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BORON NANOTUBES: STRUCTURE AND PROPERTIES

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The study of the surface structure hydrogenation is promising for its application as a storage for molecular hydrogen. ... P. 91 96. Boron and hydrogen ... – PowerPoint PPT presentation

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Title: 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_at_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 (0,00050,005
    ) increase its hardness!
  • Boron better than any other element removes
    gases from copper that improve 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. ?. 99. ?. 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. EEC quasi-planar boron.
  • Table 1. Main properties of quasi-planar
    hexagonal boron (by applying the IB-CCC and MNDO
    4).

The number of atoms ? EEC Atom charge Ionazation potential, eV ?Eg,, eV
64 0 7.31 0.00
80 0 7.25 0.00
88 0 7.95 0.01
96 0 8.08 0.02
108 0 8.13 0.01
  • 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 be constructed by rolling of
hexagonal quasi-planar boron
  • S. Ismail-Beigi 5
  • Due to three-center bonding
  • boron nanotubes are mainly of triangular and
    hexagonal types.
  • 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. ?.
    99. ?. 115501.

6
Table 2. Main characteristics of boron nanotubes
(n,n) and (n,0) types n the number of hexagons
along the perimeter of a boron nanotube, d the
tubullene diameter, ?Eg band gap energy, ?str
strain energy.

Tubullene type N d, Å ?Eg,, eV ?str, eV
(n, n) 4 5.25 0.07 0.45
(n, n) 5 6.90 0.04 0.35
(n, n) 6 8.25 0.90 0.34
(n, n) 9 12.42 0.02 0.31
(n, n) 11 15.18 0.35 0.17
(n, n) 12 16.56 0.22 0.16
(n, 0) 4 3.03 0.27 0.04
(n, 0) 5 3.99 0.00 0.07
(n, 0) 6 4.77 0.20 0.12
(n, 0) 8 6.35 0.01 0.11
(n, 0) 12 9.57 0.02 1.73

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 ?str on a
    tubullene (n, n) diametre (d).
  • Fig. 3. Dependence of strain energy ?str on a
    tubullene (n, 0) diametre (d).

8
Boron nanotubes with defects Fig. 4. EEC of
boron nanotube (6,6) with substitution defects
(either neutral carbon atom (?), or positively
(?) and negatively (?) charged carbon ions)





9
Fig. 5. Single electron energy spectra of boron
tubullenes (6, 6), calculated with molecular
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.
  • ?, eV


10
Table 3. Charges on substitution defects and the
distribution of caused by defects charges on B
atoms according to the directions interaction (?
along the axis, B along the circle).
Defect Charge on a defect directions of interaction 1 2 3 4
? -1.03 ? 0.12 0.11 0.00 -
? -1.03 B 0.08 0.04 0.03 0.00
?- -0.44 ? 0.11 0.10 0.00 -
?- -0.44 B 0.09 0.03 0.02 0.00
? -0.48 ? 0.24 0.14 0.00 -
? -0.48 B 0.25 0.06 0.01 0.00
11
Boron nanotube (6,6) with a vacancy Fig. 6.
Potential energy pattern of the vacancy formation
process in a boron nanotube (6,6), ?act 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 properties of
    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., // ?chemical
    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 mechanism of hydrogen, fluorine,
chlorine and oxygen atoms on the outside surface
of a boron nanotube (6,6)-type Fig. 7. Three
variants of adatoms orientation towards the
surface of a boron nanotube I) above a boron
atom, II) above the centre bonding ?-?, 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
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
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
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
Table 4. 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 ?-?
bonding, III) above the centre of a boron
hexagon ?a adsorption energy, eV Ra
adsorption distance, Å ??g a width of energy
gap, eV qF,H,O,Cl charges on atoms of F, O,
H, Cl, q? charges on atoms of ?.
I II III
Cl ?a, eV -0,02 - -
Cl Ra, Å 2 - -
Cl ??g, eV 1,5 - -
Cl qcl -0,15 - -
Cl q? -0,08
H ?a, eV -1,43 -0,07 -1,74
H Ra, Å 1,2 2,1 2
H ??g, eV 1,2 1,3 1,5
H q? 0,17 0,18 0,23
H q? -0,32 0,17 0,15
O ?a, eV -3,7 -1,64 -
O Ra, Å 1,4 1 -
O ??g, eV 0,7 0,8 -
O qo -0,16 -0,02 -
O q? -0,06 0,08
F ?a, eV 1,86 - -
F Ra, Å 1,6 - -
F ??g, eV 1,3 - -
F qF -0,25 - -
F q? -0,01
20
Study of regular hydrogenation of boron
nanotube Fig. 9. Extended extended elementary
cells of boron nanotubes (6, 6) with indications
of hydrogen atoms location on the surface ?)
variant 1 b) variant 2.
The difference in total energies of these
variants ?? 4 eV and 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 mechanism along the
nanotube surface between the two stationary
states of an adsorbed particle H 1)
hopping mechanism, when a proton ? 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 ? 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 ?) (6, 6) way II b) (6,
6) way - I c) (8, 8) way II d) (8, 8) way
I.

?)

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).
??kt (I), eV ??kt (II), eV
(6, 6) 0,77 0,22
(8, 8) 0.35 0,34
Thus, the process of migration ? 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 ??II 0,18 eV for
migration on way II, ??I (1) 0,46 eV for
migration on the way I (the first minimum) and
??I(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
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