Nanoscale
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Large-scale synthesis and functionalization of hexagonal boron nitride nanosheets† Ganesh R. Bhimanapati,ac Daniel Kozuchab and Joshua A. Robinson*ac
Received 3rd April 2014 Accepted 4th August 2014 DOI: 10.1039/c4nr01816h www.rsc.org/nanoscale
A simple and inexpensive method to functionalize hexagonal boron nitride (hBN) was achieved by using an acid mixture of phosphoric and sulphuric acid. This functionalization induced the exfoliation of the layered structure of hBN into monolayer to few-layer sheets where the sizes of the sheets were dependent on the parent hBN powder used. Exfoliated hBN was shown to be stable in solvents such as ethanol, acetone, deionized water and isopropyl alcohol, and this stability was linked to sulfur functionalization that was induced during the exfoliation process. Further evidence of the functionalization was observed using transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS). By deconvoluting the high resolution peaks for B 1s, the bonding of boron to oxygen and sulfur was confirmed. The exfoliated hBN nanosheets were crystalline as confirmed from X-ray diffraction and they also exhibited an optically active defect related to sulfur functionalization at 320 nm (3.9 0.1 eV).
Introduction Boron nitride (BN) is the isoelectric and isostructural analog to graphite with alternating boron and nitrogen atoms in the structure.1–3 Within each layer, boron and nitrogen are bound by strong covalent bonding while weak van der Waals bonding exists between the layers. Similar to graphite, BN materials exist as layered (2D), cubic and tubular structures (1-D). Hexagonal BN (hBN) exhibits many unique properties such as extraordinary thermal stability (up to 2950 C), mechanical strength, and intrinsic electrical insulation because of the large band gap of a
Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania, 16803, USA. E-mail: [email protected]
b
Department of Chemical Engineering, The Pennsylvania State University, University Park, 16803, Pennsylvania, USA
c Department of Material Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, 16803, USA
† Electronic supplementary information (ESI) available: More information on the details for characterization, thickness measurements by AFM, FTIR analysis, XRD analysis, Raman characterization and XPS survey scans. See DOI: 10.1039/c4nr01816h
This journal is © The Royal Society of Chemistry 2014
5.9 eV.3,4 Because of these unique properties, hBN can be used effectively as a solid lubricant at extreme temperatures,5 UVlight emitters,6–10 insulating llers in composite materials11–13 and as dielectric materials.14 Exfoliation and functionalization strategies are quite useful when forming single layer hBN materials. The rst method for synthesis of monolayer hBN utilized the micromechanical cleavage technique and chemical treatment of bulk BN crystals.15,16 Following these initial reports, several other methods such as mechanical exfoliation via etching,17 lithium ion intercalation18 and low-energy ball milling16 were used to produce small quantities of high quality hBN nanosheets. Chemical exfoliation of hBN was later performed as an alternative to the mechanical routes, leading to a simple and cost effective way for the bulk exfoliation of hBN nanosheets. These were performed by treating the bulk hBN powder in various organic solvents,19 N,N-dimethylformamide (DMF),20 methane sulfonic acid (MSA),21 and molten metal hydroxides.22 These approaches yield a low-concentration of hBN nanosheets (0.5–1.0 mg per gram of bulk hBN) even aer extensive sonication. Functionalization of hBN was previously reported by Connell et al.,23 however the process yielded hBN nanosheets with dimensions less than 100 nm for monolayers. This process was effective in obtaining a high yield (10–20%) but involved treating the sample for extended periods of time (6–8 days). Finally, although chemical vapor deposition (CVD)24–30 yields high quality, large area lms, it requires vacuum and high temperature environments for the synthesis, which increases complexity and manufacturing costs. Hence, a low cost method to produce large scale and high quality hBN needs to be explored. In this paper, a simple and cost effective method for the exfoliation of large scale functionalized sheets of hBN using an acid mixture of phosphoric + sulphuric acid is reported. The sheet size is dependent on the maximum particle size in the hBN powder used for this process (6 mm was achieved in this report). The chemically exfoliated hBN is generally 1–4 layers thick, with exfoliation efficiency as high as 25% and has high crystallinity and majority of the akes oriented in the (002) direction when
Nanoscale, 2014, 6, 11671–11675 | 11671
View Article Online
Nanoscale
deposited on a substrate. With the addition of functional groups an improved dispersion and stability of hBN exists in a variety of solvents. A new optically active defect was created at 3.9 eV because of this functionalization. This method is highly scalable and can be used for a variety of applications.
Published on 07 August 2014. Downloaded by Iowa State University on 18/04/2017 07:08:10.
Experimentation To synthesize mono- and few-layer hBN, 1 g of hBN powder (99.5%, particle size 1–5 mm, Alpha Aesar) was mixed with 6 g of potassium permanganate (KMnO4, A.C.S Reagent, J.T. Baker) in a 1 L glass beaker. An acid mixture of 135 ml was prepared separately by mixing phosphoric acid (H3PO4, 85% w/w, EMD chemicals) and sulphuric acid (H2SO4, 95%, EMD chemicals) in the 1 : 8 ratio and was added to the hBN dry mixture. The resultant reaction is slightly exothermic, leading to a temperature rise in the solution to 40 C. The solution was then heated on a hot plate to 75 C under constant mixing for 12 hours. Subsequently, 6 ml of hydrogen peroxide (H2O2, 30% w/w, A.C.S Reagent, J.T. Baker) and 120 ml deionized water (DI water) were added to this mixture to halt the oxidation. This approach was similar to the synthesis of graphene oxide by Marcono et al.,31 with conditions tuned for the hBN material system. The resultant suspension was cooled to room temperature and centrifuged at 6000 rpm for 30 minutes, and the supernatant was pipetted out from the solution. Subsequently, 45 ml DI water was added to this solution and centrifuged again at 6000 rpm for 30 minutes. The non-exfoliated material was removed during the centrifugation and the supernatant solution was then subjected to a series of washing and centrifugation stages with DI water, ethanol and HCl. These washings were performed until a pH >3 was reached, which ensures the complete removal of the metal ions. The supernatant solution is then drop-cast or spin coated on Si wafers and dried to obtain the exfoliated and functionalized hBN sheets.
Results and discussion The exfoliation process results in mono- and few-layer hBN sheets that are distributed uniformly throughout the suspension. The initial hBN powder (before exfoliation) exhibits a variety of particle clusters, with varying thicknesses of hBN platelets (Fig. 1a). The thickness of the hBN platelets can be controlled by carefully centrifuging the hBN in solvent. Various centrifugation speeds were used to obtain hBN platelets of different thicknesses. Lower centrifugation speeds (1500– 2000 rpm) result in thicker platelets (100–200 nm) while higher centrifugation speeds yield thinner akes. In order to obtain the thinnest akes, the solution was centrifuged at 6000 rpm for 30 minutes. The resultant supernatant was pipetted out and dropcast onto a substrate for further analysis. Aer separating out the exfoliated akes by centrifugation, a highly concentrated solution of exfoliated akes was obtained. From the weight balance, the suspension typically consisted of about 12–25% (see ESI S0†) of the starting hBN weight, providing a signicantly higher yield than that previously reported.23
11672 | Nanoscale, 2014, 6, 11671–11675
Communication
Following the exfoliation process, most of the sheets are transparent to the electron beam at 2 kV clearly indicating a near atomic thickness of the hBN sheets (Fig. 1b and c). Importantly, while the sheet thickness is reduced to
Published on 07 August 2014. Downloaded by Iowa State University on 18/04/2017 07:08:10.
COMMUNICATION
Cite this: Nanoscale, 2014, 6, 11671
View Article Online View Journal | View Issue
Large-scale synthesis and functionalization of hexagonal boron nitride nanosheets† Ganesh R. Bhimanapati,ac Daniel Kozuchab and Joshua A. Robinson*ac
Received 3rd April 2014 Accepted 4th August 2014 DOI: 10.1039/c4nr01816h www.rsc.org/nanoscale
A simple and inexpensive method to functionalize hexagonal boron nitride (hBN) was achieved by using an acid mixture of phosphoric and sulphuric acid. This functionalization induced the exfoliation of the layered structure of hBN into monolayer to few-layer sheets where the sizes of the sheets were dependent on the parent hBN powder used. Exfoliated hBN was shown to be stable in solvents such as ethanol, acetone, deionized water and isopropyl alcohol, and this stability was linked to sulfur functionalization that was induced during the exfoliation process. Further evidence of the functionalization was observed using transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS). By deconvoluting the high resolution peaks for B 1s, the bonding of boron to oxygen and sulfur was confirmed. The exfoliated hBN nanosheets were crystalline as confirmed from X-ray diffraction and they also exhibited an optically active defect related to sulfur functionalization at 320 nm (3.9 0.1 eV).
Introduction Boron nitride (BN) is the isoelectric and isostructural analog to graphite with alternating boron and nitrogen atoms in the structure.1–3 Within each layer, boron and nitrogen are bound by strong covalent bonding while weak van der Waals bonding exists between the layers. Similar to graphite, BN materials exist as layered (2D), cubic and tubular structures (1-D). Hexagonal BN (hBN) exhibits many unique properties such as extraordinary thermal stability (up to 2950 C), mechanical strength, and intrinsic electrical insulation because of the large band gap of a
Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania, 16803, USA. E-mail: [email protected]
b
Department of Chemical Engineering, The Pennsylvania State University, University Park, 16803, Pennsylvania, USA
c Department of Material Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, 16803, USA
† Electronic supplementary information (ESI) available: More information on the details for characterization, thickness measurements by AFM, FTIR analysis, XRD analysis, Raman characterization and XPS survey scans. See DOI: 10.1039/c4nr01816h
This journal is © The Royal Society of Chemistry 2014
5.9 eV.3,4 Because of these unique properties, hBN can be used effectively as a solid lubricant at extreme temperatures,5 UVlight emitters,6–10 insulating llers in composite materials11–13 and as dielectric materials.14 Exfoliation and functionalization strategies are quite useful when forming single layer hBN materials. The rst method for synthesis of monolayer hBN utilized the micromechanical cleavage technique and chemical treatment of bulk BN crystals.15,16 Following these initial reports, several other methods such as mechanical exfoliation via etching,17 lithium ion intercalation18 and low-energy ball milling16 were used to produce small quantities of high quality hBN nanosheets. Chemical exfoliation of hBN was later performed as an alternative to the mechanical routes, leading to a simple and cost effective way for the bulk exfoliation of hBN nanosheets. These were performed by treating the bulk hBN powder in various organic solvents,19 N,N-dimethylformamide (DMF),20 methane sulfonic acid (MSA),21 and molten metal hydroxides.22 These approaches yield a low-concentration of hBN nanosheets (0.5–1.0 mg per gram of bulk hBN) even aer extensive sonication. Functionalization of hBN was previously reported by Connell et al.,23 however the process yielded hBN nanosheets with dimensions less than 100 nm for monolayers. This process was effective in obtaining a high yield (10–20%) but involved treating the sample for extended periods of time (6–8 days). Finally, although chemical vapor deposition (CVD)24–30 yields high quality, large area lms, it requires vacuum and high temperature environments for the synthesis, which increases complexity and manufacturing costs. Hence, a low cost method to produce large scale and high quality hBN needs to be explored. In this paper, a simple and cost effective method for the exfoliation of large scale functionalized sheets of hBN using an acid mixture of phosphoric + sulphuric acid is reported. The sheet size is dependent on the maximum particle size in the hBN powder used for this process (6 mm was achieved in this report). The chemically exfoliated hBN is generally 1–4 layers thick, with exfoliation efficiency as high as 25% and has high crystallinity and majority of the akes oriented in the (002) direction when
Nanoscale, 2014, 6, 11671–11675 | 11671
View Article Online
Nanoscale
deposited on a substrate. With the addition of functional groups an improved dispersion and stability of hBN exists in a variety of solvents. A new optically active defect was created at 3.9 eV because of this functionalization. This method is highly scalable and can be used for a variety of applications.
Published on 07 August 2014. Downloaded by Iowa State University on 18/04/2017 07:08:10.
Experimentation To synthesize mono- and few-layer hBN, 1 g of hBN powder (99.5%, particle size 1–5 mm, Alpha Aesar) was mixed with 6 g of potassium permanganate (KMnO4, A.C.S Reagent, J.T. Baker) in a 1 L glass beaker. An acid mixture of 135 ml was prepared separately by mixing phosphoric acid (H3PO4, 85% w/w, EMD chemicals) and sulphuric acid (H2SO4, 95%, EMD chemicals) in the 1 : 8 ratio and was added to the hBN dry mixture. The resultant reaction is slightly exothermic, leading to a temperature rise in the solution to 40 C. The solution was then heated on a hot plate to 75 C under constant mixing for 12 hours. Subsequently, 6 ml of hydrogen peroxide (H2O2, 30% w/w, A.C.S Reagent, J.T. Baker) and 120 ml deionized water (DI water) were added to this mixture to halt the oxidation. This approach was similar to the synthesis of graphene oxide by Marcono et al.,31 with conditions tuned for the hBN material system. The resultant suspension was cooled to room temperature and centrifuged at 6000 rpm for 30 minutes, and the supernatant was pipetted out from the solution. Subsequently, 45 ml DI water was added to this solution and centrifuged again at 6000 rpm for 30 minutes. The non-exfoliated material was removed during the centrifugation and the supernatant solution was then subjected to a series of washing and centrifugation stages with DI water, ethanol and HCl. These washings were performed until a pH >3 was reached, which ensures the complete removal of the metal ions. The supernatant solution is then drop-cast or spin coated on Si wafers and dried to obtain the exfoliated and functionalized hBN sheets.
Results and discussion The exfoliation process results in mono- and few-layer hBN sheets that are distributed uniformly throughout the suspension. The initial hBN powder (before exfoliation) exhibits a variety of particle clusters, with varying thicknesses of hBN platelets (Fig. 1a). The thickness of the hBN platelets can be controlled by carefully centrifuging the hBN in solvent. Various centrifugation speeds were used to obtain hBN platelets of different thicknesses. Lower centrifugation speeds (1500– 2000 rpm) result in thicker platelets (100–200 nm) while higher centrifugation speeds yield thinner akes. In order to obtain the thinnest akes, the solution was centrifuged at 6000 rpm for 30 minutes. The resultant supernatant was pipetted out and dropcast onto a substrate for further analysis. Aer separating out the exfoliated akes by centrifugation, a highly concentrated solution of exfoliated akes was obtained. From the weight balance, the suspension typically consisted of about 12–25% (see ESI S0†) of the starting hBN weight, providing a signicantly higher yield than that previously reported.23
11672 | Nanoscale, 2014, 6, 11671–11675
Communication
Following the exfoliation process, most of the sheets are transparent to the electron beam at 2 kV clearly indicating a near atomic thickness of the hBN sheets (Fig. 1b and c). Importantly, while the sheet thickness is reduced to
