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Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury
Journal Pre-proofs Full Length Article Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury Basant Roondhe, Prafulla K. Jha, Rajeev Ahuja PII: DOI: Reference:
S0169-4332(19)33677-3 https://doi.org/10.1016/j.apsusc.2019.144860 APSUSC 144860
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Applied Surface Science
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25 July 2019 13 November 2019 22 November 2019
Please cite this article as: B. Roondhe, P.K. Jha, R. Ahuja, Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144860
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Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury Basant Roondhe1, Prafulla K Jha1*, Rajeev Ahuja2,# 1
Department of Physics, Faculty of Science, The M. S. University of Baroda, Vadodara-390 002, India 2 Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala, 75120, Sweden *
Email: [email protected]
#
Email: [email protected]
Corresponding Authors *
(P.K.J.)E-mail:[email protected]
#
(R.A.)E-mail: [email protected]
1
ABSTRACT A new two-dimensional (2D) member in the boron nitride family, haeckelite BN (haeck-BN) is proposed as a nanosensor for the detection of environmental hazardous material, methyl mercury (CH3Hg). Using first principles-based dispersion corrected density functional calculations, we have studied interaction of CH3Hg with haeck-BN to understand its role as toxic gas sensor. The interaction of CH3Hg with haeck-BN is validated by calculating the adsorption energy and electronic density of states (DOS). The change in DOS and work function (WF) upon adsorption of CH3Hg molecules further confirms the sensing ability of haeck-BN. In the pristine form, haeck-BN shows weak interaction towards CH3Hg with adsorption energy of -0.45 eV. To tune the sensing and electronic properties of haeck-BN, we have also doped haeck-BN with Al, S, Si, and P atoms. The substitutions were found to alter the electronic properties of haeck-BN, turning it from semiconducting to metallic. However, with the substitution of heteroatom, a significant increment in the adsorption energy is observed in the range of -1.9 eV to -3.8 eV. Our finding suggests that haeck-BN can be utilized as a sensor for the detection of hazardous toxins.
Keywords: haeckelite BN; methyl mercury; electronic properties; adsorption; sensor; density functional theory.
1. Introduction The environment has suffered significantly due to the enormous increase of pollutants released from the fast-growing industrial units. The toxins’ level in the environment has increased up to a dangerous extent. One of the most hazardous toxins are the mercury-based pollutants due to their severe effect on environment as well as human health [1-3], among which their organic form being the most dangerous ones. Methyl mercury (CH3Hg) is one of the organic mercury-based compounds formed by inorganic ions of mercury, Hg2+, which is highly hazardous to humans because of its lipophilic tendency causing severe damage to the whole body and central nervous system [4-5]. Several industries are responsible for direct or indirect production of methyl mercury, among which 2
the acetaldehyde manufacturing industry is one of them. Other major factors behind the increased level of methyl mercury in the environment are the burning of fossil fuels and waste materials containing inorganic mercury. There exist many studies which report the adverse effect of methyl mercury on humans such as loss of IQ, attention deficiency in children [6] and risk of cardiovascular disease in adults [7]. The major pathway due to which humans get affected by methyl mercury is through the aquatic food chain. The increasing concentration of methyl mercury in the aquatic food chain is difficult to eliminate since it is typically accumulated in tissues of organism [8]. Being the apex predator of the aquatic food chain, its consumption results in severe neurological damages in humans [9]. Accordingly, there should be a technique to track as well as filter methyl mercury from the entire biota and environment. This challenge can be easily achieved by highly selective and sensitive sensors. There exist many experimental and theoretical studies tha describe the methods to detect and sense elemental mercury (Hg) [10-15], however, in the case of CH3Hg, only experimental studies are accomplished using fluorescence spectroscopy [16-20]. In the study of Yan et al. [13], detection of CH3Hg has been developed based on the restriction of intermolecular rotations which aggregates induced emission on interacting with CH3Hg. Lin et al.[14] reported detection of CH3Hg by lysozyme type VI-stabilized gold nano cluster where they found that it is highly sensitive and its sensitivity can be modulated by changing the concentration of lysozyme. Recently, a new method was developed for the detection of methyl mercury based on colorimetric detection by Xie et al. [20]. However, this method is still in development and need further improvement. Therefore, a fast, reliable and cost-efficient method, not only for detection but also for sensing and filtration of CH3Hg is much needed. There are many conventional metal oxide semiconductor-based sensors are available in the market at low cost with many advantages like low power consumption, high sensitivity and easy fabrication [21-22]. However, apart from many advantages, it possesses some limitations like long recovery time, high operating temperature and low accuracy [23-24]. The limitation of these conventional sensor materials motivates scientists to explore other materials that 3
can overcome these drawbacks and satisfy the high demand for sensitive material. Two dimensional (2D) materials are possible alternatives for sensor material. Graphene, a mono-atomic single sheet of carbon became an active area of research in the field of nanomaterials and nanotechnology, due to its potential for integration in next-generation electronic and energy conversion devices [2526].The high surface-to-volume ratio feature of 2D materials provides large active surface for reaction with gas molecules, which enhance the sensor performance down to parts-per-billion (ppb) levels and make it frontier material for gas sensing applications [21]. The prime reason for researchers’ exploration towards other similar 2D materials is the attention-seeking properties of graphene. Following the discovery of graphene, many 2D materials were isolated from their bulk counterparts[27-32], however, apart from all, two dimensional boron nitride (BN) gathered much attention [27] due to its better thermal and chemical stability than carbon, despite both being isomorphic [33-35]. Unique properties of hexagonal BN (h-BN) make it useful in many applications such as insulator in electronic devices, dielectric materials, lubricants, etc. [36-37]. Recently, a new planar member with square and octagonal ring haeckelite BN (haeck-BN) has been reported theoretically and experimentally [38-40]. This new form of BN has totally different electronic properties from conventional h-BN, as it has much lower bandgap as compared to h-BN and it is also capable as a biosensor [38]. The existence of this type of periodic structure has also been observed in GaN and ZnO [41-42]. Apart from its superior electronic properties as well as biosensing capability as compared to h-BN, the possibility of this haeck-BN as a toxin sensor has not been considered yet to the best of our knowledge. It is further observed that the aluminium(Al), silicon (Si), phosphorus (P) and sulfur (S) dopants improves the electronic, structural properties and adsorption capability of nanostructures [43-47].Moreover, the 3p elements Al, Si, P, and S have atomic radii close to boron and nitrogen atoms which form haeck-BN. Therefore, it will be easier for these atoms to form a stable covalent structure with haeck-BN. Thus, motivated with these facts, in the present study, we performed first principle calculations based on
4
density functional theory to check the possibility of pristine and 3p elements (Al, Si, P, and S) doped haeck-BN as a CH3Hg toxic gas scraper.
2. Computational Methodology The present study was performed by employing state-of-the-art first principles calculation under the framework of density functional theory (DFT) as implemented in Quantum Espresso [48].For the treatment of exchange-correlation functional, generalized gradient approximation (GGA) proposed by Perdew–Burke–Ernzerhof (PBE) with ultrasoft pseudopotential was employed [49]. For the ground state optimization of structures, the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method was used [50]. The energy cut-off and charge density cut-off for single-particle functions were set as 80 Ry and 800 Ry respectively in the plane-wave basis set to expand the Kohn–Sham orbitals which were sufficient to fully converge the total energy. The Monkhorst–Pack [51] scheme for the integration of the Brillouin zone (BZ) was performed with k-mesh of 16 x16x1. To solve the KohnSham equation, we employed the iterative Davidson-type diagonalization method with an electronic energy convergence of 10-8 Ry. We analyzed the interaction of haeck-BN with CH3Hg by performing structural optimization, adsorption energy calculation and electronic density of states (DOS) plots. Accounting of the dispersion forces has been emphasized in the literature by a precise quantum mechanical description that explains the interaction of a molecule with a nanosurface [52]. Therefore, for the description of a long-range electron correlation, it is important to choose a suitable computational method. We employed Grimme’s dispersion correction (DFT-D2) [53] to consider the long-range van der Waals (vdW) interaction between the CH3Hg and the haeck-BN monolayer. This type of vdW interaction has been widely used and found very successful in predicting the long-range interaction of variety of materials for adsorption applications [54-55]. The adsorption energy Ead is calculated using the following equation: 𝑬𝒂𝒅 = 𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵+𝑪𝑯𝟑𝑯𝒈) − {𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵) + 𝑬(𝑪𝑯𝟑𝑯𝒈) } (1)
5
where𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵+𝑪𝑯𝟑𝑯𝒈) is the total energy of haeck-BN adsorbed by CH3Hg, 𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵) is the total energy of the adsorbent haeck-BN monolayer and 𝑬(𝑪𝑯𝟑𝑯𝒈) is the total energy of CH3Hg molecule. Their energies were obtained from their fully optimized geometries. The formation energy of all dopants (Al, Si, S, and P) for haeck-BN sheet has been calculated through: 𝒅𝒐𝒑𝒆𝒅
𝒑𝒓𝒊𝒔𝒕𝒊𝒏𝒆
𝑬𝒇 = 𝑬𝒕𝒐𝒕𝒂𝒍 − 𝑬𝒕𝒐𝒕𝒂𝒍
+ 𝒏𝑩 𝝁 𝑩 + 𝒏𝑵 𝝁 𝑵 − 𝒏𝒅 𝝁 𝒅
𝒅𝒐𝒑𝒆𝒅
(2) 𝒑𝒓𝒊𝒔𝒕𝒊𝒏𝒆
where 𝑬𝒕𝒐𝒕𝒂𝒍 is the total energy of haeck-BN sheet substituted with dopant, 𝑬𝒕𝒐𝒕𝒂𝒍
is the total
energy of pristine haeck-BN sheet, 𝝁𝑩 , 𝝁𝑵 , and 𝝁𝒅 is the chemical potential of boron, nitrogen and dopant atom, respectively and n is the number of individual atom in the sheet.
3. Result and Discussion In this section, we present the results of our calculation on the structural and electronic properties of CH3Hg adsorbed haeck-BN. This section also discusses the changes in the electronic structure, binding energy, and work function of haeck-BN sheet due to heteroatoms (Al, S, P and Si) doping on haeck-BN sheet. The key factor for the determination of a good sensor is the changes that arise in the bandgap of the material having gas/chemical exposure which is considered in this work. Therefore, finally, we present and discuss the ability of pristine and doped haeck-BN sheet towards sensing and detection of CH3Hg.
3.1 Pristine haeck-BN monolayer (haeck-BN-ML)
Fig. 1(a) shows the optimized structure of haeck-BN sheet. It has two rings, square and octagonal in the optimized structure of the BN sheet. The bond length between boron and nitrogen atoms in the square ring is 1.402 Å while the same in the octagonal ring is 1.475 Å. The measured B-N-B bond angles in the square ring are 84.21° and 95.78° while these are 132.10° and 137.11° in the octagonal ring. The stability of haeck-BN sheet has been investigated in our previous study [38]. 6
Fig. 1(b) presents the density of states (DOS) for haeck-BN which depicts its semiconducting nature with 3.9 eV bandgap. We have also calculated the partial density of states (PDOS) and presented them in Fig. 1(c) for a better view of the atomic orbital contribution in the DOS.
Doping Sight
) (c)
y
(b)
x a=9.85 Å
Figure 1: (a) Optimized structure, (b) Density of States (DOS and (c) Projected density of states (PDOS) of haeck-BN.
For the investigation of gas sensing ability of haeck-BN towards hazardous CH3Hg, we have calculated the adsorption energy of the CH3Hg adsorbed haeck-BN sheet. The CH3Hg molecule is kept in parallel orientation with haeck-BN due to its large size and equal participation of C, H and Hg atoms of CH3Hg molecule with the sheet and it was observed that there was no significant variation in structure of the system during energy minimization (Fig. 2(a)). The calculated adsorption 7
energy for CH3Hg over haeck-BN is -0.49eV indicating its physisorption characteristic with haeckBN sheet. The effect of CH3Hg adsorption on the electronic properties of haeck-BN sheet can be observed from the DOS of adsorbed haeck-BN which is presented in Fig. 2(b).
(a)
(c)
(b)
Figure 2: (a) Optimized structure (b) Density of states (DOS) and (c) Projected density of states (PDOS) of CH3Hg over haeck-BN.
It can be clearly seen from Fig. 2(b) that the adsorption of the CH3Hg molecule on the haeck-BN sheet introduces impurity states at Fermi level making the sheet metallic in nature. The CH3Hg molecule participates majorly in the valence band region. The density at -7 eV in the valence band region increases after the adsorption of CH3Hg due to C (p) orbital contribution (Fig. 8
2(c)) while the peak at -13 eV in the lower valence band region is due to Hg (d) orbital. The sensitivity of haeck-BN sheet towards CH3Hg can be further confirmed by work function calculation. Work function is the energy required by an electron to get free from the Fermi level to vacuum level and is expressed as WF(F) = Evac - EF
(a)
(2)
(b)
Figure 3: Work function plots of (a) Pristine haeck-BN (b) CH3Hg adsorbed haeck-BN.
Here, Evac and EF denote the energy of the vacuum level and Fermi level respectively. Fig. 3 (a-b) presents work function of pristine and CH3Hg adsorbed haeck-BN sheet. An appreciable change in work function of haeck-BN sheet is observed after the adsorption of CH3Hg molecule. The work function of pristine haeck-BN reduces from 5.31 eV to 3.76 eV after CH3Hg adsorption. This large modulation in the work function of haeck-BN sheet further confirms the interaction of the molecule with the sheet, reinforcing the gas-sensitive ability of haeck-BN sheet.
9
3.2. Haeck-BN-ML with Heteroatom Substitution
(a)
(b)
(c)
(d)
Figure 4: Optimized structure of adatoms haeck-BN systems (a) Aluminum (Al) (b) Silicon (Si) (c) Phosphorus (P) and (d) Sulphur (S). Among many, the substitution of atoms is utilized as a successful strategy to enhance the adsorption energy and sensitivity of any sensor. For this purpose, we have considered Al, Si, P and 10
S as a substitution for N in haeck-BN sheet for the enhancement of adsorption energy of CH3Hg over haeck-BN sheet. The elements for the substitution are chosen on the basis of their comparability of size with nitrogen. The calculated energy gap (Eg), adsorption energy (Ead), Fermi energy (EF), the distance between molecule and haeck-BN (d) and work function (Φ) are presented in Table 1 where an effect of dispersion on the listed properties is evident. The optimized structures are presented in Fig. 4(a-d), where a considerable structural change can be seen after the substitution of the doping elements. In order to check the stability of these doped systems, we have calculated the formation energy by using Eq. 2., utilizing the chemical potential instead of electronic energy [52]. We found that the S-haeck-BN have higher (negative) formation energy (14.3 Ry) in comparison with P-haeck-BN (-7.1Ry) and Si-haeck-BN (-2.67 Ry), confirming its
(a)
(b)
(c)(a
(d)
Figure 5: Density of states of (a) Al-doped haeck-BN (b) Si-doped haeck-BN (c) Pdoped haeck-BN and (d) S-doped haeck-BN. 11
higher stability. Al-haeck-BN has positive formation energy which indicates that doping of Al requires higher energy, which implies that the formation energies follows the sequence: S > P > Si > Al. Substitution at the place of N by considered elements causes a large change in the electronic properties. All elements, except the phosphorus ones, convert the semiconducting nature of haeckBN sheet into metallic.
(a)
(b)
(c)
(d)
Figure 6: Projected density of states of (a) Al-doped haeck-BN (b) Si-doped haeckBN (c) P-doped haeck-BN and (d) S-doped haeck-BN.
The DOS and PDOS, respectively shown in Fig. 5(a-d) and Fig. 6(a-d) evidently demonstrates the modulation in the electronic properties and the respective contribution of the 12
considered elements in haeck-BN sheet. We now systematically discuss the effect of substitution of Al, Si, S and P atoms on the adsorption as well as electronic properties of haeck-BN sheet in the following section. 3.2.1. Al-doped haeck-BN-ML(Al-haeck-BN-ML) Initially, we considered Al atom for the substitution in place of N atom of haeck-BN-ML. In Al-haeck-BN-ML (Al substituted haeck-BN monolayer), the B-Al bond length and the B-Al-B angles are found to be 2.12 Å and 87.79° respectively. The Al atom in the haeck-BN-ML induces a buckling of the sheet by 1.8 Å at the site of substitution, which can be observed from Fig. 4(a). The electronic properties of haeck-BN sheet change from semiconductor to metallic after the substitution of Al atom as can be seen from DOS and PDOS plots presented in Fig. 5(a) and Fig. 6(a), respectively.
Figure 7: Work function plots of Al-doped haeck-BN. We have also calculated the charge transport due to the difference in electronegativity of adatom by calculating the Lowdin charge. The Al atom has the lowest electronegativity (1.6 e)
13
among the entire dopants considered in our system and gives more than half of its charge (0.91e) to the B atom in the haeck-BN sheet.
(c)
(a)
(b)
(d)
Figure 8: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed Al-doped haeck-BN. Moreover, we have calculated the work function of Al-haeck-BN-ML and found that the substitution of Al in haeck-BN sheet reduces the work function by the magnitude of 1.11 eV as depicted in Fig. 7, which further confirms the donation of charge by Al to the sheet. We have also investigated the interaction of CH3Hg molecule with Al-haeck-BN-ML and observed chemisorption of CH3Hg due to the formation of a bond between Hg and Al (Fig. 8(a)) and value of the calculated 14
adsorption energy is -1.98 eV. On adsorption of CH3Hg, a significant transfer of charge (0.03e) occurs from the sheet towards the molecule but it does not have a significant effect on electronic properties, as the metallic nature after the adsorption remains unchanged. This removal of charge from the sheet by CH3Hg can be attributed to the oxidizing nature of the CH3Hg molecule. Fig.8 (bc) representing DOS and PDOS clearly illustrates the effect of charge transfer, as there is a decrease in the peak intensity in the conduction band region at around 2 eV. From the PDOS it is obvious that the major contributions of the molecule are close to the Fermi level. The Al(p) and Hg(p) contribute to both conduction as well as valence bands region at 1.5 eV and -2 eV respectively, while C(p) and Hg(d) contribute in valence band region at -1.5 eV and at -7 eV. The large contribution of Al(p) in the case of Al-haeck-BN-ML at 2 eV in the conduction band reduces after the adsorption of CH3Hg. This reduction is consistent with the significant transfer of charge between the Al-haeck-BN sheet and CH3Hg molecule. The adsorption of CH3Hg also alters the work function as shown in Fig. 8(d).
3.2.2. Si-doped haeck-BN-ML(Si-haeck-BN-ML) Another element that we have considered for substitution in the N site is Si. In Si-haeck-BNML (Si substituted haeck-BN monolayer) , the B-Si bond length and B-Si-B bond angle is found to be 1.98 Å and 94.28°, respectively on optimization. Similar to the case of Al, Si atom also induces a buckling at the site of substitution as shown in Fig. 4(b). However, the buckling height is 0.1 Å lower than that of the one observed in the case of Al substitution. The substitution of Si atom in the sheet also converts the electronic properties of haeck-BN sheet as Al does. Further, the semiconductor to metal transition after the substitution of Si atom can be observed from the DOS and PDOS plots presented in Fig. 5(b) and Fig. 6(b), respectively. There is a significant transfer of charge (0.8e) between Si and surrounding B atoms, which consequently reduces the work function of haeck-BN sheet by 0.64 eV as can be seen from Fig. 9
15
Figure 9: Work function plots of Si-doped haeck-BN.
The sensitivity of the Si-haeck-BN sheet towards CH3Hg was investigated by calculating the adsorption energy and the effect on the structure is presented in Fig 10 (a). There is significantly high adsorption energy of -3.26 eV indicating a strong chemisorption. The adsorption energy, in this case, is higher than that obtained for Al substitution. Comparatively significant transfer of charge from haeck-BN sheet to CH3Hg molecule (0.18e) is the reason behind this high adsorption energy, in contrast to the case of Al substitution. This large charge transfer converts the system again to the semiconductor state as can be noted from DOS and PDOS plots presented in Fig. 10(b-c).There is a shift in the density of states in the valence band region after the adsorption of CH3Hg, reflecting the removal of charge from the sheet by the molecule. The contribution of Si (p) orbital was initially present at the Fermi level in the Si-haeck-BN shifts to the valence band region around -2 eV. After the adsorption of CH3Hg, the impurity states of Si (p) at 2 eV in the conduction band region is found to disappear totally.
16
(c)
(a)
(b)
(d)
Figure 10: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed Si-doped haeck-BN. The major contribution in the conduction band region is due to the Hg (p) and B (p) orbitals. The work function of Si-haeck-BN also reduced (~ 0.71eV) on the adsorption of CH3Hg (Fig. 10(d)) which further validates the interaction of CH3Hg molecule with haeck-BN.
17
3.2.3. P doped haeck-BN-ML (P-haeck-BN-ML)
Figure 11: Work function plots of P-doped haeck-BN.
The substituted haeck-BN shows different electronic behavior, though the transformation of the structure has a similar out-of-plane trend with the previously considered cases in this study. The optimized B-P bond length is 1.88 Å and the B-P-B bond angle 101.45°. Fig. 4(c) shows that the substituted P atom is 1.5 Å above the plane of haeck-BN-ML. The electronic property of haeck-BN sheet does not get affected by the substitution of P as the semiconducting nature remains preserved. The DOS and PDOS plots shown in Fig. 5(c) and Fig. 6(c), respectively depicts the unaffected electronic properties of the haeck-BN sheet on P substitution. We further checked the charge transfer between the P and its surrounding B atoms and found that it shares a 0.66e charge. Fig. 11 shows an increment in the work function (~0.69 eV) after the substitution of P in haeck-BN-ML. This new P-haeck-BN-ML system is then investigated for the sensing of CH3Hg molecule. The adsorption energy calculation reveals that the CH3Hg molecule gets physisorbed (observed from
18
(c)
(a)
(d)
(b)
Figure 12: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed P-doped haeck-BN. Fig. 12(a)) over P-haeck-BN-ML with the adsorption energy of -0.36 eV which falls in the range of weak chemisorption and good physisorption. The reason behind this low adsorption energy is that the molecule interacts non-covalently with the P-haeck-BN sheet. The adsorption of CH3Hg molecule alters the electronic properties of P-haeck-BN from semiconducting to metallic, due to the C (p) and Hg (p) contributions at Fermi level (Fig. 12(b-c)). The density at Fermi level which arises due to the P (p) orbital contribution in P-haeck-BN shifts to valence band region around -2 eV after
19
the adsorption of CH3Hg. There is a large reduction in the work function (~2.7eV) of P-haeck-BN after the adsorption of CH3Hg which can be seen from Fig. 12(d). 3.2.4. S doped haeck-BN-ML(S-haeck-BN-ML)
Figure 13: Work function plots of S-doped haeck-BN.
Finally, we substituted S in haeck-BN-ML to see its effect on CH3Hg. The S-haeck-BN-ML also follows the same structural transformation of haeck-BN as previous substitutions did. From the optimized structure shown in Fig. 4(d), we can see that the S atom is 1.4 Å out-of-plane of haeckBN sheet. The optimized S-B bond length is 1.85Å while the B-S-B bond angle is 94.73°
20
(a)
(c)
(b)
(d)
Figure 14: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed S-doped haeck-BN.
21
Table 1: Calculated energy gap (Eg), adsorption energy (Ead), Fermi energy (EF), the distance between molecules and haeck-BN (d) and work function (Φ). All the energy values are in eV. Eg (eV)
Ead (eV)
Ef (eV)
d (Å)
Φ (eV)
3.9
-
-3.542
-
5.317
haeck-BN+CH3Hg
metallic
-0.496
-1.710
2.3
3.764
Al-haeck-BN
metallic
-
-2.473
-
4.2
Al-haeck-BN+CH3Hg
metallic
-1.986
-1.538
attached
3.69
Si-haeck-BN
metallic
-
-2.849
-
4.668
Si-haeck-BN+CH3Hg
2.5
-3.261
-1.769
attached
3.957
P-haeck-BN
3.98
-
-4.170
-
6.011
P-haeck-BN+CH3Hg
metallic
-0.365
-1.167
2.8
3.3
S-haeck-BN
metallic
-
-0.807
-
2.726
2
-2.80
-2.345
attached
4.511
System haeck-BN
S-haeck-BN+CH3Hg
There is a significant effect on the electronic properties of haeck-BN after the substitution of S atom. The semiconducting nature of haeck-BN sheet is found to be disappeared on S substitution. The S (p) orbital contributes to the density at the Fermi level as can be seen from DOS and PDOS presented in Fig. 5(d) and Fig. 6(d) respectively. The charge transfer analysis shows that the S atom shares a 0.79e charge with B atoms in the haeck-BN sheet. The substitution of S causes large decrement in the work function (~2.59eV) of haeck-BN among all other considered substituted atoms as shown in Fig. 13. This large decrement in the work function of haeck-BN after S substitution can be attributed to the fact of plenty of charge share between the S and neighboring B atoms. Further, the interaction of CH3Hg over the S-haeck-BN system has been investigated and found that the adsorption of CH3Hg is chemisorption (Fig. 14(a)). The adsorption energy is found to be -2.8 eV which is larger than Al-haeck-BN, P-haeck-BN and pristine haeck-BN. Fig. 14 (b-c) which presents DOS and PDOS clearly depicts that the density shifts to the valence band region, which points out that a significant change in the electronic properties is introduced to the S-haeckBN sheet after the adsorption of CH3Hg. The impurity states at the Fermi level are due to the C(p) 22
and Hg (p) orbital electrons of CH3Hg after adsorption. The metallicity of the system has undergone no significant variation on the adsorption of CH3Hg molecule. A large change (increased ~ 1.78 eV) in the work function is observed after the adsorption of CH3Hg (Fig. 14(d)). This can be attributed to the oxidizing nature of CH3Hg molecule. From these results, we can deduce that the haeck-BN and adatom haeck-BN sheets can be considered as promising materials to remove as well as to sense the toxic hazard CH3Hg from the environment.
4. Conclusion
First principle calculations based on density functional theory (DFT) were carried out with the inclusion of dispersion correction to investigate the surface reaction chemistry of haeck-BN towards methyl mercury (CH3Hg). Effect of adatoms (Al, Si, P and S) on the electronic and structural properties along with the sensing ability of haeck-BN was inspected in detail. It was observed that the CH3Hg shows moderate adsorption over pristine and P doped haeck-BN (between -0.3 to -0.45 eV). Significant enhancement in the adsorption energy of CH3Hg is observed with the substitution of Al, Si and S in haeck-BN. We found that the key mechanism behind the large adsorption of CH3Hg is chemisorption, which is due to large charge transfer and its accumulation. A good sensing ability and great filterability of haeck-BN towards CH3Hg were observed on substitution with the considered atoms. The CH3Hg molecule binds with the substituted atom in all cases except for P atom. Since Al, Si, P and S are abundant as well as inexpensive metals, these metal-doped haeckBN systems can have large scale applications in future. Finally, we can conclude from our results that the modified haeck-BN sheet with Al, Si and S substitution is suitable for the removal of hazardous CH3Hg molecule from the environment while P substituted and pristine haeck-BN can act as a good sensor for the same.
23
Acknowledgments The study is financially supported by a research project from the Science and Engineering Research Board (SERB) (SB/S2/CMP-0005/2013), Govt. of India. The computations were carried out using high-performance computer cluster provided under DST-FIST program and SERB project.
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Highlights Surface reaction chemistry of haeckelite-BN (haeck-BN) as sensor is investigated. Strong interaction with hazardous methyl mercury is observed with haeck-BN Effect on the electronic properties of haeck-BN after adatoms is studied. Improvement in the adsorption energy of CH3Hg after Al, Si and S substitution is found.
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Graphical Abstract
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Authors Contribution Statment Basant Roondhe: Original draft preparation, Writing, Visualization, Investigation. Prafulla K. Jha: Supervision, Validation, Editing. Rajeev Ahuja: Reviewing and Validation.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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S0169-4332(19)33677-3 https://doi.org/10.1016/j.apsusc.2019.144860 APSUSC 144860
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
25 July 2019 13 November 2019 22 November 2019
Please cite this article as: B. Roondhe, P.K. Jha, R. Ahuja, Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144860
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Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury Basant Roondhe1, Prafulla K Jha1*, Rajeev Ahuja2,# 1
Department of Physics, Faculty of Science, The M. S. University of Baroda, Vadodara-390 002, India 2 Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala, 75120, Sweden *
Email: [email protected]
#
Email: [email protected]
Corresponding Authors *
(P.K.J.)E-mail:[email protected]
#
(R.A.)E-mail: [email protected]
1
ABSTRACT A new two-dimensional (2D) member in the boron nitride family, haeckelite BN (haeck-BN) is proposed as a nanosensor for the detection of environmental hazardous material, methyl mercury (CH3Hg). Using first principles-based dispersion corrected density functional calculations, we have studied interaction of CH3Hg with haeck-BN to understand its role as toxic gas sensor. The interaction of CH3Hg with haeck-BN is validated by calculating the adsorption energy and electronic density of states (DOS). The change in DOS and work function (WF) upon adsorption of CH3Hg molecules further confirms the sensing ability of haeck-BN. In the pristine form, haeck-BN shows weak interaction towards CH3Hg with adsorption energy of -0.45 eV. To tune the sensing and electronic properties of haeck-BN, we have also doped haeck-BN with Al, S, Si, and P atoms. The substitutions were found to alter the electronic properties of haeck-BN, turning it from semiconducting to metallic. However, with the substitution of heteroatom, a significant increment in the adsorption energy is observed in the range of -1.9 eV to -3.8 eV. Our finding suggests that haeck-BN can be utilized as a sensor for the detection of hazardous toxins.
Keywords: haeckelite BN; methyl mercury; electronic properties; adsorption; sensor; density functional theory.
1. Introduction The environment has suffered significantly due to the enormous increase of pollutants released from the fast-growing industrial units. The toxins’ level in the environment has increased up to a dangerous extent. One of the most hazardous toxins are the mercury-based pollutants due to their severe effect on environment as well as human health [1-3], among which their organic form being the most dangerous ones. Methyl mercury (CH3Hg) is one of the organic mercury-based compounds formed by inorganic ions of mercury, Hg2+, which is highly hazardous to humans because of its lipophilic tendency causing severe damage to the whole body and central nervous system [4-5]. Several industries are responsible for direct or indirect production of methyl mercury, among which 2
the acetaldehyde manufacturing industry is one of them. Other major factors behind the increased level of methyl mercury in the environment are the burning of fossil fuels and waste materials containing inorganic mercury. There exist many studies which report the adverse effect of methyl mercury on humans such as loss of IQ, attention deficiency in children [6] and risk of cardiovascular disease in adults [7]. The major pathway due to which humans get affected by methyl mercury is through the aquatic food chain. The increasing concentration of methyl mercury in the aquatic food chain is difficult to eliminate since it is typically accumulated in tissues of organism [8]. Being the apex predator of the aquatic food chain, its consumption results in severe neurological damages in humans [9]. Accordingly, there should be a technique to track as well as filter methyl mercury from the entire biota and environment. This challenge can be easily achieved by highly selective and sensitive sensors. There exist many experimental and theoretical studies tha describe the methods to detect and sense elemental mercury (Hg) [10-15], however, in the case of CH3Hg, only experimental studies are accomplished using fluorescence spectroscopy [16-20]. In the study of Yan et al. [13], detection of CH3Hg has been developed based on the restriction of intermolecular rotations which aggregates induced emission on interacting with CH3Hg. Lin et al.[14] reported detection of CH3Hg by lysozyme type VI-stabilized gold nano cluster where they found that it is highly sensitive and its sensitivity can be modulated by changing the concentration of lysozyme. Recently, a new method was developed for the detection of methyl mercury based on colorimetric detection by Xie et al. [20]. However, this method is still in development and need further improvement. Therefore, a fast, reliable and cost-efficient method, not only for detection but also for sensing and filtration of CH3Hg is much needed. There are many conventional metal oxide semiconductor-based sensors are available in the market at low cost with many advantages like low power consumption, high sensitivity and easy fabrication [21-22]. However, apart from many advantages, it possesses some limitations like long recovery time, high operating temperature and low accuracy [23-24]. The limitation of these conventional sensor materials motivates scientists to explore other materials that 3
can overcome these drawbacks and satisfy the high demand for sensitive material. Two dimensional (2D) materials are possible alternatives for sensor material. Graphene, a mono-atomic single sheet of carbon became an active area of research in the field of nanomaterials and nanotechnology, due to its potential for integration in next-generation electronic and energy conversion devices [2526].The high surface-to-volume ratio feature of 2D materials provides large active surface for reaction with gas molecules, which enhance the sensor performance down to parts-per-billion (ppb) levels and make it frontier material for gas sensing applications [21]. The prime reason for researchers’ exploration towards other similar 2D materials is the attention-seeking properties of graphene. Following the discovery of graphene, many 2D materials were isolated from their bulk counterparts[27-32], however, apart from all, two dimensional boron nitride (BN) gathered much attention [27] due to its better thermal and chemical stability than carbon, despite both being isomorphic [33-35]. Unique properties of hexagonal BN (h-BN) make it useful in many applications such as insulator in electronic devices, dielectric materials, lubricants, etc. [36-37]. Recently, a new planar member with square and octagonal ring haeckelite BN (haeck-BN) has been reported theoretically and experimentally [38-40]. This new form of BN has totally different electronic properties from conventional h-BN, as it has much lower bandgap as compared to h-BN and it is also capable as a biosensor [38]. The existence of this type of periodic structure has also been observed in GaN and ZnO [41-42]. Apart from its superior electronic properties as well as biosensing capability as compared to h-BN, the possibility of this haeck-BN as a toxin sensor has not been considered yet to the best of our knowledge. It is further observed that the aluminium(Al), silicon (Si), phosphorus (P) and sulfur (S) dopants improves the electronic, structural properties and adsorption capability of nanostructures [43-47].Moreover, the 3p elements Al, Si, P, and S have atomic radii close to boron and nitrogen atoms which form haeck-BN. Therefore, it will be easier for these atoms to form a stable covalent structure with haeck-BN. Thus, motivated with these facts, in the present study, we performed first principle calculations based on
4
density functional theory to check the possibility of pristine and 3p elements (Al, Si, P, and S) doped haeck-BN as a CH3Hg toxic gas scraper.
2. Computational Methodology The present study was performed by employing state-of-the-art first principles calculation under the framework of density functional theory (DFT) as implemented in Quantum Espresso [48].For the treatment of exchange-correlation functional, generalized gradient approximation (GGA) proposed by Perdew–Burke–Ernzerhof (PBE) with ultrasoft pseudopotential was employed [49]. For the ground state optimization of structures, the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method was used [50]. The energy cut-off and charge density cut-off for single-particle functions were set as 80 Ry and 800 Ry respectively in the plane-wave basis set to expand the Kohn–Sham orbitals which were sufficient to fully converge the total energy. The Monkhorst–Pack [51] scheme for the integration of the Brillouin zone (BZ) was performed with k-mesh of 16 x16x1. To solve the KohnSham equation, we employed the iterative Davidson-type diagonalization method with an electronic energy convergence of 10-8 Ry. We analyzed the interaction of haeck-BN with CH3Hg by performing structural optimization, adsorption energy calculation and electronic density of states (DOS) plots. Accounting of the dispersion forces has been emphasized in the literature by a precise quantum mechanical description that explains the interaction of a molecule with a nanosurface [52]. Therefore, for the description of a long-range electron correlation, it is important to choose a suitable computational method. We employed Grimme’s dispersion correction (DFT-D2) [53] to consider the long-range van der Waals (vdW) interaction between the CH3Hg and the haeck-BN monolayer. This type of vdW interaction has been widely used and found very successful in predicting the long-range interaction of variety of materials for adsorption applications [54-55]. The adsorption energy Ead is calculated using the following equation: 𝑬𝒂𝒅 = 𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵+𝑪𝑯𝟑𝑯𝒈) − {𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵) + 𝑬(𝑪𝑯𝟑𝑯𝒈) } (1)
5
where𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵+𝑪𝑯𝟑𝑯𝒈) is the total energy of haeck-BN adsorbed by CH3Hg, 𝑬(𝒉𝒂𝒆𝒄𝒌−𝑩𝑵) is the total energy of the adsorbent haeck-BN monolayer and 𝑬(𝑪𝑯𝟑𝑯𝒈) is the total energy of CH3Hg molecule. Their energies were obtained from their fully optimized geometries. The formation energy of all dopants (Al, Si, S, and P) for haeck-BN sheet has been calculated through: 𝒅𝒐𝒑𝒆𝒅
𝒑𝒓𝒊𝒔𝒕𝒊𝒏𝒆
𝑬𝒇 = 𝑬𝒕𝒐𝒕𝒂𝒍 − 𝑬𝒕𝒐𝒕𝒂𝒍
+ 𝒏𝑩 𝝁 𝑩 + 𝒏𝑵 𝝁 𝑵 − 𝒏𝒅 𝝁 𝒅
𝒅𝒐𝒑𝒆𝒅
(2) 𝒑𝒓𝒊𝒔𝒕𝒊𝒏𝒆
where 𝑬𝒕𝒐𝒕𝒂𝒍 is the total energy of haeck-BN sheet substituted with dopant, 𝑬𝒕𝒐𝒕𝒂𝒍
is the total
energy of pristine haeck-BN sheet, 𝝁𝑩 , 𝝁𝑵 , and 𝝁𝒅 is the chemical potential of boron, nitrogen and dopant atom, respectively and n is the number of individual atom in the sheet.
3. Result and Discussion In this section, we present the results of our calculation on the structural and electronic properties of CH3Hg adsorbed haeck-BN. This section also discusses the changes in the electronic structure, binding energy, and work function of haeck-BN sheet due to heteroatoms (Al, S, P and Si) doping on haeck-BN sheet. The key factor for the determination of a good sensor is the changes that arise in the bandgap of the material having gas/chemical exposure which is considered in this work. Therefore, finally, we present and discuss the ability of pristine and doped haeck-BN sheet towards sensing and detection of CH3Hg.
3.1 Pristine haeck-BN monolayer (haeck-BN-ML)
Fig. 1(a) shows the optimized structure of haeck-BN sheet. It has two rings, square and octagonal in the optimized structure of the BN sheet. The bond length between boron and nitrogen atoms in the square ring is 1.402 Å while the same in the octagonal ring is 1.475 Å. The measured B-N-B bond angles in the square ring are 84.21° and 95.78° while these are 132.10° and 137.11° in the octagonal ring. The stability of haeck-BN sheet has been investigated in our previous study [38]. 6
Fig. 1(b) presents the density of states (DOS) for haeck-BN which depicts its semiconducting nature with 3.9 eV bandgap. We have also calculated the partial density of states (PDOS) and presented them in Fig. 1(c) for a better view of the atomic orbital contribution in the DOS.
Doping Sight
) (c)
y
(b)
x a=9.85 Å
Figure 1: (a) Optimized structure, (b) Density of States (DOS and (c) Projected density of states (PDOS) of haeck-BN.
For the investigation of gas sensing ability of haeck-BN towards hazardous CH3Hg, we have calculated the adsorption energy of the CH3Hg adsorbed haeck-BN sheet. The CH3Hg molecule is kept in parallel orientation with haeck-BN due to its large size and equal participation of C, H and Hg atoms of CH3Hg molecule with the sheet and it was observed that there was no significant variation in structure of the system during energy minimization (Fig. 2(a)). The calculated adsorption 7
energy for CH3Hg over haeck-BN is -0.49eV indicating its physisorption characteristic with haeckBN sheet. The effect of CH3Hg adsorption on the electronic properties of haeck-BN sheet can be observed from the DOS of adsorbed haeck-BN which is presented in Fig. 2(b).
(a)
(c)
(b)
Figure 2: (a) Optimized structure (b) Density of states (DOS) and (c) Projected density of states (PDOS) of CH3Hg over haeck-BN.
It can be clearly seen from Fig. 2(b) that the adsorption of the CH3Hg molecule on the haeck-BN sheet introduces impurity states at Fermi level making the sheet metallic in nature. The CH3Hg molecule participates majorly in the valence band region. The density at -7 eV in the valence band region increases after the adsorption of CH3Hg due to C (p) orbital contribution (Fig. 8
2(c)) while the peak at -13 eV in the lower valence band region is due to Hg (d) orbital. The sensitivity of haeck-BN sheet towards CH3Hg can be further confirmed by work function calculation. Work function is the energy required by an electron to get free from the Fermi level to vacuum level and is expressed as WF(F) = Evac - EF
(a)
(2)
(b)
Figure 3: Work function plots of (a) Pristine haeck-BN (b) CH3Hg adsorbed haeck-BN.
Here, Evac and EF denote the energy of the vacuum level and Fermi level respectively. Fig. 3 (a-b) presents work function of pristine and CH3Hg adsorbed haeck-BN sheet. An appreciable change in work function of haeck-BN sheet is observed after the adsorption of CH3Hg molecule. The work function of pristine haeck-BN reduces from 5.31 eV to 3.76 eV after CH3Hg adsorption. This large modulation in the work function of haeck-BN sheet further confirms the interaction of the molecule with the sheet, reinforcing the gas-sensitive ability of haeck-BN sheet.
9
3.2. Haeck-BN-ML with Heteroatom Substitution
(a)
(b)
(c)
(d)
Figure 4: Optimized structure of adatoms haeck-BN systems (a) Aluminum (Al) (b) Silicon (Si) (c) Phosphorus (P) and (d) Sulphur (S). Among many, the substitution of atoms is utilized as a successful strategy to enhance the adsorption energy and sensitivity of any sensor. For this purpose, we have considered Al, Si, P and 10
S as a substitution for N in haeck-BN sheet for the enhancement of adsorption energy of CH3Hg over haeck-BN sheet. The elements for the substitution are chosen on the basis of their comparability of size with nitrogen. The calculated energy gap (Eg), adsorption energy (Ead), Fermi energy (EF), the distance between molecule and haeck-BN (d) and work function (Φ) are presented in Table 1 where an effect of dispersion on the listed properties is evident. The optimized structures are presented in Fig. 4(a-d), where a considerable structural change can be seen after the substitution of the doping elements. In order to check the stability of these doped systems, we have calculated the formation energy by using Eq. 2., utilizing the chemical potential instead of electronic energy [52]. We found that the S-haeck-BN have higher (negative) formation energy (14.3 Ry) in comparison with P-haeck-BN (-7.1Ry) and Si-haeck-BN (-2.67 Ry), confirming its
(a)
(b)
(c)(a
(d)
Figure 5: Density of states of (a) Al-doped haeck-BN (b) Si-doped haeck-BN (c) Pdoped haeck-BN and (d) S-doped haeck-BN. 11
higher stability. Al-haeck-BN has positive formation energy which indicates that doping of Al requires higher energy, which implies that the formation energies follows the sequence: S > P > Si > Al. Substitution at the place of N by considered elements causes a large change in the electronic properties. All elements, except the phosphorus ones, convert the semiconducting nature of haeckBN sheet into metallic.
(a)
(b)
(c)
(d)
Figure 6: Projected density of states of (a) Al-doped haeck-BN (b) Si-doped haeckBN (c) P-doped haeck-BN and (d) S-doped haeck-BN.
The DOS and PDOS, respectively shown in Fig. 5(a-d) and Fig. 6(a-d) evidently demonstrates the modulation in the electronic properties and the respective contribution of the 12
considered elements in haeck-BN sheet. We now systematically discuss the effect of substitution of Al, Si, S and P atoms on the adsorption as well as electronic properties of haeck-BN sheet in the following section. 3.2.1. Al-doped haeck-BN-ML(Al-haeck-BN-ML) Initially, we considered Al atom for the substitution in place of N atom of haeck-BN-ML. In Al-haeck-BN-ML (Al substituted haeck-BN monolayer), the B-Al bond length and the B-Al-B angles are found to be 2.12 Å and 87.79° respectively. The Al atom in the haeck-BN-ML induces a buckling of the sheet by 1.8 Å at the site of substitution, which can be observed from Fig. 4(a). The electronic properties of haeck-BN sheet change from semiconductor to metallic after the substitution of Al atom as can be seen from DOS and PDOS plots presented in Fig. 5(a) and Fig. 6(a), respectively.
Figure 7: Work function plots of Al-doped haeck-BN. We have also calculated the charge transport due to the difference in electronegativity of adatom by calculating the Lowdin charge. The Al atom has the lowest electronegativity (1.6 e)
13
among the entire dopants considered in our system and gives more than half of its charge (0.91e) to the B atom in the haeck-BN sheet.
(c)
(a)
(b)
(d)
Figure 8: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed Al-doped haeck-BN. Moreover, we have calculated the work function of Al-haeck-BN-ML and found that the substitution of Al in haeck-BN sheet reduces the work function by the magnitude of 1.11 eV as depicted in Fig. 7, which further confirms the donation of charge by Al to the sheet. We have also investigated the interaction of CH3Hg molecule with Al-haeck-BN-ML and observed chemisorption of CH3Hg due to the formation of a bond between Hg and Al (Fig. 8(a)) and value of the calculated 14
adsorption energy is -1.98 eV. On adsorption of CH3Hg, a significant transfer of charge (0.03e) occurs from the sheet towards the molecule but it does not have a significant effect on electronic properties, as the metallic nature after the adsorption remains unchanged. This removal of charge from the sheet by CH3Hg can be attributed to the oxidizing nature of the CH3Hg molecule. Fig.8 (bc) representing DOS and PDOS clearly illustrates the effect of charge transfer, as there is a decrease in the peak intensity in the conduction band region at around 2 eV. From the PDOS it is obvious that the major contributions of the molecule are close to the Fermi level. The Al(p) and Hg(p) contribute to both conduction as well as valence bands region at 1.5 eV and -2 eV respectively, while C(p) and Hg(d) contribute in valence band region at -1.5 eV and at -7 eV. The large contribution of Al(p) in the case of Al-haeck-BN-ML at 2 eV in the conduction band reduces after the adsorption of CH3Hg. This reduction is consistent with the significant transfer of charge between the Al-haeck-BN sheet and CH3Hg molecule. The adsorption of CH3Hg also alters the work function as shown in Fig. 8(d).
3.2.2. Si-doped haeck-BN-ML(Si-haeck-BN-ML) Another element that we have considered for substitution in the N site is Si. In Si-haeck-BNML (Si substituted haeck-BN monolayer) , the B-Si bond length and B-Si-B bond angle is found to be 1.98 Å and 94.28°, respectively on optimization. Similar to the case of Al, Si atom also induces a buckling at the site of substitution as shown in Fig. 4(b). However, the buckling height is 0.1 Å lower than that of the one observed in the case of Al substitution. The substitution of Si atom in the sheet also converts the electronic properties of haeck-BN sheet as Al does. Further, the semiconductor to metal transition after the substitution of Si atom can be observed from the DOS and PDOS plots presented in Fig. 5(b) and Fig. 6(b), respectively. There is a significant transfer of charge (0.8e) between Si and surrounding B atoms, which consequently reduces the work function of haeck-BN sheet by 0.64 eV as can be seen from Fig. 9
15
Figure 9: Work function plots of Si-doped haeck-BN.
The sensitivity of the Si-haeck-BN sheet towards CH3Hg was investigated by calculating the adsorption energy and the effect on the structure is presented in Fig 10 (a). There is significantly high adsorption energy of -3.26 eV indicating a strong chemisorption. The adsorption energy, in this case, is higher than that obtained for Al substitution. Comparatively significant transfer of charge from haeck-BN sheet to CH3Hg molecule (0.18e) is the reason behind this high adsorption energy, in contrast to the case of Al substitution. This large charge transfer converts the system again to the semiconductor state as can be noted from DOS and PDOS plots presented in Fig. 10(b-c).There is a shift in the density of states in the valence band region after the adsorption of CH3Hg, reflecting the removal of charge from the sheet by the molecule. The contribution of Si (p) orbital was initially present at the Fermi level in the Si-haeck-BN shifts to the valence band region around -2 eV. After the adsorption of CH3Hg, the impurity states of Si (p) at 2 eV in the conduction band region is found to disappear totally.
16
(c)
(a)
(b)
(d)
Figure 10: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed Si-doped haeck-BN. The major contribution in the conduction band region is due to the Hg (p) and B (p) orbitals. The work function of Si-haeck-BN also reduced (~ 0.71eV) on the adsorption of CH3Hg (Fig. 10(d)) which further validates the interaction of CH3Hg molecule with haeck-BN.
17
3.2.3. P doped haeck-BN-ML (P-haeck-BN-ML)
Figure 11: Work function plots of P-doped haeck-BN.
The substituted haeck-BN shows different electronic behavior, though the transformation of the structure has a similar out-of-plane trend with the previously considered cases in this study. The optimized B-P bond length is 1.88 Å and the B-P-B bond angle 101.45°. Fig. 4(c) shows that the substituted P atom is 1.5 Å above the plane of haeck-BN-ML. The electronic property of haeck-BN sheet does not get affected by the substitution of P as the semiconducting nature remains preserved. The DOS and PDOS plots shown in Fig. 5(c) and Fig. 6(c), respectively depicts the unaffected electronic properties of the haeck-BN sheet on P substitution. We further checked the charge transfer between the P and its surrounding B atoms and found that it shares a 0.66e charge. Fig. 11 shows an increment in the work function (~0.69 eV) after the substitution of P in haeck-BN-ML. This new P-haeck-BN-ML system is then investigated for the sensing of CH3Hg molecule. The adsorption energy calculation reveals that the CH3Hg molecule gets physisorbed (observed from
18
(c)
(a)
(d)
(b)
Figure 12: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed P-doped haeck-BN. Fig. 12(a)) over P-haeck-BN-ML with the adsorption energy of -0.36 eV which falls in the range of weak chemisorption and good physisorption. The reason behind this low adsorption energy is that the molecule interacts non-covalently with the P-haeck-BN sheet. The adsorption of CH3Hg molecule alters the electronic properties of P-haeck-BN from semiconducting to metallic, due to the C (p) and Hg (p) contributions at Fermi level (Fig. 12(b-c)). The density at Fermi level which arises due to the P (p) orbital contribution in P-haeck-BN shifts to valence band region around -2 eV after
19
the adsorption of CH3Hg. There is a large reduction in the work function (~2.7eV) of P-haeck-BN after the adsorption of CH3Hg which can be seen from Fig. 12(d). 3.2.4. S doped haeck-BN-ML(S-haeck-BN-ML)
Figure 13: Work function plots of S-doped haeck-BN.
Finally, we substituted S in haeck-BN-ML to see its effect on CH3Hg. The S-haeck-BN-ML also follows the same structural transformation of haeck-BN as previous substitutions did. From the optimized structure shown in Fig. 4(d), we can see that the S atom is 1.4 Å out-of-plane of haeckBN sheet. The optimized S-B bond length is 1.85Å while the B-S-B bond angle is 94.73°
20
(a)
(c)
(b)
(d)
Figure 14: (a) Optimized structure (b) DOS (c) PDOS and (d) work function plot of CH3Hg adsorbed S-doped haeck-BN.
21
Table 1: Calculated energy gap (Eg), adsorption energy (Ead), Fermi energy (EF), the distance between molecules and haeck-BN (d) and work function (Φ). All the energy values are in eV. Eg (eV)
Ead (eV)
Ef (eV)
d (Å)
Φ (eV)
3.9
-
-3.542
-
5.317
haeck-BN+CH3Hg
metallic
-0.496
-1.710
2.3
3.764
Al-haeck-BN
metallic
-
-2.473
-
4.2
Al-haeck-BN+CH3Hg
metallic
-1.986
-1.538
attached
3.69
Si-haeck-BN
metallic
-
-2.849
-
4.668
Si-haeck-BN+CH3Hg
2.5
-3.261
-1.769
attached
3.957
P-haeck-BN
3.98
-
-4.170
-
6.011
P-haeck-BN+CH3Hg
metallic
-0.365
-1.167
2.8
3.3
S-haeck-BN
metallic
-
-0.807
-
2.726
2
-2.80
-2.345
attached
4.511
System haeck-BN
S-haeck-BN+CH3Hg
There is a significant effect on the electronic properties of haeck-BN after the substitution of S atom. The semiconducting nature of haeck-BN sheet is found to be disappeared on S substitution. The S (p) orbital contributes to the density at the Fermi level as can be seen from DOS and PDOS presented in Fig. 5(d) and Fig. 6(d) respectively. The charge transfer analysis shows that the S atom shares a 0.79e charge with B atoms in the haeck-BN sheet. The substitution of S causes large decrement in the work function (~2.59eV) of haeck-BN among all other considered substituted atoms as shown in Fig. 13. This large decrement in the work function of haeck-BN after S substitution can be attributed to the fact of plenty of charge share between the S and neighboring B atoms. Further, the interaction of CH3Hg over the S-haeck-BN system has been investigated and found that the adsorption of CH3Hg is chemisorption (Fig. 14(a)). The adsorption energy is found to be -2.8 eV which is larger than Al-haeck-BN, P-haeck-BN and pristine haeck-BN. Fig. 14 (b-c) which presents DOS and PDOS clearly depicts that the density shifts to the valence band region, which points out that a significant change in the electronic properties is introduced to the S-haeckBN sheet after the adsorption of CH3Hg. The impurity states at the Fermi level are due to the C(p) 22
and Hg (p) orbital electrons of CH3Hg after adsorption. The metallicity of the system has undergone no significant variation on the adsorption of CH3Hg molecule. A large change (increased ~ 1.78 eV) in the work function is observed after the adsorption of CH3Hg (Fig. 14(d)). This can be attributed to the oxidizing nature of CH3Hg molecule. From these results, we can deduce that the haeck-BN and adatom haeck-BN sheets can be considered as promising materials to remove as well as to sense the toxic hazard CH3Hg from the environment.
4. Conclusion
First principle calculations based on density functional theory (DFT) were carried out with the inclusion of dispersion correction to investigate the surface reaction chemistry of haeck-BN towards methyl mercury (CH3Hg). Effect of adatoms (Al, Si, P and S) on the electronic and structural properties along with the sensing ability of haeck-BN was inspected in detail. It was observed that the CH3Hg shows moderate adsorption over pristine and P doped haeck-BN (between -0.3 to -0.45 eV). Significant enhancement in the adsorption energy of CH3Hg is observed with the substitution of Al, Si and S in haeck-BN. We found that the key mechanism behind the large adsorption of CH3Hg is chemisorption, which is due to large charge transfer and its accumulation. A good sensing ability and great filterability of haeck-BN towards CH3Hg were observed on substitution with the considered atoms. The CH3Hg molecule binds with the substituted atom in all cases except for P atom. Since Al, Si, P and S are abundant as well as inexpensive metals, these metal-doped haeckBN systems can have large scale applications in future. Finally, we can conclude from our results that the modified haeck-BN sheet with Al, Si and S substitution is suitable for the removal of hazardous CH3Hg molecule from the environment while P substituted and pristine haeck-BN can act as a good sensor for the same.
23
Acknowledgments The study is financially supported by a research project from the Science and Engineering Research Board (SERB) (SB/S2/CMP-0005/2013), Govt. of India. The computations were carried out using high-performance computer cluster provided under DST-FIST program and SERB project.
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Highlights Surface reaction chemistry of haeckelite-BN (haeck-BN) as sensor is investigated. Strong interaction with hazardous methyl mercury is observed with haeck-BN Effect on the electronic properties of haeck-BN after adatoms is studied. Improvement in the adsorption energy of CH3Hg after Al, Si and S substitution is found.
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Graphical Abstract
31
Authors Contribution Statment Basant Roondhe: Original draft preparation, Writing, Visualization, Investigation. Prafulla K. Jha: Supervision, Validation, Editing. Rajeev Ahuja: Reviewing and Validation.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
33