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Electrochemistry Communications 8 (2006) 1211–1214

Synthesis of B50N2 nanorods by electrolysis of organic solutions

Jing Guo, Hao Wang *, ManKang Zhu, JingSen Zhu, Hui Yan

The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, China

Received 11 April 2006; received in revised form 29 May 2006; accepted 31 May 2006Available online 11 July 2006

Abstract

B50.06N1.87 nanorod was successfully synthesized by the technique of electrolysis, using a solution of mixed dimethylformamide(HCON(CH3)2) and boric acid (H3BO3) as the electrolyte. The sample was characterized by XPS, TEM and HRTEM. These nanorodshave uniform diameters of about 60 nm. HRTEM investigation demonstrates that these nanorods have a uniform crystalline structure oftetragonal B50.06N1.87 single crystal, and the growth direction of the nanorod is along [001]. It was proposed that other boron nitridesmay be prepared using electrolysis method.� 2006 Elsevier B.V. All rights reserved.

Keywords: Boron nitride; Electrolysis; Nanorods

1. Introduction

Nanostructured materials are drawing considerableattention for potential applications in electronic, optical,and magnetic devices due to their peculiar structural char-acteristics, size/shape effects, and physical properties thatare different from the bulk materials [1]. One-dimensionalstructures such as 1D nanowires or nanorods of carbides(TiC, SiC, and BCx), silicon nitrides (Si3N4 and Si2N2O)and boron nitrides (BN, (BN)x and BxNy), etc. [2–4] exhibitmany interesting properties due to their low dimensionalityand have potential use in the interconnects and functionalblocks that are used for fabricating nanoscale devices [5].The boron nitrides are extraordinary topics in the area ofmaterials science. The boron nitrides are analogous to car-bons in many ways. They can form both hard, diamond-like sp3-bonded phase and soft, graphite-like sp2-bondedphases. Due to the special bonding behaviors of boronand nitrogen, boron nitrides exist in many different struc-tures. The well-defined crystallographic structures are hex-agonal BN (h-BN), rhombohedral BN (r-BN), wurtziticBN (w-BN), and cubic BN (c-BN) [6]. Additionally, other

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doi:10.1016/j.elecom.2006.05.025

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

crystalline and amorphous structures exist. Recently,mixed boron–nitrogen clusters have been a topic of sub-stantial interest because of their importance as precursorsfor thin solid films of b-boron nitride (which scratches dia-mond) [7,8]. Such molecules as B2N, BN2, B2N2, and BxNy

etc. have been characterized by an interaction between the-oretical predictions [9–11] and pulsed laser evaporation fol-lowed by matrix infrared spectroscopy [11,12].

Boron nitrides – in all their various structures – are syn-thetic products and not found in nature. A variety of syn-thesis techniques adopted by researchers include pyrolysis[13], ion beam deposition [14], RF sputtering [15],plasma-enhanced chemical vapor deposition (PECVD)[16] and ECR-PECVD [17]. Now, there has been a con-certed effort by several groups to deposit boron nitridesat lower pressures and temperatures by physical vapordeposition (PVD) [18] or CVD [19] methods with ion bom-bardment on a growing surface.

Electrodeposition provides a cost-effective means of pro-ducing fully dense nanocrystalline metals, alloys, andmetal–matrix composites as coatings or free-standingforms (foil, sheet, wire, complex shapes). The electrosyn-thesis approach is highly adaptable to conventional indus-trial material processes, yielding significant materialand process improvements from relatively small capital

186 188 190 192 194 196 198 200

800

1000

1200

1400

1600

1800

2000

2200

Inte

nsi

ty (

CP

S)

Binding energy (eV)

B1s192.20 eV

394 396 398 400 402 404 4061400

1500

1600

1700

1800

1900

2000

2100

Inte

nsi

ty (

CP

S)

Binding energy (eV)

N1s400.37 eV

Fig. 1. High-resolution XPS spectra of the sample in the B1s and N1sregion.

1212 J. Guo et al. / Electrochemistry Communications 8 (2006) 1211–1214

equipment and process modifications [20]. Electrodeposit-ion technique has demonstrated some advantages overvapor phase deposition, such as the simplicity and low costof experimental apparatus, possibility of low temperaturesynthesis and ease of experimental control to producematerials with diverse properties, etc. [21]. To sum up, thistechnique is economical and environmentally benign, andproduces uncontaminated nanoparticles which may be use-ful in many applications.

Recently, efforts are underway to synthesis diamond-likecarbon (DLC) and carbon nitride films by electrodeposit-ion technique [22–28], which is cheap and scalable. Thesegroups reported on the application of substantially higherapplied potential (0–4 kV) between the electrodes for depo-sition of DLC and carbon nitride films. Specific efforts werecarried out by using a series of different electrolytes.

2. Experimental

In this letter, nanocrystallites are attempted in organicsolutions by the technique of electrolysis. A solution ofmixed dimethylformamide (HCON(CH3)2 99.5%) andboric acid (H3BO3), with purity larger than 99% was usedas a stating electrolyte. The mole ratio of the H3BO3 andHCON(CH3)2 is 1:5.2. The deposition was carried out atroom temperature. Two titanium sheets were used as elec-trodes. The distance between the two electrodes was set to8 mm. When a potential of 400 V was applied to the elec-trodes, straw yellow precipitates were formed and sepa-rated from the solution by centrifugation. Theprecipitates were washed with pure ethanol and distilledwater respectively to remove the residual organic solutionand impurity ions, and dried in air at 60 �C.

3. Results

The composition of the sample was determined from theX-ray photoelectron spectroscopy (XPS) spectra. The sur-vey spectrum indicates the existence of B and N elements.The high-resolution XPS spectra (Fig. 1) in the B1s andN1s region show symmetry peaks at about 192.20 and400.37 eV, respectively. It is known that binding energyand intensity of boron in boron nitrides strongly dependon nitrogen concentration – so does the peak position fornitrogen. Peak positions of B(1s) and N(1s) for boron nit-rides with lower nitrogen concentration resulted in shift inpeak position to higher energy. These binding energieswere in good agreement with the report in the literature[29]. The qualification of the peak intensities reveals thatthe atomic ratio of B to N is about 50.1:1.9 (S. F.:B,0.20; N, 0.41) which presents the chemical stoichiometricrelation between B and N. The XPS analysis shows thatthe synthetic product is a B-rich nitride approximated toB50N2.

The morphology of the nitride powders was investigatedby TEM. Fig. 2 shows a typical TEM image of the B50N2

sample. It reveals that the B50N2 sample consists of uni-

form rod-shaped particles with a diameter of about60 nm. The selected area transmission electron diffraction(SAED) pattern shown in the inset of Fig. 2 indicates thatthe powders are highly crystalline nanorods. No diffractionfeatures of the nanotube structure were identified. The lat-tice parameters of the sample were calculated from themeasured distance D between spots and the diffraction cen-ter in the picture:

dhkl ðAÞ ¼ K=D

Here, K = k/L is the microscope camera constant with a24.5 A mm value for the microscope used (wavelength, k,in angstrom by camera length, L, in millimeters). D corre-sponds to the measured distance in millimeter. The latticespace dhkl obtained (see Table 1) were compared with thecrystallographic data of B50.06N1.87 (JCPDS card No. 71-0098). It was found that the crystallographic data are inagreement with a tetragonal B50.06N1.87 structure, with anerror smaller than 3% with respect to the reference data.

Fig. 3 shows a typical high-resolution transmission elec-tron microscopy (HRTEM) image of an individual B50N2

nanorod with a diameter of about 60 nm. The HRTEMimage reveals that the nanorod is single crystalline in

Fig. 2. TEM image of the sample, the inset shows the SAED pattern ofthe crystallites.

Table 1Lattice spaces, dhkl exp, calculated from diffraction pattern

dhkl exp (A) dhkl ref (A) Error (%) hk l

3.89 3.927 0.94 1113.00 3.085 2.76 2112.85 2.878 0.97 3002.59 2.564 1.01 0022.40 2.410 0.41 3112.10 2.136 1.69 2121.95 1.963 0.66 2221.85 1.869 1.02 3121.70 1.677 1.37 103

The error was calculated with respect to the reference crystallographicdata.

Fig. 3. HRTEM image of a rod-shaped object.

J. Guo et al. / Electrochemistry Communications 8 (2006) 1211–1214 1213

nature and free of dislocation and structural defects. Fur-thermore, it confirms that the nanorod is of a solid natureon its inside and not a nanotube. The resolved spacing ofapproximately 0.26 nm is in well accordance with the lat-tice parameter of the (002) plane of B50.06N1.87, whichshows that the growth direction of the nanorod is along[001].

The reaction process of synthesizing B50N2 nanorodsfrom electrolyte is not very clear, a preliminary mechanismof electrochemical deposition in this experiment is pro-posed to be as follows:

(1) In DMF, two equivalent methyl groups are linkedwith a N atom conjugated with a carbonyl group.The methyl end of the molecule display part positive

charge due to its low electronegativity, and the dipolemoment of the molecule is enhanced under a highpotential. The conjugation between N and C=Oalso weakens the strength of the N–CH3 bond, thenfacilitates the heterolytic cleavage of the bond andthe formation of N�.

(2) The decomposition of H3BO3 in the liquid phase bythe electrochemical process:

H3BO3 ! 3Hþ þ BO3�3

Under high potentials, BO3�3 brought about reductive

reaction and produced B�.(3)

xB� þN� ! BxN

where B� and N� represent the highly reactive atoms.

4. Conclusions

B50N2 nanorods were successfully synthesized by thetechnique of electrolysis, using a solution of mixed dimeth-ylformamide (HCON(CH3)2) and boric acid (H3BO3) asthe electrolyte. HRTEM investigation demonstrates thatthese nanorods have a uniform crystalline structure oftetragonal B50N2 single crystal, and the growth directionof the nanorod is along [001]. Although the productionis not yet adequate for industrial application, electrolysisis much softer than conventional pyrolysis processes used

1214 J. Guo et al. / Electrochemistry Communications 8 (2006) 1211–1214

before. On the other hand, the electrodeposition techniquein liquid phase is very easy to control. This experimentshows us strong evidences that other boron nitrides maybe prepared using this method. We believe that more kindsof boron nitrides will be obtained in the liquid phase byimproving electrolysis technology in the near future.

Acknowledgement

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

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