Size- and Shape-Controlled Synthesis of Bismuth Nanoparticles

Size- and Shape-Controlled Synthesis of Bismuth Nanoparticles

Fudong Wang,§,‡ Rui Tang,§,‡ Heng Yu,§,† Patrick C. Gibbons,#,‡ and William E. Buhro*,§,‡

Department of Chemistry and Physics and Center for Materials InnoVation, Washington UniVersity,Saint Louis, Missouri 63130-4899

ReceiVed February 13, 2008. ReVised Manuscript ReceiVed April 2, 2008

Near-monodisperse Bi dots in the diameter range of 3-115 nm are synthesized by a simple, solution-based one-step approach by varying the amounts of Bi[N(SiMe3)2]3, Na[N(SiMe3)2], and a polymersurfactant, poly(1-hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33, employed. The reaction conditions arefurther modified to produce Bi nanorods and nanoplates. Alternatively, near-monodisperse Bi dots in thediameter range of 30-45 nm are synthesized by a secondary-addition technique. With a slight modificationof this technique, nanoribbons are obtained. The roles of polymer and Na[N(SiMe3)2] in the size andshape control of these Bi nanoparticles are discussed.

Introduction

Metal nanoparticles are of great interest due to their uniquesize- and shape-dependent optical,1–4 magnetic,5–8 andcatalyticproperties,9–11andpotentialapplicationsinbiosensing,12,13

information storage,5,8 catalysis,9–11 and surface-enhancedRaman scattering (SERS).14,15 Despite the extensive researchactivities and enormous progress in this field, the mainchallenges have been and remain size and morphologycontrol, and the lack of sufficient mechanistic understandingto achieve this control.10,16–18 Here we describe syntheticprocedures for preparation of size-controlled, spherical Bi

nanoparticles (dots), and strategies for generating rod-, plate-,and ribbon-shaped Bi nanoparticles. This study may alsoprovide a basis for gaining mechanistic insights into size andshape control of metal nanoparticles.

We chose to study the Bi system for two reasons. Firstand most importantly, Bi dots are the best catalysts for thesolution-liquid-solid (SLS) growth of diameter-controlledsemiconductor quantum wires and rods.19–24 We now usesuch Bi nanoparticles almost exclusively for the SLS growthof semiconductor nanowires and expect that they will begenerally useful to others. The motivation for this study,therefore, is to provide a diameter-controlled synthesis ofnear-monodispersed Bi dots over a wide diameter range, forSLS growth of near-monodispersed (in diameter) semicon-ductor nanowires over a similarly wide diameter range.

Others are also interested in the electronic properties ofBi nanoparticles. Bulk Bi is a semimetal with unusualelectronic properties (i.e., magnetoresistance, thermoelec-tronic characteristics) due to its highly anisotropic Fermisurface, low carrier densities (105 times smaller thanconventional metals at 4.2 K), small carrier effective masses,and long carrier mean free path (as long as a millimeter at4.2 K).25–28 The electronic properties of Bi are highlysusceptible to size-induced quantum confinement effects. For

* To whom correspondence should be addressed. E-mail: [email protected].§ Department of Chemistry, Washington University.‡ Center for Materials Innovation, Washington University.† Current address: MagArray, Inc., 450 El Escarpado, Stanford, CA 94305-

8431.# Department of Physics, Washington University.

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3656 Chem. Mater. 2008, 20, 3656–3662

10.1021/cm8004425 CCC: $40.75 2008 American Chemical SocietyPublished on Web 05/08/2008

example, Bi undergoes a semimetal to semiconductor transi-tion in nanowires when the wire diameter is decreased toabout 50 nm.29 Theoretical studies suggest that Bi nanowiresmay exhibit an enhanced thermoelectric figure of merit, ZT,at 300 K.28 An even larger thermoelectric effect might beachieved under dimensionally more-restricted conditions suchas in Bi dots and rods.28,30,31

Xia and co-workers have reported the synthesis of mono-disperse Bi dots in the diameter range of 100-600 nm,32

which, however, is not in the generally useful range for thegrowth of semiconductor quantum wires and rods.19–24 Foosand co-workers reported the synthesis of Bi dots withdiameters in the range of 3-10 nm but did not achievemonodispersity.33 In a related study, Wang, Ren, and co-workers prepared Bi dots, nanocubes, nanoplates, andnanobelts (nanoribbons) but did not address size control.34

We previously reported the preparation of near-monodis-persed Bi dots using a seeded-growth method, in which verysmall (d ≈ 1.5 nm) Au nanoclusters served as heterogeneousnucleants for nanoparticle growth.35 This method, however,was somewhat laborious and provided a limited size range(8.5 nm < d < 12.5 nm). We subsequently discovered aconvenient one-step synthesis of near-monodisperse Bi dotsby using the thermal decomposition of Bi[N(SiMe3)2]3 in thepresence of Na[N(SiMe3)2] but did not develop diametercontrol for a wide range of diameters.36 In the present study,we have modified this one-step method to afford Bi dots inthe diameter range of 3-115 nm, with standard deviationsin the diameter distributions of 4-19% of the nanoparticlemean diameters. The Bi dots can be made on a large scaleand stored for at least a few years under an inert atmospherefor use as needed.

The diameters and diameter distributions of the Bi dotsare highly dependent on the relative quantities of Bi[N-(SiMe3)2]3, Na[N(SiMe3)2], and a polymer stabilizer em-ployed. Under certain conditions, nanoribbons, nanorods, andhexagonal nanoplates are also generated. The possible rolesof Na[N(SiMe3)2] and the polymer stabilizer in the size andmorphology control are discussed. We expect that suchconvenient, optimized syntheses of Bi dots will benefitresearch in the field of semiconductor quantum wires andmay provide opportunities for studying the size- and shape-dependent electronic properties of Bi nanostructures.

Experimental Section

Materials. Bi[N(SiMe3)2]3 was prepared according to a literaturemethod.37 Na[N(SiMe3)2] (as a 1.0 M THF solution) was obtained

from Aldrich packaged under N2 in Sure/Seal bottles. Poly(1-hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33 (also known as poly(1-vinylpyrrolidone)-graft-(1-hexadecene) by Aldrich) was used asreceived from Aldrich. The solvent 1,3-diisopropylbenzene (DIPB)was purchased from Aldrich, shaken with concentrated sulfuric acidto remove thiophene, neutralized with K2CO3, washed with water,and distilled over Na.36 A 25 wt % poly(1-hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33 solution in DIPB (polymer-DIPB solution)was dried over molecular sieves at least one week with frequentshaking prior to use.36 Polymer-DIPB solutions with lowerconcentrations were prepared by diluting the 25 wt % stock solutionwith DIPB. Other reagents were used as received.

Synthesis of Bi Dots. All synthetic steps were conducted underdry, O2-free N2(g). The reaction conditions (i.e., quantities ofreagents, reaction temperature, and reaction time) for a variety ofBi dot diameters are recorded in Table 1. In a typical synthesisof 7.1 nm diameter (standard deviation ) (11%) Bi dots,Bi[N(SiMe3)2]3 (303 mg, 0.44 mmol) and Na[N(SiMe3)2] (1608mg of 1.0 THF solution, 1.78 mmol) were combined with the 25wt % polymer-DIPB solution (10 g) in a Schlenk reaction tube togenerate a pale-red solution. This solution was then inserted into atemperature-controlled oil bath at 180 °C with stirring, whereuponthe solution turned red then black very quickly (∼1-2 min). Thefinal solution (after 17 h) contained a deep black dispersion of Bidots.

The Bi dots were stored without isolation in the synthesis mixtureunder inert atmosphere and remained stable for a few years. Forthe subsequent use in the synthesis of semiconductor quantum wiresand rods, the Bi dots were typically not isolated from the reactionmixture. Rather, the mixture was used as a stock solution. The Bidots could precipitate from the solution but were readily redispersedupon gentle shaking.

Synthesis of Bi Nanorods and Nanoplates. The nanorods andnanoplates were synthesized using the same procedure as above.The quantities of reagents and reaction conditions used are recordedin Table S1 (Supporting Information).

Synthesis of Bi Dots Using the Secondary-Addition Tech-nique. A mixture containing Bi[N(SiMe3)2]3 (105 mg, 0.15 mmol),Na[N(SiMe3)2] (1023 mg of 1.0 M THF solution, 1.13 mmol), and25 wt % polymer solution (10 g) was preheated at 210 °C for 2 hbefore the dropwise addition of a mixture of Bi[N(SiMe3)2]3 (200mg, 0.29 mmol) and the 25 wt % polymer solution (2 g) from asyringe. The addition was finished in 1 h. Stirring at 210 °C wascontinued for an additional 15 h, resulting in the 32.1 nm diameter(standard deviation ) (6%) Bi dots.

Similarly, for the synthesis of the 44.2 nm diameter (standarddeviation ) (6%) Bi dots, a mixture containing Bi[N(SiMe3)2]3

(110 mg, 0.16 mmol), Na[N(SiMe3)2] (205 mg of 1.0 M THFsolution, 0.23 mmol), and 25 wt % polymer solution (8 g) waspreheated at 210 °C for 1 h before the dropwise addition of amixture of Bi[N(SiMe3)2]3 (200 mg, 0.29 mmol), and the 25 wt %polymer solution (4 g) from a syringe. The addition was finishedin 2.3 h. Stirring at 210 °C was continued for an additional 16 h.

Synthesis of Bi Nanoribbons. A mixture containing Bi[N-(SiMe3)2]3 (16 mg, 0.023 mmol), Na[N(SiMe3)2] (203 mg of 1.0M THF solution, 0.22 mmol) and 12.5 wt % polymer solution (8g) was preheated at 210 °C for 35 min before the dropwise additionof a mixture of Bi[N(SiMe3)2]3 (336 mg, 0.49 mmol) and the 12.5wt % polymer solution (4 g) from a syringe. The addition wasfinished in 2.2 h. Stirring at 210 °C was continued for an additional17 h, resulting in a grayish black suspension.

(29) Dresselhaus, M. S.; Lin, Y.-M.; Rabib, O.; Jorio, A.; Souza Filho,A. G.; Pimenta, M. A.; Saito, R.; Samsonidze, G. G.; Dresselhaus, G.Mater. Sci. Eng., C 2003, 23, 129–140.

(30) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 16631–16634.(31) Heremans, J. S.; Thrush, C. M.; Morelli, D. T.; Wu, M.-C. Phys. ReV.

Lett. 2002, 88, 216801.(32) Wang, Y.; Xia, Y. Nano Lett. 2004, 4, 2047–2050.(33) Foos, E. E.; Stroud, R. M.; Berry, A. D.; Snow, A. W.; Armistead,

J. P. J. Am. Chem. Soc. 2000, 122, 7114–7115.(34) Wang, W. Z.; Poudel, B.; Ma, Y.; Ren, Z. F. J. Phys. Chem. B 2006,

110, 25702–25706.(35) Yu, H.; Gibbons, P. C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem.

Soc. 2001, 123, 9198–9199.(36) Yu, H.; Gibbons, P. C.; Buhro, W. E. J. Mater. Chem. 2004, 14, 595–

602.(37) Carmalt, C. J.; Compton, N. A.; Errington, N. J.; Fisher, G. A.;

Moenandar, I.; Norman, N. C. Inorg. Synth. 1996, 31, 98–101.

3657Chem. Mater., Vol. 20, No. 11, 2008Bi Nanoparticles

TEM Analysis. The isolation and purification steps wereconducted in the ambient atmosphere. The black Bi nanoparticles(dots, rods, plates, or ribbons) were isolated from the reactionmixture (0.2-0.5 mL) by adding toluene (ca. 1 mL) and methanol(ca. 3 mL) to the mixture, followed by centrifugation of the mixtureand decanting of the supernatant. The precipitate was purified byredispersion in a mixture of toluene (ca. 1 mL) and methanol (ca.3 mL) upon sonication in a cleaning bath, followed by centrifugationand decanting the supernatant. After purification the precipitatecould be redispersed in pure toluene to form uniform blackdispersions for TEM analysis.

Carbon-coated copper grids were dipped in the toluene solutionand then immediately taken out in air to evaporate the solvent. TheTEM images were obtained using a JEOL 2000 FX microscopeoperated at 200 kV and a magnification of 500K × . The particlesizes were measured using the Image Pro Express (version 4.5)software at the 2× zoom. Generally, 300-500 particles were usedin the statistical analysis for each sample.

Selected-area electron-diffraction (SAED) patterns were takenwith a camera length of 120 cm.

X-ray Powder Diffraction (XRD). XRD patterns were obtainedusing a Rigaku Dmax A vertical powder diffractometer with CuKR radiation (� ) 1.5418 Å) and Materials Data Incorporated(MDI) automation and Jade software.

Results

Synthesis of Bi Dots (d ) 3-30 nm). The Bi dots in thediameter range of 3-30 nm are the most useful for growingsemiconductor quantum wires and rods in the quantum-confinement regime.19–24 Bi dots in this range were synthe-sized by the thermal decomposition of Bi[N(SiMe3)2]3 in the25 wt % polymer-DIPB solution. The syntheses wereconducted by adding Na[N(SiMe3)2] in varying amounts toa constant amount of Bi[N(SiMe3)2]3 to tune the Bi nano-particle sizes. Representative TEM images are shown inFigure 1. The standard deviations in the Bi dot diameterdistributions were 4-18% of the dot mean diameters andthus approached near monodispersity (see Figure S1 forrepresentative diameter-distribution histograms). The electron-diffraction and XRD patterns (Figure S2) established therhombohedral crystal structure of the Bi dots, with theexception of smaller Bi dots (∼4-10 nm) which exhibitedthe cubic crystal structure. The transition from the cubic torhombohedral structure occurring within the range of 10-12nm may be a surface-pressure phenomenon. Cubic Bi is ahigh-pressure phase,38 and the internal pressure exerted bya nanocrystal surface is known to increase with decreasingnanocrystal size.39,40

As noted above, varying amounts of Na[N(SiMe3)2](Figure 2) were added to Bi[N(SiMe3)2]3 to adjust theprecursor ratio. The Bi dot mean diameter decreased withincreasing amount of Na[N(SiMe3)2] or increasing Na[N-(SiMe3)2]/Bi[N(SiMe3)2]3 (molar) ratio, to a limiting size of3-4 nm. This size minimum was achieved at a Na[N-(SiMe3)2]/Bi[N(SiMe3)2]3 ratio of ∼ 6 under the reactionconditions employed. The dashed curve (Figure 2) showsthat the Bi dot sizes were also dependent on the initialconcentration of Bi[N(SiMe3)2]3.

(38) Schaufelberger, P.; Merx, H.; Contre, M. High Temp. High Press.1973, 5, 221–230.

(39) Baldinozzi, G.; Simeone, D.; Gosset, D.; Dutheil, M. Phys. ReV. Lett.2003, 90, 216103.

(40) Magomedov, M.N. Tech. Phys. Lett. 2005, 31, 13–17.

Figure 1. Representative TEM images of 3-30 nm diameter Bi dots.Diameter ((one standard deviation in the diameter distribution expressedas a percentage of the mean diameter) ) (a) 4.7 nm ((15%); (b) 7.1 nm((11%); (c) 9.4 nm ((8%); (d) 15.1 nm ((6%); (e) 20.4 nm ((5%); (f)25.2 nm ((5%).

Figure 2. Bi nanoparticle diameter as a function of Na[N(SiMe3)2]/Bi[N(SiMe3)2]3 molar ratio at various temperatures. The solid lines refer toexperiments using 0.44 mmol of Bi[N(SiMe3)2]3 and the dashed line toone using 0.15 mmol of Bi[N(SiMe3)2]3. Additional ∼1-3 nm diameterBi dots were formed at the low Na[N(SiMe3)2]/Bi[N(SiMe3)2]3 ratios (<2)at 160 °C (not shown).

3658 Chem. Mater., Vol. 20, No. 11, 2008 Wang et al.

The Bi dot diameters were not significantly influenced bythe reaction temperature, in the range of 160-210 °C.However, at the lowest growth temperature of 160 °C andlowest Na[N(SiMe3)2]/Bi[N(SiMe3)2]3 ratios (<2; points notshown in Figure 2), the average particle size was smallerthan that obtained at higher temperatures, due to thecoexistence of small Bi dots (1-3 nm). Such low-temper-ature reaction conditions should be avoided for the synthesisof mondispersed Bi dots.

Synthesis of Bi Dots (d ) 30-115 nm). As shown above,varying the Na[N(SiMe3)2]/Bi[N(SiMe3)2]3 ratio at a fixedpolymer concentration allowed diameter control in the rangeof 3-30 nm. By reducing the polymer concentration tovarious degrees, Bi dots having diameters of up to 115 nmwere produced. Other Bi nanoparticle morphologies werealso obtained at decreased polymer concentrations.

The Bi dot diameter was increased from 20.4 to 39.7 nm(compare Figure 1e and Figure 3a) by lowering the polymerconcentration from 25 to 12.5 wt %, while holding theamounts of Na[N(SiMe3)2] (0.22 mmol) and Bi[N(SiMe3)2]3

(0.44 mmol) constant. As the polymer concentration wasfurther lowered to 2.5 wt % and the amount of Na[N-(SiMe3)2] was reduced to 0.017 mmol, the dot diameterincreased to 54.5 nm (Figure 3b). The dot diameter reached69.8 nm when no Na[N(SiMe3)2] was added (2.5 wt %

polymer solution). Under these conditions, the Bi dotdiameter was varied from 68.8-115 nm by varying theamount of Bi[N(SiMe3)2]3 employed (Figure 3c-f). Thediameter distributions also varied significantly and were quitenarrow when the larger amounts of Bi[N(SiMe3)2]3 (g0.6mmol) were used.

By lowering the polymer concentrations even further to0.25 wt % (in the presence of 0.057 mmol Na[N(SiMe3)2]),the dot diameters could be increased to as large as 173 nm,but good morphology control was not retained (Figure S3).Larger crystallites of various morphologies were producedalong with the Bi dots. In the absence of polymer, bulk Biwas generated,36 indicating that a threshold concentrationof polymer is essential for the controlled growth of Bi dots.

Alternatively, large Bi dots (diameter >30 nm) weresynthesized by secondary addition of Bi[N(SiMe3)2]3, in aslow dropwise manner, to a solution containing a dispersionof smaller, preformed Bi dots. However, the timing of thesecondary addition of Bi[N(SiMe3)2]3 was required to bewhile the smaller, seed dots were in a growth regime.Otherwise, polydispersed dots and/or other morphologiesresulted (Figure S4). Secondary addition of Bi[N(SiMe3)2]3

while the seed dots were in the early, nucleation stagebroadened the diameter distributions and in some casesgenerated nanoribbons. Secondary addition of Bi[N(SiMe3)2]3

after the seed dots finished growing broadened the diameterdistributions. The synthesis of near-monodisperse 32.1 and44.2 nm diameter Bi dots by secondary addition is shownin Figure 4; however, this method was found to be unreliablycontrolled and thus less synthetically useful than the one-step, large-dot syntheses above.

Synthesis of Bi Nanorods, Hexagonal Nanoplates,and Nanoribbons. Rod-like and hexagonal-plate shapeswere synthesized in 2.5-5.0 wt % polymer solutions in thepresence of a small amount of Na[N(SiMe3)2] (0.06-0.22mmol) (Figure 5). The rods were truncated hexagonal plates,having truncation at two opposing plate edges. As the amountof Na[N(SiMe3)2] was decreased from 0.22 to 0.06 mmol,the rods changed sizes from ∼20 × ∼50 nm (diameter ×length) to ∼50 × ∼80 nm, and the hexagonal nanoplateschanged in edge-to-edge lateral dimension from ∼50 nm to∼100 nm. The percentage of plates increased from 40% to60% and that of rods decreased from 30 to 15%. Theremainder of the sample consisted of dots (about 30%). Thepercentage of dots increased to ∼80% with a significant

Figure 3. Representative TEM images of 40-115 nm diameter Bi dots.Diameter ((one standard deviation in the diameter distribution expressedas a percentage of the mean diameter) ) (a) 39.7 nm ((6%); (b) 54.5 nm((5%); (c) 68.8 nm ((7%); (d) 78.2 nm ((4%); (e) 91.2 nm ((7%); and(f) 114.7 nm ((9%).

Figure 4. Representative TEM images of Bi dots prepared by secondaryaddition (see text). Diameter ((1 standard deviation in the diameterdistribution expressed as a percentage of the mean diameter) ) (a) 32.1nm ((6%); (b) 44.2 nm ((6%).


decrease in rods and plates when larger amounts of Bi[N-(SiMe3)2]3 were used without changing other conditions.Larger amounts of Na[N(SiMe3)2] (i.e., 1.11 mmol) generatedpolydisperse dots that tended to aggregate (Figure S5). Theaggregation tendency presumably reflected reduced stericstabilization as a result of the decreased amount of polymerfor surface passivation. In the presence of a smaller amountof Na[N(SiMe3)2] (i.e., 0.017 mmol), Bi dots were formed(see above and Figure 3b).

The representative XRD pattern (Figure S6) revealed therhombohedral crystal structure of these rod- and plate-shapedparticles. The SAED patterns (Figure 6a and b) establishedthat the hexagonal plates were oriented with their plate facesperpendicular to the c-axis (the [001] direction) of the crystalstructure. The rod faces (parallel to the field of view) weresimilarly [001] oriented, establishing that the rods are notmerely plates with their hexagonal faces rotated 90° fromthe field of view. The long axis of the rods was oriented inthe [110] direction.

The Bi nanoribbons (Figure 7) were synthesized bymodifying the secondary-addition technique mentioned above.The initial mixture contained a small amount of Bi[N-(SiMe3)2]3 (0.023 mmol) and Na[N(SiMe3)2] (0.22 mmol),

in comparison to the relatively large amount of Bi[N-(SiMe3)2]3 (0.16 mmol) and Na[N(SiMe3)2] (0.30 mmol) usedin the synthesis of the 44.2 nm diameter dots. In addition, alower-concentration polymer solution (12.5 wt %, insteadof 25 wt %) was employed to increase the yield ofnanoribbons by decreasing the yield of Bi dots. This mixturewas preheated in the reaction temperature for a short period(0.5 h) before the dropwise addition of a secondary mixtureof Bi[N(SiMe3)2]3 (0.49 mmol), Na[N(SiMe3)2] (0.37 mmol),and the 12.5 wt % polymer solution (4 g).

The nanoribbons fell into two categories: thin nanoribbonshaving widths in the range of ∼60-200 nm, and thicknanoribbons having width × thickness dimensions of lessthan ∼50 × ∼50 nm. The thin ribbons were twistable (Figure7b) and occasionally had steps on their edges (Figure 7c).The thick ribbons had triangular, hexagonal, or rectangularcross-sections (Figure 7d-f). These ribbons were generallylonger than 10 µm. The crystalline ribbons exhibited therhombohedral crystal structure and grew in the [110]direction (Figure 6c,d).

Figure 5. Representative TEM images of Bi nanorods and hexagonalnanoplates synthesized using 0.44 mmol of Bi[N(SiMe3)2]3 in a 2.5 wt %polymer solution in the presence of small amount of Na[N(SiMe3)2]: (a)0.22 mmol, (b) 0.11 mmol, and (c) 0.06 mmol.

Figure 6. Electron diffraction patterns of a Bi hexagonal nanoplate (a),nanorod (b), nanoribbon (c) viewed in the [001] zone, and nanoribbon (d)in the [221] zone. The observed diffraction spots were indexed, establishingthat the plate plane was perpendicular to the [001] direction, and the rodand ribbon long axes were oriented in the [110] direction.

Figure 7. Representative TEM images of Bi nanoribbons, synthesized bysecondary addition of Bi[N(SiMe3)2]3 into a preheated mixture containingsmall amounts of Bi[N(SiMe3)2]3 and Na[N(SiMe3)2], and a low-concentra-tion polymer solution (see text). Thin ribbons having rectangular cross-sections (b) or steps (c) are indicated by pink and green arrows, respectively.Insets (d-f) show the shapes of the cross-sections for thick ribbons asindicated by the red or blue arrows; in some cases the inset images wereobtained after sample tilting, as indicated. The scale bars are 200 nm forimages b-f.


Discussion

As clearly revealed by the results, the three importantsynthetic parameters in this work were the amounts ofthe Bi[N(SiMe3)2]3 and Na[N(SiMe3)2], and the concentra-tion of the polymer (poly(1-hexadecene)0.67-co-(1-vi-nylpyrrolidinone)0.33) employed. The influence of theseparameters is plotted in the synthesis-space diagram inFigure 8. This three-dimensional graph falls short of acomplete morphological phase diagram, because much ofthe synthesis space was not explored. The graph is useful,however, for summarizing and analyzing the experimentsperformed.

The top surface of the synthesis space (at high polymerconcentration) contains a large area where near-monodis-persed dots formed. Within this area, the size of the dots isdependent on the amount of Na[N(SiMe3)2] used, as indicatedby the blue arrow in Figure 8. The blue line connecting aseries of points on the top surface identifies the syntheticconditions we recommend for the growth of dots in the sizerange of 3-30 nm.

Two orange triangles are also plotted on the top surfaceof Figure 8, representing the formation of wire-dot mixturesat high polymer concentrations and low Na[N(SiMe3)2]/Bi[N(SiMe3)2]3 ratios. We suspect there may be a wire-formation area or volume lying along the upper-front-rightedge of the synthesis space. Indeed, the black triangle nearthe orange triangles represents our previously publishedconditions for the synthesis of Bi wires.36 However, we did

not investigate the wire-formation region systematically here,as our primary goal was the synthesis of near-monodispersed,diameter-controlled dots.

A second region of near-monodispersed dot formation isfound along the lower-front-right edge of the synthesis spaceat very low or zero amounts of Na[N(SiMe3)2] and lowpolymer concentrations. Here dots with diameters in therange of 40-115 nm were grown, along a line orthogonalto the blue line discussed above, and indicated by the redarrow in Figure 8.

We now speculate about the roles of reagent quantitiesin controlling the dot-growth behavior depicted in Figure8. The decrease in dot diameter with increasing amountof Na[N(SiMe3)2] (along the blue line) strongly suggeststhat Na[N(SiMe3)2] promotes dot nucleation. Increasingnumbers of dot nuclei are presumably formed as theamount of Na[N(SiMe3)2] is increased, leading to a smallermean dot diameter at a fixed amount of Bi[N(SiMe3)2]3.Such nucleation may be of a classical or nonclassical kind.We have evidence, not presented here, that nanoparticlegrowth occurs by an initial burst formation of a very largenumber of small, primary Bi nanocrystallites (d e 2 nm),which subsequently undergo aggregation to set the numberof viable, growing dots. This proposed, nonclassical,aggregative nucleation process, and the mechanism bywhich Na[N(SiMe3)2] may serve as a nucleation promoterwill be discussed in a subsequent manuscript.

Figure 8. Synthesis-space diagram for Bi nanoparticles, showing the dependence of particle size and morphology on the amounts of Bi[N(SiMe3)2]3,Na[N(SiMe3)2], and polymer employed. The blue arrow on the upper-right-rear edge shows the trend of increasing dot size (3-30 nm) with decreasingamount of Na[N(SiMe3)2] at a fixed (25 wt %) polymer concentration (170-210 °C). The blue line on the top surface indicates the recommended syntheticconditions for Bi dots in the size range of 3-30 nm. The red arrow on the lower-right edge shows the trend of increasing dot size (40-115 nm) withincreasing amount of Bi[N(SiMe3)2]3 at very low or zero amounts of Na[N(SiMe3)2] and low polymer concentrations (200 °C). Ribbons (pink pentagons)were obtained at 210 °C, wires (orange and black36 triangles) at 210 and 203 °C, respectively, polydispersed dots (purple star) at 200 °C, and plates and rods(green diamonds) at 200-210 °C. The dashed lines and small points are the projections of the synthetic conditions (large points) to guide the eye.


In contrast, with a fixed amount of Na[N(SiMe3)2], or noadded Na[N(SiMe3)2] (along the red line in Figure 8), thedot diameters increased with the amount of Bi[N(SiMe3)2]3

employed. Under these conditions, the number of dot nucleiformed is presumably held nearly constant, such that largeramounts of Bi[N(SiMe3)2]3 lead to larger mean dot diameters.The larger overall diameters formed along the red line incomparison to those formed along the blue line in Figure 8presumably reflect the decreased availability of polymer forsurface passivation, resulting in smaller stabilized surfacefractions and therefore larger mean particle volumes.

Rod and plate morphologies were observed in a small areanear the projected intersection of the red and blue lines(Figure 8), with very small but nonzero amounts of Na[N-(SiMe3)2]. We speculate that, under the conditions of lowpolymer concentration and small amounts of added Na[N-(SiMe3)2], bis(trimethylsilyl)amide ligands may selectivelybind Bi (001) faces, leading to the anisotropic crystal growthobserved. Additionally, surface energies may strongly influ-ence the anisotropic growth patterns of nanocrystals.41 The(001) and (110) faces of Bi have surface energies of 0.356and 0.507 eV atom-1, respectively.42 Consequently, Bi platesand rods may grow along the [110] (or equivalent) directionto reduce the relative area of (110) faces, to increase therelative area of the (001) faces, and thus to minimize totalsurface energy.

Two points are added to Figure 8 to describe the initialconditions for ribbon growth. However, because the ribbons

were grown by secondary addition the initial conditions maynot be particularly informative. We include the ribbon pointsfor completeness but offer no speculations about theirsignificance.

Conclusion

We have demonstrated simple, solution-based, one-potsyntheses of Bi dots, nanorods, nanoplates, and nanoribbons.Near-monodisperse Bi dots in the diameter range of 3-30nm were synthesized at high polymer concentrations, andthose in the range of 30-115 nm were synthesized at lowpolymer concentrations. These Bi dots are particularly usefulfor the SLS growth of semiconductor nanowires.19–24 Weare currently conducting mechanistic studies of Bi dot growthunder conditions related to those identified here.

Acknowledgment. We thank Prof. Richard A. Loomis andDr. Shawn P. Shields for helpful discussions. We are gratefulto the National Science Foundation for funding this work undergrant no. CHE-0518427.

Supporting Information Available: Tables listing detailedsynthetic conditions, diameter-distribution histograms of Bi dots,an SAED pattern of Bi dots, XRD patterns of Bi dots, nanorods,nanoplates, and nanoribbons, TEM images of 128.3 and 173.1 nmdiameter Bi particles, TEM images of Bi dots formed underuncontrolled conditions, and a TEM image of a mixture of Bi wiresand dots. This material is available free of charge via the Internetat http://pubs.acs.org.

CM8004425

(41) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310–325.(42) Vitos, L.; Ruban, A. V.; Skiver, H. L.; Kollar, J. Surf. Sci. 1998, 411,

186–202.

Table 1. Representative Synthetic Conditions and Resulting Diameters and Diameter-Distributions for Bi Dots

Bi[N(SiMe3)2]3 NaN(SiMe3)2 polymer-DIPB soln temp time diameter (nm)

(mmol) (mmol) (wt%, 10 g) (°C) (h) (Standard deviation)0.446 0.057 0.25 200 17 173.1 ( 32.8 (19%)1.14 0 25 (8 g) 205 15 128.3 ( 23.1 (18%)1.45 0 2.5 200 18.3 114.7 ( 9.8 (9%)1.11 0 2.5 200 17 91.2 ( 6.3 (7%)0.820 0 2.5 200 17 78.2 ( 2.9 (4%)0.588 0 2.5 200 17 68.8 ( 4.8 (7%)0.438 0.017 2.5 200 17 54.5 ( 2.9 (5%)0.434 0.221 12.5 210 16 39.7 ( 2.4 (6%)0.437 0.123 25 210 23 25.2 ( 1.3 (5%)0.300 0.123 25 210 23 22.6 ( 1.2 (5%)0.435 0.222 25 210 17 20.4 ( 1.0 (5%)0.437 0.444 25 210 17 18.3 ( 0.9 (5%)0.437 0.227 25 170 17 18.1 ( 0.8 (4%)0.434 0.455 25 170 17 15.1 ( 0.9 (6%)0.434 0.669 25 170 17 14.8 ( 0.8 (5%)0.443 0.885 25 170 17 12.9 ( 0.8 (6%)0.437 1.11 25 170 17 11.6 ( 0.8 (7%)0.441 1.34 25 170 17 9.4 ( 0.8 (8%)0.437 1.58 25 170 17 9.0 ( 1.2 (13%)0.440 1.78 25 180 17 7.1 ( 0.8 (11%)0.435 1.78 25 170 17 6.4 ( 0.7 (11%)0.435 2.21 25 170 17 4.7 ( 0.7 (15%)0.435 2.65 25 180 17 4.3 ( 0.8 (19%)0.438 2.89 25 170 17 3.3 ( 0.6 (18%)


Size- and Shape-Controlled Synthesis of Bismuth Nanoparticles

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