Fabrication of
␦-Bi
2O
3Nanowires
C. C. Huang, I. C. Leu, and K. Z. FungzDepartment of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan
␦-Bi2O3nanowires were successfully fabricated using a thermal enhanced process on electroplated Bi nanowires. Bi was first prepared using template-assisted electroplating. After thermal modification at 350°C for 12 h in air, Bi2O3 nanowires with high-temperature phase共␦-phase兲 were obtained. A Bi/Bi2O3core-shell structure was examined by high-resolution transmission electron microscopy 共HRTEM兲 after the Bi nanowires heated at 250°C for 12 h. The oxidation reaction of Bi occurred at temperatures below the melting point of Bi共271.3°C兲. According to the analysis of HRTEM, the formation of high-temperature ␦-phase is caused by the coherent relationship between Bi and Bi2O3nanowires.
© 2005 The Electrochemical Society. 关DOI: 10.1149/1.1865632兴 All rights reserved. Manuscript submitted October 9, 2004; revised manuscript received November 24, 2004. Available electronically March 1, 2005.
Bi2O3 was used as a good dielectric material in many applica-tions such as optical coatings and capacitors.␦-Bi2O3 was also of the highest oxygen ionic conductivity共1 S/cm at 800°C兲 for high-temperature electrochemical application such as solid oxide fuel cells 共SOFCs兲 and oxygen pumps. However, the presence of the highest ionic conductivity was due to the fluorite structure with 25% anionic vacancies. Hence,␦-phase was the only favored structure for solid electrolyte. Owing to its defective structure, the ␦-phase is only stable at 723 to 823°C and may be stabilized by adding some rare earth oxide.1-5Solid-state synthesis of doped bismuth oxide was widely investigated to obtain a stable ␦-phase with fluorite structure.6-11 For SOFC application, a nanostructure established at the anode/electrolyte interface would increase the triple phase boundary共TPB兲 to minimize the concentration polarization causing the degradation of voltage. To obtain an anode/electrolyte interface with nanostructure, a facile process was studied here to form nanow-ires of solid electrolyte using anodic alumina oxide共AAO兲 template. Electroplating was a useful method for filling materials into nanoporous AAO templates. Based on Swizter’s research, single-crystal ␦-phase Bi2O3 was obtained by electrodeposition at low temperature.12,13 Thus, nanostructured ␦-Bi2O3 may be fabricated using an electrodeposition process with an AAO template. However, according to Swizter’s work, the electrolyte solution used was con-sisted of 2.5 M KOH which would dissolve AAO membrane. In this work, metallic Bi nanowires were fabricated by electroplating in an ethylene glycol/water electrolyte solution containing Bi共NO3兲3·5H2O. ␦-Bi2O3 nanowires were obtained by heat-treatment at temperatures greater than 250°C. These Bi2O3 nanow-ires were characterized by high-temperature X-ray diffraction 共HTXRD兲, transmission electron microscopy 共TEM兲, and high-resolution transmission electron microscopy共HRTEM兲.
Experimental
Preparation of AAO template.—The nanoporous Al2O3 mem-brane was fabricated by anodization of Al metal. An Al sheet 共99.997% purity; Alfa aluminum foil; Johnson Matthey, Ward Hill, MA兲 of 1 mm thick was used as the starting material for anodiza-tion. One-step anodization was then conducted in 0.3 M oxalic acid solution under a constant voltage of 40 V in a thermostated bath. The anodized specimens were then immersed in a saturated HgCl2 solution to remove the remaining aluminum substrates, and pore opening was then conducted by chemical etching in 10 wt % phos-phoric acid solution at 30°C to remove the barrier layer. A 20 nm thick Pt layer was deposited on the backside of the porous alumina as the working electrode.
Electroplating of Bi nanowires.—The deposited Pt layer served as the working electrode in a conventional three-electrode cell for
electrodeposition using a potentiostat/galvanostat共263A兲. A graphite plate was used as the counter electrode. The electrolyte solution was composed of 48.57g Bi共NO3兲3·5H2O 700 mL ethylene glycol and 300 mL distilled water. To ensure a stable electrodeposition process, the solution was stirred for 1 day to remove the bubbles. Owing to the neutral solution, the AAO template would not be dissolved dur-ing deposition process and remained its nanoporous structure. Hence, the electrochemical method was applied at ⫺0.15 V vs. Ag+/AgCl reference electrode for 10 h to overfill the AAO template with Bi. After rinsing with distilled water, the deposited samples were pasted on a solid substrate共Au coated Si wafer兲 by Ag adhe-sive. The specimen was bathed in 3 M aqueous NaOH for 2 h to dissolve the AAO template. To remove the residual NaOH solution, the samples were also immersed in water for 30 min and rinsed with distilled water several times.
Thermal modification of Bi nanowires.—In this study, the nanowires of Bi2O3 were obtained by oxidizing Bi nanowires. HTXRD共Rikagu Multuflex兲 was used for characterizing the oxida-tion of metallic Bi nanowires at various temperatures in air. The specimen was placed in the high-temperature attachment for HTXRD analysis. With the heating rate of 10°C/min, the tempera-tures of specimens were held at 50, 150, 250, 350, 450, and 550°C for 30 min. The scanning condition was 2°/min from 20 to 80° of 2 angles with 30 kV and 20 mA Cu k␣ radiation. According to the HTXRD results, as-fabricated specimens were also annealed at 250, 350, and 450°C for 12 h in air atmosphere. To collect the nanowires for TEM analysis, the nanowires were scraped off and dispersed in ethanol. Then, several drops of alcohol were to put on the 325 mesh copper grid. A transmission electron microscope共TEM, JEOL-3010兲 and electron diffraction 共ED兲 attached to HRTEM were used for structural analysis of single nanowire.
Results and Discussion
Fabrication of metallic bismuth nanowires array.—Although, Bi nanowires have been successfully prepared in nitric acid solutions,16-20 in this study, to avoid the dissolution of AAO template, an organic solvent, ethylene glycol, was chosen to dissolve bismuth nitrate prentahydrate. After applying a voltage of ⫺0.15 V vs. Ag+/AgCl reference electrode for 10 h, Bi was also successfully deposited by electroplating and filled into the nanopores of AAO in the neutral electrolyte solution. Figure. 1 shows the SEM of Bi nanowires after removal of AAO template. Consequently, the Bi metal deposited in the pores of AAO template was exposed and became free-standing Bi nanowires. The inset clearly shows the Bi nanowire array with an average diameter of 47 nm that is close to the pore diameter of AAO template.
Oxidation of metallic bismuth nanowires.—HTXRD was used for characterizing the phase transformation of bismuth nanowires in air atmosphere. The in situ HTXRD was taken from an as-fabricated
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specimen after heated from 50 to 550°C at interval of every 100°C for 30 min. Figure 2a-f shows the XRD patterns of bismuth nanow-ires heated at temperatures ranging from 50° to 550°C. In Fig. 2a, the reflections of bismuth with rhombohedral lattice were observed, while the reflections of Si, Ag, Au, and Pt are from the substrate, adhesive, conductive layer and retained electrode. Figure 2a-d show the XRD patterns of heat-treated bismuth nanowires. In Fig. 3c, the reflections representing the cubic lattice of␦-Bi2O3were observed. Thus, the oxidation of bismuth nanowires began around 250°C.
The increasing intensity of cubic reflections indicates that the extent of Bi oxidation increased with increasing temperature. Al-though it is known that the equilibrium phase of Bi2O3at tempera-tures below 723°C is␣-Bi2O3,␦-Bi2O3was formed due to the oxi-dation of nanosized bismuth. Similar phase stabilization of ␦ -Bi2O3was also observed in the electrodeposition␦-Bi2O3film.12,13 In addition, ␦-Bi2O3 film could also be synthesized by thermal evaporation method.14,15However, owing to some restricted condi-tion of AAO template, thermal enhanced method was a preferred way to fabricate␦-Bi2O3nanowires array. The reflection intensity of ␦-phase decreased as the temperature increased up to 550°C as Fig. 2e and f show. This implies that the as-oxidized ␦-Bi2O3 was a metastable phase. Consequently, the phase transformation of Bi2O3 nanowire from␦ to ␣ phase occurred as the temperature increased to
Figure 1. SEM of Bi2O3nanowires array.
Figure 2. HTXRD traces of metallic Bi nanowires at共a兲 50, 共b兲 150, 共c兲 250,
共d兲 350, 共e兲 450, and 共f兲 550°C.
Figure 3. TEM images and SAED patterns of共a兲 as-fabricated Bi nanowire
共b兲 ␦-Bi2O3nanowire.
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550°C. Hence, the oxidation reaction of metallic bismuth caused the formation of metastable␦-Bi2O3. As the temperature increased, the equilibrium phase of␣-Bi2O3was obtained.
TEM analyses of Bi and Bi2O3 nanowires.—According to HTXRD results, the oxidation of Bi nanowires was clearly observed at 350°C. Hence, the Bi nanowires were annealed at 350°C for 12 h. Figure 3a shows the TEM image of single Bi nanowire. Again, the diameter of Bi nanowire is⬃47 nm. The inset shows the select area diffraction pattern共SAED兲 of Bi nanowires along 关001兴 zone axis of a rhombohedral lattice 共R3m兲. The electron diffraction pattern shown in Fig. 3b was corresponding to the 关100兴 zone axis of cubic ␦-Bi2O3 共Fm3m兲. Compared to Fig. 3a, the diameter of the ␦-Bi2O3 nanowire 共50 nm兲 is greater than that of Bi nanowire 共42 nm兲. That is because the density of Bi2O3 共8.9 g/cm2兲 is less than that of Bi metal 共9.8 g/cm2兲. The TEM results suggested that Bi2O3 nanowires with metastable ␦-phase could be synthesized at 350°C.
HRTEM observation of core-shell Bi/Bi2O3nanowire.—In prepa-ration of nanostructure materials, the effect of nanoscale on the crystal structure was very important. Some studies pointed out that ␦-Bi2O3 film could be synthesized by oxidation of thermal evaporated Bi.14,15 In addition, ␦-Bi2O3 film was also prepared by electrochemical method.12,13 Hence, the formation of ␦ -Bi2O3nanowires was not due to the nanostructure. To investigate the interface between Bi and ␦-Bi2O3, nanowires with core-shell structure was prepared. According to the HTXRD result as shown in Fig. 2c, both Bi and Bi2O3, were present. To further investigate the oxidation process of Bi nanowires, the specimen was annealed at 250°C for 12 h. From the TEM images shown in Fig. 4, a core-shell structure was formed after oxidation. An obvious interface of Bi/Bi2O3 was observed. From the HRTEM image, obvious lattice image with d-spacing of 1.95 Å was found in the center of the nanowire. The SAED pattern from the same zone axis is also shown on the HRTEM image. According to the identification of pattern, the diffraction was along关124兴 zone axis. Hence, based on the orientation and its d-spacing, the lattice line shown in Fig. 5 was imaged as 共210兲 planes. In addition, the 共210兲 plane in Bi was coherent with the 共110兲 plane in Bi2O3. A schematic diagram of the lattice relationship between
Bi and Bi2O3is shown in Fig. 5. As Fig. 5 showed, the arrangement of Bi atoms was similar to that in Bi2O3. It was suggested that the formation of ␦-Bi2O3 was due to the following two steps. First, Bi atoms reacted with oxygen and formed Bi-O ionic bonding. Second, the lattice was adjusted to favor the formation of ␦-Bi2O3. Although the equilibrium phase of Bi2O3 at low temperature was monoclinic, the formation of metastable ␦-Bi2O3 was clearly enhanced by the coherent relationship at Bi/Bi2O3 interface. Once the atomic diffusion is favored at high temperature, the rearrangement of cations and anions caused the transformation of ␦-phase to ␣-Bi2O3.
Conclusions
With the assistance of AAO template, metallic Bi nanowires were first obtained by electroplating at applied voltage ⫺0.15 V. After heat-treatment at 350°C, Bi transformed to␦-Bi2O3. Because little volume change was occurred during the thermal modification, the nanowires still remained freestanding. According to the crystal structure, the cationic position in the Bi and␦-Bi2O3structures ex-hibited similar symmetry. Hence, rhombohedral Bi was of a coher-ent relationship with cubic␦-Bi2O3. From the structure character-ization of Bi/␦-Bi2O3interface in this work, it was suggested that the formation of the␦-phase was due to the presence of coherent planes of Bi共210兲 and Bi2O3共110兲.
Acknowledgments
This research was supported by the National Science Council, grant no. NSC 93-2120-M-006-004.
National Cheng Kung University assisted in meeting the publication costs of this article.
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Figure 5. A schematic diagram of the lattice relation between Bi and Bi2O3.
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