Fabrication of bismuth oxide-tin oxide nanowires by direct thermal oxidation of Bi-Sn eutectic nanowires

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Fabrication of bismuth oxide

–tin oxide nanowires by direct thermal oxidation of

Bi

–Sn eutectic nanowires

Shih-Hsun Chen

a

, Chiu-Yen Wang

b

, Tzeng-Feng Liu

a

, Chuen-Guang Chao

a,

⁎

a

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan

b_{Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan}

a b s t r a c t

a r t i c l e i n f o

Article history: Received 26 July 2010 Accepted 6 August 2010 Available online 19 August 2010 Keywords:

Thermal oxidation Bi–Sn eutectic nanowires BiOx–SnOxnanowires

Bismuth oxide–tin oxide (BiOx–SnOx) heterostructure nanowires with a diameter of 70 nm were fabricated by directly annealing Bi–Sn eutectic nanowires synthesized by the vacuum hydraulic pressure injection process. After removal of AAO (Anodic Aluminum Oxide) template with an etching solution, a spontaneous oxide was formed on nanowires to enclose the Bi–Sn eutectic alloys. While these nanowires went through the annealing process with the proper heating rate of 50 °C/min, the well-annealed oxide nanowires remain solid, straight and segmental. The results of cathodoluminescence (CL) spectrum and photoresponse proved that the products consisted of bismuth oxide and tin oxide. This fabrication methodology provides a simple way to produce one-dimensional oxide nanomaterials.

1. Introduction

Functional metal oxides are used in many applications due to their unique physical properties. Among them, tin oxide (SnO2) and

bismuth oxide (Bi2O3) are two important strategic materials in

modern solid-state technology. As semiconductors with large and wide energy band gaps, they are widely applied to gas sensors[1,2], optoelectronic devices[3,4], photovoltaic cells[5,6], and transparent conduction electrodes[7]. However, it is generally accepted that the chemical and physical properties of these materials are inﬂuenced by their dimensions. One-dimensional materials, such as nanowires, nanorods and nanotubes, are considered ideal materials for both fundamental and applied research.

Due to the scienti_{ﬁc and technological importance of SnO}2and

Bi2O3, many studies have published synthesis reports on their

one-dimensional nanostructures. Chemical vapor deposition (CVD)[8,9], chemical method[10,11], laser ablation[12], and thermal evaporation

[13]are common bottom-up approaches to the preparation of one-dimensional self-assembly materials in the literature. Even so, to seek a simpler, cheaper, and more stable process is always the top objective for industrial applications.

In this study, we present a fabrication method to obtain BiOx–SnOx

nanowires by directly oxidizing Bi–Sn eutectic nanowires in an air furnace. The Bi–Sn eutectic nanowires are of a segmental microstruc-ture, and are synthesized by a template-assisted casting process[14]. The as-injected nanowires have a regular structure; thus, the BiOx–

SnOx nanowires are simultaneously obtained through a thermal

oxidation process; and retain the segmental microstructure. The optical properties are evaluated as the evidence for both oxides. 2. Experimental procedures

The segmental Bi–Sn eutectic nanowires were synthesized by the vacuum hydraulic pressure injection process [14]; the as-injected nanowires had an average diameter of 70 nm and lengths of several microns. Details of the injection process have been reported previously [15,16].The prepared materials were put into an air furnace and directly annealed at 700 °C for 1 h. After cooling to room temperature, the BiOx–SnOxnanowires were obtained.

The morphology and appearance of metal and oxide nanowires were characterized byﬁeld-emission scanning electron microscopy (FESEM, JSM-6500) and transmission electron microscopy (TEM, JEM-2010) combined with energy-dispersive X-ray spectrometry (EDS).

Spectroscopy of the oxide nanowires was performed by SEM-cathodoluminescence (CL) at room temperature with a commercial Gatan MonoCL2 system using an alkali halide photomultiplier detector attached to the SEM described above. To characterize the photocurrent, an ultraviolet (UV) lamp (λ=325 nm) was used as a light source; the power density of the light was 10 mW/cm2_{. The}

photocurrent was measured by a two-probe I–V system using a

Keithley 237 source measurement unit. 3. Results and discussion

The nanostructure materials synthesized by this injection process formed an oxide shell during the removal of the AAO template. This

Materials Letters 64 (2010) 2502–2504

⁎ Corresponding author.

E-mail address:[email protected](C.-G. Chao).

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spontaneous oxide shell was stable and served as a container to preserve the alloys inside during the annealing process [17]. To prevent the metastable phase, the annealing temperature must be no higher than 729 °C[18]; thus, the temperature was 700 °C.

Fig. 1(a) shows an SEM image of as-injected Bi–Sn nanowires after removal of the AAO template with an etching solution (0.4 M H3PO4+ 0.2 M CrO3) at 60 °C for 2 h and washing with ethanol and

deionized water. A thin oxide layer could be observed on the products. These spontaneous oxides formed a core–shell structure and enclosed the alloys within the nanowires for the next annealing treatment. However, the initial oxides were not dense enough for rapid expansion of liquid alloys. If the heating rate was higher than 100 °C/min, the alloys would not convert to oxides within the shell and would instead leak out of it. InFig. 1(b), the SEM image shows the residual oxide shells after annealing with an excessive heating rate. The fragmentation of the annealed wires under fast heating was mainly caused by the brittleness of initial oxides. The original oxide on bismuth segments was thicker and incomplete[16]. Upon subsequent annealing, liquid alloys leak out the oxide shell from locations such as defects and interfaces, which results in a broken segmental structure.

Fig. 2shows TEM images of annealed nanowires at the excessive and proper conditions respectively. InFig. 2(a), most alloys leaked out, and only fractured oxide nanowires were obtained after the excessive annealing process. Moreover, some complete segmental oxide nano-wires could be still discovered. This was due to the remaining AAO, which served as dense oxide shells of Bi–Sn nanowires and restrained

the alloys from leaking. As the heating rate was reduced to a proper value of 50 °C/min, the morphology of oxide nanowires revealed a solid rod with a segmental structure, as shown inFig. 2(b). The differences in the segments originally resulted from precipitation of bismuth and tin in sequence. Thus, the oxide nanowires followed the array of Bi–Sn eutectic nanowires.

While Bi–Sn eutectic nanowires were annealed at 700 °C for 1 h with a heating rate of 50 °C/min, the one-dimensional nano-compounds made of bismuth oxide and tin oxide were produced in the shape of straight wires. The TEM image inFig. 3(a) shows that the morphology of well-annealed nanowires retained the segmental structure. Further-more, compositions of nanowires were also determined by EDS analysis, presented in Fig. 3(b). The observation indicates that BiOx–SnOx

nanowires were successfully produced through the annealing process. The Mo and Ta peaks came from the TEM grid and heating holder, respectively.

To ensure the types of oxide nanowires, an SEM-CL spectrum was utilized to inspect the radiative property of oxides at room temperature. InFig. 4, the pulsating line, spreading from 200 nm to 800 nm, represents the raw CL spectrum of oxide nanowires, and its ﬁtting curve is also shown. According to previous studies[8,13,19], it could be decomposed into two sub-curves. One is sharper and

ranges from 300 nm to 530 nm. Leonite [13] reported that the

absorption curve of the Bi–O system mainly resulted from the band gaps ofα-Bi2O3and Bi2O3(Eg= 2.29–3.31 eV). The other is consistent

Fig. 1. (a) SEM image of as-injected Bi–Sn eutectic nanowires after removal of the AAO template with an etching solution. The core–shell of the segmental structure can be observed. (b) SEM image of nanowires annealed with an excessive heating rate.

Fig. 2. TEM images of annealed Bi–Sn eutectic nanowires under (a) excessive and (b) proper heating rates. The oxides were not dense enough to conﬁne the melting alloys under an excessive heating rate, or the proper annealing condition could result in stiff and complete nanowires. The residual AAO would help to maintain the integrity of melting alloys.

2503 S.-H. Chen et al. / Materials Letters 64 (2010) 2502–2504

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with the hypothesis that the transition is due to deep levels in the band gap due to surface states induced by oxygen vacancies of SnO2

nanowire, presented by the authors[8,19]. Thus, the CL spectrum clariﬁes the types of oxides and supports the existence of BiOx–SnOx

nanowires.

Fig. 5shows the photoresponse of BiOx–SnOxnanowires at a bias

voltage of 0.8 V in air under modulated illumination of 325 nm wavelength light. The photocurrent of the oxide nanowires was

increased rapidly when the light was switched on, and it saturated. After the light was switched off, the current decayed to one-tenth of the saturated value within 300 s. This phenomenon indicated that the photoresponse of n-type oxide semiconductor is controlled by surface effects, such as oxygen absorption under dark conditions by trapping electrons (O2

(g)+ e−= O2−(ad)) and photodesorption of oxygen ion

by capturing photogenerated holes (h+_{+ O}

2−(ad)= O2(g)) [20,21].

Recent studies[22,23]have also focused on functional oxides; and identiﬁed them with this optical mechanism.

4. Conclusions

This work demonstrates that the compound nanowires of bismuth oxide and tin oxide were produced by directly annealing Bi–Sn eutectic nanowires synthesized by the vacuum hydraulic pressure injection process. The well-annealed oxide nanowires were straight and solid, and retained the segmental structure. Moreover, the CL spectrum and photoresponse analyses conﬁrmed that the types of oxides were BiOx–SnOx nanowires. The characteristics of this

fabrication methodology provide a simple way to produce one-dimensional oxide nanomaterials.

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Fig. 3. (a) TEM image of well-annealed Bi–Sn eutectic nanowires. (b) EDS was used to identify the composition of nanowires. The Mo and Ta peaks came from the TEM grid and heating holder, respectively.

Fig. 4. CL spectrum from BiOx–SnOxnanowires with an average diameter of 70 nm. The

curve was composed of two emission resources: the band gap of Bi2O3and the oxygen

vacancies in SnO2[8,13,19].

Fig. 5. Photoresponse of Bi2O3–SnO2nanotubes with periodic irradiation of light under

continuous illumination of 325 nm in air at a bias voltage of 0.8 V.