BORON NANOTUBES : STRUCTURE AND PROPERTIES

1. BORON NANOTUBES: STRUCTURE AND PROPERTIES ZAPOROTSKOVA Irina, professor, Director of the Research Center «Nanotecnology and nanomaterials» Volgograd State University, RUSSIA E-mail: sefm@volsu.ru

2. «We need to know much to understand how little we know» (Socrates) • Scientists used to think know everything about this element, and since it was not in great demand in industry boron was out of the focus of attention. • But in recent decades, boron and its compounds has found application in different industries such as atomic, rocket-building, metal processing, chemical and many others. • Boron atoms are capable of forming both ion and covalent bonds. They can make chains, frames, nets etc.Still, we do not know much about this element. • Boron has found application in many fields of modern technology. • small addition of boron to steel (0,0005…0,005 %) increase its hardness! • Boron better than any other element removes gases from copper thatimprove its properties. • Saturation of metals with boron forms hard borids!

3. There is no consensus about how many boron modifications exist[1,2]. • Researchers (Boris Yakobson)anticipated the existence of a fullerene consisting of 80 boron atoms. • Boron nanotubes were synthesised recently[3]and their properties and nature have not been fully revealed. Research into possible configurations of boron is vital! __________________________________________________________________________________________________ 1. Xiaobao Yang, Yi Ding, and Jun Ni. Ab initio prediction of stable boron sheets and boron nanotubes: Structure, stability and electronic properties. // Phys. Rev. B 77, 041402(R). - 2008. 2.H. Tang and S. Ismail-Beigi. Novel Precursors for Boron Nanotubes: The Competition of Two-Center and Three-Center Bonding in Boron Sheets. // Physical Review Letters. – 2007. – Т. 99. – С. 115501. 3.Dragos Ciuparu, Robert F. Klie, Yimei Zhu, and Lisa Pfefferle. Synthesis of Pure Boron Single-Wall Nanotubes // J. Phys. Chem. B 2004, 108, 3967-3969.

4. One of the configurations of boron – hexagonal boron Fig. 1. EECquasi-planar boron. • Table 1.Main properties of quasi-planar hexagonal boron (by applying the IB-CCC and MNDO [4]). • 4. Litinsky A.O., lebedev N.G., Zaporotskova I.V. // Journal of physical chemistry. – 1995. – V. 69. № 1. – P. 189.

5. By analogy with carbon nanitubes we assumed that boron nanotubes can beconstructed by rolling of hexagonal quasi-planar boron • S. Ismail-Beigi [5]: Due to three-center bonding boron nanotubes are mainly of triangular and hexagonaltypes. We based our research on this assumption 5. H. Tang and S. Ismail-Beigi. Novel Precursors for Boron Nanotubes: The Competition of Two-Center and Three-Center Bonding in Boron Sheets. // Physical Review Letters. – 2007. – Т. 99. – С. 115501.

6. Table 2.Main characteristics of boron nanotubes (n,n) and (n,0) types: n – the number of hexagons along theperimeter of a boron nanotube, d – the tubullene diameter, ∆Eg– band –gap energy, Еstr – strain energy.

7. Strain energy decreases with an increase in the diameter of (n, n)-nanotube (fig. 2). In (n, 0)-tubes strain energy increases with an increasing number of n (and, accordingly, the diameter of a tube) (fig. 3). It allowed us to draw a conclusion that formation process of a «zig-zag»-nanotube from hexagonal boron structures is energetically less favoured. Fig. 2. Dependence of strain energy Еstron a tubullene (n, n) diametre (d). Fig. 3. Dependence of strain energy Еstron a tubullene (n, 0) diametre (d).

8. Boron nanotubes with defectsFig. 4. EECof boron nanotube (6,6) with substitution defects(either neutral carbon atom (С), or positively (С+) and negatively (С–) charged carbon ions)

9. Fig. 5. Single electron energy spectra of boron tubullenes (6, 6), calculated withmolecular cluster technique: 1) substitution defect with a carbon atom; 2) substitution defect with a positive carbon ion; 3) substitution defect with a negative carbon ion; 4) pure nanotube; twice filled and vacant levels are shown. • Е, eV

10. Table 3.Charges on substitution defects and the distribution of caused by defects charges on B atoms according to the directions interaction (А – along the axis, B – along the circle).

11. Boron nanotube (6,6) with a vacancy Fig. 6. Potential energy pattern of the vacancy formation process in a boron nanotube (6,6), Еact = 0,68 eV

12. Sorption properties of boron nanotubes • It is known that carbon nanotubes have unique soption properties. Much research into the mechanism of atom and molecular adsorption on their surface has been carried out. Some of the papers are presented here [4-7]. • The study of the surface structure hydrogenation is promising for its application as a storage for molecular hydrogen. • The search for structures with well-developed surfaces capable of adsorbing gases (including hydrogen) remains in the focus of attention. In this respect research of sorption propertiesof boron nanotubes is important. ______________________________________________ • 4. Zaporotskova I.V., Litinsky A.O., Chernozatonsky L.A. // The letters to JETPh. – 1997. - V. 66. - P. 799 - 802. • 5.Lebedev N.G., Zaporotskova I.V., Chernozatonskii L.A. Single and regular hydrogenation and oxidation of carbon nanotubes: MNDO calculations // International Journal of Quantum Chemistry. – 2003. - V. 96, № 2. - P. 149 - 154. • 6.Lebedev N.G., Zaporotskova I.V., Chernozatonskii L.A. Fluorination of carbon nanotubes: quantum chemical investigation within MNDO approximation // International Journal of Quantum Chemistry. – 2003. - V. 96, № 2. - P. 142 - 148. • 7. Zaporotskova I.V., Lebedev N.G., // Хchemical physics, 2006. - V. 25, № 5. - P. 91 – 96.

13. Boron and hydrogen complex (borane) might find application in developing new kinds of borane fuels.While burning, boron produces twice as much heat as carbon (14 170 kcal/kg), so aviation will gain much from using borane fuels. Firstly, the size of a plane can be smaller so that it will gain higher speed. Secondly, a plane can carry more cargo. Finally, this will help to reduce take-off run. That is why we consider the study of hydrogen adsorption on the surface of a boron nanotube as important.

14. Adsorption mechanismof hydrogen, fluorine, chlorine and oxygen atoms on the outside surface of a boron nanotube(6,6)-typeFig. 7. Three variants of adatoms orientation towards the surface of a boron nanotube: I) above a boron atom, II) above the centre bonding В-В, III) above the centre of a hexagon

15. I – In the first case adsorption process was simulated by step-by-step approach (with a step 0,1 Å) of adatom to a boron atom of surfase along a perpendicular to axis of nanotube and passing through the B atom on which adsorption takes place.

16. Н над атомом В 3 2 1 Е,эВ 0 0,5 1,0 1,5 2,0 2,5 -1 -2 R,А Fig. 8.Energy curves of H, F, Cl, O atoms interaction with a boron nanotube (6,6) surface; variant I – above a boron atom.

17. Н над центром связи В-В 1,0 0,8 0,6 Е,эВ 0 0,2 0,0 0,5 1,0 1,5 2,0 2,5 3,0 -0,2 R,A Fig. 9.Energy curves of H, F, Cl, O atoms interaction with a boron nanotube (6,6) surface; variant II – above the centre bonding B-B

18. О над центром В-гексагона Н над центром В-гексагона 5 0,5 4 0,0 0,5 1,0 1,5 2,0 2,5 3 -0,5 Е,эВ Е,эВ 2 -1,0 1 -1,5 0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 -1 -2,0 R,A R,A Fig. 10. Energy curve of H, F, Cl, O atoms interaction with a boron nanotube (6,6) surface; variant III – above the centre of a hexagon

19. Table4.The main electron- energetic characteristics of the adsorption process of H, O, Cl, F atoms on the nanotube surface: I) above a boron atom, II) above the center of the В-В bonding, III) above the centre of a boron hexagon; Еa – adsorption energy, eV; Ra – adsorption distance,Å; ∆Еg –a width of energy gap, eV; qF,H,O,Cl– charges on atoms of F, O, H, Cl, qв – charges on atoms of В.

20. Study of regular hydrogenation of boron nanotubeFig. 9. Extended extended elementary cellsof boron nanotubes (6, 6) with indications of hydrogen atoms location on the surface: а) variant 1; b) variant 2. The difference in total energies of these variants ΔЕ = 4 eVand the second position of the hydrate boron nanotube structures appears energetically more favoured.Thus, it is possible to form the hydrogen composites on the basis of boron nanotubes!

21. The study of proton conductivity on the surface of a single-walled nanotube One of the ptiorities of modern physics is studing of the materials with special properties, in particular, materials with proton conductivity. They can be used as effective fuel elements using the reaction of hydrogen oxidation, electrolysers of water vapour, sensor control of hydrogen and ect. This has defined a search and research into new firm proton conductive materials. Two variants of proton migration along the surface of a carbon nanotube has been proved. [Zaporotskova I.V., Lebedev N.G., Zaporotskov P.A. // Physics of solid state. - V. 48 . – 2006- № 4.- P. 756 – 760]. Is proton conductivity in boron nanotubes possible? Our calculation show, that an H atom is adsorbed on the surface of a boron nanotube thus a transfer of electron density from H to B takes place and as a result hydrogen becomes a proton.

22. Single proton H+ migration mechanismalong the nanotube surface between the two stationary states of an adsorbed particle H+: 1) «hopping» mechanism, when a proton Н+moves from one boron atom on the surface to another over two following one after another hexagons (way I); 2) «relay» mechanism, when a proton Н+ moves from one boron atom on the surface to another along the cohesive bonding (way II).

23. The energy curve of proton transport along the surface of a nanotube: а) (6, 6) – way II; b) (6, 6) – way - I; c) (8, 8) – way II; d) (8, 8) – way I. а) b) c) d)

24. Table 5.The activation energies for proton migration along the ways I and II for boron nanotubes (6, 6) and (8, 8). Thus, the process of migration Н+ along way II is more preferable in comparison with variant I for (6, 6) tube. For (8, 8) tube proton transport is equally probable in both ways: way I and way II.

25. All the curves have minima close to the centre of bonding B-B in case of “relay” mechanism. In case of hopping mechanism the proton is drawn to the centre of the two closest B-B bonds. So, we can observe minima on the energy curves. This fact can be explained by the proved possibility of a hydrogen atom adsorption over the centre of B-B bonding (table 4). However, for tubes (8, 8) a proton is “drawn” stronger towards the location where its state is stable: ∆ЕII = 0,18 eV for migration on way II, ∆ЕI(1) = 0,46 eV for migration on the way I (the first minimum) and ∆ЕI(2) = 0,31 eV (the second minimum). Hence, a smaller nanotube diameter provides the best proton conductivity of the system.

26. CONCLUSION • We have studied two possible structural modifications of boron – hexagonal boron and boron nanotube - and proved their stability. All boron nanotubes are narrow-gapped semi-conductors irrespective of its diameter. • Single defects in the structure of boron nanotubes do not change their conductive, which proves the stability of their conductive properties. These results can find application in nanoelectronics. • We have studied adsorption mechanism for hydrogen, oxygen, fluorine and chlorine atoms on the external surface of boron nanotubes. • The process of regular adsorption of hydrogen atoms on the surface of nanotubes has been studied. In hydrates of boron nanotubes the type of conductivity does not change. We have confirmed the possibility of borane fuels. • The two mechanisms of proton migration along the surface of a single-walled nanotube has been investigated namely “hopping” and “relay”. Both mechanisms are possible. Nanotube with smaller diameter have better proton conductivity.

27. Thank you for your attention