Synthesis of the borazine oligomer
In order to synthesize borazine oligomer as precursor, borazine was prepared via dehydrogenation reaction of ammonia borane (BH3NH3) with nickel nanoparticles (NiNPs) at 80 °C. Ammonia borane is a reactant and nickel nanoparticles were used as catalyst in dehydrogenation reaction. The borazine (2.43 g, 30.1 mmol) was transferred into an evacuated Fischer-Porter glass pressure reaction vessel. The flask was allowed to warm to room temperature and then heated at 70 °C with the oil bath. The reaction was allowed to continue, with periodic degassing, until the liquid became sufficiently viscous for 48 h to remove the volatile materials at the room temperature at the nitrogen atmosphere for 48 h. Instead of removing with high vacuum system in the Sneddon’s paper22,39, leaving a white translucent gel (2.27 g, 93% yield based on reacting borazine). After completion of the reaction and removing the volatile materials, borazine oligomer (2.27 g) was dissolved in the chlorobenzene (24.49 g, 0.2 mol) with continuously stirring for 48 h while keeping the ratio of about 9 wt.%. And then the solution was filtrated with filter-paper at nitrogen atmosphere and room temperature for separating the practically insoluble materials. The filtrated solution was kept in a freezer at −45 °C for at least 3 weeks to have aging period in order to optimize its viscosity.
Synthesis of the h-BN Films
Chlorobenzene solution of borazine oligomer was spin-coated onto an electrochemically polished nickel foil (99%, Nilaco Corp.) at 9000 rpm for 50 s. Since borazine oligomer is moisture sensitive, spin-coating was carried out inside a glove box that is continually purged with nitrogen. Then, the spin coated specimen was transported to the annealing reactor with container filled with nitrogen. The sample was then heated up to 1026 °C for 45 minutes under argon atmosphere and kept for 60 minutes for annealing. The reactor pressure was kept at 613 mtorr with the argon flow rate of 500 sccm for the annealing. After the synthesis, the chamber was slowly cooled down to room temperature under argon atmosphere. The h-BN film on nickel foil then was transferred to SiO2/Si or quartz substrate using graphene transfer process for characterization40.
Synthesis of the h-BN Films under a Ni catalyst layer
In that method, borazine oligomer was spin-coated onto a silicon substrate with 1000 rpm inside a glove box, and a nickel catalyst layer was deposited onto the coated substrate by electron beam evaporator. The thickness of the film was about 300 nm. After deposition, annealing was followed at 1026 °C for 60 min under argon atmosphere, and nickel film was then etched by Iron (III) chloride (FeCl3). Thus, h-BN films were obtained directly on the silicon substrates.
Device fabrication and analysis
The h-BN as the gate dielectric was transferred onto a 300 nm-thick SiO2/n+ Si substrate through a conventional transfer method using Polymethyl methacrylate (PMMA). In order to fabricate GFETs, the CVD-grown graphene films were formed onto a h-BN/SiO2/n+ Si substrate. Then, the channel region with a size of 3 μm × 3 μm was defined by photolithography and an inductively coupled plasma reactive ion etching system using O2 gas. As a final step, Cr (5 nm)/Au (50 nm) metals for the source/drain contacts were deposited using electron beam evaporator. Device performances were measured at room temperature under vacuum in a probe station system (Lakeshore 7500 series) using a Semiconductor Parameter Analyzer.
The transferred h-BN film was analyzed using Raman spectroscopy (Horiba, LabRAM HR, Ar laser, λ = 514 nm), Fourier transform infrared spectroscopy (FT-IR, Nicolet IS10), X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-ALPHA), X-ray diffraction (XRD, Rigaku, copper target), UV-Visible spectroscopy (UV-Vis, Jasco, V-670), and Transmission electron microscopy (TEM, Tecnai F20, 200 kV).
Borazine is an inorganic compound with the chemical formula (BH)3(NH)3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. Like benzene, borazine is a colourless liquid. For this reason borazine is sometimes referred to as "inorganic benzene".
The compound was reported in 1926 by the chemists Alfred Stock and Erich Pohland by a reaction of diborane with ammonia.
Borazine is synthesized from diborane and ammonia in a 1:2 ratio at 250–300 °C with a conversion of 50%.
- 3 B2H6 + 6 NH3 → 2 B3H6N3 + 12 H2
An alternative more efficient route begins with lithium borohydride and ammonium chloride:
- 3 LiBH4 + 3 NH4Cl → B3H6N3 + 3 LiCl + 9 H2
In a two-step process to borazine, boron trichloride is first converted to trichloroborazine:
- 3 BCl3 + 3 NH4Cl → Cl3B3H3N3 + 9 HCl
The B-Cl bonds are subsequently converted to B-H bonds:
- 2 Cl3B3H3N3 + 6 NaBH4 → 2 B3H6N3 + 3 B2H6 + 6 NaCl
Borazine is a colourless liquid with an aromatic smell. In water it hydrolyzes to boric acid, ammonia, and hydrogen. Borazine, with a standard enthalpy change of formation ΔHf of −531 kJ/mol, is thermally very stable.
Borazine is isoelectronic with benzene and has similar connectivity, so it is sometimes referred to as "inorganic benzene". This comparison is not rigorously valid due to the electronegativity difference between boron and nitrogen. X-ray crystallographic structural determinations show that the bond lengths within the borazine ring are all equivalent at 1.429 Å, a property shared by benzene. However, the borazine ring does not form a perfect hexagon. The bond angle is 117.1° at the boron atoms and 122.9° at the nitrogens, giving the molecule distinct symmetry.
The electronegativity of boron (2.04 on the Pauling scale) compared to that of nitrogen (3.04) and also the electron deficiency on the boron atom and the lone pair on nitrogen favor alternative mesomer structures for borazine.
Boron behaves as a Lewis acid and nitrogen behaves as a Lewis base.
Main article: Aromaticity
Due to its similarities to benzene, there have been a number of computational and experimental analyses of borazine's aromaticity. The number of pi electrons in borazine obeys the 4n + 2 rule, and the B-N bond lengths are equal, which suggests the compound may be aromatic. The electronegativity difference between boron and nitrogen, however, creates an unequal sharing of charge which results in bonds with greater ionic character, and thus it is expected to have poorer delocalization of electrons than the all-carbon analog.
Natural Bond Orbitals (NBO)
Natural Bond Orbital (NBO) analysis suggests weak aromaticity in borazine. In the NBO model, B-N bonds in the ring are slightly displaced from the nuclear axes, and B and N have large differences in charge. Natural chemical shielding (NCS) analysis provides some further evidence for aromaticity based on a contribution of the B-N π bond to magnetic shielding. Computations based on NBO orbitals show that this π bond allows for weak ring current which somewhat counteracts a magnetic field simulated at the center of the borazine ring. A small ring current does suggest some delocalization.
Atoms in Molecules (AIM)
Atoms in Molecules (AIM) analysis can examine degree of covalency in bonding through examination of critical points in the negative Laplacian of electron density for a given molecule. In the case of borazine, bond critical points are found to lie extremely close to the boron atoms, indicating electrons are concentrated around the nitrogens, and thus there is a large difference in charge between B and N. Furthermore the Laplacians of these bond critical points are positive, implying that these bonds are largely ionic. AIM analysis thus suggests poor covalency and thus poor electron sharing, indicating only weak aromaticity, if any.
Electron Localization Function (ELF)
Topological analysis of bonding in borazine by the Electron Localization Function (ELF) indicates that borazine can be described as a π aromatic compound. However, the bonding in borazine is less delocalized than in benzene based on a difference in bifurcation values of the electron basins. Larger bifurcation values indicate better electron delocalization, and it is argued that when this bifurcation value is greater than 0.70, the delocalization is sufficient to designate a compound aromatic. For benzene, this value is 0.91, but the borazine π system bifurcates at the ELF value 0.682. This is caused by the difference in electronegativity between B and N, which produces a weaker bond interaction than the C-C interaction in benzene, leading to increased localization of electrons on the B-H and N-H units. The bifurcation value is slightly below the limit of 0.70 which suggests moderate aromaticity.
Although often compared with benzene, borazine is far more reactive. With hydrogen chloride it forms an adduct, whereas benzene is unreactive toward HCl.
- B3N3H6 + 3 HCl → B3N3H9Cl3
- Addition reaction of borazine with hydrogen chloride
- B3N3H9Cl3 + NaBH4 → (BH4N)3
- Reduction with sodium borohydride
The addition reaction with bromine does not require a catalyst. Borazines undergo nucleophilic attack at boron and electrophilic attack at nitrogen. Heating borazine at 70 °C expels hydrogen with formation of a borazinylpolymer or polyborazylene, in which the monomer units are coupled in a para fashion by new boron-nitrogen bonds. Boron nitride can be prepared by heating polyborazylene to 1000 °C. Borazines are also starting materials for other potential ceramics such as boron carbonitrides. Borazine can also be used as a precursor to grow boron nitride thin films on surfaces, such as the nanomesh structure which is formed on rhodium.
Polyborazylene has been proposed as a recycled hydrogen storage medium for hydrogen fuel cell vehicle applications, using a "single pot" process for digestion and reduction to recreate ammonia borane.
Among other B-N type compounds mixed amino-nitro substituted borazines have been predicted to outperform carbon based explosives such as CL-20.
Main article: Heterocyclic Compounds
Carborazine is another six-membered aromatic ring with two carbon atoms, two nitrogen atoms and two boron atoms in opposing pairs.
Media related to borazine at Wikimedia Commons
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