1. Introduction

Recently bismuth contained oxides serve as a promising candidate for a variety of applications in optoelectronics and microelectronics devices. Among them, nanostructral-photocatalyst bismuth oxide possesses an effective function for self-cleaned activity on architecture, assistance in germ and virus sterilization as well as for green and environmental protection use.

Thin film Bi₂O₃ usually forms a nanostructure with different polymorphisms of α-, β-, γ-, δ-, ε-, and ω-phases [1, 2], which may be a potential high-efficiency photocatalyst with a band gap below 3 eV. The values of direct band gap for the commonly forming phases of α- and β-Bi₂O₃ obtained by optical absorption [3–5] revealed lower energy than that of anatase TiO₂ with an indirect gap of ~3.32 eV [6]. The smaller band gap of Bi₂O₃ means that the bismuth oxide nanostructures can generate electron-hole pairs more efficiently under visible light’s excitation beneath the sunlight shiny.

For the crystal structure of bismuth oxide, α-Bi₂O₃ is a stabilized low-temperature phase with monoclinic structure in the Bismites. The most stable form of the room temperature variety in Bi₂O₃ is the α monoclinic polymorph [1]. The high temperature phase of bismuth oxide is usually a cubic form, δ- Bi₂O₃. The polymorph of δ- Bi₂O₃ only stabilizes between 730 °C and the melting point 824 °C [2]. The γ-phase Bi₂O₃ appears in a body centered cubic structure formed below 639 °C. The β-Bi₂O₃ is a tetragonal structure formed below 650 °C. The formation of the β- polymorphism depends on impurities and reaction condition in the oxide. Both γ and β are metastable phases, which can be obtained by cooling during the growth process. If the crystalline nanomaterial was tempered at some established lower temperatures, the stabilization form is usually the α-Bi₂O₃ phase. Among all the polymorphisms of Bi₂O₃, α and β phases have been proven to be effective and sensitive photcatalysts operated in visible region [3, 7, 8]. α-Bi₂O₃ is the most stable phase existed in the Bismite photocatalysts, however, optical characteristic and band-edge structure of α-Bi₂O₃ have not yet been comprehensively studied hereto.

In this paper, we have characterized the band-edge optical property of α-Bi₂O₃ thin film nanowires using thermoreflectance (TR) and photoluminescence (PL) experiments. The α-Bi₂O₃ nanowires were grown by ambient controlled vapor transport process using vapor-liquid-solid (VLS) mechanism. The growth direction of the individual nanowire was along <$\overline{1}22>$. The TR results show direct semiconductor behavior of α-Bi₂O₃ with a direct gap close to 2.91 eV at 300 K. PL measurements identify the band-edge emission and defect luminescence for the α-Bi₂O₃ nanowires. Temperature dependent TR measurements at several temperatures between 30 and 300 K were respectively carried out. The temperature dependences of energies and broadening parameters for the direct band gap of α-Bi₂O₃ nanowires are analyzed and discussed.

2. Experimental details

The growth of Bi₂O₃ nanowires was carried out using a simple and effective ambient controlled vapor transport process. A horizontal tube furnace with three independent heating zones was used for the thin film growth. Bismuth powder (1.5 g, 99.95%) was the source material. A thin layer (5 nm) of Au was pre-deposited on Si (100) substrate by DC sputtering using for catalyst of the VLS growth. The tube furnace was initially heated to 600 °C and then oxygen and argon mixtures (100 SCCM) were fed into the quartz tube. The growth temperature was set at ~400 °C with background pressure of 0.3 Torr. The growth time was about two hours. After the growth, the chamber tube was cooled down to room temperature under ambient pressure. X-ray diffraction measurement revealed α phase of the as-grown Bi₂O₃ nanowires. The lattice constants were analyzed and determined to be a = 5.85 Å, b = 8.17 Å, c = 7.51 Å, and β = 113°, respectively.

TR experiments were carried out by an indirect heating manner with a gold-evaporated quartz plate as the heating element [9, 10]. The thin sheet-type sample was closely attached on the heating element by silicone grease. The on-off heating disturbance was uniformly modulated the individual nanorods periodically. An 150 W xenon-arc lamp filtered by a PTI 0.2-m monochromator provided the monochromatic light. The incident light was focusing onto the sample with a spot size of ~100 μm². The reflected and scattering lights from the thin-film nanorods were collected and detected by a photomultiplier tube. The signal was detected and recorded via an EG&G 7265 lock-in amplifier and personal computer. A RMC model 22 closed-cycle cryogenic refrigerator with model 4075 thermometer controller facilitated the temperature-dependent measurements. PL spectra of the nanowires samples were detected by a QE65000 charge-couple-devices imaging spectrometer. A Q-switched diode-pumped solid-state laser (λ = 266 nm) acted as the pumping light source. The measurements were done from 1.25 to 4.5 eV at 300 and 30 K, repectively.

3. Results and discussion

Figure 1(a) shows the scanning electron microscopy (SEM) image of the top view of the Bi₂O₃ nanowires. The cross section view of the thin-film nanowires is also displayed in Fig. 1(b), which indicates the Bi₂O₃ thin film is consisted of a compact continuous film (~960 nm) initially deposited on Si (100) substrate, and then the nanowires of high density were grown on the continuous film. The average thickness of the Bi₂O₃ thin-film nanowires was about 24.5 μm. In order to verify crystal structure and stochiometry of the nanowires, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were carried out. Figures 1(c) and 1(d) show the TEM image and EDS spectrum of one single nanowire of Bi₂O₃. The wire width was ~298 nm and the EDS result depicted some related X-ray peaks coming from the Bi and O elements which can approximately verify the stoichiometric content of the Bi₂O₃ nanowire after analysis [see Fig. 1(d)]. Figure 1(e) shows the high resolution TEM image of the Bi₂O₃ nanowire derived from Fig. 1(c). The picture reveals that the interplanar spacing of c-axis estimated from the TEM image is d₍₀₀₁₎ = 0.75 nm, which is identical with that of the α-phase Bi₂O₃. The TEM image of Fig. 1(e) also indicates the axial direction of the single nanowire is grown along $<\overline{1}22>$ direction similar to that of the previous nanowires obtained by a reaction of trimethylbismuth and oxygen mixture on Si (100) substrate [11]. Some of the α-Bi₂O₃ nanowires were usually found to grow along the other direction of <010> depending on growth condition [12, 13]. The selected-area electron diffraction (SAED) pattern of the single nanowire {i.e. with [210] zone axis} in Fig. 1(f) also indicates α crystalline phase of the Bi₂O₃ nanowire with the growth direction along [$\overline{1}22$].

 

Fig. 1 SEM images of (a) top view and (b) cross-section view for as-grown α-Bi₂O₃ nanowires on Si substrate. (c) TEM image of a single nanowire with diameter of ~298 nm, the corresponding EDS spectrum and the content data are shown in (d). (e) High-resolution TEM image of the α-Bi₂O₃ nanowire, the interplannar distance of d₍₀₀₁₎ is indicated. (f) The related SAED pattern of the α-Bi₂O₃ nanowire with zone axis along [210].

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To further verify the structure of the Bi₂O₃ nanowires, Raman spectroscopy has been implemented at 300 K. Figure 2 shows the Raman spectrum of the as-grown Bi₂O₃ nanowires between 100 and 700 cm⁻¹. A lot of peak features observed due to the α-Bi₂O₃ is a double refraction biaxial crystal with a lower symmetry of monoclinic structure [14]. Ten Raman peaks above 100 cm⁻¹ (see Fig. 2) have been observed in good agreements with the Raman modes seen in previous results [12–14], which further confirm the nanowires are comprising in monoclinic α-Bi₂O_3. As shown in Fig. 2, the 119 cm⁻¹ mode is coming from A_g symmetry caused by mainly the Bi atoms’ participations. Modes of 138 (A_g) and 153 cm⁻¹ (B_g) may come from the displacements of both Bi and O atoms in the α-Bi₂O₃ lattice. The Raman peaks of the higher frequency modes 183, 211, 279, 313, 410, 446, and 521 cm⁻¹ are attributed to the displacements of the O atoms in α-Bi₂O₃. These vibration modes possess A_g, B_g or both symmetries, which can be defined and distinguished by tensor selection rule using polarized micro-Raman spectroscopy [12, 14]. The Raman peak pattern in Fig. 2 also identifies the low-temperature α phase of the as-grown bismuth oxide nanowires.

 

Fig. 2 Raman peak pattern of α-Bi₂O₃ thin-film nanowires with the vibration frequencies between 100 and 700 cm⁻¹.

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Figure 3(a) shows the temperature-dependent TR spectra of the α-Bi₂O₃ thin-film nanowires from 300 K down to 30 K near the band edge. TR has been proven to be more effective for the characterization of optical properties in semiconductor nanostructures [9, 15]. The TR can keep uniformity of periodically thermal modulation of each nanowire to result in easily resolved derivative line-shape spectrum that emphasizes at direct critical-point transition [16]. As shown in Fig. 3(a) by dashed lines are the experimental TR spectra of band-edge transition of α-Bi₂O₃ nanowires. The solid lines are the least-square fits of the experimental data using a first derivative Lorentzian line-shape function appropriate for direct band gap transition expressed as [17]:

 

Fig. 3 (a) Temperature-dependent TR spectra of α-Bi₂O₃ nanowires between 300 and 30 K near band edge. (b) Temperature dependence of direct band gaps of α-Bi₂O₃ from 30 to 300 K. (c) Temperature-dependent broadening parameter of the TR feature for the direct band gap derived from spectral analysis in (a). The solid line is fitted to a Bose-Einstein expression containing electron-phonon interaction.

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To evaluate below- and near-band-edge electronic structure of α-Bi₂O₃ nanowires, PL measurements of 30 and 300 K are respectively carried out and the PL spectra are shown in the lower part of Fig. 4 with dashed line (30 K) and solid line (300 K). For comparison purpose, the corresponding TR spectra at 30 and 300 K are also included in the upper part of Fig. 4 as a reference. The energy position of direct band gap E_g (i.e. 2.91 eV at 300 K and 3.144 eV at 30 K) measured by TR is close to a shoulder peak in the corresponding PL spectrum at 300 and 30 K. The shoulder peak may correlate with direct band-edge emission coming from E_g of α-Bi₂O₃ nanowires. It also verifies the direct band-edge character of α-Bi₂O₃. A broadened PL peak E_d that centered at ~2.53 eV at 300 K and ~2.645 eV at 30 K was also detected in the PL spectra in Fig. 4. The broadened peak may be a defect luminescence (E_d) comprising a lot of defect states (by oxygen vacancy V_O) to valence band emissions. The oxygen vacancies in the oxide nanostructures usually form a defect donor band such as that observed in the other oxide nanostructure of Ga₂O₃ [15]. The V_O states in Bi₂O₃ may also have the possibility to contain Bi⁺, Bi²⁺, or Bi³⁺ centers, which can emit visible luminescences from red to blue region [20]. The defect emission E_d in Fig. 4 therefore appears in a broadened emission peak with wider line width. As shown in Fig. 4, the temperature-energy shift of E_d between 30 and 300 K is ~115 meV, which is approximately one half of the energy separation (~230 meV) of band-edge emission E_g. This result verifies that E_g is a band-to-band transition and E_d is a defect to valence-band recombination. The temperature insensitive energy shift (only one half of E_g) is a general character for a defect transition with dangling bond such as the oxygen vacancies in the crystals. The representative band-edge scheme of α-Bi₂O₃ nanowires is also depicted in the inset of Fig. 4 for comparison. The α-Bi₂O₃ is a direct semiconductor, which can easily emit and absorb a photon with hν = E_g. The defect luminescence (hν = E_d) from the α-Bi₂O₃ nanowire may be a broadened emission band caused by defect donor band consisted of V_O [15]. Both TRand PL results show α-Bi₂O₃ nanowire not only a visible-active semiconductor but also a white-light luminescent material with broad-band defect emissions.

 

Fig. 4 The PL and TR spectra of α-Bi₂O₃ nanowires at 300 and 30 K. The inset depicts a representative band-edge scheme of α-Bi₂O₃ nanowire derived from the experimental analysis of TR and PL results.

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4. Conclusions

We prove and evaluate, for the first time, the accurate direct band-edge nature of α-Bi₂O₃ thin-film nanowires grown by vapor transport method. TEM and Raman spectroscopy confirm the α-monoclinic phase of the nanowire with an axial direction grown along $<\overline{1}22>$. TR measurement determines a direct band gap of E_g = 2.91 eV for the α-Bi₂O₃ nanowires at 300 K. Temperature dependences of energies and broadening parameters of direct band gap in α-Bi₂O₃ verify optoelectronic semiconducting properties of the oxide nanowires. The temperature-energy shift of the direct band gap has been characterized. Both PL and TR measurements verify direct band-edge nature of the α-Bi₂O₃ nanowires, and a broad-band white light luminescence occurs owing to a defect donor band caused by oxygen vacancies may exist in the oxide nanowires. On the basis of PL and TR results, a probable near-band-edge scheme of α-Bi₂O₃ nanowires is proposed. It shows that α-Bi₂O₃ nanostructure is a visible active semiconductor, which may also be applied in solid-state white lightening optoelectronics.