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Electrochemistry Communications 9 (2007) 1824–1827

Boron nitride synthesized at ambient pressure and roomtemperature by plasma electrolysis

Jing Guo a, Hao Wang a,*, JingSen Zhu a, Kun Zheng b, ManKang Zhu a, Hui Yan a,Masahiro Yoshimura c

a The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, Chinab Institute of Microstructure and Properties of Materials, Beijing University of Technology, Beijing 100022, China

c Materials and Research Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

Received 21 March 2007; received in revised form 9 April 2007; accepted 11 April 2007Available online 30 April 2007

Abstract

A mixture of cubic boron nitride (c-BN) and extra-diamond boron nitride (E-BN) has been synthesized at ambient pressure and roomtemperature by plasma electrolysis. The formation of c-BN was characterized by FTIR and TEM measurements. This method may notonly offer a facile technique for c-BN production, but also provide a new research field in c-BN thermodynamics.� 2007 Elsevier B.V. All rights reserved.

Keywords: Boron nitride; Plasma electrolysis; Ambient atmosphere

1. Introduction

Cubic boron nitride (c-BN) offers a number of highlydesirable mechanical, thermal, electrical, and optical prop-erties and has many attractively potential applications.However, it has not found the attention it deserves, dueto the easy preparation route not available yet [1]. Since,Wentorf first synthesis c-BN in 1957 under high pressureand high-temperature with catalyst [2], decrease of thepressure and temperature in the c-BN synthesis has beena key target for its development. Despite almost fifty yearsof trying, commercial c-BN is also mainly obtained fromthe conversion of h-BN to c-BN at high temperature(1200–2000 �C) and high pressure (2.5–7.5 GPa), just inthe region where c-BN is a thermodynamic stable phase,with a help of catalyst or solvent [3]. Furthermore, manyother methods have been developed to create c-BN, suchas chemical vapor deposition (CVD), pulsed laser deposi-

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.04.005

* Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392412.E-mail address: haowang@bjut.edu.cn (H. Wang).

tion (PLD) and physical vapor deposition (PVD), and soon, which depend on vacuum equipment (a few Torr,500–1300 K) to form plasma vapour and ion bombardment[3–6]. These methods make growing c-BN as a thermody-namic meta-stable phase not just an imagination and resultin a new annotation of boron-nitride thermodynamic equi-librium phase diagram [7]. Here we report the formation ofa mixed phase of boron nitride with c-BN and E-BN(extra-diamond boron nitride) at ambient pressure androom temperature by plasma electrolysis. This discoverymay be an important milestone for c-BN synthesis tech-nique and eventually lead to a novel substitute for boron-nitride thermodynamic equilibrium phase diagram.

2. Experimental

Our synthesis was attempted in organic solutions by thetechnique of plasma electrolysis. The experimental setup issimilar to that of Ref. [8]. A solution of dimethylformam-ide (HCON(CH3)2, DMF, 99.5%) and boric acid(H3BO3), with purity larger than 99%, was chosen as theelectrolytes. The mole ratio of boron atom and nitrogen

500 1000 1500 2000

Abs

orbt

ance

(arb

.uni

t)

Wavenumber cm-1

1191

946802

1635

1399

1075

2000 V

1500 V

1000 V

453

Fig. 2. FTIR spectra of samples deposited at different applied potentials.

J. Guo et al. / Electrochemistry Communications 9 (2007) 1824–1827 1825

atom is 1:1. A thin tungsten wire (the diameter is about0.5 mm) and a titanium sheet are used as cathode andanode, respectively. The tungsten wire with a pin-shapeend of 5 lm diameter was bent to form a structure thatthe pin was vertically directed to the Ti plate. The distancebetween the pin and the plate was set about 10 mm and ahigh voltage was applied between them using a DC sourcewith a maximum voltage of 4000 V. During the process, aspark which could even be observed by the naked eye wasinitiated by the high electric field. To avoid the probableexplosion once the spark happened and for overall systemcooling, nitrogen gas was introduced during the experi-ments. At the end of the process, hundreds of straw yellowprecipitates were formed in the bulk solution. Before driedin air at 60 �C, the precipitates were separated from thesolution by centrifugation, then washed with pure ethanol,hydrofluoric acid and distilled water, respectively toremove the residual organic solution, impurity oxides andions.

3. Results

The composition of the sample was determined from theX-ray photoelectron spectroscopy (XPS) spectra. The sur-vey spectrum is shown in Fig. 1. Four primary peaks arecorresponding to B1s, C1s, N1s and O1s, respectively.The binding energies centered at 399.65 eV for N1s and192.05 eV for B1s are in good agreement with the reportedvalues of BN [9]. Quantification of B1s and N1s peaks givesaverage B: N atomic ratio of 0.998:1, which agrees wellwith the stoichiometric composition of BN. In addition,the carbon peak located at 286 eV is also present at highintensity. The contaminations are either from source mate-rials or from the chamber atmosphere. The samples showno evidence of any other elements, indicating boron nitrideis indeed the main ingredient of the powder.

Fourier transformed infrared (FTIR) spectroscopy,which serves as a quick non-destructive process, could beused as a major tool for phase identification of BN.

0 200 400 600 800 1000

Inte

nsit

y (C

PS)

Binding energy (eV)

B1sC1s

N1s

O1s

O(KLL)

Fig. 1. XPS spectrum of the sample.

Fig. 2 displays the FTIR transmission spectra of the sam-ples deposited at different applied potentials, showing amixed phase with peaks present for both c-BN andE-BN. The main feature of the 2000 V sample is describedby a peak centered at 1075 cm�1, which is referred to as thec-BN peak [10]. As can be seen in the figure, an apparentlyhighly cubic phase is revealed inside the focused area.Other relatively weak absorption peaks (453, 802, 946,1191, 1399 and 1635 cm�1) could be entirely assigned tothe corresponding phase of E-BN, whose full name wasextra-diamond boron nitride and was generally synthesizedby explosive shock compression of turbostratic boronnitride [11,12]. Fig. 2 also shows that, depending on theapplied potential, the sample may be deposited with differ-ent absorption intensity of c-BN phase in our study.

Transmission electron microscopy (TEM) measure-ments indicate that the sample deposited at 2000 V con-tains two different morphological grains. Fig. 3a showsthe TEM image of grains with irregular nano-sheet mor-phology, the diameter of which is about 60–70 nm. Theselective area electron diffraction (SAED) pattern(Fig. 3b) obtained from the nano-sheets indicates that theyare perfect crystals with a lattice parameter of 3.62 A,which matches with that of bulk cubic boron nitride. Thespots in the SAED pattern could be indexed to the (11 1),(200), ð1�1�1Þ diffractions of c-BN (JCPDS Card No. 26-0818). The crystalline structure of the c-BN nano-crystalswere further confirmed by high-resolution TEM investiga-tion. Fig. 3c displays a representative HRTEM image of anindividual c-BN nano-crystal, which reveals that the nano-sheet is single crystalline in nature and free of dislocationand structure detects. The resolved spacing of approxi-mately 0.21 nm is in well accordance with the latticeparameter of the (111) plane of c-BN. Fig. 4a shows theimage of grains with particle-like morphology whose diam-eter is about 25–30 nm. The corresponding SAED patternof the particle is shown in Fig. 4b. The obtained diffraction

Fig. 3. (a) TEM image of c-BN; (b) SAED pattern of a c-BN single crystal shown in part (a) and (c) HRTEM image.

1826 J. Guo et al. / Electrochemistry Communications 9 (2007) 1824–1827

spots are in agreement with the (021), (23 1), (230) diffrac-tions of E-BN (JCPDS Card No. 18-0251). The TEMresults adequately validate the FTIR analysis.

The feature of plasma electrolysis is the formation ofspecific surface structures such as meta-stable high-temper-ature phases, non-equilibrium solid solutions, complex

Fig. 4. (a) TEM image of E-BN an

mixed-compounds, etc. These substances are formed as aresult of plasma thermochemical reactions at the near-elec-trode region [13]. We have previously shown that plasmaelectrolysis is an attractive technique for synthesis and pat-terning of diamond-like carbon (DLC) [8,14]. Here we con-duct a further experimentation on the possibility of

d (b) SAED pattern of E-BN.

J. Guo et al. / Electrochemistry Communications 9 (2007) 1824–1827 1827

synthesizing nano-sized cubic phase of boron nitride byelectrolysis of boron and nitrogen containing solvents.The mechanism for the meta-stable formation of c-BNinvolved is not very clear yet, a preliminary mechanism isproposed to be as follows: The surface area of the cathodewas much lower than that of the anode. Hence, the currentdensity was concentrated around the cathode, leading tothe formation of a vapor sheath around the cathode byboiling of the solution. A glow discharge was then initiatedby the high electric field in the gas sheath around this elec-trode [8,12]. The plasma produced by glow discharge leadsto the formation of radicals N* and B*. The radicals N* andB* reacted at the near-electrode region, the nucleation of c-BN and E-BN may happen in this dense radical atmo-sphere. The as-formed c-BN and E-BN nucleus diffusedfrom the plasma zone to the bulk solution by convectionand then quenched there, thus the nanostructured boronnitride were retained.

Furthermore, the titanium anode seemed to play a sig-nificant role in the electrochemical process. In the case ofusing graphite or conductive glass as an anode, the yieldwas so poor to make a collection or could not endure wash-ing with hydrofluoric acid. All these phenomena are con-trary to using titanium as the anode and quite the sameas what happened in synthesis of nano-carbons [15,16].

4. Conclusions

Our results demonstrate that the synthesis of c-BN canbe reached under macroscopically ambient pressure androom temperature. Besides its intrinsic significance for fac-ile production of c-BN in future, it may provide a newresearch field in c-BN thermodynamics. The further study

should be mainly focused on producing pure cubic boronnitride by electrolysis technique and explore the detailedreaction process.

Acknowledgement

This work is supported by National Natural ScienceFoundation of China (No. 20021003).

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