Low-dimensional boron nitride nanomaterials · 2018-02-14 · Boron nitride (BN) low-dimensional...

JUNE 2012 | VOLUME 15 | NUMBER 6256

Low-dimensional boron nitride nanomaterials

“Low-dimensional materials” refers to a new class of material with

reduced dimensionality, i.e., with one or more physical dimension(s)

constrained to the nanometer scale. Two-dimensional (2D) nanosheets,

one-dimensional (1D) nanotubes, and zero-dimensional (0D) fullerenes

represent typical examples of such materials. When compared

to three-dimensional (3D) bulk substances, low-dimensional

structures are anticipated to exhibit new properties due to quantum

confinement and/or surface and interfacial effects. Therefore in recent

years, materials scientists have drawn particular attention to these

nanosystems to make sense of their unusual physical and chemical

properties that can promote novel applications in engineering.

Boron nitride (BN) low-dimensional materials are among the most

promising inorganic nanosystems explored so far. BN is a chemical

compound, consisting of equal numbers of boron (B) and nitrogen (N)

atoms, which is not found in nature and is therefore produced synthetically.

BN is isostructural to carbon (C) and exists in various crystalline forms.

The hexagonal form (h-BN) is analogous to graphite with a layered

structure. Four years after the identification of C nanotubes (CNTs)1, BN

nanotubes (BNNTs) were successfully synthesized in 19952. Subsequently

other 1D BN nanomaterials such as nanowires, nanoribbons, nanofibers,

and nanorods were synthesized3-6. Moreover, inspired by C60 buckyballs7,

0D nested and single-layered octahedral BN fullerenes were produced in

19988,9. In addition, after the rise of graphene in 200410 and the research

progress on layered 2D nanostructures, free-standing 2D BN flakes were

peeled off from a BN crystal in 200511. However, the initial growth of

BN nanosheets (BNNSs) in the form of so-called nanomeshes on metallic

substrates had been reported a year earlier12. Structural models of 0D, 1D,

and 2D BN nanomaterials are illustrated in Fig. 1.

In this review, a concise research history of low-dimensional boron nitride (BN) nanomaterials followed by recent developments in their synthesis, morphology, properties, and applications are presented. Seventeen years after the initial synthesis of BN nanotubes, research on BN nanomaterials has developed far enough to establish them as one of the most promising inorganic nanosystems. In this regard, it is envisaged that the unique properties of low-dimensional BN systems, such as superb mechanical stiffness, high thermal conductivity, wide optical bandgap, strong ultraviolet emission, thermal stability and chemical inertness will play a key role in prospective developments.

Amir Pakdela,b,*, Chunyi Zhia, Yoshio Bandoa, and Dmitri Golberga,b,*aWorld Premier International (WPI) Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), aNamiki 1-1, Tsukuba, Ibaraki 305-0044, JapanbGraduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai 1, Tsukuba, Ibaraki 305-0005, Japan

*E-mail: [email protected], [email protected]

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ISSN:1369 7021 © Elsevier Ltd 2012 Open access under CC BY-NC-ND license. .

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Low-dimensional boron nitride nanomaterials REVIEW

JUNE 2012 | VOLUME 15 | NUMBER 6 257

Synthesis of BNNTs Most of the techniques known for the growth of CNTs have been

modified to synthesize BNNTs as well. These methods can be classified

into two broad categories: (1) high temperature synthesis (> 2000 °C),

such as arc discharge and laser ablation; (2) medium/low temperature

synthesis (< 2000 °C), such as carbothermal methods, ball milling, and

chemical vapor deposition (CVD).

Arc discharge The first successful synthesis of BNNTs was reported in 1995 by Chopra

et al.2. They used a tungsten electrode filled with h-BN powder as anode

and a copper (Cu) electrode as cathode to produce multi-walled BNNTs

which included metallic particles at their tips. Subsequently, single- and

double-walled BNNTs were synthesized by arcing Hafnium diboride

(HfB2) electrodes in an inert atmosphere13. Double-walled BNNTs were

mass-produced by arcing B electrodes containing nickel (Ni) or cobalt

(Co) in an N2 atmosphere14. Later in 2006, a continuous process using an

arc-jet technique at high temperature (5000 – 20 000 K) was developed

as a promising method for the large-scale production of single- and

multi-walled BNNTs15. A mixture of h-BN and catalyst metal powders

was used as precursor and a combination of argon (Ar) and N2 formed

the plasma gas. A similar technique had been employed earlier in our

group, but the obtained product was a mixture of BN, C and composite

nanotubes of BN−C16.

Laser ablationA laser assisted technique was used by Golberg et al.17 in 1996 to

generate multi-walled BNNTs. Single-crystal samples of cubic or

hexagonal BN (c-BN, h-BN) were laser heated for a short time in a

diamond anvil cell under high pressures of N2. However a large quantity

of BN flakes and particles were also included in the product. Further

development of this method resulted in the synthesis of single-, double-,

and triple-walled BNNTs by using a mixture of BN, Ni, and Co powders

in an inert atmosphere18. Later, a rotating catalyst-free BN target in a

continuous laser ablation reactor under N2 was used to produce bulk

quantities of BNNTs19. Subsequently, high yields of single- and multi-

Fig. 1 Structural models of low-dimensional BN nanostructures: (a) single-layered nanosheet (2D), (b) single-walled nanotube (1D), (c) single-shelled fullerene (0D).

(b)(a) (c)

Fig. 2 (a) HRTEM images of nanotubes with 1 – 6 walls. (b) Schematic of the substitutional reactions in CNTs, developed with temperature (T) and time (t). (i) Void formation in C layers due to oxidation of C with O2 or metal oxides (MOs). (ii) More voids formation and outer layers peeling off. (iii) Reaction of B2O3 and N2 with C to form BN domains. (iv) End of the substitution reactions. (v) Shrinkage of the BN layers by the rearrangement of B and N atoms to eliminate large voids. Reprinted with permission from25. © 2006 American Institute of Physics.

(a)

(b)

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REVIEW Low-dimensional boron nitride nanomaterials


walled BNNTs were obtained based on the vaporization of h-BN targets

via a continuous CO2 laser under N2 flow at ~3500 K20. However, the

product contained unwanted morphologies besides tubular structures.

Carbothermal methodsAn alternative method to synthesize BNNTs was developed by Han et al.21

in 1998. It was based on the idea of turning CNTs to BNNTs through

the substitution of C atoms with B and N atoms. Boron trioxide (B2O3)

powders covered by CNTs were under N2 flow at 1500 ºC inside an

induction furnace. While the CNTs were oxidized by B2O3, B and N

atoms filled the generated vacancies in the hexagonal structure. As a

result, multi-walled BNNTs were produced with diameters similar to

those of the initial CNTs22. This technique was further optimized to

produce single-walled BNNT bundles by using pure single-walled CNTs

as the precursor23. However, the product included B−C and B−C−N

compound single-walled nanotubes as well. Further work demonstrated

that C content in the product can be reduced by adding an extra oxidizing

agent to the system24. This technique could also lead to the generation

of BNNTs with smaller diameters than those of the starting CNTs25.

Transmission electron microscope (TEM) images of BNNTs with 1 to 6

shells and a schematic of the substitution process are illustrated in Fig. 2.

In another approach, Bartnitskaya et al.26 employed boron carbide

(B4C) as the C source instead of CNTs. The carbothermal reduction

of BxOy and the simultaneous nitriding at 1100 – 1450 °C resulted in

the production of BN tubes with large diameters and a bamboo-like

structure. Also, synthesis of multi-walled BNNTs by using boric acid

(H3BO3) and active carbon or CNTs in the presence of iron compounds

under ammonia (NH3) atmosphere has been reported27.

Mechanothermal methodsThe initial mechanothermal method for the production of BNNTs

was pioneered by Chen et al.28 in 1999. It included ball milling of B

powder in NH3 gas, followed by thermal annealing at 1000 – 1200 ºC

under N2 or Ar. The BN products had a bamboo-like structure with

50 – 75 nm external diameters. They also applied this process to h-BN

powder, obtained bamboo-like nanotubes with diameters ranging from

~11 to ~280 nm, and suggested that ball milling of h-BN powder

could generate highly disordered or amorphous nanostructures, which

could promote nucleation and growth of BNNTs after annealing at

high temperatures29. It has been also proposed that surface diffusion in

milled powder could be the key factor for growing BNNTs through a tip-

growth model, due to the presence of Fe particles at the tip of the BN

tubes (Fig. 3)30. Further development of this method by other research

groups resulted in synthesis of BNNTs with diameters of 5 – 30 nm31

and 40 – 60 nm32, and lengths of several μm, depending on the chemical

composition of initial powders and synthesis temperature. High yields of

multi-walled BNNTs with diameters < 10 nm were synthesized by Yu et

al.33 using a ball milling-annealing process in which amorphous B was

ball-milled for 150 h and annealed at 1200 °C under NH3. The produced

nanotubes had no trace of metallic particles, which may suggest a root-

growth mechanism for their growth.

Chemical vapor deposition (CVD)The pioneering work on the CVD synthesis of BNNTs, by using borazine

(B3H6N3) as precursor and Ni2B particles as catalyst at 1000 – 1110 °C,

was carried out by Lourie et al.34 in 2000. The nanotubes often possessed

bulbous, flag-like, and/or club-like tips. An efficient CVD route to

synthesize significant amounts of BNNTs was developed in our group

through heating a mixture of B, MgO, and FeO powders to 1300 °C.

The growth vapors were transported by an Ar flow to the reaction

chamber, where a substrate was placed and heated to 1550 °C under

NH3 flow. BNNTs (both cylindrical and bamboo-like) with diameters of

20 – 100 nm were produced by this method35. Recently this technique

has successfully been applied to generate high yields of thin BNNTs with

an average diameter of 10 nm by using a mixture of Li2O and B powders

as precursor36. There are several reports on the development of various

CVD techniques and using different precursor materials to obtain BNNTs

at 450 – 1200 °C in different shapes and sizes37-40.

An interesting feature of the CVD process is the growth of thin

films and coatings made of nanostructured materials. Fe-particle-

functionalized substrates were used by Yap’s group41 for growing BNNT

bundles at 600 °C. They used a plasma-enhanced pulsed-laser deposition

technique with a negative substrate bias voltage to generate the reactive

condition for tubes growth perpendicular to the substrates (Fig. 4). A

plasma technique was also used to produce highly-ordered multi-walled

BNNT arrays42. Later, conventional horizontal tube furnaces were used to

synthesize BNNT films by thermal CVD43. Very recently a comprehensive

study by Pakdel et al.44 demonstrated the effect of process variables on

the CVD growth of films of BNNTs with different sizes and morphologies

(Fig. 5). In spite of the success of CVD techniques in the production of

milligram-scale quantities of BNNTs to date, these methods still need

further development to be comparable with CVD-grown hundreds of

grams of CNTs in a single experimental run45.

Synthesis of BNNSs The formation of a highly regular mesh of h-BN on a rhodium

(Rh) single crystalline surface by self-assembly was performed by

Fig. 3 TEM image of a bamboo-like BNNT, containing a metallic tip, synthesized by ball milling. Reprinted with permission from30. © 1999 Elsevier.

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Fig. 4 SEM images of BNNT bundles grown at a substrate bias of (a) −380 V and (b) −450 V and (c) their corresponding bundling configurations (left and right, respectively). (d) Patterned growth of BNNTs. Reprinted with permission from41. © 2005 American Chemical Society.

(b)(a)

(c) (d)

Fig. 5 Effect of growth temperature and catalyst content on the morphology of BNNT films, as revealed by SEM. From reference44.

(a)

(b)

(c) (e)

(d)

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Corso et al.46 in 2004. The preparation procedure consisted of

exposing the atomically clean Rh(111) surface at 800 °C to B3H6N3

vapor inside an ultrahigh vacuum chamber and consecutive cooling

to room temperature. The regular mesh structure was observed by

scanning tunneling microscopy (STM), as shown in Fig. 6. It was

suggested that the hole formation is driven by the lattice mismatch

of the film and the Rh substrate.

Synthesis and characterization of 2D nanosheets in general, and

BNNSs in particular, are rather new topics in materials science and the

related reports are still very limited, as categorized in this section.

Micromechanical cleavage The pioneering procedure to obtain atomic sheets of h-BN was the

micromechanical cleavage technique, reported by Novoselov et al.11 in 2004.

In this method, layers of h-BN are peeled off with adhesive tapes and attached

to a substrate. Atomic force microscopy (AFM) and TEM investigations of

BNNSs prepared by this method in Zettl’s group revealed the clean well-

ordered nanosheets, with thicknesses between 3.5 and 80 nm47.

Chemical exfoliation (sonication)Preparation of mono- and few-layered nanosheets from a single-crystalline

h-BN by a chemical-solution-derived method was first accomplished

by Han et al.48 in 2008. The h-BN crystal was sonicated in an organic

solution to break up into few-layered h-BN sheets. Fig. 7 displays typical

TEM images of the obtained BNNSs. Later, the exfoliation of nanosheets

from BN particles dispersed in a strong polar solvent was performed in

our group via vigorous sonication-centrifugation49. The solvent facilitated

the exfoliation, due to strong interactions between its polar molecules

and the BN surface. Milligram levels of pure BNNSs with 2 − 10 nm

thicknesses were achieved. Different solvents have been employed in

chemical exfoliation methods to promote BNNSs formation50,51.

High-energy electron irradiation Fabrication of freestanding single-layered BNNSs by controlled electron

irradiation through a layer-by-layer sputtering process in situ inside a

TEM was reported by Jin et al.52 and Meyer et al.53 in 2009. First, h-BN

nano-flakes and powders underwent mechanical cleavage to obtain

h-BN sheets with a reduced number of layers. Then, the h-BN nano-

flakes were further thinned down to monolayers by focusing an intensive

Fig. 6 A constant-current STM image of the BN nanomesh formed on a Rh(111) surface. Two steps on the Rh(111) surface cross the image. The black features are defects in the mesh, one of which is shown with different contrast in the inset. Reprinted with permission from46. © 2004 The American Association for the Advancement of Science.

Fig. 7 (a) Low-magnification and (b) HRTEM image of a BN sheet with two atomic layers; (c) HRTEM image of a multi-layered BN sheet. Inset III is a simulated image based on a structural model in which a 13.8° rotated BN sheet is overlapped with another BN sheet. Reprinted with permission from48. © 2008 American Institute of Physics.

(b)(a) (c)

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electron beam onto the specimen. By scanning the electron beam under

a manual control, h-BN nano-flakes were burnt layer-by-layer until h-BN

monolayers were obtained.

Wet chemical reactions BNNSs were prepared through chemical reactions of boric acid (H3BO3)

and urea (CO(NH2)2) at 900 °C under N2 in Rao’s group in 201054. Few-

layered BN sheets formed due to the following reactions:

2H3BO3 → B2O3 + 3H2OCO(NH2)2 → NH3 + HNCOB2O3 + 2NH3 → 2BN + 3H2O

The authors claimed to have controlled the number of layers through

variations in the concentration of the reacting materials. Namely, the

thickness of the sheets decreased with increasing the urea concentration

in the reaction mixture.

Ball millingHigh yields of BNNSs were obtained from ball milling of h-BN

powders under an N2 atmosphere by Li et al.55 in 2011. They tailored the

ball milling conditions to generate gentle shear forces that resulted in

production of high-quality BNNSs. The milled samples were diluted with

benzyl benzoate and sonicated for 0.5 h to produce thin sheets of BN

(Fig. 8). These authors mentioned that their method could be applicable

to produce nanosheets from any layered materials.

Chemical vapor depositionAn alternative technique to prepare BNNSs is CVD. Relatively large

quantities of thick h-BN sheets were synthesized via a catalyst-free CVD

process at 1100 – 1300 °C by Gao et al.56 in 2009. B2O3 and melamine

powders were mechanically mixed and placed in an induction furnace. The

temperature was raised to 1000 – 1300 °C under N2 flow and after ~1 h

BN sheets with thicknesses of 25 – 50 nm (depending on the synthesis

temperature) were obtained. Later, a multi-step thermal catalytic CVD

method was employed to fabricate h-BN films, consisting of 2 – 5 atomic

layers, under ammonia borane (NH3−BH3) flow on a Cu foil as the

substrate57. Few-layered h-BN films were also synthesized by ambient

pressure CVD on polycrystalline Ni films with thicknesses of ~5 – 50 nm,

depending on the growth conditions58. The h-BN grew continuously on the

entire Ni surface and the regions with uniform thicknesses were up to 20

μm in lateral size. In another set of experiments, reaction of polyborane

decaborane (B10H14) with NH3 formed BNNSs on either polycrystalline Ni

or Cu foils at 1000 °C59. Very recently, monolayer h-BN was grown on Cu

foils by using NH3−BH3 via low pressure CVD with two heating zones60.

Vertically-aligned BNNSs on silicon (Si) substrates were prepared

by Yu et al.61 via microwave plasma CVD from a gas mixture of

BF3−N2−H2. The thickness of the nanosheets could be controlled

between ~5 and 20 nm by changing the growth conditions. In another

approach, Pakdel et al.62,63 developed a thermal CVD technique to

manufacture vertically-aligned pure and C-doped BNNSs on Si/SiO2

substrates by using B, MgO, and FeO powders under NH3 flow in a

horizontal tube furnace (Fig. 9). The majority of the nanosheets was less

than 5 nm in thickness, and by increasing the growth time and/or the

growth temperature, larger-sized nanosheets were formed.

Fig. 8 SEM images and corresponding diagrams illustrating two observed exfoliating mechanisms under the shear force created by milling balls: (a), (b) cleavage from the edge of an h-BN particle; (c), (d) thin sheets peeling off the top surface of an h-BN particle. Reprinted with permission from55. © 2011 Royal Society of Chemistry.

(b)

(a) (c)

(d)

Table 1. The crystallographic information of h-BN and graphite

Material Crystal structure

Nearest neighbor distance

Lattice parameters

Inter-layer spacing

h-BN hexagonal 0.144 nm a: 0.250 nmc: 0.666 nm

0.333 nm

Graphite hexagonal 0.142 nm a: 0.246 nmc: 0.670 nm

0.335 nm

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StructureGenerally, BN nanomorphologies can be considered as h-BN layers

formed in different ways. BNNTs can be defined as cylinders of concentric

h-BN layers (from one to many layers), with diameters in the nanometer

scale. The structure of multi-walled BN tubes can be described by two

different models. In the “Russian Doll” model, h-BN sheets are arranged

in separate concentric cylinders (e.g., a single-walled nanotube within a

larger one, and so on), and in the “Parchment” model, a single sheet of

h-BN is rolled around itself, resembling a scroll of parchment or a rolled

newspaper64. However, multi-walled nanotubes are usually believed to

be made of concentric tubes, rather than a spiraling layer, due to energy

considerations. As for BNNSs, they may be depicted as graphene layers,

in which C atoms have been fully substituted by alternating B and N

atoms. Within each h-BN layer, atoms are bound together by strong

covalent bonds, while there are weak van der Waals forces between

different layers. The crystallographic parameters of h-BN and graphite

are almost identical, as summarized in Table 165.

Unlike the popular monolayer graphenes and single-walled CNTs,

their corresponding BN sister systems have rarely been observed66,67,

due to the peculiar B−N stacking characteristics. The hexagons of

neighboring planes in h-BN are superposed, i.e., B and N atoms are in

succession along the c-axis, while in graphite, they are shifted by half a

hexagon. Moreover, due to the difference in electronegativity of B and

N, the B−N bonds in BN materials are partially ionic, in contrast with

the purely covalent C−C bonds in graphitic structures. These can lead

to the so-called “lip−lip” interactions between neighboring layers/shells

in BN nanostructures, i.e., chemical bonds form as bridges or “spot-

welds” between the atoms of adjacent layers/shells. This phenomenon

contributes to a metastable energy minimum through decreasing the

number of dangling bonds at the edges/tips and reducing the “frustration”

effect (i.e., when B−B and N−N bonds form instead of the energetically

more favorable B−N bonds). Therefore, formation of multi- layers/shells

stabilizes the whole structure68,69.

Ab initio calculations have demonstrated that the folding of an isolated

h-BN sheet onto a tubular BN is slightly more favorable than that of

graphene onto a CNT of the same radius, owing to a “buckling” effect

which stabilizes the BN tubular structure. First-principles calculations

indicate that in the minimum energy structure, all B atoms are arranged

in one cylinder and all N atoms in a larger concentric one, and due

to charge transfer from B to N, the buckled tubular structure forms a

dipolar shell. As a result, each B atom is virtually located on a plane

formed by its three neighboring N atoms, so that the sp2 environment

Fig. 9 SEM images of the BNNS coatings synthesized at 900 – 1200 °C for 30 and 60 minutes. The scale bar is 200 nm. From reference62.

(b)(a) (c) (d)

(f)(e) (g) (h)

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for the B atom in the planar hexagonal structure is restored. This buckling

effect significantly reduces the occupied band energy in the case of BN

compounds; thus, folding an h-BN sheet into a tube is energetically more

favorable than in the case of graphene. Also, the calculated energy of

dangling bonds associated with opened BN strips is much larger than that

of tubes, that it is energetically more favorable to preserve the cylindrical

geometry despite the elastic energy resulting from the curvature70.

Polygonization of cross sections is favorable in multi-walled BNNTs

since the B−N−B−N stacking order across a BN tube could be better

preserved within multiple polygonized shells with flat facets71. Such

stacking can be broken in cylindrical multi-walled nanotubes due to

the existence of different circumferences in consecutive shells. In fact,

a multi-walled nanotube with a polygonal cross section can have a

lower energy than a nanotube with a circular cross section. This energy

reduction occurs because the flat sections of the nanotube can have a

coherent interface with no tensile strain. But, when the cross section

is circular it is not possible to maintain a coherent interface without

an excessive amount of tensile strain, due to the difference in the

circumference of consecutive shells72.

Properties and applicationsBulk h-BN has been a matter of interest for a long time due to its low

density, high thermal conductivity, electrical insulation, superb oxidation

resistance, excellent inertness (i.e., passivity to reactions with acids,

alkalis, and melts), and low friction coefficient. BNNTs and nanosheets,

as inheritors of h-BN, also possess these advantageous properties73.

Electronic and optical propertiesBN nanostructures are generally recognized as insulators with wide

bandgaps of 5.0 – 6.0 eV. The theoretical analysis of electronic properties of

BNNTs and nanosheets reviewed by Arenal et al.74 showed their bandgap

to be first-order independent of the details of the atomic structure.

This explains the so-called stability of the bandgap in BNNTs which

indeed hardly depends on curvature and helicity70, except in the limit

of very small diameters. Recent experimental studies by Lee et al.75and

Pakdel et al.63 have pointed out optical bandgaps of ~6 and ~5.7 eV for

BNNTs and BNNSs, respectively, which approach that of h-BN single

crystals. This insulating behavior of BN nanostructures encourages their

applications as protective shields encapsulating nanomaterials. In this

regard, considerable research has been performed to fill BNNTs during

or after the synthesis with fullerenes or with crystalline compounds, such

as cobalt, iron-nickel and magnesium oxide76-78. However, Terrones et

al.79 have theoretically and experimentally demonstrated that BN

nanoribbons with zigzag edges can show metallic behavior. Their porous

BN nanospheres showed stable field emission properties at low turn-

on voltages (e.g., 1 – 1.3 V/μm) due to the presence of these ribbons

protruding from the surface of the spheres. Such BN nanostructures may

find applications as catalysts or field-emitters in the future. Moreover BN

nanostructures exhibit distinctive violet or ultraviolet (UV) luminescence

emissions, which promote their application in downsized UV lasing

devices for sterilization, surgery, photocatalysis, and optical storage45.

Thermal propertiesh-BN possesses remarkable thermal conductivity and high specific heat.

Theoretical calculations by Xiao et al.80 demonstrated high specific heat

of BNNTs and predicted that their thermal conductivity is higher than

that of CNTs. They also showed that thermal conductance of single-

walled BNNTs at low temperatures is independent of the tube diameter

and chirality81. A recent study indicates that the strong phonon-phonon

scattering in h-BN is the cause for its lower thermal conductivity

compared to graphite, however, reduction in such scattering in a

single layer BN sheet leads to a substantial increase in its conductivity

(>600 W/m/K at room temperature)82. Experimentally, Chang et al.83

measured the thermal conductivity values for BNNTs to be ~350 W/m/K

Fig. 10 (a) Water contact angles of BNNS coatings synthesized at 900 − 1200 °C, indicating the change in their wettability from partial wetting to nonwetting. (b) Photograph of spherical water droplets on a BNNS film. From reference62.

(b)(a)

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at room temperature and demonstrated that if they were mass-loaded

externally and heterogeneously with heavy molecules (e.g., C9H16Pt)

they would possess asymmetric axial thermal conductance properties.

Thus, BNNT thermal rectifiers were suggested to have substantial

implications for diverse nanoscale calorimeters, microelectronic

processors, macroscopic refrigerators, and energy-saving buildings.

Thermal stability experiments on multi-walled BNNTs were performed

by Golberg et al.84 and indicated excellent oxidation resistance up to

830 °C in air. Moreover, thermogravimetric analysis showed that the

onset temperature for the oxidation of BNNTs (800 °C) was much higher

than that of CNTs (400 °C) under the same conditions. Later it was

shown that thin BNNTs with diameters smaller than 20 nm can resist

oxidation up to 900 °C85.

Mechanical propertiesTheoretical studies by Hernandez et al.86 revealed the elastic modulus of

BNNTs to vary between ~0.84 and ~0.91 TPa with diameters ranging from

0.81 to 2.08 nm. Chopra and Zettl87 estimated the elastic modulus of a

cantilevered individual double-walled BNNT by measuring the amplitude

of the thermal-induced vibrations in a TEM at room temperature. They

found a value of 1.22 ± 0.24 TPa, which is similar to the elastic modulus

of CNTs. Subsequently, an electric-field-induced resonance method in

TEM was utilized to calculate elastic modulus values between 0.51 and

1.03 TPa88. More recently, Golberg’s group performed extensive in situ

TEM bending and tensile experiments on individual multi-walled tubes

with diameters of 40 – 100 nm and estimated their elastic modulus as

0.5 – 0.6 TPa71 or up to 1.3 TPa89, and their tensile strength as 33 GPa.

However, in the case of bamboo-like BNNTs, the elastic modulus and

tensile strength were estimated as 225 GPa and 8 GPa, respectively90.

Superior mechanical properties and thermal conductivity of BN

nanostructures make them attractive as nanofillers in composite materials. For

instance, BNNTs can reinforce polymer matrices and improve their thermal

conductivity, while transparency of the polymers is preserved. Zhi et al.91

initiated research on polymer matrix composites reinforced by BNNTs and

showed an effective improvement in elastic modulus of polystyrene and

polymethyl methacrylate (PMMA) by ∼20 % with addition of only 1 wt.% of nanotubes. A very recent work on nanomechanical characterization of

single-walled BNNTs revealed their axially strong, but radially supple,

characteristics and suggested that they might be superior to single-walled

CNTs as reinforcing additives for nanocomposite applications92.

Wetting propertiesWhile h-BN films are partially wetted by water with contact angle

of ~50°, BNNT films can achieve superhydrophobic state with water

contact angle exceeding 150°93,94. In a systematic approach, the present

authors succeeded in growing partially vertically aligned BNNSs on Si

substrates with controllable water-repellency levels62. As a result, water

contact angles from ~51° to ~159° were obtained, indicating a significant

change from hydrophilicity to superhydrophobicity. Due to outstanding

chemical intentness of BN, the pH value of water did not affect the

wetting characteristics of BNNS and BNNT films62,93. Therefore, BN

nanostructure films are anticipated to find industrial applications in

water-repelling, anti-fouling, self-cleaning, and anti-corrosion systems.

Conclusion and prospectsAn overview of research on low dimensional BN nanomaterials in

terms of synthesis, properties, and prospective applications has been

presented. Despite the interesting properties and promising outlook of

BN nanostructures, the research progress on the fundamental physical

aspects and potential applications of these nanomaterials has been

rather limited so far, especially when one compares them with the well-

explored C nanosystems (i.e., CNTs, graphene, and fullerene family). This

might be attributed to the difficulties involved in the mass-production

of low dimensional BN nanomaterials with high purity and good

crystallinity, which hinders their immediate applications in industry.

However, the possibility of morphological manipulation in some

synthetic routes (e.g., CVD) can motivate further work toward new

functions and applications, such as chemically inert surfaces with

controllable levels of water repellency. Moreover, aside from the

unique properties of BN nanostructures such as homogeneous electrical

insulation, wide optical bandgap and deep UV emission, excellent

thermal stability and oxidation resistance, good thermal conductivity

and outstanding stiffness, new properties are likely to be discovered in

tailored BN nanostructures.

h-BN has been considered as one of the best substrates for maintaining

the excellent transport behavior of graphene, because it provides

extraordinarily flat graphene layers that significantly reduce electron−

hole puddles as compared to SiO295. By reducing the charge fluctuations,

the low density regime and the Dirac point can be more readily accessed.

Graphene devices on h-BN substrates have demonstrated mobilities and

carrier inhomogeneities that are almost an order of magnitude better than

devices on SiO296. A hybrid structure consisting of h-BN and graphene

would enable the physical properties of graphene-based structures to be

tailored97,98. Such a system could be useful for various applications, such

as field-effect transistors for radio-frequency apparatus99.

Further applications of BN nanomaterials could be in nanomedicine

due to their good biocompatibility, scanning probe microscopy as ultrathin

stiff tips, new generations of semiconducting materials with adjustable

bandgap through doping of other elements into BN structures, and

manufacturing hybrid nanostructures through surface functionalization

of BN nanomaterials with nanoparticles of other materials. Recently,

functionalized BNNTs have been tested in vitro on fibroblast cells, and

demonstrated optimal cytocompatibility even at high concentrations in

the culture medium100.

Acknowledgments The authors acknowledge the financial support of the World Premier

International (WPI) Center for Materials Nanoarchitectonics (MANA) of

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the National Institute for Materials Science (NIMS), Tsukuba, Japan. D.G.

acknowledges a funding Mega-Grant award for leading scientists tenable

at the National University of Science and Technology, Moscow, Russian

Federation, under the agreement No. 11.G34.31.0061.

MT156p256_265.indd 265 12/06/2012 10:33:44

Low-dimensional boron nitride nanomaterialsSynthesis of BNNTsArc dischargeLaser ablationCarbothermal methodsMechanothermal methodsChemical vapor deposition (CVD)

Synthesis of BNNSsMicromechanical cleavageChemical exfoliation (sonication)High-energy electron irradiationWet chemical reactionsBall millingChemical vapor deposition

StructureProperties and applicationsElectronic and optical propertiesThermal propertiesMechanical propertiesWetting properties

Conclusion and prospectsAcknowledgmentsREFERENCES

Low-dimensional boron nitride nanomaterials · 2018-02-14 · Boron nitride (BN) low-dimensional...

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