IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 335604 (6pp) doi:10.1088/0957-4484/18/33/335604

In situ formation of bismuth nanoparticlesthrough electron-beam irradiation in atransmission electron microscopeS Sepulveda-Guzman, N Elizondo-Villarreal, D Ferrer,A Torres-Castro, X Gao, J P Zhou and M Jose-Yacaman1

Chemical Engineering Department and The Texas Materials Institute, The University of Texasat Austin, Austin, TX 78712, USA

E-mail: yacaman@che.utexas.edu

Received 1 May 2007, in final form 12 June 2007Published 25 July 2007Online at stacks.iop.org/Nano/18/335604

AbstractIn this work, bismuth nanoparticles were synthesized when a precursor,sodium bismuthate, was exposed to an electron beam at room temperature ina transmission electron microscope (TEM). The irradiation effects wereinvestigated in situ using selected-area electron diffraction, high-resolutiontransmission electron microscopy and x-ray energy dispersive spectroscopy.After the electron irradiation, bismuth nanoparticles with a rhombohedralstructure and diameter of 6 nm were observed. The average particle sizeincreased with the irradiation time. The electron-induced reduction isattributed to the desorption of oxygen ions. This method offers a one-steproute to synthesize bismuth nanoparticles using electron irradiation, and theparticle size can be controlled by the irradiation time.

1. Introduction

Considerable research has been focused on the synthesisof uniform metal nanoparticles due to their potential useas pigments, recording materials and catalysts [1–3]. Agreat number of bismuth submicron structures have beenobtained using wet chemical methods, such as the precipitationprocess [4, 5]. However, over the past years, bismuthnanostructures have attracted great attention due to their veryinteresting properties. Nanotubes, nanowires and nanobeltshave been synthesized using hydrothermal processes [6, 7].In addition, bismuth nanoparticles and nanoplates havebeen synthesized by the solution-phase technique usingdifferent reducing agents [8, 9], whereas clusters of bismuthnanoparticles were prepared by inert-gas aggregation by Wrulet al [10]. Many of the resulting nanostructured materialshave several advantages when compared with bulk bismuth.For instance, bismuth nanowires could have thermoelectricalproperties [11], and recent studies suggest that a semimetal–semiconductor transition occurs in bismuth nanostructures asthe crystallite size decreases [11, 12]. This makes bismuth

1 Author to whom any correspondence should be addressed.

nanoparticles a potential material for optical and optoelectronicapplications.

As the new nanostructures have been mainly studiedby transmission electron microscopy, important research intoelectron irradiation effects on the nanostructures has been con-ducted using novel in situ characterization techniques. Ya-mamoto et al [13], reported the modification of the conduc-tivity in an insulating BaTiO3 using electron holographic stud-ies of irradiated sample in a transmission electron microscope(TEM). The stability and the crystallization of amorphous al-loys by electron irradiation have also been studied in situ [14].In a different approach, the mechanism of formation of carbononion-like structures by electron irradiation of Pt, Al or Aunanoparticles deposited on a ultra-thin carbon film has beenelucidated [15]. Little attention has been paid, however, tostudying the electron-stimulated oxidation and/or reduction inthe high-vacuum system of the electron microscope. Liu et al[16] observed the oxidation of Fe4N(001) to Fe3O4 in a TEMand attributed this process to absorption of residual oxygen un-der electron irradiation in the microscope. On the other hand,the reduction of orthorhombic V2O5 to cubic VO by electronirradiation was studied by spectroscopic techniques in an an-alytical TEM [17]. Very recently, the synthesis of bismuth

0957-4484/07/335604+06$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 335604 S Sepulveda-Guzman et al

Figure 1. Low-resolution TEM images of the NaBiO3 sample (a) at the initial point of irradiation and (b) after 30 min of irradiation.Inset: corresponding SAED pattern.

nanoparticles by the reduction of BiCl3 under electron-beamirradiation in a TEM was reported [18]. However, a pretreat-ment involving harsh reaction conditions is required before theelectron irradiation of the sample. In addition, a high dose anda high accelerating voltage (400 kV) are needed. In this work,we report an easier route to synthesize bismuth nanoparticlesby electron-stimulated reduction of sodium bismuthate withoutpretreatments and using considerably lower irradiation condi-tions. The phase transition, the effect of irradiation time andthe formation mechanism of the metal nanoparticles were in-vestigated in situ using selected-area electron diffraction, high-resolution transmission electron microscopy, and x-ray energydispersive spectroscopy.

2. Experimental procedure

Sodium bismuthate (NaBiO3) was purchased from Aldrich andused without further purification. The sodium bismuthate waspowdered and placed over a carbon-film copper grid. Theexperiments were carried out in a FEI Tecnai G20 TEM and aJEOL 2010F (200 kV) TEM equipped with field emission gun(FEG) and an ultrahigh-resolution observation system. Theobservation was carried out under an accelerating voltage of200 kV, at a column vacuum pressure of 10−9 Torr and abeam current density of 14 pA cm−2. Selected-area electrondiffraction (SAED), x-ray energy dispersive spectroscopy(EDS) and TEM images were recorded in situ at roomtemperature.

3. Results and discussion

The initial state of the NaBiO3 was analysed using SAEDto define the diffraction pattern of the sample before theirradiation process. Figure 1 shows the TEM images andthe corresponding SAED patterns of the NaBiO3 sampleat the beginning and after irradiation. The SAED patternshows only a characteristic ring without any diffraction spots(figure 1(a) inset), which indicates the amorphous state ofthe NaBiO3 sample. In addition, no clusters were observed

Figure 2. (a) EDS spectrum acquired from the NaBiO3 sample at theinitial point of irradiation and (b) EDS spectrum acquired from anisolated particle formed after irradiation.

in the corresponding TEM image at this stage. However,when the sample was irradiated for 30 min small particles andcrystalline clusters developed (figure 1(b)), along with severaldiffraction spots randomly distributed in the correspondingSAED pattern (figure 1(b) inset). Both the cluster size and thebrightness of the diffraction spots increased with the irradiationtime. The resulting crystalline structure is related to metallicBi with rhombohedral structure, similar to bulk Bi. Thereciprocal distances of the stronger reflections observed inthe SEAD were measured and correspond to the diffractionindices (0 0 6), (2 0 8), (0 1 8), and (1 0 10) of therhombohedral structure of bismuth, according to the JCPDS

2

Nanotechnology 18 (2007) 335604 S Sepulveda-Guzman et al

Figure 3. TEM images of the NaBiO3 sample (a) after 5 min of irradiation and (b) after 10 min of irradiation.

database [19]. Some diffraction spots are weaker and werenot identified; however, they could be related to by-products,such as sodium or bismuth salts. The bright-field (BF) imagesand SAED clearly show the low stability of NaBiO3 underelectron irradiation and the induced reduction of Bi5+ ionsby electron irradiation. In order to study the change incomposition, the sample was analysed by EDS at differentstages during the irradiation process. The spectrum acquired atthe beginning of the experiment displays the peaks of Bi, Na,and O (figure 2(a)), as well as those peaks related to the carbonfilm (Cu and C). At this point, the semi-quantitative analysiswas consistent with the atomic composition of NaBiO3. After30 min of sample irradiation, the EDS spectrum of anisolated particle shows that the bismuth to sodium ratio hasincreased considerably, which indicates that the particle ismainly composed of Bi (figure 2(b)). These results suggest thatbesides the decomposition of the NaBiO3 under the electron-beam irradiation and the phase transformation from Bi5+to Bi0 in the sample, sodium and oxygen were segregatedor volatilized under the vacuum environment. Changes incomposition have been also observed in inorganic samplesunder electron irradiation [16].

The electron irradiation of the NaBiO3 and thenanoparticle formation were followed up by observation athigher magnification. The effect of the irradiation time onthe particle size distribution is shown in figure 3. When thesample was irradiated for 5 min, nanoparticles with an averagediameter of 4 nm were observed (figure 3(a)). The particlesshowed different morphologies, ranging from spherical torhombohedral shape; the latter morphology has been reportedfor bismuth nanoparticles [10, 20]. After 10 min of irradiation,the number of large particles increased to an average size of6 nm diameter (figure 3(b)), and as the irradiation continued,the particles coalesced, leading to irregularly shaped particles.After 60 min of irradiation time, significant broadening in theparticle size distribution was observed. This process could beexplained as follows. The synthesis of the smallest bismuthnanoparticles occurred during the earlier minutes of irradiation.Once formed, these nanoparticles acted as seeds and grewinto crystals. Some of these particles were able to grow into

Figure 4. A series of bright-field TEM images of the sample exposedto different electron irradiation doses. (a) 4.2 nC cm−2,(b) 8.4 nC cm−2, (c) 16.8 nC cm−2 and (d) 37 nC cm−2.

large crystals by the Ostwald ripening process [21]. Figure 4shows a series of BF images which illustrate the dynamicsof this process as a function of the irradiation dose. Thesmaller nanoparticles were progressively dissolved into thebigger ones resulting in rhombohedral-shaped particles afterbeing exposed to an electron dose of 37 nC cm−2. Sincethe reaction was carried out in the solid state, the absence ofcapping organic molecules or stabilizers, which could preventthe continuous dissolving of the smaller particles into thebigger ones, makes this process easier. Once the particleswere synthesized, their stability under the TEM is criticalfor all the potential applications. During the earlier reactionminutes, the particles were too small, and showed a rapidmotion under the electron beam; however, when the particle

3

Nanotechnology 18 (2007) 335604 S Sepulveda-Guzman et al

Figure 5. HRTEM image of an isolated bismuth particle. Inset: FFTof the image.

size increased to ∼6 nm, the motion became slower andmore stable to the observation conditions. At this moment,a high-resolution TEM (HRTEM) image of a single particlewas recorded (figure 5). The characteristic lattice fringesof the rhombohedral structure of bismuth were observed andmeasured as 0.23 and 0.33 nm accordingly. In addition, a fastFourier transform (FFT) was conducted on the image, and thespots in the corresponding FFT were indexed to the (1 2 2),(1 1 0), and (0 1 2) diffraction planes, with a lattice spacingof 1.43 A, 2.26 A, and 3.27 A, respectively (figure 4 inset).These values are in agreement with the simulated diffractionpattern of the rhombic structure of the bismuth [1 1 1] zoneaxis. In a previous work, Oshima et al [20] observed atransition from rhombohedral to cubic structure in 6–7 nmdiameter Bi particles under an electron beam; however, inour case, no structural transition was observed. Instead ofa structural transition a coalescence process was observedduring the irradiation of the Bi nanoparticles (figure 6). Unlikethe Ostwald ripening process, which occurs during the early

Figure 7. Bright-field TEM image of irradiated sample after 45 minof irradiation. Arrows indicate the resulting structures after thecoalescence between the particles: (a) agglomerate and (b) snake-likestructure.

minutes of irradiation and is related to smaller particles, thecoalescence involves the diffusion of an entire particle whichcollides with other particles. During coalescence, adjacentparticles first form an agglomerate and then merge into asingle particle. As a result of the bulk transport and extendedcoalescence of several nanoparticles in the sample, snake-likestructures were finally observed (figure 7). This diffusionmechanism is most likely caused by continuous irradiation andthe weak interaction between the particles and the support.This mechanism is in agreement with recent studies whichhave shown that electron-beam irradiation can trigger thecoalescence of nanoparticles, even those highly stabilized by acapping agent. The process depends on several factors, suchas the sensitivity of the passivating ligands to the electronbeam, the support characteristics, the size of the nanoparticles,and the irradiation dose [22]. On the other hand, Kim et al[23] reported the synthesis of gold nanoparticles by electron-

Figure 6. (a) HRTEM image of Bi particles after 30 min of irradiation. (b) Magnification of selected area. Inset: FFT of the image.

4

Nanotechnology 18 (2007) 335604 S Sepulveda-Guzman et al

Figure 8. (a) High-angle annular dark-field (HAADF) image of the synthesized bismuth nanoparticles and (b) composition profile of C, O andBi extracted from the line-scan microanalysis of one nanoparticle.

beam irradiation and found that the particle size can be partlycontrolled by the accelerating voltage. According to previousstudies done in the field, it is clear that several parameters,such as temperature, electron dose, accelerating voltage, timeand precursor particle size, should influence the growth andmorphology of the bismuth nanoparticles synthesized throughelectron-beam irradiation.

Although there are well-known aspects of the electronirradiation effects discussed in the literature, such asadsorption, desorption, diffusion and dissociation, each systemhas particular considerations. The complete mechanism of thereduction of NaBiO3 by electron-beam irradiation is still notfully understood. However, we believe it could include theionization of the sodium bismuthate, i.e., the electron beambreaks the bonding between the elements under the observationconditions in the TEM (200 kV). Similar ionization has beenobserved during the oxidation of Fe4N and InP in an ultrahigh-vacuum (UHV) chamber [16, 24, 25]. Nevertheless, in ourcase, as the irradiation continues the collisions between theelectrons and the bismuthate sample result in the formationof Bi metal and simultaneous desorption of oxygen atoms.Thus the oxygen is lost to the vacuum, particularly in thearea where the transformation takes place completely. Thisis further supported by the absence of an oxygen peak inthe EDS spectrum acquired from an isolated Bi nanoparticleformed after 30 min (37 nC cm−2) of irradiation (figure 2(b)).However, in order to rule out any noise signal effect andto verify the accuracy of the EDS, a line-scan microanalysiswas done on a single bismuth nanoparticle comparing boththe bismuth and oxygen signal to that of carbon, which isused as a reference (figure 8). The results revealed that thebismuth signal is originated by the bright particle seen on theimage. As expected, the intensity increases at the centre ofthe particle. In contrast, the intensity of the oxygen signalis considerably lower than that of Bi and the intensity is notlimited to the particle boundaries, whereas the intensity of thecarbon signal fluctuates around an average value, indicatingthat the sensitivity of the acquisition was constant throughoutthe process. These results confirm the desorption of oxygen

from the irradiated sample. The induced reduction in theTEM by the desorption of oxygen atoms under electronirradiation has been observed in other materials, such as V2O5

and NiO–MgO, resulting in the formation of VO and Nimetal, respectively [17, 26]. A more detailed explanationof the mechanism of formation is needed; nevertheless, thesynthesis of Bi nanoparticles under electron irradiation ishighly interesting for nanotechnology purposes, and could findapplications in areas like electron-beam nanolithography andmight also serve as an example for the synthesis of othersemiconductor nanoparticles.

4. Conclusions

Bismuth metal nanoparticles were easily synthesized bythe electron irradiation of a bismuth precursor (NaBiO3)

under observable conditions (200 kV) in a TEM. In situcharacterization was used to study the reduction of the NaBiO3

and the growth mechanism of the Bi nanoparticles. Thedecrease of oxygen peak in the EDS spectrum suggests thatthe reduction of Bi5+ results from the desorption of oxygenions under electron irradiation. The Bi particles showed arhombohedral structure which is common in bulk bismuth. Theparticles size could be controlled by the irradiation time. Inaddition, the bismuth particles were unstable under the electronbeam and showed a diffusion–collision process during the lastirradiation period.

Acknowledgments

The authors want to thank ICNAM of the University Of TexasAustin, the Welch Foundation and NSF materials division, andIndustrias Penoles for financial support. S Sepulveda-Guzmanacknowledges the support received from CIQA-Mexico.

References

[1] Jose-Yacaman M, Rendon L, Arenas J and Serra-Puche M C1996 Science 273 223

[2] Majetich S A and Jin Y 1999 Science 284 470

5

Nanotechnology 18 (2007) 335604 S Sepulveda-Guzman et al

[3] Persson K, Jansson K and Jaras S G 2007 J. Catal. 245 401[4] Wang Y and Xia Y 2004 Nano Lett. 4 2047[5] Goia C, Matijevic E and Goia D V 2005 J. Mater. Res. 20 1507[6] Li Y, Wang J, Deng Z, Wu Y, Sun X, Yu D and Yang P 2001

J. Am. Chem. Soc. 123 9904[7] Wang J, Wang X, Peng Q and Li Y 2004 Inorg. Chem. 43 7552[8] Wang Y W, Hong B H and Kim K S 2005 J. Phys. Chem. B

109 7067[9] Fu R, Xu S, Lu Y-N and Zhu J-J 2005 Cryst. Growth Des.

5 1379[10] Wrul A, Hyslop M, Brown S A, Hall B D and Monot R 2001

Eur. Phys. J. D 16 205[11] Sun X, Zhang Z and Dresselhaus M S 1999 Appl. Phys. Lett.

74 4005[12] Zhang Z, Sun X, Dresselhaus M S, Ying J Y and Heremans J P

1998 Appl. Phys. Lett. 73 1589[13] Yamamoto T, Hirayama T, Fulkunaga K and Ikuhara Y 2004

Nanotechnology 15 1324[14] Nagase T and Umakoshi Y 2003 Mater. Sci. Eng. A 343 13[15] Troiani H E, Camacho-Bragado A, Armendariz V,

GardeaTorresday J L and Jose-Yacaman M 2003 Chem.Mater. 15 1029

[16] Liu Z Q, Hashimoto H, Sukedai E, Song M, Mitsuishi K andFuruya K 2003 Phys. Rev. Lett. 90 255504

[17] Su D S, Wieske M, Beckman E, Blume A, Mestl G andSchlogl R 2001 Catal. Lett. 75 81

[18] Kim S H, Choi Y-S, Kang K and Yang S I 2007 J. AlloysCompounds 427 330

[19] JCPDS International Centre for Diffraction Data 2003 PDF85-1330; PCPDFWIN v. 2.4

[20] Oshima Y, Takayanagi K and Hirayama H 1997 Z. Phys. D40 534

[21] Roosen A R and Carter W C 1998 Physica A 261 232[22] Chen Y, Palmer R E and Wilcoxon J P 2006 Langmuir

22 2851[23] Kim J-U, Cha S-H, Shin K, Jho J Y and Lee J-C 2005 J. Am.

Chem. Soc. 127 9962[24] Bouslama M, Lounis Z, Ghaffour M, Ghamnia M and

Jardin C 2002 Vacuum 65 185[25] Liu Z Q, Hashimoto H, Song M, Mitsuishi K and

Furuya K 2004 Acta Mater. 52 1669[26] Davies P K and Mackinnon I D R 1986 J. Am. Ceram. Soc.

69 C-124

6