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Synthesis of Bi2O3 nanocones over large areas by magnetron sputtering
SCT-20087; No of Pages 6 Surface & Coatings Technology xxx (2015) xxx–xxx
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Synthesis of Bi2O3 nanocones over large areas by magnetron sputtering Li-Chia Tien ⁎, Ying-Hong Liou Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan
a r t i c l e
i n f o
Article history: Received 25 September 2014 Accepted in revised form 30 January 2015 Available online xxxx Keywords: Bismuth oxide 1D nanostructures Physical vapor deposition
a b s t r a c t Large areas of bismuth oxide (Bi2O3) nanocones were deposited onto Si(001) substrates by magnetron sputtering. The structural and optical properties of the nanocones were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The obtained tapered nanostructures consist of high-density nanocones with diameters approximately 70–130 nm and lengths of 1–3 μm. XRD results reveal that the Bi2O3 nanocones can undergo a phase transition from the α to the β phase at growth temperatures over 450 °C. This phase transition was confirmed by TEM and PL. The growth mechanism of Bi2O3 nanocones was identified as grain boundary-assisted growth, in which a Bi seeding layer is crucial to the formation of the nanostructures. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Significant interest exists in the use of bismuth oxide (Bi2O3) as a photocatalyst. The reported energy band gaps of α-Bi2O3 (2.91 eV) and β-Bi2O3 (2.58 eV) are significantly lower than that of TiO2 (anatase, 3.32 eV), suggesting that excitation by visible light can generate electron–hole pairs more efficiently in Bi2O3 than in TiO2 [1–3]. A particular phase of Bi2O3 may be synthesized by controlling the conditions and methods of growth, and particularly the growth temperature [4,5]. The ability to control the phase formation of Bi2O3 nanostructures is critical to their potential applications, such as development of fast oxygen conductors for solid oxide fuel cells (SOFC) [6–8]. Owing to their high surface-to-volume ratio, one-dimensional (1D) Bi2O3 nanostructures may be used as efficient visible light-driven photocatalysts [9–13]. Various methods for synthesizing 1D Bi2O3 nanostructures have been proposed; they include template synthesis, hydrothermal methods, the vapor transport process, and oxidative metal vapor transport deposition [14–18]. The main drawbacks of these synthetic methods are the small yield and high fabrication cost, which may become problematic when 1D Bi2O3 nanostructures are required for large-scale applications. Owing to the complexity of Bi2O3 compounds, investigations of the phase-selective synthesis of Bi2O3 nanostructures are very limited. Magnetron sputtering is an extensively used thin film deposition method because it can provide high deposition rates and yields dense and highly adhesive thin films with large areas. Efforts have made to synthesize various 1D nanostructures of zinc oxide (ZnO), titanium oxide (TiO2), ruthenium oxide (RuO2), and indium tin oxide (ITO), for example, by magnetron sputtering [19–23]. However,
⁎ Corresponding author. Tel.:+886 3 863 4208; fax: +886 3 863 4200. E-mail address: [email protected] (L.-C. Tien).
relatively little research has been conducted to understand the mechanism of growth of 1D nanostructures by magnetron sputtering. This work reports on the synthesis of α- and β-Bi2O3 nanocones by RF magnetron sputtering. The samples were examined comprehensively using a variety of characterization tools. The structural, compositional, and optical properties of obtained nanostructures were analyzed and discussed. The growth temperature can be controlled to synthesize αBi2O3 and β-Bi2O nanocones in high yield on 4 × 4 cm Si(001) substrate. The growth mechanism of Bi2O3 nanocones is discussed. 2. Materials and methods Bi2O3 nanocones were grown using a two-step RF magnetron sputtering process. Before deposition, the silicon (001) substrates (4 × 4 cm) were ultrasonically cleaned with acetone, and then methanol, before being dried in compressed N2. A high-purity bismuth target (99.95%) with a diameter of 2 in. was used as the source material and the substrate-to-target distance was 14 cm. Before sputtering, the chamber was pumped down to a base pressure of 3 × 10−6 Torr. The target was pre-sputtered in pure Ar for 20 min and then in a working atmosphere for 5 min prior to each run. The two-step sputtering process consisted of 10 min of magnetron sputtering at room temperature, followed by 60 min of reactive sputtering at elevated temperature (400–550 °C). To form Bi2O3 nanocones, a thin (~ 50 nm) seeding layer of Bi metal was deposited on Si(001) substrates at room temperature before growth. The growth was performed in the temperature range of 400–550 °C in an O2/Ar (15/20 sccm) atmosphere. During sputtering, the RF power was 50 W, and the sputtering pressure was 5 mTorr. The typical growth time was 1 h. The as-grown samples were characterized using X-ray diffraction (XRD, Rigaku D/Max 2500), field emission scanning electron microscopy (FE-SEM, JEOL-7000 F), energy-dispersive X-ray spectroscopy (EDX),
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and transmission electron microscopy (TEM, JEOL-3010). For TEM observation, the nanocones were ultrasonically dispersed in methanol, and then several drops of the suspension were placed on amorphous carbon films supported by copper grids and dried in air. The optical properties of the samples were studied using photoluminescence (PL). PL measurements were made using a spectrometer (Horiba J-Y, iHR 550) as the optical dispersion unit, while a CCD (Horiba J-Y, Synapse) was used for optical detection. The excitation source was an He–Cd (325 nm) laser. 3. Results and discussion
density nanocones with a top diameter of less than 100 nm and a bottom diameter of more than 500 nm. Fig. 3(c) displays an optical photograph of as-deposited nanocones on a 4 × 4 cm substrate at 500 °C, demonstrating that this synthetic method yields nanocones with excellent uniformity over a large area. The chemical composition of nanocones was determined using EDX. Fig. 3(d) presents the EDX spectrum of the sample that was deposited on Si(001) at 500 °C. The spectrum indicates that the obtained nanocones comprise of only Bi and O, and no other impurity was detected. The FE-SEM results clearly show that 1D growth of Bi2O3 nanocones on the Si(001) substrate was achieved by the addition of a thin Bi seeding layer. The mechanism of formation of Bi2O3 nanocones will be discussed below.
3.1. Surface morphology 3.2. Structural properties To study the effect of Bi seeding layers on the growth of nanostructures, the Si(001) substrates with and without the Bi seeding layer were used under the same growth conditions. FE-SEM observations were made to investigate the evolution of the surface morphology of the samples. Fig. 1 shows top-view FE-SEM images of samples that were deposited on Si(001) without a Bi seeding layer at temperatures from 400 to 550 °C. All of the deposited samples were continuous, dense, thin films and the grain size increased slightly with the growth temperature. No noticeable morphological change of the sample that was deposited on Si(001) without the Bi seeding layer was observed as the temperature increased. To facilitate 1D growth, a Bi thin film (~ 50 nm) was deposited on Si(001) at room temperature. Fig. 2 shows the top-view FE-SEM images of samples that were deposited on Bi/Si(001) at various temperatures under the same growth condition. Many 1D nanostructures were observed to be distributed over the entire Si(001) substrate. For sample deposited at 400 and 450 °C, the nanostructures were roughly 70–90 nm in diameters and lengths of 1–2 μm. The morphology shows slightly larger diameters (90–130 nm) and longer lengths (1.5–3 μm) for sample deposited at 500 and 550 °C. Fig. 3(a) shows the cross-sectional FE-SEM images of a sample deposited on Si(001) at 500 °C. The high-density growth of 1D nanostructures is highly oriented in the direction normal to the substrate. Fig. 3(b) demonstrates that the nanostructures were high-
The crystalline structures of the nanocones were determined by XRD. Fig. 4(a) depicts the XRD patterns of the as-grown sample deposited on Si(001) without a Bi seeding layer at temperatures from 400 to 550 °C. All the diffraction peaks can be indexed as monoclinic α-Bi2O3 (JCPDS 27-0053) for samples deposited at 400 and 450 °C. By further increasing growth temperature to 500 °C, a tetragonal structural βBi2O3 (JCPDS, 27-0050) was observed. Fig. 4(b) shows XRD patterns of as-deposited nanocones on Si(001) at temperatures from 400 to 550 °C. The patterns of the samples that were deposited at 400 and 450 °C were indexed to monoclinic α-Bi2O3 and a trace amount of minority phase. The (− 121) peak is the dominant peak, indicating that each sample is strongly oriented. A possible assignment for the minority phase is the triclinic Bi4O7 (JCPDS 47-1058). Although trivalent bismuth is the most stable oxidation state of Bi in the oxygen lattice, the presence of the Bi4O7 phase demonstrates that trace amounts of Bi were oxidized to Bi5+ during sputtering [24]. Notably a small diffraction peak originated from β-Bi2O3 in the sample that was deposited at 450 °C, suggesting the presence of two phases. The sample grown at 450 °C was mainly composed of α-Bi2O3 and small amount of β-Bi2O3. When the growth temperature was further increased to 500 °C and 550 °C, a complete phase transition from α-Bi2O3 to β-Bi2O3 was observed. All of the diffraction peaks for sample deposited at 500 and 550 °C can be indexed to β-
Fig. 1. FE-SEM images of samples deposited on Si(001) at (a) 400 °C, (b) 450 °C, (c) 500 °C, and (d) 550 °C.
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Fig. 2. FE-SEM images of samples deposited on Si(001) with a Bi seeding layer at (a) 400 °C, (b) 450 °C, (c) 500 °C, and (d) 550 °C.
Bi2O3. The high intensity of the (201) diffraction peak suggests that the samples that were deposited at 500 and 550 °C were highly (201)-oriented β-Bi2O3. The XRD results indicate that the synthesis of α- and β-Bi2O3 nanocones could be achieved by controlling the growth temperature. TEM was used to characterize the structure of a single nanocone. Fig. 5(a) displays a TEM image of nanocones that were synthesized at 400 °C with a top diameter of 50 nm and a bottom diameter of 100 nm. The inset shows the selected-area electron diffraction (SAED)
patterns of a single nanocone, verifying the monoclinic α-Bi2O3 structure. The diffuse diffraction pattern reveals that the nanocone may have had various growth orientations owing to the low growth temperature. Fig. 5(b) displays a TEM image of nanocones that were deposited at 500 °C, showing that the nanocone had a top diameter of 50 nm and a bottom diameter of 80 nm, with few observable structural defects, such as stacking faults or dislocations. The spot diffraction pattern of a single nanocone verifies the single crystal tetragonal structure of β-Bi2O3.
Fig. 3. (a) (b) Cross-sectional FE-SEM images of samples deposited with a Bi seeding layer at 500 °C. (c) Photograph of Bi2O3 nanocones grown on a 4 × 4 cm Si(001). (d) EDX spectrum of sample deposited at 500 °C.
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3.3. Growth mechanism As demonstrated above, the Bi seeding layer is crucial to the growth of bismuth oxide nanostructures. The growth mechanism of Bi2O3 nanocones can be explained using the growth model that is illustrated in Fig. 6. First, a thin Bi seeding layer was sputtered onto Si(001) substrate at room temperature. As the Bi seeding layer was heated to a temperature above than its melting point (271.4 °C), Bi droplets formed because of surface tension. The mobility of the Bi atoms increased with temperature, causing large droplets to grow. As a result, Bi nanoislands were formed. Since the oxygen concentration in the chamber was low, Bi2O3 could not be formed in this stage. The Bi nanoislands acted as low surface energy nucleation sites for vapor adsorption. The Vshaped notch above the grain boundaries may have resulted in a higher adsorption rate than the smooth surfaces. Driven by the high adsorption rate at the grain boundaries, the Bi and O atoms were adsorbed on specific sites with a high probability. As the reactive vapor species were supplied by sputtering process, the growth occurred initially from the grain boundaries of Bi nanoislands, and the Bi2O3 nanostructures continued to grow unidirectionally. Apparently the morphology of the obtained Bi2O3 nanocones differs from that of typical vapor–liquid–solid (VLS) grown oxide nanowires. In general, VLS grown nanowires are structurally straight with metal catalysts on the tips, which confine the vapor species where unidirectional growth can occur. In this study, the obtained nanocones are tapered structures. Hence, their mechanism of growth may not be the conventional VLS mechanism. We suggest that the Bi2O3 nanocones grow by a grain boundary-assisted growth mechanism in which the grain boundaries serve as nucleation sites and facilitate the adsorption of Bi and O vapor onto the surface. The growth mechanism is driven by reducing surface energy of substrate, and the specific phase (α, β) is formed by the phase stability at different temperatures, as confirmed by XRD and TEM. 3.4. Optical properties Fig. 4. XRD patterns of samples deposited on (a) Si(001) and (b) Si(001) with a Bi seeding layer at 400–550 °C.
The optical properties of Bi2O3 nanocones were examined using PL spectroscopy. The PL spectra of the Bi2O3 nanocones were obtained at 300 K, using an He–Cd laser as the excitation source, and are shown in Fig. 7. The samples that were deposited at 400 and 450 °C emit broad and visible emission from 400 nm to 800 nm. This emission is attributable to various inter-band transitions between defects, impurity and
Fig. 5. (a) TEM image of α-Bi2O3 nanocones. Inset shows SAED patterns from a single α-Bi2O3 nanocone grown at 400 °C. (b) TEM image of a β-Bi2O3 nanocone. Inset shows SAED patterns from a single β-Bi2O3 nanocone grown at 500 °C.
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Fig. 6. Model of growth of Bi2O3 nanocones by magnetron sputtering.
different valence states of bismuth ions (Bi+, Bi2+, and Bi3+) [3,13]. The oxygen vacancies (VO) form a defect donor band and may induce radiative transitions from donor levels to the valence band, as is commonly observed in oxide materials. Moreover, the observed energy of visible emission is red-shifted as the growth temperature increases. The shift of the emission peak originates from the different phases in Bi2O3. As discussed above, the energy band gap of α-Bi2O3 (2.91 eV) is slightly larger than that of β-Bi2O3 (2.58 eV). Therefore, the phase transition that occurs as the growth temperature is increased is responsible for the change in PL spectra.
4. Conclusions In conclusion, this work presented a simple method for the synthesis of Bi2O3 nanocones over large areas. Bi2O3 nanocones were grown in high yield on a 4 × 4 cm Si(001) substrate by magnetron sputtering. Characterizations using XRD, TEM and PL confirm that α-Bi2O3 and βBi2O3 nanocones were formed when the growth temperature was properly controlled. The Bi2O3 nanocones were suggested to undergo grain boundary-assisted growth, in which the Bi seeding layer is crucial. The results herein suggest that introducing a surface seeding layer may provide an effective way to grow various 1D nanostructures over large areas in high yield by magnetron sputtering. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 102-2112-M-259-001. References
Fig. 7. Room temperature PL spectra of Bi2O3 nanocones grown on Si(001) at (a) 400 °C, (b) 450 °C, (c) 500 °C, and (d) 550 °C.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Synthesis of Bi2O3 nanocones over large areas by magnetron sputtering Li-Chia Tien ⁎, Ying-Hong Liou Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan
a r t i c l e
i n f o
Article history: Received 25 September 2014 Accepted in revised form 30 January 2015 Available online xxxx Keywords: Bismuth oxide 1D nanostructures Physical vapor deposition
a b s t r a c t Large areas of bismuth oxide (Bi2O3) nanocones were deposited onto Si(001) substrates by magnetron sputtering. The structural and optical properties of the nanocones were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The obtained tapered nanostructures consist of high-density nanocones with diameters approximately 70–130 nm and lengths of 1–3 μm. XRD results reveal that the Bi2O3 nanocones can undergo a phase transition from the α to the β phase at growth temperatures over 450 °C. This phase transition was confirmed by TEM and PL. The growth mechanism of Bi2O3 nanocones was identified as grain boundary-assisted growth, in which a Bi seeding layer is crucial to the formation of the nanostructures. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Significant interest exists in the use of bismuth oxide (Bi2O3) as a photocatalyst. The reported energy band gaps of α-Bi2O3 (2.91 eV) and β-Bi2O3 (2.58 eV) are significantly lower than that of TiO2 (anatase, 3.32 eV), suggesting that excitation by visible light can generate electron–hole pairs more efficiently in Bi2O3 than in TiO2 [1–3]. A particular phase of Bi2O3 may be synthesized by controlling the conditions and methods of growth, and particularly the growth temperature [4,5]. The ability to control the phase formation of Bi2O3 nanostructures is critical to their potential applications, such as development of fast oxygen conductors for solid oxide fuel cells (SOFC) [6–8]. Owing to their high surface-to-volume ratio, one-dimensional (1D) Bi2O3 nanostructures may be used as efficient visible light-driven photocatalysts [9–13]. Various methods for synthesizing 1D Bi2O3 nanostructures have been proposed; they include template synthesis, hydrothermal methods, the vapor transport process, and oxidative metal vapor transport deposition [14–18]. The main drawbacks of these synthetic methods are the small yield and high fabrication cost, which may become problematic when 1D Bi2O3 nanostructures are required for large-scale applications. Owing to the complexity of Bi2O3 compounds, investigations of the phase-selective synthesis of Bi2O3 nanostructures are very limited. Magnetron sputtering is an extensively used thin film deposition method because it can provide high deposition rates and yields dense and highly adhesive thin films with large areas. Efforts have made to synthesize various 1D nanostructures of zinc oxide (ZnO), titanium oxide (TiO2), ruthenium oxide (RuO2), and indium tin oxide (ITO), for example, by magnetron sputtering [19–23]. However,
⁎ Corresponding author. Tel.:+886 3 863 4208; fax: +886 3 863 4200. E-mail address: [email protected] (L.-C. Tien).
relatively little research has been conducted to understand the mechanism of growth of 1D nanostructures by magnetron sputtering. This work reports on the synthesis of α- and β-Bi2O3 nanocones by RF magnetron sputtering. The samples were examined comprehensively using a variety of characterization tools. The structural, compositional, and optical properties of obtained nanostructures were analyzed and discussed. The growth temperature can be controlled to synthesize αBi2O3 and β-Bi2O nanocones in high yield on 4 × 4 cm Si(001) substrate. The growth mechanism of Bi2O3 nanocones is discussed. 2. Materials and methods Bi2O3 nanocones were grown using a two-step RF magnetron sputtering process. Before deposition, the silicon (001) substrates (4 × 4 cm) were ultrasonically cleaned with acetone, and then methanol, before being dried in compressed N2. A high-purity bismuth target (99.95%) with a diameter of 2 in. was used as the source material and the substrate-to-target distance was 14 cm. Before sputtering, the chamber was pumped down to a base pressure of 3 × 10−6 Torr. The target was pre-sputtered in pure Ar for 20 min and then in a working atmosphere for 5 min prior to each run. The two-step sputtering process consisted of 10 min of magnetron sputtering at room temperature, followed by 60 min of reactive sputtering at elevated temperature (400–550 °C). To form Bi2O3 nanocones, a thin (~ 50 nm) seeding layer of Bi metal was deposited on Si(001) substrates at room temperature before growth. The growth was performed in the temperature range of 400–550 °C in an O2/Ar (15/20 sccm) atmosphere. During sputtering, the RF power was 50 W, and the sputtering pressure was 5 mTorr. The typical growth time was 1 h. The as-grown samples were characterized using X-ray diffraction (XRD, Rigaku D/Max 2500), field emission scanning electron microscopy (FE-SEM, JEOL-7000 F), energy-dispersive X-ray spectroscopy (EDX),
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and transmission electron microscopy (TEM, JEOL-3010). For TEM observation, the nanocones were ultrasonically dispersed in methanol, and then several drops of the suspension were placed on amorphous carbon films supported by copper grids and dried in air. The optical properties of the samples were studied using photoluminescence (PL). PL measurements were made using a spectrometer (Horiba J-Y, iHR 550) as the optical dispersion unit, while a CCD (Horiba J-Y, Synapse) was used for optical detection. The excitation source was an He–Cd (325 nm) laser. 3. Results and discussion
density nanocones with a top diameter of less than 100 nm and a bottom diameter of more than 500 nm. Fig. 3(c) displays an optical photograph of as-deposited nanocones on a 4 × 4 cm substrate at 500 °C, demonstrating that this synthetic method yields nanocones with excellent uniformity over a large area. The chemical composition of nanocones was determined using EDX. Fig. 3(d) presents the EDX spectrum of the sample that was deposited on Si(001) at 500 °C. The spectrum indicates that the obtained nanocones comprise of only Bi and O, and no other impurity was detected. The FE-SEM results clearly show that 1D growth of Bi2O3 nanocones on the Si(001) substrate was achieved by the addition of a thin Bi seeding layer. The mechanism of formation of Bi2O3 nanocones will be discussed below.
3.1. Surface morphology 3.2. Structural properties To study the effect of Bi seeding layers on the growth of nanostructures, the Si(001) substrates with and without the Bi seeding layer were used under the same growth conditions. FE-SEM observations were made to investigate the evolution of the surface morphology of the samples. Fig. 1 shows top-view FE-SEM images of samples that were deposited on Si(001) without a Bi seeding layer at temperatures from 400 to 550 °C. All of the deposited samples were continuous, dense, thin films and the grain size increased slightly with the growth temperature. No noticeable morphological change of the sample that was deposited on Si(001) without the Bi seeding layer was observed as the temperature increased. To facilitate 1D growth, a Bi thin film (~ 50 nm) was deposited on Si(001) at room temperature. Fig. 2 shows the top-view FE-SEM images of samples that were deposited on Bi/Si(001) at various temperatures under the same growth condition. Many 1D nanostructures were observed to be distributed over the entire Si(001) substrate. For sample deposited at 400 and 450 °C, the nanostructures were roughly 70–90 nm in diameters and lengths of 1–2 μm. The morphology shows slightly larger diameters (90–130 nm) and longer lengths (1.5–3 μm) for sample deposited at 500 and 550 °C. Fig. 3(a) shows the cross-sectional FE-SEM images of a sample deposited on Si(001) at 500 °C. The high-density growth of 1D nanostructures is highly oriented in the direction normal to the substrate. Fig. 3(b) demonstrates that the nanostructures were high-
The crystalline structures of the nanocones were determined by XRD. Fig. 4(a) depicts the XRD patterns of the as-grown sample deposited on Si(001) without a Bi seeding layer at temperatures from 400 to 550 °C. All the diffraction peaks can be indexed as monoclinic α-Bi2O3 (JCPDS 27-0053) for samples deposited at 400 and 450 °C. By further increasing growth temperature to 500 °C, a tetragonal structural βBi2O3 (JCPDS, 27-0050) was observed. Fig. 4(b) shows XRD patterns of as-deposited nanocones on Si(001) at temperatures from 400 to 550 °C. The patterns of the samples that were deposited at 400 and 450 °C were indexed to monoclinic α-Bi2O3 and a trace amount of minority phase. The (− 121) peak is the dominant peak, indicating that each sample is strongly oriented. A possible assignment for the minority phase is the triclinic Bi4O7 (JCPDS 47-1058). Although trivalent bismuth is the most stable oxidation state of Bi in the oxygen lattice, the presence of the Bi4O7 phase demonstrates that trace amounts of Bi were oxidized to Bi5+ during sputtering [24]. Notably a small diffraction peak originated from β-Bi2O3 in the sample that was deposited at 450 °C, suggesting the presence of two phases. The sample grown at 450 °C was mainly composed of α-Bi2O3 and small amount of β-Bi2O3. When the growth temperature was further increased to 500 °C and 550 °C, a complete phase transition from α-Bi2O3 to β-Bi2O3 was observed. All of the diffraction peaks for sample deposited at 500 and 550 °C can be indexed to β-
Fig. 1. FE-SEM images of samples deposited on Si(001) at (a) 400 °C, (b) 450 °C, (c) 500 °C, and (d) 550 °C.
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Fig. 2. FE-SEM images of samples deposited on Si(001) with a Bi seeding layer at (a) 400 °C, (b) 450 °C, (c) 500 °C, and (d) 550 °C.
Bi2O3. The high intensity of the (201) diffraction peak suggests that the samples that were deposited at 500 and 550 °C were highly (201)-oriented β-Bi2O3. The XRD results indicate that the synthesis of α- and β-Bi2O3 nanocones could be achieved by controlling the growth temperature. TEM was used to characterize the structure of a single nanocone. Fig. 5(a) displays a TEM image of nanocones that were synthesized at 400 °C with a top diameter of 50 nm and a bottom diameter of 100 nm. The inset shows the selected-area electron diffraction (SAED)
patterns of a single nanocone, verifying the monoclinic α-Bi2O3 structure. The diffuse diffraction pattern reveals that the nanocone may have had various growth orientations owing to the low growth temperature. Fig. 5(b) displays a TEM image of nanocones that were deposited at 500 °C, showing that the nanocone had a top diameter of 50 nm and a bottom diameter of 80 nm, with few observable structural defects, such as stacking faults or dislocations. The spot diffraction pattern of a single nanocone verifies the single crystal tetragonal structure of β-Bi2O3.
Fig. 3. (a) (b) Cross-sectional FE-SEM images of samples deposited with a Bi seeding layer at 500 °C. (c) Photograph of Bi2O3 nanocones grown on a 4 × 4 cm Si(001). (d) EDX spectrum of sample deposited at 500 °C.
Please cite this article as: L.-C. Tien, Y.-H. Liou, Surf. Coat. Technol. (2015), http://dx.doi.org/10.1016/j.surfcoat.2015.01.072
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3.3. Growth mechanism As demonstrated above, the Bi seeding layer is crucial to the growth of bismuth oxide nanostructures. The growth mechanism of Bi2O3 nanocones can be explained using the growth model that is illustrated in Fig. 6. First, a thin Bi seeding layer was sputtered onto Si(001) substrate at room temperature. As the Bi seeding layer was heated to a temperature above than its melting point (271.4 °C), Bi droplets formed because of surface tension. The mobility of the Bi atoms increased with temperature, causing large droplets to grow. As a result, Bi nanoislands were formed. Since the oxygen concentration in the chamber was low, Bi2O3 could not be formed in this stage. The Bi nanoislands acted as low surface energy nucleation sites for vapor adsorption. The Vshaped notch above the grain boundaries may have resulted in a higher adsorption rate than the smooth surfaces. Driven by the high adsorption rate at the grain boundaries, the Bi and O atoms were adsorbed on specific sites with a high probability. As the reactive vapor species were supplied by sputtering process, the growth occurred initially from the grain boundaries of Bi nanoislands, and the Bi2O3 nanostructures continued to grow unidirectionally. Apparently the morphology of the obtained Bi2O3 nanocones differs from that of typical vapor–liquid–solid (VLS) grown oxide nanowires. In general, VLS grown nanowires are structurally straight with metal catalysts on the tips, which confine the vapor species where unidirectional growth can occur. In this study, the obtained nanocones are tapered structures. Hence, their mechanism of growth may not be the conventional VLS mechanism. We suggest that the Bi2O3 nanocones grow by a grain boundary-assisted growth mechanism in which the grain boundaries serve as nucleation sites and facilitate the adsorption of Bi and O vapor onto the surface. The growth mechanism is driven by reducing surface energy of substrate, and the specific phase (α, β) is formed by the phase stability at different temperatures, as confirmed by XRD and TEM. 3.4. Optical properties Fig. 4. XRD patterns of samples deposited on (a) Si(001) and (b) Si(001) with a Bi seeding layer at 400–550 °C.
The optical properties of Bi2O3 nanocones were examined using PL spectroscopy. The PL spectra of the Bi2O3 nanocones were obtained at 300 K, using an He–Cd laser as the excitation source, and are shown in Fig. 7. The samples that were deposited at 400 and 450 °C emit broad and visible emission from 400 nm to 800 nm. This emission is attributable to various inter-band transitions between defects, impurity and
Fig. 5. (a) TEM image of α-Bi2O3 nanocones. Inset shows SAED patterns from a single α-Bi2O3 nanocone grown at 400 °C. (b) TEM image of a β-Bi2O3 nanocone. Inset shows SAED patterns from a single β-Bi2O3 nanocone grown at 500 °C.
Please cite this article as: L.-C. Tien, Y.-H. Liou, Surf. Coat. Technol. (2015), http://dx.doi.org/10.1016/j.surfcoat.2015.01.072
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Fig. 6. Model of growth of Bi2O3 nanocones by magnetron sputtering.
different valence states of bismuth ions (Bi+, Bi2+, and Bi3+) [3,13]. The oxygen vacancies (VO) form a defect donor band and may induce radiative transitions from donor levels to the valence band, as is commonly observed in oxide materials. Moreover, the observed energy of visible emission is red-shifted as the growth temperature increases. The shift of the emission peak originates from the different phases in Bi2O3. As discussed above, the energy band gap of α-Bi2O3 (2.91 eV) is slightly larger than that of β-Bi2O3 (2.58 eV). Therefore, the phase transition that occurs as the growth temperature is increased is responsible for the change in PL spectra.
4. Conclusions In conclusion, this work presented a simple method for the synthesis of Bi2O3 nanocones over large areas. Bi2O3 nanocones were grown in high yield on a 4 × 4 cm Si(001) substrate by magnetron sputtering. Characterizations using XRD, TEM and PL confirm that α-Bi2O3 and βBi2O3 nanocones were formed when the growth temperature was properly controlled. The Bi2O3 nanocones were suggested to undergo grain boundary-assisted growth, in which the Bi seeding layer is crucial. The results herein suggest that introducing a surface seeding layer may provide an effective way to grow various 1D nanostructures over large areas in high yield by magnetron sputtering. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 102-2112-M-259-001. References
Fig. 7. Room temperature PL spectra of Bi2O3 nanocones grown on Si(001) at (a) 400 °C, (b) 450 °C, (c) 500 °C, and (d) 550 °C.
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