{A}}\), and \(\Delta Q\sim -0.214e\). The energy of adsorption for the AGNR structure doped with ZrO2 for NH3 gas was 22.2 times greater than that of the undoped structure. Moreover, AGNR structures doped with Zr and ZrO have also exhibited high adsorption energy for NH3. The results of this investigation demonstrate that do** of AGNR with ZrOx is a high potential approach towards production of sensitive NH3 sensors.