# High-Yield Synthesis of Cubic and Hexagonal Boron Nitride Nanoparticles by Laser Chemical Vapor Decomposition of Borazine

Authors: A. Hidalgo; V. Makarov; G. Morell; B. R. Weiner
Journal: Dataset Papers in Nanotechnology
 (2013)

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## Abstract

We report a new method for the synthesis of boron nitride nanostructures (nBN) using laser chemical vapor decomposition (LCVD). Borazine was used as precursor and excited with two simultaneous radiations, the fundamental and second YAG laser harmonics. If only one of the two radiations is employed, no reaction takes place. Abundant BN powder is obtained after one hour of laser radiation. The BN yield obtained with the LCVD technique is about 83% by weight. The BN material was characterized using scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. They all indicate that the BN powder consists of a mixture of hexagonal and cubic BN nanostructures. No other BN phases or stoichiometries were found. The size of the resulting BN nanostructures is in the range of 20–100 nm and their B : N composition is 1 : 1. A simplified mechanism involving laser-excited states followed by photoinduced removal of hydrogen is proposed to understand the synthesis of BN nanopowder by LCVD of borazine.


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## Body

## 1. Introduction

The cubic and hexagonal allotropes of boron nitride (BN) are structurally analogous to diamond and graphite, respectively. Moreover, BN nanostructures are structural analogues of carbon nanostructures, such as fullerenes. However, unlike carbon nanostructures, BN nanostructures are electrically insulating, with energy gap of ~5.5 eV [1–3], and resistant to oxidation up to 800°C [4, 5]. Due to these key characteristics, BN nanostructures are more useful in structural applications, such as reinforcing industrial ceramics (e.g., quartz, alumina, and silicon nitride) to improve their tolerance to thermal shocks [6, 7].There is a wide variety of methods reported for the synthesis of BN nanostructures. They have succeeded in making BN nanofibers [8], nanotubes [9–12], nanocapsules [13], nanowires [14], nanoparticles [15], nanohorns [16], nanosheets [17], and films [18].  However, these methods suffer from low yields, significant amounts of impurities, B segregation, and N deficiency [19, 20]. Only hexagonal BN nanoparticles are obtained with these methods.The application of the pulsed laser radiation for deposition of different materials is a very active area, where highpower pulsed laser radiation is used [21–23]. The borazine molecule (B3N3H6) is isostructural and isoelectronic with benzene and has high vapor pressure, the exact 1 : 1 proportion of B : N, and three B-N bonds, all of which favor the stoichiometric synthesis of BN nanostructures [24]. In this paper, we report the high-yield synthesis of cubic and hexagonal BN nanostructures by the LCVD technique using borazine as precursor.


## 2. Methodology

The reactor employed is schematically represented in Figure1. The reactor body comprises a quartz tube with an external diameter of 1.5′′ and wall thickness of 0.04′′. This tube was installed on stainless steel frames and connected to a vacuum system. In the input side of reactor, the quartz lens with focus length of 10 cm was installed. The laser radiation was focused by this lens in the reactor center. The total reactor volume is about 211 cm3. The reactor was evacuated by a mechanical pump to about 10−2 Torr, and then it was evacuated by turbopump down to 10−6 Torr. It was filled with high-purity nitrogen (99.99%) to atmospheric pressure and evacuated again to the base pressure 10−6 Torr. This process was repeated three times. The fundamental YAG laser harmonic (1064 nm) and second laser harmonic (532 nm) radiations were used to decompose the borazine (Boro Science, Canada, Inc.) gas. The relationship between radiation energy density of both these YAG laser harmonics is 8.9 : 0.051 (=175=ρ1(E)/ρ2(E)). The repetition laser rate was 10 Hz; the beam diameter was 3 mm; the pulse duration was about 10 ns.Figure 1
Schematic representation of the LCVD reactor.Figure 2
TOFMS of borazine showing four bands corresponding to different combinations of the B isotopes in the borazine molecule. Below each band is the corresponding molecular mass in atomic mass units.Figure 3
Raman spectrum of the BN-nanostructured material showing the bands corresponding to a mixture of hexagonal and cubic BN particles.Figure 4
FTIR spectrum of the BN-nanostructured material showing the bands corresponding to a mixture of hexagonal and cubic BN particles.Figure 5
X-ray diffractogram of the BN-nanostructured material showing the peaks corresponding to a mixture of hexagonal and cubic BN particles.The borazine purity was analyzed using time-of-flight mass spectrometry (TOFMS) by multiphoton photoionization of borazine molecule by radiation of the excimer ArF laser (193 nm; PSL-100; MPB Inc.). A mixture of Ar and Borazine (1%) at the input pressure of 760 Torr was passed through the pulsed electrodynamics valve and supersonic nozzle (diameter is 0.5 mm). The gas jet was expanded to the vacuum chamber and collimated by skimmer with an inlet diameter of 1.5 mm. The collimated molecular beam was passed through the ionization area of the commercial TOFMS machine (TOF Electronics Inc.), where the borazine molecules were photoionized by radiation of the ArF excimer laser. The positive ions were detected by a multichannel plate detector (MCP-Hamamatsu), signal from which was collected by the digital oscilloscope (LeCroy-9310A). The averaged waveform was transferred from oscilloscope to PC computer using the Scope Explorer commercial software (LeCroy). The original TOFMS is represented in the time scale, which can be converted to ion mass scale using the relationshipm=(t/93)2, where 93 is the empirical machine’s constant.The Raman spectra were measured using micro-Raman spectrometer with triple monochromator (ISA Jobin Yvon Model T64000) with 1 cm−1 spectral resolution and 514.5 nm Ar-ion laser excitation. The spectra were recorded using the 80X objective that gives a probe area of 1-2 μm2.  FTIR spectroscopy was carried out on a standard FTIR spectrometer (Bruker, Tensor 27 Helios). The X-ray diffraction patterns were measured using a Siemens D5000 X-Ray spectrometer. Images of the BN materials were obtained by Scanning Electron Microscopy (JSM-5800LV JEOL) and Energy-Filtered Transmission Electron Microscopy (LEO 922 OMEGA).


## 3. Dataset Description

The dataset associated with this Dataset Paper consists of 7 items which are described as follows.1
TOFMS of borazine. The time scale can be converted to ion mass scale using the relationshipm=(t/93)2, where 93 is the empirical machine’s constant. We employed TOFMS to analyze the purity of the borazine gas to be used as raw material to synthesize BN powder. The TOFMS spectrum (Figure 2) consists of 4 lines corresponding to ion masses of 78, 79, 80, and 81, which correspond to B3N3H6+ ions with different B isotopes. B has two stable isotopes 10B and 11B. Their relative abundance is 19:81. The relationship among the intensities of the TOFMS lines is in good agreement with what is expected from the natural abundance of B isotopes. No other lines were detected from 10 to 400 a.m.u. We can conclude that the borazine employed is more than 99.9% pure. The LCVD reactor was filled with borazine vapor to an initial pressure of 40 mbar. When either the fundamental YAG laser harmonic radiation (1064 nm) or second laser harmonic radiation (532 nm) was used, no decomposition of borazine took place after waiting for one hour. Only when the two radiations were passed simultaneously, the pressure increased indicating that decomposition of borazine was taking place. The pressure increased to about 70 mbar in 30 min, but the reaction was left to continue for 60 min and the pressure remained essentially constant. The walls of the reactor were covered with BN white dust. The BN white powder was collected from the reactor walls for further characterization. We estimated the yield of BN to be about 0.83±0.02 by taking the pressure change, the total reactor volume, and mass of the BN product. The ideal gas equation is employed. Notice that this yield is relatively high [25]. From this result, it appears that the synthesis of BN from borazine involves a primary process for the electronic excitation of borazine molecules and borazine fragments, followed by a secondary process for the photoinduced removal of H from the excited borazine and borazine fragments. The process goes on until all hydrogen is detached, leaving behind BN powder and increasing the chamber pressure. This is a simplified mechanism that captures the essential features that a computational model of the LCVD of borazine should contain.
161
2
0
Time (μs)
Relative Intensity2
SEM image  of the BN powder. The structure of the BN material was analyzed by SEM and TEM microscopy. From the SEM image, it can be seen that the BN material is covering the whole substrate and has a powdered appearance.
x20003
TEM image of the BN nanostructures. The TEM images show that the BN powder consists of BN nanostructures with sizes in the 20 to 100 nm range.4
A close-up TEM image of the BN nanostructures. The interplanar spacing was measured to be 0.35 nm, very close to that of cubic zinc blende BN, which is 0.36 nm [26], and the EELS mapping (not shown) reveals a homogenous distribution of B and N atoms in 1 : 1 in all sampled materials5
Raman spectrum of the BN-nanostructured material. The Raman spectrum (Figure3) consists of two characteristic bands at 803 and 1356 cm−1, which correspond to hexagonal BN. The A2u (805 cm−1) mode is near 803 cm−1 and the E1v (1367 cm−1) mode is near 1356 cm−1 [22, 23]. In addition, there are two broad bands at around 1000 cm−1 and 1400 cm−1 that can be assigned to cubic BN [27] with a large defect density that causes band broadening. Together, these bands indicate that the nanostructured BN material obtained from LCVD is a mixture of hexagonal and cubic BN nanostructures. a.u. means arbitrary units.
1605
2
0
Wavenumber
Intensity6
FTIR spectrum of the BN-nanostructured material. The FTIR spectrum (Figure4) consists of bands at 802.23 and 1367.19 cm−1, which can also be related to hexagonal BN-nanostructured materials, and the c-BN absorption band at ~1068cm−1 that can be related to cubic BN-nanostructured materials [22, 23, 28, 29]. Other bands observed in the FTIR spectrum have much lower intensity. The FTIR data give additional proof that the product obtained is a mixture of cubic and hexagonal BN material.
727
2
0
Wavenumber
Transmittance7
X-ray diffractogram of the BN-nanostructured material. The X-ray diffractogram (Figure5) contains the (002), (100), and (104) peaks indicating the presence of hexagonal BN material and the (004) peak corresponding to cubic BN. a.u. means arbitrary units.
3526
2
0
2θ
Intensity


## 4. Concluding Remarks

We developed a new method for the synthesis of cubic and hexagonal boron nitride nanostructures (nBN). This method involves the irradiation of borazine with the fundamental and second YAG laser harmonics simultaneously. This technique gives high yields up to 83% by weight. The multilateral characterizations done to the BN nanopowder indicate that it consists of a mixture of hexagonal and cubic BN nanostructures with sizes in the range of 20–100 nm and 1 : 1  B : N composition. The essential aspects of the synthesis mechanism to be consistent with the experimental facts should involve laser-excited states followed by photoinduced removal of hydrogen.



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*Source: 281672-2012-08-29.xml*