Orientation-controlled phase transformation of Bi2O3 during oxidation of electrodeposited Bi film

Article abstract of DOI:10.1016/j.materresbull.2005.07.043

High-temperature fluorite structure Bi₂O₃ is a well-known solid electrolyte owing to its high oxygen ion conductivity. In this study, Bi₂O₃ thin film was prepared by the oxidation process of the electrodeposited metallic Bi film. The crystal structures of the oxidized Bi films varied with the applied voltages during the electroplating process. Pure α-Bi₂O₃ was obtained when the oxidation was conducted for the metallic Bi film electrodeposited at -0.1 V. Only β-Bi₂O₃ was observed as a -0.5 V electrodeposited Bi film was oxidized. The crystal structure of Bi ₂O₃ obtained by oxidation of metallic Bi film may dominantly be affected by the orientation of as-electrodeposited Bi film. Such kind of process is favorable to the preparation of functional ceramic with specific crystal structure.

Full text of DOI:10.1016/j.materresbull.2005.07.043

Materials Research Bulletin 41 (2006) 110–118
www.elsevier.com/locate/matresbu
Orientation-controlled phase transformation of Bi2O3
during oxidation of electrodeposited Bi film
*
Chaur-Chi Huang , Teng-Yi Wen, Kuan-Zong Fung
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan
Received 14 April 2005; received in revised form 16 July 2005; accepted 20 July 2005
Available online 31 August 2005
Abstract
High-temperature fluorite structure Bi2O3 is a well-known solid electrolyte owing to its high oxygen ion
conductivity. In this study, Bi2O3 thin film was prepared by the oxidation process of the electrodeposited metallic
Bi film. The crystal structures of the oxidized Bi films varied with the applied voltages during the electroplating
process. Pure a-Bi2O3 was obtained when the oxidation was conducted for the metallic Bi film electrodeposited at
ꢀ0.1 V. Only b-Bi2O3 was observed as a ꢀ0.5 Velectrodeposited Bi film was oxidized. The crystal structure of Bi2O3
obtained byoxidation of metallicBi film may dominantly be affected bytheorientation of as-electrodeposited Bi film.
Such kind of process is favorable to the preparation of functional ceramic with specific crystal structure.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: B. Crystal growth; C. X-ray diffraction; D. Crystal structure
1. Introduction
Bi2O3 is recognized as the best oxygen ionic conductor. Its high conductivity is highly dependent on its
crystal structure and the concentration of oxygen vacancies. Many studies investigated the phase
transformation of the Bi2O3 [1–4]. d-Bi2O3, which is only stable at high temperature (723–823 8C),
exhibits defective fluorite structure with the concentration of oxygen vacancies as high as 25 mol%. At
temperatures lower than 723 8C, monoclinic a-Bi2O3 was obtained. In addition, b- and g-phases were also
examined at low temperature (about 350 8C) while cooling from a high-temperature with a rapid cooling
* Corresponding author.
E-mail address: sceruti@cubic.mat.ncku.edu.tw (C.-C. Huang).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.07.043

C.-C. Huang et al. / Materials Research Bulletin 41 (2006) 110–118
111
rate. Except for the heat treatment conditions, some other parameters may also be changed to control the
growth behaviors of the material. For example, the single crystalline d-Bi2O3 film could be obtained by
electrodeposition on a Au single crystal [5]. The single crystalline sapphire is usually used as the substrate
for the preparation ofthe metal oxide single crystal in the deposition process because the lattice constants of
sapphire is very close to that of substrates. In some previous studies, the effect of the metal surface
characteristics on the growth behaviors of the oxide was pointed out [6]. It was suggested that the surface
oxygen atoms would take part in the formation of oxide; the mass transport of oxygen and metal ions would
limit the growth of oxide layer. For this reason, an appropriate orientation of the substrate can guide the
growthofthematerialwithaspecificorientation. Inthisstudy, theBi2O3 thinfilmwiththespecificpreferred
orientation was prepared by oxidation of electrodeposited Bi film. The underlying mechanisms of the oxide
growth behaviors are also discussed through the structural relationship between Bi and Bi2O3.
2. Experimental procedure
A Pt thin film (20 nm thick) was deposited by e-beam evaporation on the n-type Si as the working
electrode, and a Pt sheet was used as the counter electrode. The reference electrode was silver/silver
Fig. 1. XRD traces of electroplated Bi thin films at: (a) ꢀ0.1 V, (b) ꢀ0.2 V, (c) ꢀ0.3 V, (d) ꢀ0.4 V and (e) ꢀ0.5 V.

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chloride. The electrodeposition was conducted in a conventional three-electrode cell (potentiostat/
galvanostat meter, EG&G 263A). The electrolyte solution was composed of 48.57 g Bi(NO3)3ꢁ5H2O,
700 ml ethylene glycol and 300 ml distilled water. To ensure electrodeposition process more stable, the
solution was stirred for 1 day to dismiss the additional bubbles produced during the mixing of the
electrolyte. The electrochemical process was applied at voltages ranging from ꢀ0.1 to ꢀ0.5 V for 30 min
to prepare metallic Bi thin film. To obtain bismuth oxide, the electroplated Bi thin films were heat-treated
at 350 8C for 12 h in air. In addition, the heating rate was controlled at l 8C/min to minimize the thermal
stress of the material during the oxidation process. In order to examine structure variation and oxidation
reaction of the electroplated metallic Bi obtained at various applied voltages, the crystal structures of the
samples were analyzed using Regaku Multi Flex X-Ray system. The Cu Ka radiation was used and the
scanning rate was set at 2 8C/min at a 2u range between 20 and 608. The lattice parameters were refined
using the least-square method. The microstructure of the samples was examined using SEM.
3. Results and discussion
3.1. Prefer orientation of metallic Bi thin film
Fig. 1 showed the XRD traces of electroplated Bi thin films obtained at applied voltages ranging from
ꢀ0.1 to ꢀ0.5 V. All the patterns reveal pure metallic Bi with rhombohedral structure. The reflection of Pt
¯
was also found at 39.88. The intensities of certain reflections of the metallic Bi, such as (1 0 2), (1 0 4) and
(2 0 2) varied with the applied voltages. It is well-known that the electroplating process was significantly
affected by the applied voltage because the nucleation and growth rate as well as the size of the critical
¯
Fig. 2. Texture coefficients of (1 0 2), (1 0 4) and (2 0 2) planes of electroplated Bi thin films at ꢀ0.1 V to ꢀ0.5 V.

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113
¯
Fig. 3. Crystal structure of rhombohedral Bi and the atomic configurations of (1 0 2), (1 0 4) and (2 0 2) planes.
¯
nucleus are varied with the over-potential [7,8]. As shown in Fig. 1, the intensity of (1 0 2) increased but
(1 0 4) and (2 0 2) decreased as the applied voltages became more negative. For a polycrystalline film, the
relatively higher intensity indicated the specific planes are well-aligned structure.
¯
According to Harris method, texture coefficient values (TC) of (1 0 2), (1 0 4) and (2 0 2) planes were
estimated and drawn versus the applied voltages as shown in Fig. 2. The (2 0 2) and (1 0 4) planes are of
¯
the higher TC values than (1 0 2) at ꢀ0.1 to ꢀ0.5 V. As we knew, the prefer orientation during deposition
process is mainly determined by the surface energy. Consequently, the growth orientation of the thin film
is based on the reducing of surface and strain energies, which are mainly determined by the atomic
configuration of the specific plane. Rhombohedral Bi can be drawn in a hexagonal symmetry with the
lattice parameters ah = bh = 0.455 nm and ch = 1.186 nm, as shown in Fig. 3 [9]. Take other symmetry
into consideration, rhombohedral Bi can be also regarded as a pseudo-tetragonal lattice with the
parameter at = 0.455 nm, bt = 0.474 nm, ct = 0.657 nm, at = 93.378, bt = 90.038 and gt = 908, as shown
2
¯
in Fig. 3. Based on the atomic density, (1 0 2) (2=at ) is more compact than (1 0 4) (2/at ꢂ ct) and (2 0 2)
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
¯
(2=at ꢂ bt þ ct ). Hence, (1 0 2) is of a higher surface energy than (1 0 4) and (2 0 2) owing to the more
broken bonds. As shown in Fig. 2, TC(2 0 2) increases at ꢀ0.1 to ꢀ0.2 V but decreases at ꢀ0.2 to ꢀ0.5 V.
In addition, TC(1 0 4) decreases at ꢀ0.1 to ꢀ0.3 V but increases at ꢀ0.3 to ꢀ0.5 V. It implied that the
prefer orientations of (2 0 2) and (1 0 4) are controlled by the adjustment of the surface and the intrinsic
¯
stress. However, the growth of (1 0 2) plane is mainly affected by the over-potential owing to its relative
higher surface energy. Hence, the TCð1 0 2Þ increases with the increasing applied voltages meaning that the
¯
¯
growth of (1 0 2) plane requires relative higher driving force.
3.2. Microstructure of Bi thin film
Fig. 4 shows the morphologies of metallic Bi film obtained at different applied voltages. Most of the
films are composed of isolated grains. The average grain size increases as the applied voltage became
more negative. At ꢀ0.5 V, the metallic Bi film became more continuous than those obtained at ꢀ0.1 to
ꢀ0.4 V. During the electrodeposition, the final average grain size usually determined by the growth
competition between large and small grains. The results in Fig. 4 indicate that the growth of the most

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Fig. 4. SEM micrographs of electroplated Bi thin films at: (a) ꢀ0.1 V, (b) ꢀ0.2 V, (c) ꢀ0.3 V, (d) ꢀ0.4 V and (e) ꢀ0.5 V.
small grains are retarded by the neighboring large grains under a large over-potential, leading to the larger
average grain size.
3.3. Oxidation of Bi thin film
Fig. 5(a–c) show the morphologies of oxidized Bi film electrodeposited at a low over-potential (ꢀ0.1
to ꢀ0.3), the average grain size are larger and many parts of the oxidized film became more continuous
(compared with the results in Fig. 4(a–c)). Because the heat treatment temperature is higher the melting
point of metallic Bi (271.3 8C), the results in Fig. 5(a–c) implies that grain growth occurs during the heat
treatment. Similar growth behaviors should occur during oxidation of metallic Bi film electrodeposited at
a higher over-potential (ꢀ0.4 to ꢀ0.5). Nevertheless, the average grain size shown in Fig. 5(d and e) seem
to be smaller than those shown in Fig. 4(d and e). Though the heating rate is controlled at l 8C/min to
minimize the thermal stress of the material during the oxidation process, some additional stresses will

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Fig. 5. SEM micrographs of oxidized Bi thin films electroplated at: (a) ꢀ0.1 V, (b) ꢀ0.2 V, (c) ꢀ0.3 V, (d) ꢀ0.4 V and (e)
ꢀ0.5 V.
still be induced because of the volume expansion when metallic Bi transform to Bi2O3. Besides, the
intrinsic stresses within the metallic Bi film electrodeposited at a more negative voltage should be higher
as a result of the higher growth rate of the deposit under a larger over-potential. These stresses will be
released by the formation of cracks along the grain boundary or passing through the grain, leading to the
smaller grain size shown in Fig. 5(d and e).
Fig. 6 shows the XRD traces of oxidized Bi films electrodeposited at applied voltages. Pure a-Bi2O3
was obtained when the oxidation was conducted for the metallic Bi film electrodeposited at ꢀ0.1 V, as
shown in Fig. 6(a). The a-Bi2O3 was of a monoclinic structure and a thermal equilibrium phase from
room temperature to 723 8C. When the ꢀ0.2 to ꢀ0.4 Velectrodeposited Bi film was oxidized, the oxide
was composed of a-Bi2O3 and b-Bi2O3, as shown in Fig. 6(b–d). Only b-Bi2O3 was observed as a ꢀ0.5 V
electrodeposited Bi film was oxidized as shown in Fig. 6(e). In the previous literatures, b-Bi2O3 of
tetragonal structure was generally prepared by rapid cooling after heat treatment [4,10,11]. In the present
study, pure b-Bi2O3 could easily be produced just by the oxidation of metallic Bi film electrodeposited at

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Fig. 6. XRD traces of oxidized Bi thin films electroplated at: (a) ꢀ0.1 V, (b) ꢀ0.2 V, (c) ꢀ0.3 V, (d) ꢀ0.4 V and (e) ꢀ0.5 V.
a more negative voltage. According to those described in Fig. 1, the results in Fig. 6 also imply that the
¯
(2 0 2) preferred orientation of metallic Bi is favorable to the formation of a-Bi2O3 while (1 0 2) preferred
orientation of metallic Bi is favorable to the formation of b-Bi2O3.
3.4. The formation of b-Bi2O3
According to the results as mentioned above, an attractive phenomenon should be emphasized here.
Even though the heat treatment condition was fixed, the phase ratio of oxidized film still varied when the
oxidation was conducted for the metallic Bi film electrodeposited at different voltages. Consequently, the
crystal structure of Bi2O3 obtained by oxidation of metallic Bi film may dominantly be affected by the
orientation of as-electrodeposited Bi film. In order to investigate the structural relationship between the
as-electroplated Bi and the as-produced bismuth oxide during oxidation reaction, a schematic diagram
was drawn in Fig. 7. The Bi atoms are not in a close-packed arrangement in rhombohedral symmetry and
some free space is available. Since Bi2O3 is an oxygen ionic conductor, oxygen ions would diffuse
through the oxide layer easily. Hence, the structure of bismuth oxide formed may be affected by the
arrangement of Bi atoms in the prefer oriented planes. In Fig. 6, the b-Bi2O3 formed obviously from the
¯
oxidation of Bi film with (1 0 2) prefer orientation. On the other hand, a-Bi2O3 was obtained when Bi film

C.-C. Huang et al. / Materials Research Bulletin 41 (2006) 110–118
117
¯
Fig. 7. Schematic diagrams of coherent relationship between (a) Bi(1 0 2)/b-Bi2O3(0 0 1) and (b)Bi(2 0 2)/a-Bi2O3(0 1 0).
¯
was preferably oriented along (2 0 2) plane. These result indicated that the Bi atoms along (1 0 2) is
closely matched with Bi ions on the plane of (0 0 1) in b-Bi2O3 as shown in Fig. 7(a). Also, the Bi atoms
along (2 0 2) plane is consist with Bi ions on the plane of (0 1 0) in a-Bi2O3 as shown in Fig. 7(b). The
crystal structure of the Bi2O3 is determined by the oxidation reaction without greater deformation on the
Bi/Bi2O3 interface. Consequently, the prefer orientations of the specific atomic planes cause the
formation of favored oxide during oxidation process.
4. Conclusions
In this study, the Bi2O3 thin film with the specific crystal structure was prepared by oxidation of
electrodeposited Bi film. During the preparation of metallic Bi film, the preferred orientation of Bi film
¯
varied from (2 0 2) to (1 0 2) as the applied voltage became more negative. After oxidation, the phase
ratio is different even though the heat treatment condition was fixed. Only b-Bi2O3 was obtained on the
¯
metallic Bi film with (1 0 2) preferred orientation, while only a-Bi2O3 was obtained on the metallic Bi
film with (2 0 2) preferred orientation. Therefore, during the oxidation, the crystal structure of the Bi2O3
can be controlled by the orientation of the metallic Bi film.
Acknowledgement
This research was supported by the National Science Council, Grant No. NSC 93-2120-M-006-004.

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