Nano lett Longitudinal Splitting of Boron Nitride Nanotubes for the Facile

LETTER

https://www.sodocs.net/doc/2d2613921.html,/NanoLett

Longitudinal Splitting of Boron Nitride Nanotubes for the Facile Synthesis of High Quality Boron Nitride Nanoribbons

Kris J.Erickson,?,?,§Ashley L.Gibb,?,?,§Alexander Sinitskii,^Michael Rousseas,?,§Nasim Alem,?,§,||James M.Tour,*,^and Alex K.Zettl*,?,§

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Department of Physics and ?Department of Chemistry,University of California at Berkeley,Berkeley,California 94720,United States §

Materials Sciences Division,Lawrence Berkeley National Laboratory,Berkeley,California 94720,United States )

Center of Integrated Nanosystems,University of California at Berkeley,Berkeley,California 94720,United States ^

Department of Chemistry,Department of Mechanical Engineering and Materials Science,and The Smalley Institute for Nanoscale Science and Technology,Rice University,Houston,Texas 77005,United States

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Supporting Information nanoribbons (BNNRs),the boron nitride structural equivalent of graphene nanoribbons (GNRs),electronic and magnetic properties.We report the synthesis of BNNRs through the potassium-intercalation-induced of boron nitride nanotubes (BNNTs).This facile,scalable synthesis results in narrow S

ince the discovery of carbon nanotubes (CNTs),1the scienti ?c community has witnessed greatly expanded interest in one-and two-dimensional allotropes of carbon,including graphene 2,3and graphene nanoribbons (GNRs)4à7(Figure 1).Although graphene and GNRs share a common sp 2-bonded carbon framework,3the di ?erent boundary conditions lead to unique properties.With sp 2-bonded boron nitride,which shares structural analogs with carbon (Figure 1),a similar discovery progression can be mapped from boron nitride nanotubes (BNNTs)8to few-layer hexagonal boron nitride (h -BN)sheets 9to a recent mounting interest in boron nitride nanoribbons (BNNRs).10à19Signi ?cantly di ?erent proper-ties also exist between these BN-based materials.For example,BNNRs are theorized to possess a wide range of electronic,optical,and magnetic properties arising from various edge structures and terminations.12,13,15,17à19These properties are of great fundamen-tal interest and they also have implications for applications within various ?elds including spintronics and optoelectronics.12à14,17à19

Numerous routes to GNRs are now well established,4à7and some of these approaches,such as the plasma etching of nanotubes imbedded within a polymer,5have been explored for the syntheses of BNNRs.18However,unlike the case for GNRs,4,6,7the facile,scalable synthesis of high quality BNNRs high quality GNRs involves potassium-intercalation-induced longitudinal splitting of CNTs.7This is the approach taken here for the synthesis of BNNRs.

Figure 2shows pictorially the mechanism for splitting BNNTs through alkali metal https://www.sodocs.net/doc/2d2613921.html,ing this approach,we ?nd that potassium vapor treatment of BNNTs yields narrow (between 20and 50nm),long (at least 1μm in length),few sheet (usually between 2and 10layer)pristine BNNRs with very uniform widths as well as minimal defects within the ribbon plane and along their edges.The synthesis process is bulk,facile,and easily scalable.Approximately 1%of treated BNNTs exhibit splitting,comparable to a common bulk route to GNRs,4and separation should be possible through established GNR puri ?cation techniques.4,7Our process allows for BNNRs that can be stabilized in a solvent and then dispersed onto various substrates to allow for further characterization and potential device fabrication.20

Precursor BNNTs were synthesized through a process similar to a previously reported route.21A mixture of solid boron,magnesium oxide,and tin oxide powders were placed in a graphite crucible that

Received:

May 4,2011

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was heated under a nitrogen atmosphere in a vertical induction furnace to 1500°C.Nitrogen carrier gas brought the powder into a graphite vessel where it reacted with ammonia to form a white,spongy material rich in BNNTs.BNNTs were taken as-synthe-sized,placed into an ampule with excess freshly cut potassium metal,evacuated to 10à6Torr,and then heated at 300°C for 72h.

After removal from the ampule,the material was brought to 600°C in air for 30min to remove any carbonaceous impurities.This material was readily dispersed under mild sonication into isopro-panol and dried onto lacey carbon TEM grids for analysis.

The synthesized BNNTs were of variable diameters rang-ing from 6to 70nm (see Supporting Information Figure S1).

Figure 1.(Left)One and two-dimensional carbon nanomaterials including a CNT,graphene,and a GNR.Carbon atoms are displayed in gray.(Right)One and two-dimensional boron nitride structural analogues including a BNNT,single sheet h -BN,and a BNNR.Boron atoms are displayed in blue and nitrogen atoms are displayed in yellow.Though local bonding con ?gurations are nearly identical for the materials in each column,boundary conditions of the selected overall geometries result in unique distinct edge states and unique characteristics.For nanotubes,the circumference de ?nes a boundary condition,whereas for a large nanosheet the circumferential boundary conditions are lifted and edge states have minimal a ?ect.For nanoribbons,the width de ?nes new boundary conditions and edge states can signi ?cantly a ?ect many material properties.

Figure 2.Schematic of the splitting process of a BNNT to form a BNNR.Boron atoms are in blue,nitrogen atoms are in yellow,and potassium atoms are in pink.The pristine BNNT (left)begins to locally unzip owing to potassium intercalation induced pressure buildup (middle),which results in further splitting of the nanotube in the longitudinal direction to form few layer nanoribbons (right).

Figure3A displays a TEM micrograph of a typical BNNT precursor.The tube displays characteristic higher contrast walls with a lower contrast inner core.22The periodic darkened regions along the length of the nanotube are common for high quality BNNTs.23Di?raction of the BNNT(Figure3A inset)displays spots that are elongated perpendicularly to the tube axis,owing to the curvature of the nanotube,and demonstrates high intrawall crystallinity.23

Figure3B displays a TEM micrograph of a BNNR likely two layers thick with a pristine,linear edge and nearly defect-free surface.This ribbon scrolls up along the top and some carbonac-eous adsorbates are noticeable on the ribbon surface.Figure3C displays a TEM micrograph of a BNNR40nm in width which has been partially split o?of its parent tube.This ribbon also appears to be two layers thick and scrolls up when near the parent tube, straightening out further from the tube.This ribbon also has minimal defects within the plane and along its edge,but the inset of Figure3C highlights a singular defect found along the edge. Triangular defects like this are commonly observed when boron nitride materials are subjected to electron beam irradiation and normally a defect of this geometry is zigzag along its edge.24 Assuming this orientation,the general ribbon edge here would be zigzag.This is likely since BNNTs often form with all their walls being mostly of armchair or mostly of zigzag orientation,22 yielding ribbons with edges de?ned by the tube chirality.The periodic contrast seen along the observed edge in the micro-graphs of Figure3should not be mistaken for the ribbon edge where bond termination occurs.It instead represents a fold where the edge of the ribbon folds back onto the plane of the ribbon.25Such edge folding or partial scrolling is common in graphitic materials,25,26including GNRs,7and it is also observed for boron nitride materials.18However,the linear nature of the observed edge likely indicates a similarly straight edge where bond termination does occur.

Brief sonication in isopropanol separates many ribbons from their parent tubes,and a lone ribbon of about5layers is seen in Figure3D.Di?raction of the BNNR(Figure3D inset)is markedly di?erent from the di?raction of a BNNT.Speci?cally, the curvature-induced elongation of di?raction spots,seen for nanotubes,is absent for the ribbon,indicating its?at geometry. This di?raction pattern further indicates that the highly crystal-line nature of the parent BNNTs is conserved during BNNR formation.It also reveals the edge orientation to be approxi-mately armchair in this ribbon.Although BNNRs,similar to the parent BNNTs,22are expected to often have successive layers of a similar orientation,our di?raction studies also show a second set of spots resulting from either another preferred orientation of some layers or folding of the ribbon along the edge.

Figure4shows more BNNRs that are well-separated from their parent tubes,one being at least1μm in length.Ribbons tend to drape along the lacey carbon and BNNTs making the full length of the ribbons di?cult to determine.The?exibility of the BNNRs is evident as a ribbon changes slightly in direction while draping along the lacey carbon support in Figure4A.Figure4B

Figure3.TEM micrographs of the following:(A)Common BNNT precursor.Scale bar is50nm.Inset is di?raction of the BNNT.Di?raction peaks elongate perpendicularly to the tube axis,indicating a cylindrical geometry.(B)A few layer BNNR that has a pristine,straight edge along the bottom and is scrolled at the top.Carbonaceous adsorbates are noticeable on the ribbon surface.Scale bar is10nm.(C)Another few layer BNNR that has unzipped o?its parent tube on the right.Arrow marks the zoomed in region found in the inset,which highlights a singular triangular defect found along the ribbon edge.Scale bars are10nm.(D)A lone BNNR about5layers thick.Scale bar is10nm.Inset shows di?raction of BNNR which lacks elongation of di?raction peaks indicating a?at geometry.

shows another ribbon with a full twist(labeled by the middle arrow).Such geometries are not possible for rigid BNNTs. Ribbons are seen to have consistent widths along their entire lengths,only changing in apparent width when accommodating geometries like the twist seen in Figure4B.

Figure5displays details of a ribbon in the process of splitting o?its parent tube,clearly displaying how a few outer walls peel o?of the nanotube to form the nanoribbon,which then separates away from the parent tube.Again,straight edges and a consistent width are observed for this ribbon.Although the intermediate stage of splitting is highlighted here,most BNNRs are seen to be fully separated from their parent BNNT following the gentle sonication used to disperse them in solution.

Potassium intercalation between walls of a nanotube is crucial for the splitting process with the previously reported GNRs synthesis7and is considered essential in our case of BNNR formation.However,unlike the case of graphitic materials where there exists a large body of research on intercalation com-pounds,27studies on the intercalation of h-BN are limited.28,29 Alkali metals are among the few experimentally reported inter-calants of h-BN,28,30and the equilibrium interplanar spacing for potassium intercalated h-BN has been calculated to be5.8?.31 For a nanotube,this signi?cant increase in wall spacing from 3.4?22would result in signi?cant bond strain circumferentially around the tube.The mechanism of splitting proposed to occur during potassium intercalation of CNTs likely also occurs for BNNTs,whereupon intercalated potassium islands grow from an initial starting point of intercalation.7This island growth con-tinues until enough circumferential strain results in bond break-age.Finally,potassium bonds to the bare ribbon edge and the hindrance arising from these moieties induces further splitting longitudinally.7If the potassium intercalation-induced splitting of BNNTs occurred randomly within the tube,the edges of the resultant ribbons would be expected to be only moderately straight.The extremely linear,high quality edges observed for our BNNRs indicate that some further order to the splitting process occurs.

To understand the mechanism of splitting,a further look at the bonding structure of BNNTs is instructive.A signi?cantly larger interplanar interaction exists between walls of a BNNT com-pared to CNT walls as polar boronànitrogen in-plane bonds result in AA0stacking between planes to allow heteroatoms from adjacent walls to align.22This often results in all walls of a BNNT being of nearly the same chirality,unlike the random chiralities found in CNTs,which maximizes the overlap between boron and nitrogen atoms in adjacent walls.22However,the commensurate stacking cannot continue around the full circumference of a tube given the changing diameter of concentric tube walls.A thorough TEM di?raction study23revealed that this geometric constraint often results in tubes that are formed in a double helical fashion. One helix consists of walls which are AA0stacked and?at with facets along the helix.A second helix completes the tube which has consecutive walls that are mismatched and not AA0stacked. The crystalline helix causes characteristic di?raction peaks of a faceted,polygonal structure whereas the lower crystallinity helix causes di?raction peaks characteristic of a cylindrical structure. Finally,within the highly crystalline faceted helix,lines of strained sp3bonds connect each facet,on the order of six facets around

Figure4.TEM micrographs of the following:(A)A long BNNR,having a consistent width along the length of the ribbon and straight edges.It spans across the image draping on top of lacey carbon to the left and over an unzipped BNNT to the right.Three arrows point to the ribbon to aid identi?cation.(B)A BNNR over1μm in length,pointed to by the arrows.The middle arrow points to a twist in the BNNR.Scale bars in(A)and(B)are 300nm.

Figure5.TEM micrographs of the following:(A)A few layer BNNR

splitting o?its parent ribbon.The arrow marks the zoomed in region

found in(B).Scale bar is50nm.(B)Zoomed-in region where the few

layer nanoribbon is seen coming o?its parent nanotube.Scale bar is5nm.

Nano Letters

LETTER

the tube,with these lines running parallel to the longitudinal axis of the tube.23

We propose that due to the observed highly linear edges of our BNNRs,potassium likely preferentially intercalates near these higher energy lines of sp 3bonds connecting facets and eventually begins splitting along these lines.Splitting then propagates down the tube following these lines of weaker bonds,also inducing splitting through the mismatched structure regions.This splitting along prede ?ned lines within the BNNTs explains the high quality,linear edges that are seen on the BNNRs.Also,boron atoms along these lines would be highly acidic with enhanced reactivity toward electropositive potassium,allowing for easier splitting along the lines.Furthermore,a larger interplanar spacing exists along these lines of sp 3bonds likely allowing for more facile intercalation into these regions.Finally,intercalation likely facilitates exfoliation of unzipped walls as it disturbs the inter-planar interaction between the BNNT walls.Splitting through this process should not introduce defects within the ribbon plane,as we have observed with our BNNRs.As with most nanomater-ials,each particular BNNT varies signi ?cantly from the next in terms of overall structure and geometry.Therefore,although this mechanism is proposed,it is understood that the splitting to form each ribbon is highly dependent upon the particular character-istics of the parent tube and much more complex mechanics may occur to allow for each individual ribbon to form.

Since BNNTs will often form in either zigzag or armchair orientation,22the longitudinal splitting of BNNTs seen in this study may result in a high proportion of ribbons with zigzag and armchair edges (Figure 3)as opposed to other edge orientations.This is especially exciting given the signi ?cant body of theoretical work done on BNNRs requiring edges with minimal defects and usually of zigzag orientation.12,13,15à17,19Predicted properties for BNNRs include metallic or semiconducting electronic states,which can be magnetically polarized,10,12,13,15,17,19as well as edges displaying ferromagnetism or antiferromagnetism,13,15,17,19all of which are dependent upon edge geometry and termination.Given this signi ?cant dependence upon BNNR edges for imbu-ing particular electronic and magnetic properties,the high like-lihood of synthesizing ribbons with zigzag edges may make them particularly suitable for addressing theoretical predictions and realizing proposed applications.It is also worthwhile to consider the possibility of functionalizing the edges of these BNNRs.GNR edges synthesized via this route are potassium terminated during synthesis and hydrogen terminated upon exposure to water or ethanol.7BNNRs synthesized through this route could be similarly terminated,and many of the predicted BNNR proper-ties necessitate hydrogen-terminated edges.13,15à17,19Further-more,the reactive potassium-terminated edge could easily be replaced with species other than hydrogen.Di ?erent chemicals could be used for quenching to impart other terminations.In addition,hydrogen could be replaced after quenching by either utilizing established boron nitride functionalization routes 32,33or by devising new routes unique to the highly reactive nanoribbon edge.

’ASSOCIATED CONTENT

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Supporting Information.

Micrograph of BNNTs,experi-mental details,and information regarding model generation.This material is available free of charge via the Internet at https://www.sodocs.net/doc/2d2613921.html,.

’AUTHOR INFORMATION

Corresponding Author

*E-mail:(A.K.Z.)azettl@https://www.sodocs.net/doc/2d2613921.html,;(J.M.T.)tour@https://www.sodocs.net/doc/2d2613921.html,.

’ACKNOWLEDGMENT

This work was supported in part by the Director,O ?ce of Science,O ?ce of Basic Energy Sciences,Division of Materials Sciences and Engineering,of the U.S.Department of Energy under Contract No.DE-AC02-05CH11231which provided for BNNT synthesis and preliminary intercalation via the sp 2and Hydrogen programs.The Center of Integrated Nanomechanical Systems (COINS)with NSF Grant EEC-0425914provided support for TEM imaging.The O ?ce of Naval Research MURI Graphene Program provided support for sample characterization and modeling.A.S.and J.M.T.further acknowledge the support by the Air Force Research Laboratory through University Technology Corporation,09-S568-064-01-C1,and the Air Force O ?ce of Scienti ?c Research FA9550-09-1-0581.’REFERENCES

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Supporting Information for:

Longitudinal Splitting of Boron Nitride

Nanotubes for the Facile Synthesis of High Quality Boron Nitride Nanoribbons

Kris J. Erickson, Ashley L. Gibb, Alexander Sinitskii, Michael Rousseas, Nasim Alem,

James M. Tour, Alex K. Zettl

This includes,

Figure S1

Materials, Methods, and Comments

Modeling Information

Figure S1. TEM micrograph of precursor BNNTs. Scale bar is 200 nm. Catalyst particles are seen in the lower left. Very straight BNNTs are formed with

diameters ranging from 6 to 70 nm with larger diameter tubes being up to tens of microns in length and smaller diameter tubes being up to microns in length.

Materials, Methods and Comments

BNNTs from different synthetic methods afforded themselves differently to potassium vapor treatment. Some BNNTs are noticed to have closed ends, and splitting was more difficult with these tubes presumably as a closed end would hinder potassium

intercalation between tube walls. If these tubes were subjected to aggressive sonication,

ends could be opened by effectively ripping apart tubes to form smaller tubes, but significant damage to nanotube walls was also caused by this process, and splitting of this material was likewise difficult. BNNTs synthesized through the vertical induction furnace method using metal oxides (specifically Mg and Sn) as catalysts with solid boron and ammonia as B and N sources, respectively, were ideal for usage as very high crystallinity, predominantly open ended BNNTs are formed.

The BNNT material (3 mg) was then taken as synthesized, and placed into the bottom of a ? inch quartz tube sealed on one end. A notch in the tube was made to separate the BNNTs from the potassium metal (30 mg), which was freshly cut in a fume hood, rinsed with dry diethyl ether to clean off mineral oil, and then placed into the quartz tube. (CAUTION: Potassium and sealing of the potassium-loaded ampoule should be handled with utmost care, due to the highly reactive nature of potassium metal. Users should wear safety glasses and a face shield, and all operations should be done in a fume hood when handling this reagent) The quartz tube was then taken to 10-6 Torr by evacuating with a turbo pump and the quartz tube was sealed with a H2/O2 torch. The created ampoule was then placed in an oven at 300 °C for 72 hours. Higher temperatures resulted in significant reduction of the quartz and sometimes resulted in broken ampoules. Lower temperatures resulted in lower yields of BNNRs. Residual carbonaceous materials, presumably from remaining mineral oil and from atmospheric adsorbates, graphitized on the surface of the BN material during the treatment making it grayish. This residual carbon was therefore calcined off at 600 °C in air for 30 minutes such that only BN material remained. Both the BNNT and BNNR material could be readily dispersed in isopropanol under mild sonication and then dried onto any surface. For our purposes, the material was dispersed in IPA and spotted onto Au mesh lacey carbon TEM grids and allowed to dry for further analysis. TEM analysis was done on a JEOL 2010 microscope operated at 100 kV for all images excepted for Figure 3 (B) and (C) at which it was operated at 80 kV.

Modeling Information

The images found in Figures 1 and 2 were made using Accelrys Materials Studio 4.3

Nano Letters

LETTER

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