Effect of the surface configuration on the oxidation of bismuth nanowire

Article abstract of DOI:10.1016/j.materresbull.2006.02.037

Incorporating nanoprocessing into the metal oxidation, it was a facile way to synthesize functional oxide with desired nanostructure. In this work, δ-Bi₂O₃ nanowires were successfully fabricated by the oxidation of electroplated Bi nanowires at 350 °C. δ-Bi₂O₃ is the high-temperature phase of Bi₂O₃ and only stable at 723-823 °C. Partially oxidized nanowires showed core-shell structure composed of metallic Bi and δ-Bi₂O₃. To investigate the mechanism of oxidation reaction, the Bi/Bi₂O₃ interface was characterized by high resolution transmission electron microscopy (HRTEM). HRTEM images showed rapid growth of oxide layer on (2 over(1, ?) 0) plane of rhombohedral Bi metal. The coherency between (10 over(2, ?)) of metallic Bi and (1 0 0) of cubic Bi₂O₃ was observed. A schematic model was also used to describe the oxidation process. The coherency Bi and Bi₂O₃ and the stabilization of high-temperature (fluorite structure) Bi₂O₃ were also discussed based on this model.

Full text of DOI:10.1016/j.materresbull.2006.02.037

Materials Research Bulletin 41 (2006) 1604–1611
www.elsevier.com/locate/matresbu
Effect of the surface configuration on the oxidation
of bismuth nanowire
*
C.C. Huang , K.Z. Fung
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan
Received 1 September 2005; received in revised form 11 January 2006; accepted 21 February 2006
Available online 29 March 2006
Abstract
Incorporating nanoprocessing into the metal oxidation, it was a facile way to synthesize functional oxide with desired
nanostructure. In this work, d-Bi2O3 nanowires were successfully fabricated by the oxidation of electroplated Bi nanowires at
350 8C. d-Bi2O3 is the high-temperature phase of Bi2O3 and only stable at 723–823 8C. Partially oxidized nanowires showed core-
shell structure composed of metallic Bi and d-Bi2O3. To investigate the mechanism of oxidation reaction, the Bi/Bi2O3 interface was
characterized by high resolution transmission electron microscopy (HRTEM). HRTEM images showed rapid growth of oxide layer
¯
¯
on ð210Þ plane of rhombohedral Bi metal. The coherency between ð102Þ of metallic Bi and (1 0 0) of cubic Bi2O3 was observed. A
schematic model was also used to describe the oxidation process. The coherency Bi and Bi2O3 and the stabilization of high-
temperature (fluorite structure) Bi2O3 were also discussed based on this model.
# 2006 Elsevier Ltd. All rights reserved.
Keywords: Metal; Oxidation; Crystal growth
1. Introduction
Bismuth oxide (Bi2O3) thin films are of significantly wide band gap, high refractive index, dielectric permittivity,
photoconductivity, and photoluminescence [1–6]. Therefore, Bi2O3 can be used for optical coatings, Schottky barrier
solar cell and metal/insulator/semiconductor (MIS) capacitors [2,7,8]. In addition, fluorite-structured d-Bi2O3 exhibits
the highest oxygen ionic conductivity that is desired (1 S/cm at 800 8C) for high-temperature electrochemical
application such as SOFC and oxygen pump [9]. The rapid ionic conduction of d-Bi2O3 was due to the fluorite
structure with 25% anionic vacancies. Hence, d-Bi2O3 has the favored structure to be used as a solid electrolyte. Owing
to its defective structure, the d-Bi2O3 is only stable at 723–823 8C [10]. However, d-phase can be stabilized by adding
rare earth oxides [11–16]. In addition to doped-Bi2O3 prepared by solid state syntheses, thin films with polycrystalline
bismuth oxide were also prepared by thermal evaporation [17], thermal oxidation [6,18], activated reactive
evaporation [19], chemical spray pyrolysis [20], and chemical vapor deposition [21], pulsed laser deposition [22].
Switzer et al. and Takeyama et al. also pointed out novel methods for preparing single crystalline d-Bi2O3 by
electroplating [23,24] and halide chemical vapour deposition (AP-HCVD) [25], respectively.
For electrochemical applications, a nanostructure established at the electrode/electrolyte interface would increase
the triple phase boundary (TPB) and then minimize the activation polarization causing the degradation of voltage.
* Corresponding author. Tel.: +886 6 2757575x62969; fax: +886 6 2380208.
E-mail address: sceruti@mse.ncku.edu.tw (C.C. Huang).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.02.037

C.C. Huang, K.Z. Fung / Materials Research Bulletin 41 (2006) 1604–1611
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Anodic alumina oxide (AAO) template-assisted method provides a facile way to prepare metal nanowires [26,27].
Based on the Leontie’s research, bismuth oxide can be prepared by oxidation of Bi under controlled condition and
revealed significant optical properties [5,6,18]. In this work, core-shelled Bi/d-Bi2O3 composite nanowires were
prepared by the similar method to investigate the mechanism of the structural relationship between metal and oxide.
The coherent relationship between rhombohedral Bi and cubic d-Bi2O3 was studied by HRTEM and select area
electron diffraction (SAED). As we know, d-Bi2O3 is not the thermodynamically equilibrium phase at temperature
<723 8C. Thus, the stabilization of this high-temperature phase without the presence of dopants is unusual and worth
of further investigation.
2. Experimental procedure
2.1. Preparation of Bi nanowires
An Al sheet (99.997% purity; Alfa aluminum foil; Johnson Matthey, Ward Hill, MA) of 1 mm thick was used as the
starting material for anodization. After a common process for AAO fabrication, large area nanopores with 47 nm in
average diameter were obtained. A Pt layer of 20 nm thick was deposited on the backside of the porous alumina as the
cathode. Bi nanowires were fabricated by electroplating method. The deposited Pt layer was served as the working
electrode in a conventional three-electrode cell for electrodeposition using potentiostat/galvanostat meter (EG&G 263
A). Graphite plate was used as the counter electrode. The electrolyte solution was composed of 48.57 g
Bi(NO3)3ꢀ5H2O, 700 ml ethylene glycol and 300 ml distilled water. To ensure a stable electrodeposition process, the
solution was stirred for 24 h to dismiss the additional bubbles. The reference electrode was soaked in a separate beaker
with a saturated NaCl solution and connected to deposition bath by a NaCl/Pt salt bridge. Owing to the neutral
solution, the AAO template would not be dissolved during deposition process and remained its nanoporous
morphology. Subsequently, a voltage of ꢁ0.15 V versus Ag/AgCl reference electrode was applied for 10 h to overfill
the AAO template with Bi. After the overfilling, the film was rinsed by distilled water, as-deposited samples were then
pasted on a solid substrate (Au coated Si wafer) by Ag adhesive. The specimen was bathed in 3 M aqueous NaOH for
2 h to dissolve the AAO template. To separate the residual solution, the samples were also immersed in water for
30 min and rinsed with distilled water several times. The morphology of Bi nanowires was examined using scanning
electron microscope (SEM, Hitachi S4100). Transmission electron microscope (TEM, JEOL-3010) and electron
diffraction (ED) were used for structural analysis of single nanowire.
2.2. Oxidation of Bi nanowires
The specimen was annealed at 250 8C to obtain a partially oxidized or core-shell nanowire. To collect the nanowires
for TEM analysis, the nanowires were scraped off and dispersed in ethanol. Then, several drops of alcohol were put on
the 325 mesh copper grid. Transmission electron microscope (TEM, JEOL-3010) and electron diffraction (ED)
attached to HRTEM were used for structural analysis of single nanowire.
3. Results and discussion
3.1. Fabrication of Bi/d-Bi2O3 core-shell nanowire
In our previous work, d-Bi2O3 nanowires were obtained after full oxidation process at 350 8C [28]. For the
investigation of the metal/oxide interface, Bi and d-Bi2O3 coexisting core-shell structure nanowires were also prepared
at 250 8C in this work. From the TEM results shown in Fig. 1(a), the Bi nanowire annealed at 250 8C for 5 h was
covered with an oxide layer. According to the SAED pattern along [0 0 1] axis of rhombohedral Bi shown in Fig. 1(b),
the center of nanowires remained to be metallic Bi with the rhombohedral structure. However, several diffraction rings
were also observed from the pattern. Obviously, the d-spaces of these rings did not match with that of any plane in the
rhombohedral Bi. Fig. 1(c) showed the SAED patterns of the outer layer, which is consistent with the pattern along
[0 0 1] zone axis of cubic d-Bi2O3. In this pattern, diffraction rings similar to those in Fig. 2(b) were also observed.
Subsequently, these rings were attributed to families of (0 0 2) and (0 0 4) of cubic d-Bi2O3 by considering the

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Fig. 1. (a) TEM image of a core-shell nanowire, (b) SAED pattern of central wire and (c) SAED pattern of outer layer.
corresponding of d-spacings. The results pointed out that the core-shell structure was composed of the metallic Bi and
covered with fine-grained d-Bi2O3.
3.2. HRTEM analyses of Bi/d-Bi2O3 interface
In order to examine the lattice relationship between rhombohedral Bi and cubic d-Bi2O3, high resolution TEM was
employed to study the corresponding lattice image and related diffraction patterns. Since the oxidation reaction

C.C. Huang, K.Z. Fung / Materials Research Bulletin 41 (2006) 1604–1611
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Fig. 2. HRTEM images and SAED patterns along (a) [1 2 0] and (b) [0 1 0] zone axis.
usually requires diffusion of metal or oxygen ions through the oxide layer, the most important mechanism of metal
oxidation is the complex interaction at metal surface [29]. From the prior works, the phases of bismuth oxide layer can
also be controlled by the prefer orientations of the metallic thin film [30]. Accordingly, the surface configuration of the
metal/oxide interface was also examined using high resolution transmission electron microscopy (HRTEM) as shown
in Fig. 2. Again, the central wire and outer layer belong to rhombohedral Bi and cubic d-Bi2O3, respectively. In
Fig. 2(a), distinct lattice lines perpendicular to the interface were observed. To identify the plane corresponding to
these lattice images, both the measurement of the d-spacing and the relative SAED pattern were carefully analyzed.
˚
˚
¯
The measured d-spacing from the lattice lines was 2.3 A representing the ð210Þ plane (2.273 A) of rhombohedral Bi.
Based on the relationship of real and reciprocal lattice, the crystal structure was characterized by the Fourier
transformation and the SAED pattern as shown in the insets. From the Fourier transformation of the lattice image, the
transformed points shown in the inset also matched the SAED pattern. Since the SAED pattern was found to be along
[1 2 0] zone axis, the corresponding diffraction planes were indexed. As shown in Fig. 2(b), two lattice lines which
˚
were of different d-spacings, 3.3 and 3.7 A, respectively, and an angel of 51.58 were observed. Correspondingly, the
¯
¯ ¯
planes were characterized to be ð102Þ and ð101Þ. From the perpendicular relationship of d- and R-spacing, the
observed angle matched theoretical plane angle (128.058) as shown in the diffraction pattern. By the characterizing the
SAED patterns and HR images, the real structure and orientations of the planes can also be examined. Hence, the
observed Bi/d-Bi2O3 interface is (0 0 3) as the reflections shown in Fig. 2.
3.3. Favored orientation of Bi for the formation of d-Bi2O3
After annealing at 250 8C for 24 h, a very distinct core-shell structure was obtained as shown in Fig. 3(a).
Obviously, most of this nanowire was oxidized except the central cylinder. From the DF image, the oxidized nanowire

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C.C. Huang, K.Z. Fung / Materials Research Bulletin 41 (2006) 1604–1611
Fig. 3. (a) BF and DF images and (b) HRTEM image and SEAED pattern of the core-shell nanowire.
showed two different phases of Bi and d-Bi2O3. In order to examine the interface of Bi and d-Bi2O3, HRTEM and
SAED pattern were also used. From Fig. 3(b), the HR image near the interface clearly shows the lattice line of Bi
˚
¯
metal. The d-spacing was measured to be 3.28 A that corresponds to the ð102Þ lateral plane of Bi metal. According to
¯
the diffraction pattern in Fig. 3(b), the Bi plane facing the axial direction would be the ð210Þ plane although the lattice
¯
image was not clearly observed. In Fig. 3(a), the oxide layer growing in the direction normal to ð102Þ plane exhibits a
¯
thickness of 13.7 nm. In addition, the oxide layer growing in the direction normal to ð210Þ plane are much thicker than
¯
that normal to ð102Þ plane. Consequently, TEM analyses including SAED patterns and HR images indicated the
¯
¯
¯
(0 0 3), ð102Þ and ð210Þ interfaces. However, the ð210Þ plane exhibits significant rapid oxidation rate than others. Such
difference was analyzed based on lattice relationship between Bi and Bi2O3 as what follows.
3.4. Lattice relationship between rhombohedral Bi and cubic d-Bi2O3
¯
A theoretical crystal structure of Bi with space group of R3m is shown in Fig. 4(a). The lattice parameter of
rhombohedral Bi was a = b = 0.455 nm ¼ c = 1.187 nm and a = b = 908 ¼ g = 1208. As the arrangement of Bi atom
was drawn based on the hexagonal symmetry. In Fig. 4(a), a monoclinic or pseudo-tetragonal subcell was selected in
the rhombohedral Bi lattice with the parameters, a = 0.455 nm, b = 0.474 nm, c = 0.657 nm, a = 93.378, b = 90.038,
¯
and g = 908. To associate with the subcell, the ð210Þ plane of rhombohedral matched well with the (1 0 0) plane of the
¯
pseudo-tetragonal lattice. It is deserved to be mentioned that the ð210Þ was also the fast-growing plane of the Bi/d-
Bi2O3 interface. Hence, the mechanism of Bi oxidation may be viewed from Fig. 5. At first, the Bi lattice exhibits about
˚
¯
48.8% open space. The oxygen may diffuse through the ð210Þ because this plane provide opening of 1.39 A for the
migration of oxygen. Subsequently, the Bi atom in center layer migrate 1/2 of unit length in the direction perpendicular
˚
¯
to ð210Þ in order to eight-fold coordination with oxygen. After oxidation, larger Bi atom (1.68 A) transformed three
electrons to oxygen and became Bi3+ ion with smaller ionic radius of 1.17 A. With the migration of Bi ions and
˚
formation of Bi–O bonding, the pseudo-tetragonal lattice slightly expands to cubic lattice. The cell volume also

C.C. Huang, K.Z. Fung / Materials Research Bulletin 41 (2006) 1604–1611
1609
¯
Fig. 4. (a) crystal structure of rhombohedral Bi and (b) atomic configuration of ð210Þ plane in rhombohedral Bi lattice.
3
changes from 128.89 to 162.77 A . In addition, the diffusion of oxygen is much easier in the direction perpendicular to
˚
¯
¯
ð210Þ plane. Thus, the oxide tends to grow in the direction normal to ð210Þ plane. From the observation of TEM
¯
micrograph, the oxidation rate along the lateral direction (normal to ð210Þ) is significantly higher than that along side
way of the Bi nanowire. More importantly, due to the similarity between the monoclinic lattice (or pseudo-tetragonal)
of Bi and the cubic lattice of Bi2O3, minimum of structure destruction/distortion is needed. Combining the results of
¯
faster growth in the direction perpendicular to ð210Þ and the atomic arrangement of Bi in a rhombohedral lattice, an
oxidation process was proposed through the atom rearrangement shown in Fig. 4(b). Owing to larger free space
˚
¯
(1.39 A) on ð210Þ, oxygen molecules dissociate, ionize and migrate into pseudo-tetragonal during the oxidation
Fig. 5. Structural schematic diagram of phase transformation from rhombohedral Bi to cubic Bi2O3.

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C.C. Huang, K.Z. Fung / Materials Research Bulletin 41 (2006) 1604–1611
reaction. Subsequently, the formation of ionic bonding results the half unit translation of Bi planes. Eight oxygen ions
were able to incorporate into the pseudo-tetragonal lattice. Due to the larger ionic radius of oxygen, the axis was
˚
expanded from 4.54 to 5.46 A. On the other hand, the strong ionic bondings also bring oxygen ion and Bi ion closer
along the c-axis direction. Consequently, a cubic fluorite lattice was obtained. Such a geometric and atomic movement
¯
consideration shows very good agreement with the faster growth of Bi2O3 in the direction normal to ð210Þ. After the
3
oxidation, the cell volume expanded from 128.89 to 162.77 A . The minimal atomic movement and limited volume
˚
expansion is the main reason for the relatively fast oxidation.
3.5. Formation of d-Bi2O3 with high-temperature defective structure
From the SAED pattern shown in Fig. 1(c), the oxide layer exhibits cubic structure and belongs to d-Bi2O3. As we
know that the thermodynamically equilibrium phase of Bi2O3 should be a-Bi2O3 with monoclinic structure. The phase
transformation of d ! a-Bi2O3 phase is basically caused by the very defective fluorite structure of Bi2O3 in which
25% of anion sites are vacant. Thus, a-Bi2O3 is constructed of layer by layer arrangement of B3+ and O2ꢁ without the
presence of oxygen vacancies. However, based on previous TEM results, the oxidation of Bi nanowires surprisingly
formed the metastable d-Bi2O3 phase that is only stable at temperatures >723 8C [28]. From the Bi/Bi2O3 interface
observed, the atomic arrangement or lattice relationship should play an important role on the stabilization of d-Bi2O3
at temperature as low as 250 8C. From previous discussion, the formation of cubic d-Bi2O3 is favored due to the limited
migration of Bi and large free space for the migration of oxygen. In addition, the cubic d-Bi2O3 with 25% anion site
vacant is known to be an excellent oxygen ion conductor. Thus, the rapid migration of oxygen ion also enhances the
growth of cubic d-Bi2O3. Therefore, when Bi nanowires were oxidized at temperatures as low as 250–350 8C, the
nucleation and growth of d-Bi2O3 is favored.
4. Conclusions
¯
It was observed that the ð210Þ plane of Bi was found to be coherent with the fast-growing (1 0 0) plane of d-Bi2O3
during the oxidation process. Based on the results of HRTEM and SAED, a schematic model through the oxidation of
¯
Bi nanowires was illustrated. By converting the rhombohedral structure to pseudo-tetragonal subcell, the ð210Þ plane
provides the larger free space for oxygen ions to migrate. The limited transformation of Bi atoms during the oxidation
favors the formation of high-temperature phase, d-Bi2O3.
Acknowledgement
This research was supported by the National Science Council, Grant No. NSC 94-2120-M-006-002.
References
[1] P.B. Clapham, Br. J. Appl. Phys. 18 (1967) 363.
¨
[2] H. Gobrecht, S. Seeck, H.-E. Bergt, A. Martens, K. Kossmann, Phys. Stat. Sol. 33 (1969) 599;
¨
H. Gobrecht, S. Seeck, H.-E. Bergt, A. Martens, K. Kossmann, Phys. Stat. Sol. 34 (1969) 569.
[3] T.S. Nikolov, M. Aroyo, E. Klein, K. Ikonopisov, Thin Solid Films 30 (1975) 37.
[4] V. Dolocan, F. Iova, Phys. Stat. Sol. A 64 (1981) 755.
[5] L. Leontie, M. Caraman, G.I. Rusu, J. Optoelectron. Adv. Mater. 2/4 (2000) 385.
[6] L. Leontie, M. Caraman, M. Alexe, C. Harnagea, Surf. Sci. 480 (2002) 507.
[7] T.A. Raju, A.A. Talwai, J. Appl. Phys. 52 (1981) 4877.
[8] E.Y. Wang, K.A. Pandelisev, J. Appl. Phys. 52 (1981) 4818.
[9] H.A. Harwig, Z. Anorg. Allg. Chem. 444 (1978) 151.
[10] H.A. Harwig, A.G. Gerards, Thermochim. Acta 28 (1979) 121.
[11] K.Z. Fung, A.V. Virkar, J. Am. Ceram. Soc. 74 (1991) 1970.
[12] K.Z. Fung, J. Chen, A.V. Vikar, J. Am. Ceram. Soc. 76 (1993) 2405.
[13] L.G. Sillen, Ark. Kemi Mineral. Geol. 12A (1937) 1.
¨
[14] G. Gattow, H.Z. Schroder, Anorg. Allg. Chem. 318 (1962) 176.
[15] G. Gattow, D.Z. Schu¨tze, Anorg. Allg. Chem. 328 (1964) 44.
[16] A.V. Joshi, N. Weber, J. Mater. Sci. 25 (1990) 1237.
[17] A.A. Agasiev, Y.Y. Guseinov, Mater. Res. Bull. 21 (1986) 765.

C.C. Huang, K.Z. Fung / Materials Research Bulletin 41 (2006) 1604–1611
[18] L. Leontie, M. Caraman, M. Delibas, G.I. Rusu, Mater. Res. Bull. 36/9 (2001) 1629.
1611
[19] J. George, K.S. Joseph, Thin Solid Films 148 (1987) 181.
[20] R.H. Misho, W.A. Murad, Z.A.R. Salmin, Solar Energy Mater. 21 (1991) 347.
[21] G. Bandoli, D. Barreca, E. Brescacin, G.A. Rizzi, E. Tondello, Chem Vap. Depos. 2 (1996) 238.
[22] M. Alexe, J.F. Scott, C. Curran, N.D. Zakharov, D. Hesse, A. Pignolet, Appl. Phys. Lett. 73 (1998) 1592.
[23] J.A. Switzer, Science 284 (1999) 293.
[24] E.W. Bohannan, J.A. Switzer, Solid State Ionics 131 (2000) 97.
[25] T. Takeyama, N. Takahashi, T. Nakamura, S. Ito, J. Phys. Chem. Solids 65 (2004) 1349.
[26] C.G. Jin, X.G. Li, J. Mater. Chem. 13 (2003) 1743.
[27] Liang Li, Lide Zhang, Mater. Lett. 59 (2005) 1223.
[28] C.C. Huang, K. Z. Fung, Electrochem. Solid State Lett., 8 (2005) 204.
[29] H. Over, A.P. Seitsonen, Science 297 (2002) 2003.
[30] C.C. Huang, K.Z. Fung, Mat. Res. Bul. 41 (2006) 110.