(5,5) BN nanotubes Dioxin interactions: Influence of Point Defect on the Structural and the Electronic Properties

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1 Mex. J. Mat. Sci. Eng. 1 (2014) (5,5) BN nanotubes Dioxin interactions: Influence of Point Defect on the Structural and the Electronic Properties A. Rodríguez Juárez, 2C. Sánchez Hernández, 2H. Hernández Cocoletzi, 2,*E. Chigo Anota Instituto Politécnico Nacional-UPIITA, Av. Instituto Politécnico Nacional Barrio Laguna Ticomán, México D.F. Benemérita Universidad Autónoma de Puebla, Facultad de Ingeniería Química, Ciudad Universitaria, San Manuel, Puebla, Código Postal 72570, Puebla, México. Author to whom correspondence should be addressed; 2,* echigoa@yahoo.es Abstract. The influence of dioxin on the structural and electronic properties of (5,5) boron nitride nanotubes is reported in this work. Both, the ends and the surface at different sites and orientations are considered in the study. The density functional theory within the Perdew-Wang approximation along with the doubly-polarized base is employed. The dioxin is mainly adsorbed on the nanotube surface increasing its solubility and dispersion. A reduction on both the reactivity and work function is also observed, important properties in the devices design. No changes are observed when the composed system is immerse in water. At the same time, point defects and carbon atom impurities improve the field emission. Keywords: Boron Nitride nanotubes; Dioxin; Point defects; DFT; Solvation Introduction Restrictions on the applicability of many systems are related to their solubility and to their surface purification. This is the case of the boron nitride nanotubes (BNNTs) [1,2]. It has been demonstrated that their functionalization (thorough their dangling bonds) may overcome some difficulties like the dispersion in water [3]. This also has permitted a reduction on their toxicity with usage as anticancer agent [4]. Functionalization with polymers gives rise applications as glucose sensors [5] and light emitting [6]. The electronic and magnetic properties have been analyzed when boron nitride nanosheets and nanotubes are functionalized with organic molecules and other functional groups [7-12]. Then, in order to find a broad range of specific expected effects and applications more theoretical and experimental work is required. Adsorbers of toxic molecules, like those generated in combustion processes where Cl is involved are needed. Dioxins are examples of these molecules [13]; they are highly toxic and in the recent years are commonly found in the environment. For this situation we investigate the interaction of these systems with the aim of proposing pollutant sensors based on armchair BN nanotubes. In general, the presence of a molecule modifies the surroundings. For example, the adsorption on a surface may modify their structural and electronic properties. In this work, the armchair nanotube or (5,5) single wall boron nitride nanotube (SW BNNT, figure 1a) is proposed as adsorbent of the dioxin C12O2H4Cl4 (Dx). The changes on the bond lengths, nanotube diameter, together with the polarity, the chemical potential 12

2 and work function are proposed as a measure of the adsorption of this dangerous molecule. The surface as well as the ends of the nanotubes is considered as an active site, where the molecule may be adsorbed. The effect of defects and doping of the nanotube is also considered. The actual environment generally involves water; this fact is also taken into account. DFT molecular modeling has been used, which has been successfully used in the description of many nitrides [14-19]. Simulation Models and Methods First principles total energy calculations were performed to study the system (5,5) SW BNNT Dx employing the Perdew-Wang [20] functional within the local density approximation (LDA) and the DNP base function; this base considers a p-orbital for H and a d-orbital for B, N, O, C and Cl atoms [21]. All these conditions are implemented in the well known DMol3 package [22]. The total energy was calculated when the Dx molecule relies parallel and perpendicular to the nanotube, and perpendicular to one of the ends (Table I). The (5,5) open nanotubes has 1.31 nm of length and 0.74 nm of diameter. The ends were monohydrogenated. A total of 120 atoms (50 N, 50 B and 20 H) to model the nanotube and a Dx molecule were used. Neutral charge and multiplicity of 1 was considered. Solvation effects have been included by using a polarized continuum model (Conductor-like Screening Model) [23-25] with a dielectric constant of 78.4 to simulate the water environment. The difference in energies is obtained with the expression Esolv1=E(BNNTsolv) E(BNNTvaccum) and Esolv2=E[(BNNT Dx)solv] E[(BNNT Dx)Vacc um) [26]. The energy difference between the HOMO and the LUMO is also reported. The adsorption energy of the Dx molecule is defined as follows: Ead=E(BNNT+Dx) E(BNNT) E(Dx). The arithmetic average (HOMO+LUMO)/2, was used to evaluate the chemical potential [27]; in a free electron gas, this quantity is equal to the Fermi level, which is considered as the center of the energy gap. The work function was calculated thorough the difference between the potential energy of the vacuum level (LUMO) and the Fermi level (chemical potential), which is the lowest energy required to get out an electron from the Fermi level. The Molecular Electrostatic Potentials (MEPs) were obtained as described elsewhere [28]. The orbital cut off for the base function was of 0.38 nm for the dioxin and 0.34 nm for the systems BNNT Dx. The convergence on the total energy was reached with a precision of Ha. The obtention of non-complex frequencies ensured the structural stability [29]. Results and discussion The electronic and magnetic properties of open BN nanotubes saturated with hydrogen atoms are different respect to the closed nanotubes [30]. Moreover, the (n,n) chirality posses the more stable gap [31]. The synthesis of open BN nanotubes [32] motivated their short length functionalization with some polymers searching for applications on biomedicine and for generating high mechanical resistance composites [3,4]. This motivated the study of open nanotubes with (5,5) chirality in the presence of the dioxin molecule; the theoretical investigation was done for pristine and defected nanotubes. 13

3 Table I. Total energy difference of configurations BNNT Dx. The reference energy corresponds to the lower energy. Configurations Energy difference ev/atom Optimization process The nanotube and the dioxin were structurally optimized by separate prior their interaction. Between the four sites studied in this work that when the dioxin molecule is parallel to the nanotube has the lower total energy, which means that this site is preferred by the molecule to be adsorbed (figure 1c), molecules like H2, O2 and H2O are adsorbed on the ends of the nanotubes due the dangling bonds [33]; the site of adsorption depends on the kind of molecule. For this configuration, the shortest distance between the nanotube and the dioxin is of 2.81 Å, which is in the non-covalent interval. The bond lengths and diameter in the nanotube do not change by the presence of the dioxin. Interestingly, the dioxin maintains its structure too. Both, the nanotube and the dioxin are highly geometrically stable, the adsorption energy is a bit weak ( 0.21 ev) which corresponds to a physisorption, attributed to the cero polarity of the dioxin and to van der Waals effects. The HOMO and the LUMO isosurfaces (figure 2), where no contribution of the pz orbitals of N and B is observed, show that the molecular orbitals are concentrated exclusively on the dioxin. Table II. Bond length (Å), HOMO LUMO gap (ev), Fermi energy level (ev), dipolar moment (ev), work function (ev) and energy adsorption (ev) for BNNT Dx pristine and with point defects. Nanotubes Bond length B N BNT Dx HOMO LUMO gap Fermi level Dipole Moment Work Function (5,5) BNNT 4.08 [32] 4.44 [35] 5.5 [36] 5.8 [36] 4.59 (5,5) BNNT Solvated water BNNT (5,5) BNNT C doped Dioxin Solvated water N vacancy B vacancy C doped Adsorption Energy

4 Electronic properties The electronic charge of the dioxin is lightly redistributed, as indicated by the MEPs surfaces (figure 1b and 1d). The Cl and O atoms mainly contain the negative zone, little amount is located around the N atom of the nanotube. The major electronic distribution is mainly on the surface of the nanotube, contrary to that suggested in the literature. This fact is very important because favors the dispersion of the nanotubes due electrostatic repulsion focusing technological applications, at the vacuum or in the presence of a solvent. The gap between the HOMO and the LUMO of the nanotube is reduced to 3.12 ev, the pristine nanotube has the value of 4.59 ev (see Table II), preserving its semiconductor character. At the same time, the dipole moment is increased from 0 to 1.23 D, along the direction of the interaction between both systems (figure 1d). A similar phenomenon is observed when BN nanotubes and nanosheets with different chirality interact with chitosan [9,10]. These results suggest that functionalizing the BNNT with dioxin their solubilization may be reached, such as when they are functionalized with methoxy-poly (ethylene gycol)-1,2-distearoylsn-glycero-3-phospoethanolamine-n immerse in water [3]. The water environment was also considered in our work. The dipole moment increased substantially, from 1.23 to 1.78 D. The solvation energy for the pristine BNNT has the value kcal/mol, when the dioxin is physisorbed on the BNNT this parameter decreases to kcal/mol. This small difference (2.24 kcal/mol) indicates a weak interaction between the dioxin molecule and the BNNT, as suggested by the adsorption energy. Solvation also increases the distance BNNT dioxin, and the BNNT diameter, 0.01 Å and nm, respectively. The work function is also diminished which means that is easier to get out electrons (the potential barrier is overcome) from the surface of the nanotubes when is attached the dioxin (1.56 ev) as compared with the pristine BNNT (2.3 ev). The field emission properties are improved, as happens when the BNNT is functionalized with water and oxygen molecules [12]. The Fermi level is increased (Table II), as indicated by the increase of the chemical potential, confirming the weak interaction BNNT dioxin system. Something similar has been observed when the BNNTs are solubilized in water (see Table II). Finally, for a given temperature (T) the electrical conductivity of a material is related with the Eg thorough the expression σ α exp (-Eg/kT) with Eg the HOMO LUMO gap, k the Boltzmann constant [34]; smaller values of Eg leads to larger values of conductivity. For this case, the Eg was reduced which improve the conductivity of the BNNT dioxin system demonstrating a notable sensitivity on the electronic properties of the BNNT in the presence of the dioxin molecule. Influence of point defects A monovacancy given by the absence of an N atom induces a concavity on the surface of the nanotube, reported previously by Zobelli et al. [35]. This point defect increases the distance BNNT dioxin by 0.51 Å (figure 1e). At the same time, the formation of new bonds is prohibited by the B atoms searching for the energetic stability. This effect promotes that the HOMO LUMO gap and the work function experiment a reduction together with an increase on the polarity, improving the field emission and the conductivity, crucial parameters for technological applications like the display fabrication. Charge transference from the nanotube to the dioxin, as shown by the MEP surface (figure 1e), was obtained. The fact that the LUMO is mainly localized around the vacancy zone (figure 2) confirms the former. When the monovacancy is due the absence of a B atom a partial surface reconstruction is 15

5 induced, the formation of N N new bonds (figure 1f) originates a pentagon and a nonagon, as obtained by Zobelli et al. [35] (figure 1 of this reference). The distance BNNT dioxin is incremented to 3.15 Å remaining in the noncovalent range. This monovacancy converts the nanotube into a semimetal, increasing substantially the conductivity, with a HOMO LUMO gap of 0.81 ev, this value of the gap confers to the nanotube applications in spintronics. The chemical reactivity is even lower (-4.65 ev), but its polarity is increased giving rise to a better localization of the charge on the N atoms around the monovacancy (figure 1f) which is confirmed by the HOMO and the LUMO localized around the monovacancies. The work function value (0.41 ev) is very appropriate for designing displays. BNNTs have been doped using the Electron-Beam Irradiation technique [34], in this work the nanotube was doped with C atoms, replacing some N atoms in the nanotube structure, near the site of the dioxin adsorption (figure 3a). Now, a covalent bond is formed between the nanotube and the dioxin whose bond distance is of 1.55 Å (figure 3a). The nanotube as well as the dioxin geometry is modified; the dioxin is non-planar with an asymmetry in the C C bond (figure 3b). The B atom also reorients the dioxin adsorption on the nanotube surface giving rise to chemisorption with an increase on the adsorption energy by a factor of 7.86 as compared with the no-doped case. A transition from semiconductor to conductor in the nanotube is also observed (Table II), in accordance to the experimental result of Golberg et al. [36]. Contrary to the monovacancy defects, the polarity is decreased and the work function becomes cero, the same value has the HOMO LUMO gap, parameters appropriate for designing devices where the field emission effects are important i. e. doping the BNNT with C atoms enhances the field emission properties, even better, than that when is solvated. The former HOMO LUMO gap values are reliable because this parameter for the pristine BNNT (4.59 Å) is in the range of experimental and theoretical results [32,25,36]. Conclusions We have presented studies of the interaction between the Dx and armchair boron nitride nanotube. The interaction is thorough a physisorption and the functionalization is better on the surface of the nanotube. A transition from semiconductor to semimetal with a B vacancy is observed, and to a semiconductor when is doped with C atoms. The C doping tends to chemisorb the dioxin on the nanotube, decreasing the gap and improving the field emission effects. The polarity is increased enhancing the solubility and dispersion of the nanotubes, confirmed by the solvation energy. The work function of the doped systems reduced to cero suggesting applications on the design of displays as well as on the spintronics. Acknowledgments This work was partially supported by projects: VIEP-BUAP (CHAE-ING14-G) and Cuerpo Académico Ingeniería en Materiales (BUAP-CA-177). 16

6 Optimized geometry and MEP Nitrogen Boron Unsolvated On Nitrogen Solvated Hydrogen a) Oxygen b) Carbon Chlorine Chlorine Oxygen Dx on BNNT c) d) Formation of a pentagon Charge on nitrogens Charge on vacancy e) f) Figure 1. Optimized geometries for a) BNNT, b) MEP for BNNT, c) BNNT Dx, d), MEP for BNNT Dx and Dx, e) BNNT Dx with vacancies of N and MEP, f) BNNT Dx with vacancies of B and MEP. In blue is the positive charge zone, in yellow the negative charge zone, N atoms in blue, B atoms in pink, Cl atoms in green, C atoms in grey, O atoms in red, H atoms in white. 17

7 HOMO LUMO BNNT BNNT Dx Dx pz orbital pz orbital BNNT Dx N Monovacancy BNNT Dx B Monovacancy Figure 2. HOMO and LUMO isosurfaces for BNNT, BNNT Dx, Dioxin, BNNT Dx with vacancies of N and BNNT Dx with vacancies of B. 18

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