One-dimensional (1D) nanomaterials with novel photoelectric, magnetic, mechanical, and electronic transport properties have long been the research focus throughout the world. Herein, the recent achievements in preparation of 1D boron nitride nanomaterials, including nanotubes, nanowires, nanoribbons, nanorods, and nanofibres are reviewed. As the most intriguing and researched polymorph, boron nitride nanotubes (BNNTs) are introduced thoroughly involving their functionalization and doping. The electronics and engineering applications of 1D boron nitride nanomaterials are illustrated in nanoscale devices, hydrogen storage, polymer composites, and newly developed biomedical fields in detail.
A nanoscience and nanotechnology revolution initiated by the observation of carbon nanotubes (CNTs) [
Hexagonal boron nitride (h-BN) with layered structure analogous to graphite is an important III-V material. Stimulated by rapid development of carbon nanomaterials, a series of boron nitride nanostructures have been synthesized, especially 1D nanomaterials such as nanotubes, nanowires, nanoribbons, nanofibers, and nanorods (Figure
Family of 1D boron nitride nanomaterials. Scanning electron microscopy (SEM) images of (a) BN nanotubes [
In this paper, the latest advances in synthesis and applications of 1D boron nitride nanomaterials in device and engineering are comprehensively introduced. Of all, BNNTs and their functionalization and doping are systematically reviewed as well as their various applications particularly in newly developed biomedical field. Other 1D BN nanomaterials are illustrated in detail as well.
BNNTs were first synthesized by Chopra et al. in a carbon-free plasma discharge between a BN-packed tungsten rod and a cooled copper electrode similar to CNTs synthesis in 1995 [
High-resolution transmission electron microscope (HRTEM) images of BNNTs prepared by arc discharge and their tube ends [
BNNTs, especially single-layer ones, can be prepared by laser ablation. BNNTs of only 1 to 3 layers with flat or polyhedral caps were synthesized by using excimer laser ablating BN target with nanosized Ni and Co powder at
Ball milling and annealing method with facile operation and mass production was successfully introduced to the synthesis of BN nanomaterials by Chen et al. in 1999 [
In view of similar structures between CNTs and BNNTs and high reaction activity of carbon at high temperature, CNTs were utilized as the template for the growth of BNNTs defined as carbon nanotube substitution reaction. Han et al. initiated this field by reacting B2O3 with nitrogen gas in the presence of multishell CNTs at
In the carbon nanotube substitution reaction, CNTs acted as both templates and reducing agent. Another commonly used template, porous anodic aluminum oxide (AAO), was utilized to fabrication of highly ordered BNNTs arrays. Borazine or related materials such as 2,4,6-trichloroborazine, polymeric borazine, borazine oligomer and BH3NH3 or B2H6 was generally selected as the starting materials [
Actually, both the above-mentioned template methods are performed in chemical vapor deposition (CVD) equipment. CVD is the widely used method for BNNTs synthesis, which usually needs catalytic growth by metal particles following typical VLS mechanism. However, available boron source suitable for CVD synthesis is severely restricted. Frequently used gaseous boron-contained compounds in the synthesis of boron nitride films, most notably BCl3 [
BNNTs obtained by oxide-assisted CVD method [
One facile and easy-controlled method based on confined reactions in a sealed autoclave developed by our groups has been successfully utilized to preparation of various nanostructures including carbon, carbide, and nitride nanomaterials from 1D to 3D at low temperature [
Typical transmission electron microscope (TEM) images of BNNTs prepared at low temperature. (a) Pure and high-yield straight BNNTs bundles [
Other methods towards BNNTs were exploited. Arc plasma jet can generate high-temperature (
Based on the above discussion, synthesis of pure multiwalled BNNTs have been achieved, although single-walled and aligned BNNTs which is of significance in device applications still remain a challenge. For industrial production, current methods developed in labs cannot meet the requirement yet. Moreover, precisely controlled synthesis of BNNTs with pure hexagonal or rhombohedral stacking and zig-zag or armchair arrangement needs further research. Remarkably, commercial BNNTs are currently available from READE Advanced Materials, Australian National University and/or other companies. Although the price is still very high now, this suggests huge application future of BNNTs undoubtedly. In order to fabricate devices using BNNTs, operation of BNNTs in solution via functionalization and doped BNNTs with tuned band gap are fulfilled, which is introduced below.
Processability of BNNTs in solution is of great significance for the fabrication of nanodevice. In the last few years, following the successful massive production of pure BNNTs, many reports about surface modification of BNNTs towards solubility in aqueous or organic media sprung out [
Covalent functionalization was achieved through forming acylamino bond between chemical reaction of COCl group of stearoyl chloride or chloroacetyl chloride and amino groups on the BNNTs [
Hydroxylated BNNTs [
BNNTs can be noncovalently functionalized by introducing carboxylate groups from perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) in aqueous solution [
A simplified strategy, where the boron site in BNNTs was referred to as Lewis acid, then enabling reaction with Lewis bases such as trialkylamine and trialkylphosphine, was proposed as an alternative way for soluble BNNTs [
Very recently, systematically theoretical study of functionalization of BNNTs by substituted carbenes (CR2) where R = H, F, Cl, CH3, CN, and NO2 was conducted using density functional theory [
Analogous to CNTs counterpart, BNNTs can be doped with other elements in the hope of novel properties including mainly modified band gaps. Owing to their similarity in structure, carbon-doped BNNTs were firstly and intensively studied from both theoretical predictions and experiments. Two types of carbon-doped BNNTs, homogeneous B-C-N nanotubes and BN/C heterostructures, are generally considered [
Tang et al. reported CVD synthesis of fluorine-doped BNNTs with a doping concentration of
Si-doped bamboo-like BNNTs were synthesized via catalyst-assisted pyrolysis of a silicon-containing polymeric precursor under N2 atmosphere [
Si-doped BNNTs [
Apart from light elements, heavy elements doped BNNTs were investigated theoretically and experimentally. BN nanobamboos doped with Eu were prepared through annealing of ball milled mixture of B and Eu powder [
Other elements, such as O, Pt, Be, Mn, and Cr [
Apart from nanotubes, boron nitride nanowires (BNNWs) are another important member of 1D boron nitride nanomaterials. They can act as insulating components in nanoscale devices. Compared with BNNTs, there are only several reports describing the synthesis of BNNWs. Maybe due to some aspects of BNNWs and BNNTs in common, such as, microscopic morphology and growth direction, BNNWs can be synthesized using slightly modified routes for BNNTs. Huo et al. prepared well crystalline BNNWs through the reaction of homemade B-rich FeB nanoparticles with the mixed nitrogen and ammonia at
Moreover, new synthetic systems were developed exclusively for BNNWs. Pure BNNWs with a uniform diameter of 20 nm were prepared through deposition of boron triiodide (BI3) on a Si (100) substrate under an ammonia atmosphere (Figure
BNNWs synthesized from BI3 [
Compared with BN nanotubes and nanowires, there are only a few reports on other 1D BN nanomaterials including nanoribbons, nanorods and nanofibers.
In view of the importance of nanoribbons as an excellent component in electric circuits and devices, BN nanoribbons received extensive research from theory to experiment. Unfortunately, most of the researchs still remained theoretic [
BN nanoribbons prepared by using ZnS nanoribbon as template [
BN nanorods or, more strictly, shortened nanowires with a aspect ratio of
As far as BN fibers are concerned, most of the methods used borazine-based polymeric precursors to produce boron nitride micrometer-scale ceramic fibers [
As to device application, nanotubes and nanowires have been the research focus in this field due to their fascinating electrical, magnetic and transport properties induced by well-known quantum confinement effects at the nanoscale. For the fabrication of devices, understanding of the electronic structures of 1D BN nanomaterials can not be sufficient enough. Due to the insulating nature, much effort has been devoted to BN nanotubes and nanowires with tuned band structure from insulators to semiconductors, which can be defined as “bandgap engineering”. As demonstrated above, doping with C, Si, F, and so forth, and covalent functionalization can lead to BNNTs with
In addition to excellent thermal stability below
(a) A histogram for comparative study of thermal conductivity of BNNT-PMMA composites showing almost 3-fold enhancement with BNNT loading fraction of 10 wt.% [
Continuous BN fibers of distinctive scientific and technological significance can be used as insulating fibrous reinforcements within ceramic composite in replacement of carbon fibers for use at high temperature in rigorous environment. The decent mechanical performance of BN fibers can be assured by well crystallinity and preferential orientation of the BN crystallites in the direction parallel to the fiber axis [
One-dimensional materials offer interesting gas adsorption properties, specially, hydrogen storage. CNTs were confirmed to possess the hydrogen uptake of up to 20 wt.%, although the reported value fluctuated markedly [
Recently, particularly in the last couples of years, BNNTs was emerging as innovative tool for nanomedicine [
BNNTs for biotechnology [
Other promising applications in field-emission devices, ultraviolet lasers, and insulating nanocables (e.g., AlN-BN, GaN-BN, SiC-BN) were also envisioned and investigated, which could be found elsewhere [
Over the last two decades, synthesis, properties and applications of 1D BN nanomaterials have been exploited and developed as above reviewed.
As for BN nanotubes, large-scale production has been achieved. However, much effort is still necessary to be made to synthesis pure nanotubes with homogeneous structure. Alternatively, development of a more efficient purification method is of great significance. Among the mass production of various kinds of BNNTs, single-walled BNNTs is still one of the research focus which are followed by comprehensive property characterization. On the other hand, doped BNNTs with modulated band structure, which may exhibit novel magnetic and electrical properties, also have large space for further development.
BN nanowires can be studied intensively as building blocks in nanoscale devices. Synthesis of BN nanoribbons still requires unceasing exploitation from both theory and experiment for the future application in nanoelectronics. As a promising candidate for super-lightweight engineering fibers with high strength and excellent thermal and chemical stability, boron nitride microscale and nanoscale fibers can progress in solving basic theoretical issues and developing cost-effective methods for industrial production.
Overall, one-dimensional BN nanomaterials can be used as promissing building blocks concerned about the electronic devices, materials engineering, biomedical engineering and other emergent and important issues.
This work was supported by the National Nature Science of China (Grant nos. 20871075 and 20971079), the 973 Project of China (no. 2005CB623601), the Independent Innovation Foundation of Shandong University (IIFSDU), and the Ph. D. Programs Foundation of Ministry of Education of China (no. 20070422046).