ARTICLE pubs.acs.org/JPCC
Aqueous Dispersions of Few-Layered and Monolayered Hexagonal Boron Nitride Nanosheets from Sonication-Assisted Hydrolysis: Critical Role of Water Yi Lin,*,† Tiffany V. Williams,‡ Tian-Bing Xu,† Wei Cao,§ Hani E. Elsayed-Ali,§ and John W. Connell‡ †
National Institute of Aerospace, 100 Exploration Way, Hampton, Virginia 23666-6147, United States Mail Stop 226, Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681-2199, United States § Applied Research Center, Old Dominion University, 12050 Jefferson Avenue, Newport News, Virginia 23606, United States ‡
bS Supporting Information ABSTRACT:
Hexagonal boron nitride (h-BN) is traditionally considered to be insoluble in water. However, here we demonstrate that water is effective to exfoliate the layered h-BN structures with the assistance of bath sonication, forming “clean” aqueous dispersions of h-BN nanosheets without the use of surfactants or organic functionalization. Besides few-layered h-BN nanosheets, there was also evidence on the presence of monolayered nanosheet and nanoribbon species. Most nanosheets were of reduced lateral sizes, which was attributed to the cutting of parent h-BN sheets induced by the sonication-assisted hydrolysis (evidenced by the ammonia test and spectroscopy results). The hydrolysis effect also assisted in the exfoliation of h-BN nanosheets in addition to the solvent polarity effect. The h-BN nanosheets in such “clean” aqueous dispersions were demonstrated to be conveniently processed via solution methods with retained physical properties. The dispersed h-BN nanosheets in water also exhibited strong affinity toward proteins such as ferritin, suggesting that the nanosheet surfaces were available for further bioconjugations. The above findings may pave the way for the applications of these novel 2-dimensional nanomaterials in various fields such as composites, electronics, and biology.
’ INTRODUCTION As the isoelectric analog of graphene, hexagonal boron nitride (h-BN) nanosheets have recently received increased attention, both theoretically1-4 and experimentally.5-17 Various methods have become available to isolate few-layered h-BN nanosheet materials in multimilligram quantities using dispersion-based approaches.9-13 One route is to simply use a solvent, such as N,N0 -dimethylformamide (DMF),10 that exhibits polar-polar interactions with the h-BN surface to effectively separate the layered structure with the assistance of sonication. An alternative route is to use Lewis bases such as amine molecules to complex with boron atoms on h-BN surface to trigger exfoliation and r 2011 American Chemical Society
solubilization of the nanosheets.11-13 While the functionalization method using amine molecules provided higher nanosheet uptake in the dispersion, the former route is attractive for many applications since the resultant dispersions were essentially free of foreign materials other than the solvent molecules. h-BN has been traditionally considered to be hydrophobic and insoluble in water despite known limited structural susceptibility to hydrolysis.18-21 However, here we demonstrate that, with the assistance of bath sonication, water can be directly used for Received: November 17, 2010 Revised: December 3, 2010 Published: January 20, 2011 2679
dx.doi.org/10.1021/jp110985w | J. Phys. Chem. C 2011, 115, 2679–2685
The Journal of Physical Chemistry C
ARTICLE
“clean” dispersions of exfoliated h-BN nanosheets and nanoribbons with various lateral sizes (up to ∼1 μm) and thicknesses (as thin as a monolayer) without the need of chemical functionalization, surfactants or organic solvents. The few-layered and monolayered h-BN species in the aqueous dispersion were of reduced lateral sizes in comparison to the pristine h-BN particles. It was proposed that it was due to the sonication-assisted hydrolysis that promoted the cutting of the pristine or partially exfoliated thicker h-BN sheet structures and yielded smaller and more exfoliated h-BN nanosheets.
’ EXPERIMENTAL SECTION Materials. h-BN powder (size -10P, Lot HZ010PA4.$06) was provided by UK Abrasives. Ferritin protein (Type I, from horse spleen, 53 mg/mL solution in 0.15 M NaCl, sterile filtered), Nessler’s reagent (K2HgI4, from e2.4% HgCl2 with 10-15% KOH and e5% KI), and DMF (anhydrous, 99.8%) were purchased from Aldrich. All chemicals were used as received. Measurements. Scanning electron microscopy (SEM) and low-magnification transmission electron microscopy (TEM) images were acquired using a Hitachi S-5200 field-emission SEM system under the secondary electron (SE) and transmitted electron (TE) modes, respectively. High resolution TEM (HRTEM) experiments were conducted on a JEOL 2100 field-emission TEM system. Atomic force microscopy (AFM) studies were conducted under the tapping mode on a Veeco Digital Instruments Multimode Scanning Probe Microscope with a Nanoscope III Controller. Optical absorption spectra were obtained using a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. FT-IR spectra were acquired on a Thermo-Nicolet FT-IR 300 spectrometer equipped with a Thunderdome Swap-Top single reflection attenuated total reflectance (ATR) module. Raman spectroscopy was performed using a Thermo-Nicolet-Almega dispersive Raman spectrometer with 532 nm excitation. A Prostat PRS-801 Resistance System was used in the surface resistivity measurements. Six column-like gold electrodes with lengths of 8 mm, widths of 1.25 mm and interval distances of 2.2 mm were sputtered on the surface of an h-BN thin film (supported on a Millipore Isopore polycarbonate filter membrane with 0.1 μm pore size). The measurements between each adjacent electrode were taken and averaged. In-plane thermal diffusivity measurements were carried out on the same supported h-BN films using a ULVAC LaserPIT system. Preparation of “Clean” h-BN Nanosheet Aqueous Dispersions. In a typical procedure, the pristine h-BN powder (20 mg) was sonicated in deionized water (10 mL) using a bath sonicator (Branson 2510, 40 kHz) for 8 h. The resultant slurry was centrifuged at ∼3000g (IEC Clinical Centrifuge). The supernatant was passed through a coarse filter paper (Whatman cellulose) and the filtrate was collected as the “clean” aqueous dispersion of h-BN nanosheets.
’ RESULTS AND DISCUSSION The starting h-BN powder has representative lateral particle sizes in the range of 1-10 μm with thicknesses over 50 nm.11,12 The preparation procedure of a “clean” h-BN nanosheet aqueous dispersion was straightforward, including sonicating the pristine h-BN powder in a certain volume of deionized water in a bath sonicator for a few hours followed by centrifugation and brief filtration. The dependence of h-BN uptake in the dispersion on the sonication time was conveniently monitored by measuring the absorbance value of an aliquot of the dispersion at 350 nm
Figure 1. TEM studies of h-BN nanosheets from “clean” aqueous dispersions: (a) a typical low-magnification TEM image (scale bar = 500 nm); (b) HR-TEM images of the folded edges of several nanosheets with 2, 4, and 8 layers, respectively (scale bars = 2 nm); (c) an HR-TEM image of a nanosheet (left, FFT shown in the inset) and the corresponding simulated image from inverse FFT (right) (scale bar = 2 nm); (d) an HR-TEM image of a few-layered nanosheet from aqueous dispersion after ∼2 min of electron beam irradiation, showing beam-induced equilateral triangular-shaped defects with opposite orientations at different areas (highlighted in insets) (scale bar = 5 nm).
(see Figure S1 in Supporting Information). It was found that the rapid increase of h-BN uptake in the first 8 h was rather consistent among batches of the same or different scales (up to 200 mg h-BN starting material). Despite further improved yield with even longer sonication, most of the samples used in the following discussions were from 8 h of sonication unless otherwise specified. The as-obtained dispersion (on the order of ∼0.05-0.1 mg/mL for 8-24 h sonication) appeared “milky” but transparent, similar in appearance to those reported for simple organic dispersions10 and the amine-functionalized h-BN nanosheet solutions.11-13 Also comparable was the apparent stability of the aqueous h-BN dispersions, with only a small amount of precipitation observed over a few days. TEM studies of the as-obtained dispersion showed the presence of h-BN nanosheets with lateral sizes of a few tens of nm to ∼1 μm (Figure 1a). When the layers at folded sheet edges (layerlayer distance ∼0.33 nm, Figure 1b) were counted using HRTEM, the layer numbers of most nanosheets were found to be less than 30, corresponding to thicknesses of
