Boron nitride and method of producing boron nitride

Abstract

BN nanosheets are prepared by a method comprising heating to a temperature of at least 500 C., a mixture comprising: (1) an alkali borohydride, and (2) an ammonium salt. NaN.sub.3 may be included to increase the yield. No catalyst is required, and the product produced contains less than 0.1 atomic percent metal impurities.

Claims

1. h-BN nanosheets, wherein the h-BN nanosheets contain less than 0.1 atomic percent metal impurities and have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.002 peak of at most 0.50 degrees, the h-BN nanosheets have an aspect ratio of at least 10:1, and the h-BN nanosheets are not nanofibers.

2. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets are few layer h-BN nanosheets.

3. The h-BN nanosheets of claim 2, wherein the h-BN nanosheets have 6 to 20 layers of BN.

4. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets do not contain r-BN, as determined by X-ray powder diffraction.

5. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.002 peak of at most 0.30 degrees.

6. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.50 degrees.

7. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.25 degrees.

8. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have a particle size of 250 to 900 nm.

9. A capacitor, comprising: (a) a substrate, (b) a first conductive layer, on the substrate (c) an insulating layer, on the conductive layer, and (d) a second conductive layer, on the insulating layer, wherein the insulating layer comprises the h-BN nanosheets of claim 1.

10. The capacitor of claim 9, wherein the h-BN nanosheets are few layer h-BN nanosheets.

11. The capacitor of claim 9, wherein the h-BN nanosheets have 6 to 20 layers of BN.

12. The capacitor of claim 9, wherein the h-BN nanosheets do not contain r-BN, as determined by X-ray powder diffraction.

13. The capacitor of claim 9, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.002 peak of at most 0.30 degrees.

14. The capacitor of claim 9, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.50 degrees.

15. The capacitor of claim 9, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.25 degrees.

16. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have 6 to 20 layers of BN, the h-BN nanosheets do not contain r-BN, as determined by X-ray powder diffraction, and the h-BN nanosheets have a particle size of 250 to 900 nm.

17. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have 6 to 20 layers of BN, and the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.25 degrees.

18. The h-BN nanosheets of claim 1, wherein the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.002 peak of at most 0.30 degrees, and the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.25 degrees.

19. The capacitor of claim 9, wherein the h-BN nanosheets have 6 to 20 layers of BN, the h-BN nanosheets do not contain r-BN, as determined by X-ray powder diffraction, and the h-BN nanosheets have a particle size of 250 to 900 nm.

20. The capacitor of claim 9, wherein the h-BN nanosheets have 6 to 20 layers of BN, and the h-BN nanosheets have a full width at half maximum (FWHM) of the X-ray powder diffraction pattern for a d.sub.100 peak of at most 0.25 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is an SEM image of BNS12 showing uniform product morphology. Nanosheets having an average diameter of 600 nm were observed with a narrow size distribution.

(2) FIG. 1B is an SEM image of BNS12 showing uniform product morphology.

(3) FIG. 1C is an SEM image of BNS12 showing uniform product morphology.

(4) FIG. 2A is a TEM image of BNS12 exhibiting a narrow size distribution.

(5) FIG. 2B is a TEM image of few-layer BN nanosheets (BNS48).

(6) FIG. 2C is a HRTEM image of BNS12 showing 6 layers.

(7) FIG. 2D is a HRTEM image of BNS48 showing 20 layers. Interlayer distance was determined to be 0.33 nm.

(8) FIG. 3A is an EEL spectra of BNS12 sample.

(9) FIG. 3B is a FT-IR spectra of BNS12 sample.

(10) FIG. 3C is a Raman spectra of BNS12 sample.

(11) FIG. 4 is an XRD pattern of BNS12. The product synthesized at 800 C. for 12 hours was indexed as d.sub.002=3.225 (27.64), d.sub.100=2.127 (42.47), d.sub.101=2.026 (44.71), d.sub.102=1.789 (50.98), d.sub.4=1.642 (55.94), d.sub.104=1.308 (72.12), d.sub.110=1.241 (76.68).

(12) FIG. 5A is a TEM image of BNS24.

(13) FIG. 5B is a TEM image of BNS48. Both materials have a good parity with the dimension of the nanosheets (average particle diameters of 960 nm and 1080 nm, respectively).

(14) FIG. 6A is an EDX spectra of BNS12 (atomic ratio of B:N=50.06:49.94). No metal impurities were found in either of the samples.

(15) FIG. 6B is an EDX spectra of BNS24 (atomic ratio of B:N=49.9:50.10).

(16) FIG. 6C is an EDX spectra of BNS48 (atomic ratio of B:N=49.66:50.34).

(17) FIG. 7A is a FT-IR spectra of BNS24 peaks at 1386 cm.sup.1 and 812 cm.sup.1.

(18) FIG. 7B is a FT-IR spectra of BNS48, peaks for h-BN at 1388 and 815 cm.sup.1.

(19) FIG. 8A is Raman Spectra of BNS24.

(20) FIG. 8B is Raman Spectra of BNS48. The spectra exhibit the Raman shift for h-BN at 1364 cm.sup.1 for BNS24 and 1366 cm.sup.1 for BNS48 respectively.

(21) FIG. 9A is UV-visible spectra of BNS12 with a band gap of 5.98 eV.

(22) FIG. 9B is UV-visible spectra of BNS24 showing the absorption band at 6.018 eV.

(23) FIG. 9C is UV-visible spectra of BNS48 exhibiting band gap of 6.048 eV.

(24) FIG. 10A is an X-ray diffraction pattern of BNS24, the product is indexed as d.sub.002=3.106 (28.72), d.sub.100=2.077 (43.51), d.sub.101=1.947 (45.73), d.sub.102=1.755 (52.05), d.sub.004=1.615 (56.98), d.sub.104=1.294 (73.14), d.sub.110=1.229 (77.68).

(25) FIG. 10B is an X-ray diffraction pattern of BNS48, the product is indexed as d.sub.002=3.239 (27.52), d.sub.100=2.132 (42.34), d.sub.101=2.031 (44.58), d.sub.102=1.792 (50.87), d.sub.004=1.645 (55.83), d.sub.104=1.310 (72.01), d.sub.110=1.245 (76.53).

(26) FIG. 11A illustrates a capacitor.

(27) FIG. 11B illustrates a capacitor.

DETAILED DESCRIPTION

(28) The present invention makes use of the discovery of a simple and inexpensive method of synthesizing BN, including BN nanosheets such as h-BN nanosheets and mixtures of h-BN nanosheets and r-BN nanosheets. The process avoids the need for metal catalysts, and therefore the product is not contaminated with metals from the catalyst. The product has excellent crystallinity, and a narrow distribution of particles size. By controlling the reaction time and temperature, particle size and the number of layers can be controlled, as well as the proportion of h-BN nanosheets and r-BN nanosheets in the product.

(29) The reaction starting materials include a borohydride, such as an alkali metal borohydrides (for example NaBH.sub.4, KBH.sub.4 or RbBH.sub.4, and mixtures thereof), and an ammonium compound, such as an ammonium salt, (for example NH.sub.4Cl, NH.sub.4Br or NH.sub.4I, and mixtures thereof). Optionally, an azide, such as NaN.sub.3 or KN.sub.3 may be included. The reaction is preferably carried out in a closed system, such as a stainless steel reactor or autoclave. Preferably, the molar ratio of borohydride to ammonium compound is 1:1; when an azide is included, it is also preferably present in an equimolar amount to the borohydride.

(30) reaction is carried out at a temperature of at least 500 C., and preferably for a time of at least 12 hours. An increase in temperature or time of the reaction will increase the particle size and the number of layers in the BN product; at the highest temperatures, an increase of reaction time will have little effect on particle size, but will increase the number of layers. Preferably, the reaction is carried out at a temperature of at least 600 C., more preferably at least 700 C., and most preferably at a temperature of at least 800 C., for example 600 to 800 C. Preferably, the reaction is carried out for at least 12 hours, or at least 24 hours, or at least 48 hours, such as 12 to 48 hours.

(31) After the reaction is completed, the product may be washed with water and/or acid to remove byproducts, such as KCl. The yield also increases with increasing temperature; preferably the yield is at least 70%, more preferably at least 80%. Increasing reaction time and temperature will also increase the proportion of h-BN nanosheets produced while reducing the proportion of r-BN nanosheets produced, allowing for the preparation of single phase h-BN nanosheets at a temperature of about 600 C. or greater.

(32) The product produced will preferably have at most 25 layers, such as 6 to 22 layers, of BN in each nanosheet. The BN produced is highly crystalline, and preferably the h-BN produced will have a full width at half maximum (FWHM) of the X-ray powder diffraction for the d.sub.002 peak of at most 0.50, more preferably at most 0.46 or 0.42, and most preferably at most 0.30, degrees. Preferably the h-BN produced will have a FWHM of the X-ray powder diffraction for the d.sub.100 peak of at most 0.50, more preferably at most 0.41 or 0.33, and most preferably at most 0.25, degrees.

(33) No catalyst is used in the preparation of the BN, thus producing a high purity product. Preferably, the amount of metal impurities is less than 0.1 atomic percent, such as the amount of magnesium, aluminum, silicon, nickel, chromium and/or iron.

(34) BN has a variety of uses. BN may be used as a high-temperature lubricant. BN is also useful has a thermal conductor, particularly in high-temperature applications. BN typically contains about 10% B.sup.10, and is effective as a neutron shielding or neutron absorbing material. h-BN nanosheets are also useful as a substrate for the growth of graphene, for example by CVD or other chemical deposition methods of forming graphene. With a particle size of less than 100 nm, h-BN nanosheets may also be used in pharmaceutical applications, such as a carrier particle for delivery of a drug (for example, an organic chemical, protein or oligonucleotide). Because BN is an insulator, it may also be used to form field effect transistors (FET), or as a dielectric material in a capacitor.

(35) An example of a capacitor is illustrated in FIG. 11(a) and FIG. 11(b). FIG. 11(a) illustrates the structure of the capacitor 10: a substrate 20, such as a metal foil or a plastic film; a first layer of conductor 30, such as a layer of graphene or carbon nanotubes, on the substrate; a layer of insulator 40, such as a layer of BN nanosheets (for example h-BN nanosheets), on the layer of conductor; and a second layer of conductor 50, such as a layer of graphene or carbon nanotubes, on the layer of insulator. The structure may then be rolled up into a cylinder, as illustrated in FIG. 11(b), where the capacitor 10 has a cylinder shape, and an optional first lead 60 is electrically connected to the first layer of conductor, and an optional second lead 70 is electrically connected to the second layer of conductor. Such a structure may be formed by forming a slurry of graphene or carbon nanotubes, to form a conductive ink, and similarly a slurry of BN nanosheets, to form a BN nanosheet ink which is otherwise identical to the conductive ink except for the substitution of the BN nanosheets for the graphene or carbon nanotubes. The conductive ink may then be painted onto the substrate. Once dried or cured, the BN nanosheet ink may be painted onto the conductive layer. Once the BN nanosheet ink is dried or cured, a layer of the conductive ink may be pained onto the insulating layer. After all layers are dried or cured, the leads may be attached to the respective layers, and the structure rolled up into a cylinder to form the capacitor.

EXAMPLES

Example 1: h-BN nanosheets

(36) Reported here is a bottom-up synthesis for few-layer h-BN nanosheets by an autoclave pyrolysis technique. The synthetic methodology involved mixing of NH.sub.4Cl, NaN.sub.3 and KBH.sub.4 in equimolar proportions, in an inert atmosphere. The resulting mixture was heated at 800 C., inside a tightly closed stainless steel autoclave to produce h-BN nanosheets. The time of the reactions was varied from 12 to 48 hours in order to optimize the reaction conditions and to analyze the time-dependency on the morphology of the product. Detailed reaction protocols are described below. The resulting h-BN nanosheets, identified as BNS12, BNS24 and BNS48, were isolated as products from 12 hours, 24 hours and 48 hours reactions, respectively. The products were vacuum dried overnight for further characterization. Scanning electron microscopy (SEM), transmission and high resolution transmission electron microscopy (TEM and HRTEM), energy-dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, X-ray powder diffraction (XRD), and UV-Visible spectroscopy are the techniques used for characterizing the product.

(37) Although this innovative technique produced few-layer h-BN nanosheets with high yield in a stainless steel autoclave, the exact mechanism of this process is unknown. One possible explanation is that the high pressure generated during the pyrolysis facilitated the formation of few-layer nanosheets of h-BN. The advantages of this methodology over the CVD methods are manifold. While almost all of the CVD techniques require temperatures higher than 1000 C., a comparatively lower temperature, used in this synthesis, is noteworthy. In addition, the catalyst-free synthetic approach involving inexpensive starting material without the continuous stream of gas made this process unique, cost-effective and ideal for large-scale production. Nonetheless, the method is simple enough in that the rigorous purification techniques can be avoided and the products can be washed with acid and deionized water to produce pristine product. The yield of the products, though in slight deviation (less than 4%), was calculated for each reaction (Table 1).

(38) TABLE-US-00001 TABLE 1 Synthesis and electrical property of h-BN nanosheets Sample Reaction Conditions Yield (%) Band gap.sup.a (eV) BNS12 800 C., 12 hrs 83 5.980 BNS24 800 C., 24 hrs 85 6.018 BNS48 800 C., 48 hrs 87 6.048 .sup.aBand gaps are calculated from the UV-visible spectroscopic data.

(39) The morphology of the BN nanosheets samples was investigated using SEM (TeScan Vega II SBH). The products were lightly coated with gold for the ease of imaging purposes. The images exhibit uniform product dimensions (FIG. 1a-c) and the average diameter of the nanosheets was found to be approximately 600 nm. A narrow size distribution of the products was observed for all of the samples at different time periods. Thus, the size of the product is independent of the reaction times.

(40) TEM (Hitachi H-600) characterization is consistent with the findings from SEM imaging for all of the samples with product dimension falling under similar data ranges (FIGS. 2a & 2b). On the other hand, the HRTEM (JEOL JEM-2100F) determined the number of layers of the h-BN nanosheets. While, BNS12 exhibited nanosheets with an average of 6-8 layers (FIG. 2c) BNS24 and BNS48 showed varied number of layers between 20 and 22 (FIG. 2d). The spacing between the layers was estimated by measuring the thickness of the sheet and dividing it by the number of layers. The resulting value of 0.33 nm matches perfectly with the previously published data as well as those collected via XRD studies. Nonetheless, the HRTEM images evidently predicted that the reaction time has a pronounced effect on the formation of h-BN layers. A controlled reaction time can reduce the number of layers without significantly changing the reaction yields.

(41) While the EDX spectra was generated over a large area of the products using an INCAx-act Analytical Standard EDS Detector, EELS data was obtained via an electron energy loss detector attached to the JEOL JEM-2100F microscope. FIG. 3a depicts the EEL spectrum of BNS12 and EDX spectra of all BN nanosheet samples are provided in FIGS. 6a-c. Both the EELS and EDX data ascertains the elemental composition and purity of h-BN nanosheets. The atomic ratio of B:N was found to be 1:1 (atomic weight percentage of B:N=50.06:49.94) by EDX analysis. The peak positions and the splitting patterns in EEL spectrum confirms the sp2 hybridization of BN in h-BN nanosheets. The energy loss around 190 eV and 400 eV corresponded to the K-shell ionization energy of the B and N atoms, respectively.

(42) Bonding patterns of the B and N atoms in nanosheets and purity of the products were further verified by FT-IR (ATI Mattson Genesis series) and Raman spectroscopy (FIGS. 3c & 3d). The FT-IR peak at 1388 cm.sup.1 is due to the stretching vibration of B-N bonds. A comparatively weaker peak at 815 cm.sup.1 can be attributed to the bending vibration of B-N-B bonds of h-BN. Raman spectroscopy of the BN nanosheet samples was performed using a Renishaw inVia Raman microscope with a 532 nm laser source. An intense sharp peak at 1364 cm.sup.1 acts as the definite confirmation of the presence of h-BN in the sample.

(43) The XRD (Rigaku MiniFlex, Cu, 30 kV, 15 mA X-ray) of the h-BN nanosheets, synthesized at 800 C., exhibited interplanar d-spacings and intensities that are indicative of h-BN crystallinity. FIG. 4 shows the spectrum of BNS12 where the indexed peaks are in good agreement with the theoretical values for h-BN (JCPDS 34-0421). The lattice constant of a=2.508 and c=6.667 were calculated using EdPCR component of FullProf Suite software. Additional XRD patterns are included in FIG. 10. Nonetheless, all of the XRD patterns suggest formation of highly ordered and pure h-BN nanosheets.

(44) In conclusion, a novel methodology to prepare few-layer h-BN nanosheets in high yields with acceptable purity has been developed. A 12 hour reaction involving inexpensive starting reagents at moderately low temperature produced nanosheets of uniform dimension and few layers. While the morphology and crystallinity of the product was thoroughly characterized, elemental composition of the products was also determined.

Synthesis of BNS12, BNS24 and BNS48

General methodology and purification

(45) Most common chemicals were obtained from Sigma-Aldrich. The reagents used were KBH.sub.4 (99.998%), NH.sub.4Cl (99.998%), and NaN.sub.3 (99.5%). The HCl used for purification was obtained from Fisher Scientific. In a typical pyrolysis experiment, the reagents were assembled in an argon filled glovebox that contained <2.00 ppm O.sub.2. The B and N-containing precursors, KBH.sub.4 (4.44 g, 82.31 mmol), NH.sub.4Cl (4.41 g, 82.45 mmol) and NaN.sub.3 (5.37 g, 82.60 mmol), were thoroughly mixed together and transferred into a 100 mL capacity autoclave that was sealed and then heated to various temperatures from 500 C. to 800 C. for various reaction times of 12 to 48 hours (12 hours for BNS12, 24 hours for BNS24, and 48 hours for BNS48) at 800 C. in a vertical furnace. After completing the reaction, the crude product from the autoclave was transferred to another vessel and washed three times with 3M HCl with sonication between each wash. The next step involved washing with deionized water until the pH of the decanted liquid became neutral. Sonication was carried out between each washing with water. The material was then washed with acetone and dried under high vacuum at room temperature overnight.

Characterization

(46) FIGS. 5-10 show TEM images, EDX spectra, FTIR spectra, Raman spectra, UV-visible spectra and X-ray diffraction patterns of the BNS12, BNS24 and BNS48.

(47) Analysis of FIGS. 6(a), 6(b) and 6(c) indicate a purity (B:N) for BNS12, BNS24 and BNS48, of 43.47:56.53 wt. % (49.90:50.10 atomic %), 43.23:56.77 wt. % (49.66:50.34 atomic %), and 43.62:56.38 wt. % (50.06:49.94 atomic %), respectively. The level of metal impurities in all the samples was less than 0.1 atomic %.

(48) FIGS. 4, 10(a) and 10(b), show the X-ray diffraction patters of BNS12, BNS24 and BNS48, respectively. The full width at half maximum of the d.sub.002 diffraction peaks were 0.42, 0.46, and 0.28, for BNS12, BNS24 and BNS48 respectively. The full width at half maximum of the d.sub.100 diffraction peaks were 0.33, 0.41, and 0.22, for BNS12, BNS24 and BNS48 respectively. This demonstrates the highly crystalline nature of the h-BN product produced.

Example 2: mixed phase r-BN/h-BN and pure h-BN

Experimental

(49) The experimental details for Example 2 were the same as for Example 1, except that the temperatures used with the samples varied from 500 C. to 800 C. for various reaction times of 12 to 48 hours, as indicated in Table 2.

(50) TABLE-US-00002 TABLE 2 Synthesis and yields of BN nanosheet reactions Sample Reaction conditions Yield (%) Phase 1 500 C. 12 hrs 40 r-BN, h-BN 2 24 hrs 67 r-BN, h-BN 3 48 hrs 83 r-BN, h-BN 4 600 C. 12 hrs 66 r-BN, h-BN 5 24 hrs 69 r-BN, h-BN 6 48 hrs 90 h-BN 7 700 C. 12 hrs 87 r-BN, h-BN 8 24 his 88 h-BN 9 48 hrs 78 h-BN 10.sup.a 800 C. 12 hrs 83 h-BN 11.sup.b 24 hrs 85 h-BN 12.sup.c 48 his 87 h-BN .sup.aCorresponds to sample BNS12 from Example 1. .sup.bCorresponds to sample BNS24 from Example 1. .sup.cCorresponds to sample BNS48 from Example 1.

(51) The threshold temperature for the formation of BN was found to be 500 C. The first appearance of predominant h-BN crystal lattice was in sample 6, for which the reaction temperature was 600 C. over a period of 48 hours. Samples 10-12 showed high purity and excellent h-BN crystallinity. The yield of the products, with slight deviation, was calculated for each reaction and was determined based on the use of KBH.sub.4. In addition, it is important to note that sample 6 resulted in a 90% yield which translates into over 1.8 g of the pure white product.

(52) In addition to the temperature and time-dependent study, the role of the chemical precursors in the synthesis was also examined. It was found that the absence of NaN.sub.3 in the precursor mixture reduced the yield to less than 40% when equimolar quantities of KBH.sub.4 and NH.sub.4Cl were reacted at 800 C. for 48 hours (Table 3). No h-BN, or any other crystalline phase formation of BN, was observed when equimolar amounts KBH.sub.4 and NaN.sub.3 reacted at the same reaction conditions (sample 14), thus indicating that when all three chemical reagents are used single phase h-BN is produced in high yield.

(53) TABLE-US-00003 TABLE 3 Effects of precursors on h-BN formation KBH.sub.4 NH.sub.4Cl NaN.sub.3 Reaction Yield Sample (mol) (mol) (mol) conditions (%) Phase 13 0.0412 0.0412 None 800 C. 48 hrs 40% r-BN, h-BN 14 0.0412 None 0.0413 800 C. 48 hrs None No BN produced

(54) Reactions run at 500 C. using KBH.sub.4, NH.sub.4Cl and NaN.sub.3 (samples 1-3) showed mixed-phase products of both r-BN and h-BN. When molar ratio of NaN.sub.3 was increased by 1.5 fold and the reaction was run at 500 C. for 48 hours, there was little to no difference in mixed-phase formation of r-BN and h-BN.

Results and discussion

(55) Morphologies and sizes of the BN nanosheet samples were investigated using TEM (Hitachi H-600). The range and average particle diameters of the nanosheets increased as the temperature and time were changed (Table 4). Desired sizes can be obtained by running a reaction at elevated temperatures for a shorter period or by lowering temperatures and doubling the reaction time. In addition, the morphology of the samples are also time and temperature dependent. At lower reaction temperatures and shorter reaction times, the morphology took on a triangular shape and then transformed into more hexagonal and irregular forms when reaction conditions increased up to 800 C. during 48 hour periods.

(56) TABLE-US-00004 TABLE 4 Range and average particle diameters of BN nanosheets Range of particle Average particle Reaction diameter diameter Sample conditions (nm) (nm) 1 500 C. 12 hrs 10-50 25 2 24 hrs 10-50 30 3 48 hrs 12-60 30 4 600 C. 12 hrs 12-200 70 5 24 hrs 60-400 200 6 48 hrs 250-500 400 7 700 C. 12 hrs 12-800 80 8 24 hrs 200-1000 300 9 48 hrs 250-1200 500 10 800 C. 12 hrs 250-900 600 11 24 hrs 200-1500 960 12 48 hrs 500-2000 1080

(57) Due to structural similarities in h-BN and r-BN, XRD was needed to verify the lattice type for each sample. Each reaction sample (1-12) was indexed. The XRD patterns of samples 1-5, and 7 showed indexed spectra of mixed phases of r-BN and h-BN. Samples 6, 8 and 9 were predominantly h-BN and indexed XRD samples 10-12 (800 C. for 12-48 hrs) exhibited interplanar d-spacings and intensities that are indicative of h-BN crystallinity. Sample 10 exhibited indexed peaks that were in close agreement with the theoretical values for h-BN (JCPDS 34-0421). The lattice constants of a=2.508 and c=6.667 were calculated using the EdPCR component of FullProf Suite software.

(58) Comparison of XRD data with the respective TEM image indicated the morphological triangle shapes present in the samples with mixed-phases of r-BN and h-BN. However, at higher temperatures, the XRD patterns indicated the presence of pure h-BN, while the TEM images show morphologies that are more hexagonal and irregularly shaped.

(59) The UV-Visible spectroscopy (Lambda XLS+) was used to determine the UV absorption properties and the corresponding band gaps of all the BN samples. The UV-visible absorption spectrum of the BN samples are provided in FIG. 9. A very dilute and well dispersed ethanolic dispersion of BN nanosheet samples were scanned between 200 and 700 nm in a quartz cell. Table 5 shows the trend of increasing band gap as the purity of h-BN samples increase. The mixed-phases of r-BN and h-BN have smaller band gaps indicating that the r-BN nanostructures affect the optical band gap of the material. A sharp absorption band at 207 nm for sample 10 (FIG. 9a) reflects a band gap of 5.98 eV which is consistent with the value reported for a single crystal of pure h-BN. A slight blue shift was noticed for samples 11 (BNS24) and 12 (BNS48), which can be attributed to the increasing number of layers in the nanosheets.

(60) TABLE-US-00005 TABLE 5 Band Gaps of BN nanosheet samples Sample Band gap (eV) 1 5.821 2 5.848 3 5.876 4 5.961 5 6.048 6 6.108 7 5.848 8 6.078 9 6.108 10 5.980 11 6.018 12 6.048

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