Microwave synthesis of bismuth nanospheres using bismuth citrate as a precursor

Article abstract of DOI:10.1016/j.jallcom.2010.03.165

Well-separated bismuth nanospheres were successfully synthesized from bismuth citrate and urea in diethylene glycol by a fast and simple microwave irradiation method. The products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In this fabrication, bismuth citrate plays a critical role as a precursor in the formation of bismuth nanospheres. The amount of urea also has an influence on the size of the bismuth nanospheres. The possible growth mechanism of bismuth nanospheres was discussed on the basis of the investigation of reaction time.

Full text of DOI:10.1016/j.jallcom.2010.03.165

Journal of Alloys and Compounds 498 (2010) L8–L11
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Microwave synthesis of bismuth nanospheres using bismuth
citrate as a precursor
Jiliang Wua, Hanmin Yangb, Hui Lia, Zhong Lua, Xianglin Yua, Rong Chena,∗
a Key Laboratory for Green Chemical Process of Ministry of Education and School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, PR China
b Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central University for Nationalities, Wuhan
430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Well-separated bismuth nanospheres were successfully synthesized from bismuth citrate and urea in
diethylene glycol by a fast and simple microwave irradiation method. The products were character-
ized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). In this fabrication, bismuth citrate plays a critical role as a precursor in the formation
of bismuth nanospheres. The amount of urea also has an influence on the size of the bismuth nanospheres.
The possible growth mechanism of bismuth nanospheres was discussed on the basis of the investigation
of reaction time.
Received 16 November 2009
Received in revised form 18 March 2010
Accepted 18 March 2010
Available online 25 March 2010
Keywords:
Bismuth
Nanospheres
Microwave synthesis
Bismuth citrate
Urea
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
mation [25–30]. Although many synthesis methods for bismuth
nanospheres have been reported, there are still the problems of
Bismuth is a fascinating semimetal with unusual properties
due to its highly anisotropic Fermi surface, low carrier densities,
small carrier effective masses and long carrier mean free path [1].
Bismuth nanostructures have been extensively studied recently,
since they play important roles in many diverse applications. For
examples, single-crystal bismuth thin films exhibited large mag-
netoresistance and finite-size effects [2]. The thermal conductivity
of single-crystal bismuth nanowire was found to be three to six
times smaller than bulk materials [3]. Size-dependent transport
and thermoelectric properties of individual poly-crystalline bis-
muth nanowires were also investigated [4]. Up to now, a wide
variety of bismuth nanostructures have been synthesized, such as
nanoparticles [5,6], nanowires [7], nanorods [8], nanotubes [9], tri-
angular nanoplates [10], nanocubes [11] as well as nanospheres
(>100 nm) [12–15]. Among the metal nanostructures, monodis-
perse metallic nanospheres have attracted considerable attention
because of their unique optical, catalytic, novel chemical and bio-
logical properties [16–22]. For instance, spherical bismuth was a
good catalyst for the growth of SnS2 nanotubes and germanium
nanowires [23,24]. Thus, much effort has been devoted to the
preparation of metal nanospheres and their functional transfor-
rigorous experimental conditions (under a stream of N2) and long
reaction time. Therefore, a simple, low-cost and rapid approach for
the fabrication of bismuth nanospheres is highly desired.
In recent years, microwave irradiation method has received con-
siderable attention as a new promising method for the one-pot
synthesis of metallic and semiconductor nanostructures in solu-
tion [31–35]. Many metallic nanomaterials have been fabricated
via microwave irradiation method [31,36]. In previous work, we
described a simple refluxing method for the preparation of bismuth
micro- and nanospheres from bismuth citrate and urea in the pres-
ence of PVP, using ethylene glycol as both solvent and reducing
agent [13]. To the best of our knowledge, studies on synthesis of
bismuth nanospheres by microwave irradiation have never been
reported. In this communication, a rapid urea-assisted microwave
irradiation method for the synthesis of bismuth nanospheres was
reported. In this synthesis, bismuth citrate was used as a bismuth
precursor.
2. Experimental
2.1. Materials and preparation
All chemicals are analytical grade and used without further purification. In
a typical synthesis, 0.299 g (0.75 mmol) of bismuth citrate (Bi(cit)) and 0.135 g
(2.25 mmol) of urea were added to a round-bottom flask which contained 50 mL of
diethylene glycol (DEG). The mixture was stirred and sonicated until all the chem-
icals were well dispersed. Then the mixed solution was treated under microwave
∗
Corresponding author. Tel.: +86 13659815698; fax: +86 2787194465.
E-mail address: rchenhku@hotmail.com (R. Chen).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.03.165

J. Wu et al. / Journal of Alloys and Compounds 498 (2010) L8–L11
L9
The nanostructures of the products were examined by scan-
ning electron microscopy (SEM). Fig. 2 displays the typical SEM
images of the as-synthesized bismuth nanospheres at different
time. As shown in Fig. 2a, only after 5 min microwave heating,
spheric morphology of bismuth nanomaterials were observed in
the sample. The diameters of these nanospheres varied from 40 to
500 nm. In this stage, bismuth nanosheets with diameters of about
500 nm were also formed. More and more bismuth nanospheres
were fabricated with the increase of reaction time. It was observed
that large quantities bismuth nanospheres were obtained (nearly
100%) when the reaction time was prolonged to 20 min (Fig. 2b).
These nanospheres were well separated and the diameters of most
nanospheres varied from 400 to 700 nm. Controlled experiments
over different reaction time demonstrated the shape evolution and
the corresponding growth process of bismuth nanospheres. Fig. 2c
and d shows the SEM images of the dispersed bismuth nanospheres
after 30 and 40 min microwave heating, respectively. It was found
that the diameter of the nanospheres changed with the increase
of reaction time. When the reaction time reached to 40 min, the
maximal diameter of these nanospheres was about 1 ␮m, and the
diameters of most nanospheres were more than 700 nm (Fig. 2d).
The results indicate that reaction time plays an important role in the
fabrication of bismuth nanospheres. Long reaction time does not
favor the formation of small sizes of bismuth nanospheres. When
the reaction time was prolonged to 30 min, the maximal diameter
of some spheres was already at micrometer scale.
Fig. 1. XRD pattern of as-synthesized bismuth nanospheres with urea/Bi(cit) molar
ratio of 3:1 under microwave heating for 20 min.
heating for 20 min with continuous vigorous stirring. After cooling down to room
temperature naturally, the obtained black product was centrifuged and washed with
deionized water for four times. Finally, the black product was dried in a desiccator
for a few days for further characterization. Other samples were prepared under
identical experiment conditions by using different bismuth precursors or different
urea/Bi(cit) molar ratios.
In this synthesis, it was considered that DEG was used both as
solvent and reducing agent. In the initial step, the reaction mix-
ture changed to black color after 3 min, microwave heating, which
suggested the reduction of Bi3+ ions to bismuth by DEG in the solu-
tion. The reduction reaction of DEG might be similar to that of EG
reported previously [37]:
2.2. Characterization
The obtained products were characterized by X-ray diffraction (XRD) (Bruker
D8 advance), scanning electron microscope (SEM) (JEOL 6700-F), and transmission
electron microscope (TEM) (Tecnai G2 20) with an accelerating voltage of 200 kV.
3. Results and discussion
ꢀ
2HOCH2CH2OCH2CH2OH−→ 2CH3CH2OCH2CHO + 2H2O
(1)
Fig. 1 shows the typical powder X-ray diffraction (XRD) pattern
of the as-synthesized product after 20 min microwave heating. All
the diffraction peaks could be readily indexed to be rhombohe-
dral phase of bismuth, which were consistent with the standard
data (JCPDS No. 05-0519). No impurity was detected in the pattern,
indicating that the product has high purity under current synthetic
conditions.
6CH3CH2OCH2CHO + 2Bi3+
ꢀ
−→ 3CH3CH2OCH2COCOCH2OCH2CH3 + 2Bi + 6H+
(2)
There are many reports concerning the formation mechanism
of inorganic nanospheres [38,39]. Generally, the formation of
Fig. 2. SEM images and size distributions estimated from the photographs of the products synthesis by the microwave heating method with urea/Bi(cit) molar ratio of 3:1
for different time: (a) 5 min, (b) 20 min, (c) 30 min and (d) 40 min.

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J. Wu et al. / Journal of Alloys and Compounds 498 (2010) L8–L11
Fig. 3. SEM images of products sampled with different urea/Bi(cit) molar ratio for 20 min microwave heating: (a) 0, (b) 6 and (d) 9. Note: (c) shows the TEM images and size
distributions (inset of “c”) of the nanospheres corresponding to “b”.
nanostructure features after fast nucleation in solution involved
in two primary mechanisms: the aggregation growth process
and the Ostwald ripening process [40,41]. Crystal growth by
aggregation can occur by two means: random aggregation and
the oriented attachment mechanism, which provide routes for
the incorporation of defects in stress-free and initially defect-free
nanocrystalline materials [42–44]. The Ostwald ripening process
refers to the growth of larger crystals at the expense of smaller
crystals, and this mode may result in the formation of faceted
particles if the difference of surface energy sufficiently existed in
different crystallographic facets [39,45]. In the present work, it was
proposed that both of the two mechanisms were involved in the
formation of bismuth nanospheres. First, DEG reduced Bi3+ ions to
bismuth nuclei in the solution. The quick aggregation of bismuth
nuclei led to the formation of bismuth nanosheets and nanospheres
in this early stage (Fig. 2b), which is in agreement with random
aggregation growth process. With the proceeding of microwave
irradiation, the primary building units will dissolve in the solvent
gradually and reprecipitate onto the larger nanocrystals, resulting
in increasing the size of the nanospheres. The preferential growth
into large crystals is determined by the oriented attachment and
Ostwald ripening process.
The influence of precursors on the formation of bismuth
nanospheres was investigated by altering the urea/Bi(cit) molar
ratio. Fig. 3a shows the SEM image of the product prepared in
the absence of urea. It was found that bismuth microspheres
were formed, and irregular morphologies of bismuth nanomate-
rials were also observed. When the urea/Bi(cit) molar ratio was
increased to 6, well-separated bismuth spherical particles with
diameter distribution ranging from ca. 100 to 600 nm were fab-
ricated, as shown in Fig. 3b and c. When the urea/Bi(cit) molar
ratio was 9, the diameters of bismuth nanospheres were in range
of 100–400 nm, which had a relatively narrow size distribution.
However, the SEM image (Fig. 4d) shows that the nanospheres
linked by the transparent films due to the excessive urea, which
can be washed off by deionized water for more than four times
(inset of Fig. 3d). The results illustrate that there is obvious dif-
ference of bismuth nanospheres diameters with the increase of
urea concentration. Otherwise, the viscosity of the reaction sys-
tem increased with the increase of the concentration of urea, which
Fig. 4. SEM images of the products synthesis by the microwave heating method with urea/Bi-salt molar ratio of 3:1 using different bismuth precursors: (a) Bi(NO3)3·5H2O
and (b) BiCl3.

J. Wu et al. / Journal of Alloys and Compounds 498 (2010) L8–L11
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made it difficult to remove the excessive urea under normal con-
dition. It illustrates that the concentration of urea plays a key role
in the size control of bismuth nanospheres, similar with the exam-
ple of poly(vinylpyrrolidone) (PVP) capped bismuth nanospheres
[13,14].
[3] A.L. Moore, M.T. Pettes, F. Zhou, L. Shi, J. Appl. Phys. 106 (2009) 034310–034317.
[4] A. Boukai, K. Xu, J.R. Heath, Adv. Mater. 18 (2006) 864–869.
[5] F. Wang, R. Tang, H. Yu, P.C. Gibbons, W.E. Buhro, Chem. Mater. 20 (2008)
3656–3662.
[6] S.H. Kim, Y.-S. Choi, K. Kang, S.I. Yang, J. Alloys Compd. 427 (2007) 330–332.
[7] Y. Xu, Z. Ren, W. Ren, G. Cao, K. Deng, Y. Zhong, Nanotechnology 19 (2008)
115602–115606.
To further investigate the function of bismuth citrate, different
bismuth precursors were introduced to the synthesis of bismuth
nanospheres. When the urea/Bi-salt molar ratio was kept at 3,
Bi(NO3)3·5H2O and BiCl3 was used as bismuth precursor under
identical conditions, respectively, instead of bismuth citrate. As
shown in Fig. 4a, bismuth nanospheres and irregular shapes of
bismuth nanomaterials were obtained by using Bi(NO3)3·5H2O as
bismuth precursor. Interestingly, bismuth nanoflowers were fab-
ricated when BiCl3 was instead of bismuth citrate (Fig. 4b). It
illustrates that the anions of bismuth salts have a great influence
on the morphologies of growing bismuth nanostructures. In fact,
citrate anion has been widely used as an effective capping agent to
modify crystalline surface [46]. As shown in Fig. 3a, bismuth micro-
and nanospheres were also obtained from bismuth citrate in the
absence of urea. In this fabrication, citrate anions could generate
from bismuth citrate during the reduction process. Citrate anion
was not only an efficient capping agent but also exerted a significant
function on the morphology of the growing bismuth nanoparticles
[47].
[8] S. Cao, C. Guo, Y. Wang, J. Miao, Z. Zhang, Q. Liu, Solid State Commun. 149 (2009)
87–90.
[9] D. Yang, G. Meng, Q. Xu, F. Han, M. Kong, L. Zhang, J. Phys. Chem. C 112 (2008)
8614–8616.
[10] R. Fu, S. Xu, Y.-N. Lu, J.-J. Zhu, Cryst. Growth Des. 5 (2005) 1379–1385.
[11] W.Z. Wang, B. Poudel, Y. Ma, Z.F. Ren, J. Phys. Chem. B 110 (2006) 25702–25706.
[12] Y. Wang, Y. Xia, Nano Lett. 4 (2004) 2047–2050.
[13] G. Cheng, J. Wu, F. Xiao, H. Yu, Z. Lu, X. Yu, R. Chen, Mater. Lett. 63 (2009)
2239–2242.
[14] J. Li, H. Fan, J. Chen, L. Liu, Colloids Surf. A 340 (2009) 66–69.
[15] J. Wang, X. Wang, Q. Peng, Y. Li, Inorg. Chem. 43 (2004) 7552–7556.
[16] N.C. Bigall, T. Hartling, M. Klose, P. Simon, L.M. Eng, A. Eychmuller, Nano Lett. 8
(2008) 4588–4592.
[17] A.M. Schwartzberg, T.Y. Olson, C.E. Talley, J.Z. Zhang, J. Phys. Chem. B 110 (2006)
19935–19944.
[18] N. Semagina, L. Kiwi-Minsker, Catal. Lett. 127 (2009) 334–338.
[19] S. Lee, H. Chon, M. Lee, J. Choo, S.Y. Shin, Y.H. Lee, I.J. Rhyu, S.W. Son, C.H. Oh,
Biosens. Bioelectron. 24 (2009) 2260–2263.
[20] N. Nitin, D.J. Javier, D.M. Roblyer, R. Richards-Kortum, J. Biomed. Opt. 12 (2007)
051505–051510.
[21] M.C. Skala, M.J. Crow, A. Wax, J.A. Izatt, Nano Lett. 8 (2008) 3461–3467.
[22] H. Chon, S. Lee, S.W. Son, C.H. Oh, J. Choo, Anal. Chem. 81 (2009) 3029–3034.
[23] A. Yella, E. Mugnaioli, M. Panthöfer, A. Helen, U. Therese, W. Kolb, Tremel,
Angew. Chem. Int. Ed. 48 (2009) 6426–6430.
[24] Y. Xiang, L. Cao, J. Arbiol, M.L.A. Brongersma, A. Fontcuberta i Morral, Appl. Phys.
Lett. 94 (2009) 163101–163103.
[25] F.L. Jia, L.Z. Zhang, X.Y. Shang, Y. Yang, Adv. Mater. 20 (2008) 1050–1054.
[26] S. Goy-López, E. Castro, P. Taboada, V.C. Mosquera, Langmuir 24 (2008)
13186–13196.
4. Conclusions
In summary, we have developed, for the first time, a fast and
economical route based on an efficient microwave assisted pro-
cess for the preparation of bismuth nanospheres at large scale. The
diameter of bismuth nanospheres was varied by altering the con-
centration of urea or the reaction time. Bismuth citrate plays a key
role in the formation of bismuth nanospheres. This method here
provided a more convenient, fast and economic way for the mass
production of metal nanospheres.
[27] C.-W. Xiao, C.-M. Shen, Z.-C. Xu, T.-Z. Yang, H.-J. Gao, Chin. Phys. B 17 (2008)
2066–2071.
[28] C.-L. Lee, L.-C. Kuo, Y.-C. Huang, Y.-W. Yen, Electrochem. Commun. 8 (2006)
697–702.
[29] C.-L. Lee, R.-B. Wu, C.-M. Syu, Electrochem. Commun. 11 (2009) 270–273.
[30] A. Gautam, S. Ram, J. Alloys Compd. 463 (2008) 428–434.
[31] M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa, T. Tsuji, Chem. Eur. J. 11
(2005) 440–452.
[32] Q. Yao, Y. Zhu, L. Chen, Z. Sun, X. Chen, J. Alloys Compd. 481 (2009) 91–95.
[33] G. Wei, W. Qin, D. Zhang, G. Wang, R. Kim, K. Zheng, L. Wang, J. Alloys Compd.
481 (2009) 417–421.
[34] A. Alarifi, N.M. Deraz, S. Shaban, J. Alloys Compd. 486 (2009) 501–506.
[35] T. Thongtem, S. Jattukul, A. Phuruangrat, S. Thongtem, J. Alloys Compd. 491
(2010) 654–657.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (Grant 20801043), Wuhan Chenguang Scheme (Grant
200850731376) established under Wuhan Science and Technology
Bureau, the Open Research Fund of Key Laboratory of Catalysis and
Materials Science of the State Ethnic Affairs Commission & Min-
istry of Education (No. CHCL09010), and Graduate Innovative Fund
of Wuhan Institute of Technology (to J. Wu).
[36] X. Hu, J.C. Yu, Chem. Mater. 20 (2008) 6743–6749.
[37] C. Tang, Y.X. Zhang, G. Wang, H.Q. Wang, G. Li, Chem. Lett. 37 (2008) 722–723.
[38] M. Mo, J.C. Yu, L. Zhang, S.-K.A. Li, Adv. Mater. 17 (2005) 756–760.
[39] W. Du, X. Qian, X. Niu, Q. Gong, Cryst. Growth Des. 7 (2007) 2733–2737.
[40] Y. Yin, A.P. Alivisatos, Nature 437 (2005) 664–670.
[41] A.P. Alivisatos, Science 271 (1996) 933–937.
[42] V. Privman, D.V. Goia, J. Park, E. Matijevic, J. Colloid Interf. Sci. 213 (1999) 36–45.
[43] A.P. Alivisatos, Science 289 (2000) 736–737.
[44] R.L. Penn, G. Oskam, T.J. Strathmann, P.C. Searson, A.T. Stone, D.R. Veblen, J.
Phys. Chem. B 105 (2001) 2177–2182.
[45] J.F. Banfield, S.A. Welch, H. Zhang, T.T. Ebert, R.L. Penn, Science 289 (2000)
751–754.
References
[46] S. Cho, J.-W. Jang, S.-H. Jung, B.R. Lee, E. Oh, K.-H. Lee, Langmuir 25 (2009)
3825–3831.
[47] X. Zou, E. Ying, S. Dong, Nanotechnology 17 (2006) 4758–4764.
[1] F.Y. Yang, K. Liu, C.L. Chien, P.C. Searson, Phys. Rev. Lett. 82 (1999) 3328.
[2] F.Y. Yang, K. Liu, K. Hong, D.H. Reich, P.C. Searson, C.L. Chien, Science 284 (1999)
1335–1337.