Identification of distinctive structural and optoelectronic properties of Bi₂O₃ polymorphs controlled by tantalum addition

Abstract

Diverse crystal phases of bismuth (Bi) oxides induced by the addition of different amounts of tantalum (Ta) were synthesized. Their optoelectronic and redox properties were quantitatively investigated using combined experimental and computational approaches. Synthesis conditions that transform α-Bi₂O₃ into β-Bi₂O₃ and δ-Bi₂O₃ in terms of the Ta quantity, as well as synthesis temperatures, are identified and demonstrated. The phase transition behavior and crystal structures were characterized by in situ high temperature X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG/DTA), X-ray absorption near edge structures (XANES), and extended X-ray absorption fine structures (EXAFS). Density functional theory calculations employing the HSE exchange–correlation functional with spin–orbit coupling were used to quantitatively simulate the optoelectronic properties and band structures of β-Bi₂O₃ and δ-Bi₂O₃. Along with the absorption coefficient and density of states, effective masses and dielectric constants were elucidated. The characterization study confirmed the distortion of Ta–O bonds in the Ta-supplemented β-Bi₂O₃ and the substitutional positions of the Bi and Ta atoms in the δ-Bi₂O₃ and Bi₃TaO₇ compounds. The reducibility of these oxides was strongly influenced by the crystal phase confirmed by temperature-programmed reduction (TPR) analysis. These findings can be used as a bismuth oxides' benchmark for optoelectronic applications as well as thermal catalysis as the redox active center or the support.

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Introduction

Bi₂O₃ is one of the polymorphous oxides, and is employed in a broad range of applications such as solid-state electrocatalysis,^1–6 inorganic orange pigment,⁷ and photocatalytic reactions.^8–27 To date, four different crystal phases have been reported: α phase (monoclinic), β phase (tetragonal), γ phase (cubic), and δ phase (cubic). Among the polymorphs, the α phase is the most stable phase at room temperature. The high-temperature–stable δ phase is transformed into the β or γ phase while cooling, which results in the further transformation into the α phase at room temperature.²⁸ While the δ phase shows a high O²⁻ ion conductivity, applicable to mid-temperature electrocatalysis at 300–500 °C,^1–5 β-Bi₂O₃ attracts attention as a photocatalyst due to a narrow bandgap of ∼2.2 eV.^8–12

Solid-state synthesis of pure Bi₂O₃ has been well reported in the literature.²⁸ Knowing the melting point of Bi₂O₃ to be ∼830 °C, phase transition temperatures have been studied in detail. Nevertheless, selective synthesis of specific phases from pure Bi₂O₃ remains challenging. By doping α-Bi₂O₃ with a trace amount of oxides, such as Ta₂O₅, CeO₂, ZrO₂, EuO_x, TiO_x, and MnO_x,^5,7,29–33 a solid-state reaction at a high temperature becomes possible to make metastable β-, γ-, and δ-phases recoverable for successive applications. Recently, Masui and coworkers reported the addition of Ta₂O₅ into α-Bi₂O₃ to synthesize β- and δ-phases, and among them, β-Bi₂O₃ was recognized to have a potential to be a orange pigment with its vivid orange color due to its unique bandgap narrowing.⁷ This bandgap with a visible light response may be suitable for solar energy conversion, but the investigation of β-Bi₂O₃ as a photocatalyst, especially for application in photosynthetic reactions, has been limited.⁹ The details of how Ta induces the crystal phase transition and narrows the bandgap also remain unclarified.

The purpose of this study is to provide not only explicit synthesis protocols for each crystal structure, but also quantitatively analyse various optoelectronic and redox properties of bismuth oxides. Different Bi₂O₃ phases and Bi₃TaO₇ were successfully achieved by varying the Ta/Bi ratio (0.020–0.333). The obtained materials were characterized by in situ X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), diffuse-reflectance ultraviolet-visible spectroscopy (DR-UV-vis), and photoelectron spectroscopy in air (PESA). The optoelectronic structure was studied both experimentally and theoretically to understand their density of states (DOS), dielectric properties, and effective masses. Photoelectrochemical (PEC) measurements and temperature-programmed reduction (TPR) analyses were conducted.

Experimental and theoretical

Synthesis

Ta₂O₅ (99.9%, Kojundo Chemical Laboratory Co., Ltd.) was mixed with Bi₂O₃ (98.0+%, FUJIFILM Wako Pure Chemical Corporation) (0.020 ≤ Ta/Bi ≤ 0.333) using a mortar and pestle. Heat treatment was usually conducted at 850 °C, with a heating rate of 10 °C min⁻¹ and was held at this temperature for 5 h in a muffle furnace in air. During the heat treatment, the mixtures were placed on an Alumina boat (Nikkato, SSA-S grade, Al₂O₃ 99.6%) to prevent contamination of silica components. Then, the samples were cooled naturally in the furnace by turning off the electric power. The samples were ground in an Alumina boat prior to characterization.

X-ray diffraction

The powder XRD patterns were collected using a Rigaku Ultima III equipped with a copper (Cu) X-ray source operated at 40 kV and 40 mA. The diffractometer was configured with a 1/2° divergence slit limited to 10 mm, an open scattering slit, an open receiving slit, and a nickel (Ni) filter to attenuate contributions from Cu-K_β fluorescence. The data sets were acquired in continuous scanning mode (20° min⁻¹) with an integration step size of 0.02°; however, when focusing on certain peaks (i.e., only around 2θ = 26–29°), these parameters were set to 1° min⁻¹ and 0.01°, respectively. The peak positions were calibrated using the secondary peak of silicon (Si) (standard, 220, 47.2660°, cubic, Fd3̄m). In situ high-temperature XRD was performed using an attachment (Reactor-X, Rigaku) equipped with an infrared heater connected to its controller (PTC EVO, Rigaku) and an external power source (Thermo plus EVO PU-4K CE, Rigaku). An infrared reflector was attached to assist in heating the sample, while the attachments were continuously cooled with circulating water and both an external and internal fan. With this setup, the peak positions were calibrated using the secondary peak of Si powder (Wako, 220, 47.2660°, cubic, Fd3̄m) at room temperature. The temperature was increased with a heating rate of 10 °C min⁻¹ from room temperature to 830 °C and 2 °C min⁻¹ from 830 °C to 850 °C to prevent the cell from overheating. The temperature was decreased with a cooling rate of 10 °C min⁻¹ to 25 °C, and the determined temperature was held for 5 min before each measurement. All measurements were conducted by flowing a mixture of 80 mL min⁻¹ argon (Ar) and 20 mL min⁻¹ O₂. The temperature was corrected by using the α to δ phase transition peak (in the heating process) of Bi₂O₃ (98.0+%, Wako Pure Chemical Corporation) at 735 °C.

X-ray absorption spectroscopy

Bi L₃-edge and Ta L₃-edge XAS were performed at the bending magnet XAFS beamline, BL01B1 at SPring-8 (Hyogo, Japan). Experiments were performed at room temperature and in the transmission mode using the ionization chambers. Bi₂O₃ and Ta₂O₅ were used as references. The powder samples were thoroughly ground with boron nitride in an agate mortar, and 10 mm–diameter pellets were made. The amount of boron nitride was ∼90 mg to ensure that the pellets could be handled without breaking. The EXAFS spectra were analyzed using the HORAE-IFEFFIT (Athena) program package. This program enabled us to normalize the absorption coefficient, μ(k), and to separate the oscillatory EXAFS, χ(k), from the absorption background. Before normalization, the pre-edge range of the spectra for Bi and Ta L₃ edges was flattened in the energy range of −210 to −60 keV and −200 to −60 keV, respectively. The data sets for Bi and Ta L₃ edges were normalized in the energy range of 50–880 keV (4.0–71.0 Å⁻¹) and 200–1000 eV (16.1–80.7 Å⁻¹), respectively. Then, the background was calculated from the data in energy ranges of 0–15 Å⁻¹ (0–857 eV) for Bi L₃ edges and 0–17 Å⁻¹ (0–1001 eV) for Ta L₃ edges. All data were Fourier-transformed using the same k range, k = 3 – 12 Å⁻¹, k weighting of 2 and a hanning k window and plotted with k weighting of 3. The theoretical radial distribution functions were calculated using the Visual Molecular Dynamics (VMD) program (version 1.9.4a49).³⁴

Band position determination

DR-UV-vis and PESA were performed to determine the band alignment of the conduction band minimum (CBM) and the valence band maximum (VBM) positions of the synthesized samples. DR-UV-vis spectroscopy was performed on a JASCO model V-770 spectrophotometer equipped with an integrating sphere. The reflectance spectra were scanned over the range of 200–1000 nm with a scan speed of 400 nm min⁻¹ and a bandwidth of 2.0 nm for the ultraviolet/visible region and 8.0 nm for the near-infrared region. The reflectance spectra were referred to the background reflectance of BaSO₄ powder. The contributions of scattering were removed by the Kubelka–Munk function, F(R_∞). Followed by the calculation of the bandgap energy, the absorption edge was determined by the intercept of the straight line extrapolated to the F(R_∞) vs wavelength.

The valence band positions were determined by PESA (AC-3 photoelectron spectrometer, Riken Keiki Co., Ltd.). The obtained spectra were calibrated based on the work function of gold (Au) (5.30 eV vs. vacuum).³⁵ The yield of the photoelectrons was measured for 30 s at every 0.1 eV step in the range of 4.0–7.0 eV of irradiation UV light. The valence band potentials, obtained as the potentials relative to the absolute vacuum potential, were converted to the standard hydrogen potentials (E_SHE = E_abs − 4.44 V). The conduction band positions were calculated from the valence band positions, and the bandgaps were determined by PESA and DR-UV-vis spectroscopy.

Photoluminescence

Photoluminescence (PL) spectroscopy was performed on a JASCO model FP-8500 fluorescence spectrometer operated at liquid nitrogen temperature under ambient pressure. The excitation and emission had a 5 nm bandwidth. The spectra were scanned at a speed of 1000 nm min⁻¹ with medium level sensitivity and 50 ms of response at every 0.1 nm interval and accumulated over 10 cycles.

Thermogravimetry/differential thermal analysis

TG/DTA was performed to investigate the phase transition processes of Ta₂O₅-supplemented Bi₂O₃in situ. TG/DTA was performed using a Thermo plus EVO2 series TG-DTA 8120 (Rigaku) in the atmosphere of the residual air in the chamber without flow. All measurements were performed with a 10 °C min⁻¹ heating rate and cooling rates of up to 850 °C. The sample (8–10 mg of the Bi₂O₃ powder or the mixture of Bi₂O₃ and Ta₂O₅ powders mixed at the stoichiometric amounts [Ta/Bi = 0.05, 0.10]) was placed in an open platinum pan. An open and empty platinum pan was used as the reference.

Scanning electron microscopy (SEM) and elemental analysis

The morphologies, particle sizes, and the layered structure of the particulate photoelectrodes were examined using a high-resolution field emission SEM (FE-SEM; JEOL, JSM-IT800) equipped with a secondary electron detector. Energy-dispersive X-ray spectroscopy (EDS) was conducted with an EDS detector (Ultim Max, Oxford Instruments).

Computational method

All calculations were performed using the Vienna Ab initio Simulation Package (VASP),^36,37 with the projector augment wave (PAW) method.³⁸ Calculations were performed with the cut-off energy set to 400 eV. Brillouin zone sampling was performed using Monkhorst and Pack schemes.³⁹ We completely relaxed the bulk unit cell of β-Bi₂O₃ in its tetragonal structure with space group P4̄21c, which contains 8 Bi atoms and 12 oxygen (O) atoms. δ-Bi₂O₃ has a cubic fluorite structure with a space group of Fm3m. The unit cell contains two Bi₂O₃ formulas. The valence atomic configurations are 5d¹⁰ 6s² 6p³ for Bi and 2s² 2p⁴ for O atoms. Geometry optimizations were terminated when the energy and force on each ion were reduced below 10⁻⁶ eV and 0.02 eV Å⁻¹, and the optimized structures were then used to calculate the electronic structures. Hybrid DFT calculations were performed at a single point at the generalized gradient approximation (GGA) lattice geometry.⁴⁰ First, the GGA scheme was employed for the fully-relaxed structural optimization. Then, the screened-exchange hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06),^41,42 representing the electronic exchange–correlation energy, was adopted for calculations of the electronic and optical properties. The spin–orbit coupling (SOC) effect was taken into consideration through the second variational method because of the relativistic effect in Bi atoms.⁴³ More details of the computational modeling of surfaces and of the methodology used to calculate the optical properties can be found in the ESI.†

Electrochemical (EC) and photoelectrochemical (PEC) measurements

To conduct EC and PEC measurements, particulate photoelectrodes were fabricated through previously reported particle transfer methods.⁴⁴ Multilayers of photocatalyst particles were deposited on a glass plate by drop-casting and drying its suspension in isopropanol several times, resulting in an approximate amount of 10 mg cm⁻². Subsequently, 200 nm-thick–Ti and 3–4 μm–thick-Sn were sequentially deposited on the particles through electron beam deposition (ULVAC) to prepare the assembly of photocatalysts with a Ti–Sn conductive layer. The obtained assembly of particles and the Ti–Sn layer adhering to them were peeled off from the glass plate, and the excess unused particles piled up on the deposited particles were removed by ultrasonication in ethanol. The film electrode was fixed on another glass plate using carbon tape. Finally, the film was connected to lead wire using an indium solder, after which its unnecessary part was covered with epoxy resin for insulation. The geometrical electrode areas of the photoelectrodes were approximately 0.20–0.30 cm². EC and PEC measurements for the particulate-photoelectrodes were performed using a three-electrode system under an argon (Ar) gas atmosphere at a temperature of 25 °C. The Hg/Hg₂Cl₂/saturated KCl reference electrode (BAS Inc., RE-2BP, 241 mV vs. RHE at 25 °C) and coiled-Pt wire counter electrode were connected to a potentiostat (SP-150, BioLogic Science Instruments). Prior to the PEC measurements, a CoO_x catalyst promoting the carrier transfer at the solid–liquid interface was electrodeposited under light irradiation on the surface of the particulate photoelectrodes. A solar simulator (XES-40S2, SAN-EI Electric Co., Ltd.) equipped with an AM 1.5G filter was used as the light source, and the light intensity was adjusted to 100 mW cm⁻² (1 sun). The electrolyte was made by saturating Co(NO₃)₂·6H₂O (0.1 M, 99.5%, Wako) in a 1 M potassium phosphate buffer solution. The pH of the buffer was adjusted to 7.0 by adding a potassium hydroxide (KOH; >85%, Sigma-Aldrich) solution into a phosphoric acid (H₃PO₄, ≥ 85 wt% in H₂O, Sigma-Aldrich) aqueous solution. Electrodeposition was performed at a constant current density of 50 μA cm⁻² for 5 min under simulated solar light (AM 1.5G). CoO_x deposited or not deposited particle-transferred working electrodes with an area of approximately 0.2–0.3 cm² were positioned in a glass cell. The electrolyte was a 1M potassium borate buffer (pH 9.0) aqueous solution. The pH was adjusted to 9.0 by adding a KOH (≥85%, Sigma-Aldrich) solution into a boric acid (H₃BO₃, ≥99.5 wt% in H₂O, Sigma-Aldrich) aqueous solution. For LSV measurements, 0.1 M of Na₂SO₃ was added as a hole scavenger. PEC measurements employed a Xe-lamp (MAX-303 Xenon Light Source, 300 W, Asahi Spectra), with a UV-Vis miler module and a cut-off filter (HOYA L42) as the light source (420 < λ < 650 nm).

Temperature-programmed reduction

TPR equipment consisted of mass flow controllers (KOFLOC, model 3660) and a U-shaped quartz reactor inside an oven. Samples were treated in flowing Ar (20 mL min⁻¹) by increasing their temperature to 400 °C at 40 °C min⁻¹ and holding for 1 h. After cooling to room temperature, the Ar stream was replaced with the reducing gas, 4% H₂ in Ar (20 mL min⁻¹). The temperature was increased with a heating rate of 10 °C min⁻¹ from room temperature to 850 °C, and the H₂ concentration was monitored using a GC (GC-8A, TCD, Shimadzu) connected to the outlet of the reactor.

Results and discussion

Synthesis and structural characterization of β- and δ-Bi₂O₃ and Bi₃TaO₇

The formation process of the β-Bi₂O₃:Ta (5%) and the δ-Bi₂O₃:Ta (10%) in the solid-state reaction was studied by DTA and in situ high temperature XRD measurements. Fig. 1 shows the DTA curves of the pure Bi₂O₃, mixture of Bi₂O₃ and Ta₂O₅ at ratios of Ta/Bi of 0.05 and 0.10. Fig. 2 shows the in situ high temperature XRD patterns for the samples at Ta/Bi = 0 (Fig. 2a and b) and 0.050 (Fig. 2c and d). Upon heating the samples, all of the samples show endothermic peaks at 735 °C (Fig. 1). Accordingly, the new peak assignable to δ-Bi₂O₃ appeared (Fig. 2a), clearly indicating the α to δ transition at this temperature. Pure Bi₂O₃ shows an additional endothermic peak at 830 °C, which can be attributed to the melting of δ-Bi₂O₃.²⁸ Lack of melting behavior with Ta addition suggests that the melting point of Bi₂O₃ is higher than 850 °C. Subsequent decreases in the temperature provided substantial differences in phase transition by varied Ta addition. For pure Bi₂O₃ (Ta/Bi = 0), in Fig. 1, the DTA curve in the cooling process shows two exothermic peaks of curding at 830 °C and at 620 °C. As the peak at 830 °C is likely ascribed to solidification, consistent with the literature,²⁸in situ XRD measurements clearly identified that a δ-to-α transition occurred (Fig. 2b), indicating that the original state was recovered during the heating–cooling cycle. For the sample with Ta/Bi = 0.050, an exothermic peak at 580 °C (Fig. 1) corresponds to the δ-to-β transition (Fig. 2d), the phase that was not obtained without Ta addition. For the sample with Ta/Bi = 0.100, the DTA curve (Fig. 1) did not show any pronounced peaks while cooling. This indicates that the δ phase of the Ta-supplemented Bi₂O₃ was maintained during the cooling process.

Fig. 1 DTA curves of the Bi₂O₃ samples with Ta/Bi ratios of 0, 0.050, and 0.100. The temperature was increased and subsequently decreased at the rate of 10 °C min⁻¹ in flowing air.

Fig. 2 In situ high-temperature XRD for the Bi₂O₃ with Ta/Bi ratios of 0 (a and b) and 0.050 (c and d). The temperature was increased (a and c) and decreased (b and d) at the rate of 10 °C min⁻¹ in static air. The assignments of the XRD peaks are indicated by coloring their backgrounds (blue: α phase; red: β phase; green: δ or cubic phase).

Fig. 3a shows the XRD patterns of the samples with a much wider range of Ta/Bi ratios after cooling from the heat treatment at 850 °C for 5 h. Fig. 3b shows the schematics of the corresponding various crystal structures. The XRD patterns of α-, β-, δ-Bi₂O₃ and Bi₃TaO₇ from the inorganic crystal structure database (ICSD) are shown in Fig. S1 (ESI†) as references. While no Ta₂O₅ addition led to the formation of the monoclinic α-Bi₂O₃ structure, a small addition of Ta (Ta/Bi ratio from 0.020 to 0.050) led to the formation of the β-Bi₂O₃ (tetragonal) structure. Further addition of Ta, to give rise to Ta/Bi ratio of 0.100, led to the formation of the δ phase of Bi₂O₃ (cubic, Pn3̄ m). The samples with Ta/Bi = 0.167 and 0.333 were identified as the Bi₃TaO₇ phase (cubic, F3̄ m). The principal XRD peaks of the obtained β-Bi₂O₃, δ-Bi₂O₃, and Bi₃TaO₇ phases are highlighted in Fig. S2 (ESI†). It is of great significance to note that the amount of Ta from 2–5% did not influence the peak position of β-Bi₂O₃, suggesting that the Bi and Ta atoms do not replace each other in the crystal structures of our Ta-supplemented β-Bi₂O₃ samples in a way that affects the lattice constant. On the other hand, for Bi_3−xTa_xO_3+y (x = 1.29) and Bi₃TaO₇ with the Ta/Bi ratios of 0.167 and 0.333 (Fig. S2b, ESI†), the main XRD peaks for the (111) plane can be explained by the replacement of a Ta⁵⁺ cation with a Bi³⁺ cation, which is accompanied by the creation of oxygen vacancies for the charge compensation, and the overall charge electroneutrality of the system is preserved through a mechanism: Bi³⁺ +mpe⁵⁺ + O²⁻.⁶ Finally, the studied samples are compiled in Table 1.

Fig. 3 (a) XRD patterns of the samples at the Ta/Bi ratio. B) Schematics of the crystal structures from ICSD 94229 (α-Bi₂O₃), 417638 (β-Bi₂O₃), 38436 (δ-Bi₂O₃), and 161407 (Bi₃TaO₇).

Table 1 Ta/Bi ratios in the prepared material, identification of the crystal phase, and the notation in this study

Ta/Bi ratio	Crystal phase	Notation
0	α-Bi2O3 (monoclinic)	α-Bi2O3 (precursor)
0.020	β-Bi2O3 (tetragonal)	β-Bi2O3:Ta (2%)
0.026	β-Bi2O3 (tetragonal)	β-Bi2O3:Ta (2.6%)
0.033	β-Bi2O3 (tetragonal)	β-Bi2O3:Ta (3.3%)
0.050	β-Bi2O3 (tetragonal)	β-Bi2O3:Ta (5%)
0.100	δ-Bi2O3 (cubic, Pn3̄m)	δ-Bi2O3:Ta (10%)
0.167	Bi3TaO7 (cubic, Fm3̄m)	Bi3+xTa1–xO7–y (x = 0.43)
0.333	Bi3TaO7 (cubic, Fm3̄m)	Bi3TaO7
—	Ta2O5 (orthorhombic)	Ta2O5 (precursor)

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Fig. S3 (ESI†) shows the SEM images and EDS mappings of the representative samples. Aggregated coarse particles with sizes on the order of micrometers somewhat thermally fused were observed in all samples. From EDX mapping, it can be observed that Bi, Ta, and O are well distributed throughout the particles for all samples, indicating the formation of a homogeneous solid solution or Ta-doped Bi₂O₃.

A small amount of added Ta was observed to play a significant role in structural determination. To identify at which positions the Ta atoms are located in the crystal structures, the local environments of Bi and Ta atoms were studied with XAS measurements, including XANES and EXAFS. All spectra and their analyses are summarized in Fig. S4 (ESI†). The Bi L₃-edge XANES spectra of α-Bi₂O₃ (reference), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇ are shown in Fig. 4a. The spectral shape of the samples basically consists of a main absorption peak (A) located at 13.445 eV, a slight shoulder-like structure just below the main peak (A1, hν ∼ 13.42 keV), and a single oscillation after the main peak (B, hν ∼ 13.46 keV). The spectra of β-Bi₂O₃:Ta (5.0%), δ-Bi₂O₃:Ta (10.0%), and Bi₃TaO₇ show a hump (B, hν ∼ 13.46 keV) followed by a negative feature at higher energies (C, hν ∼ 13.47 keV), and the spectrum of α-Bi₂O₃ presents a wide plateau in the energy region immediately after the main peak (B–C, hν ∼ 13.46–13.47 keV). This directly indicates that the local environments of Bi in the samples under study are different from those of α-Bi₂O₃. Moreover, the shift in the shoulder-like structure (A1, hν ∼ 13.42 keV) at the edge (A, hν ∼ 13.445 keV) and just below the edge (B, hν ∼ 13.46 keV) cannot be explained with the linear combination of each other. In other words, the XANES profiles of the samples cannot be explained with the presence of mixed polymorphs.

Fig. 4 (a) Comparison of the experimental Bi L₃-edge of α-Bi₂O₃ (reference), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇. (b) Ta L₃-edge XANES spectra of Ta₂O₅ (reference), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇.

Next, the Ta L₃-edge XANES spectra of α-Bi₂O₃ (reference), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇ are shown in Fig. 4b. All of the compounds show a main peak (A) at ∼9.884 keV, but our samples have a double peak corresponding to the split of 5d orbitals, whereas the Ta₂O₅ reference shows only a single peak. This difference is more apparent in the energy region immediately after the main peak because our samples show a hump (B, hν ∼ 9.906 keV), whereas the Ta₂O₅ reference only shows a shoulder-like structure (B1, hν ∼ 9.903 keV) followed by a negative feature at a higher energy (B2, hν ∼ 9.910 keV). These comparisons to the Ta₂O₅ reference show that the local environments around Ta in our studied samples are different from those in the Ta₂O₅ reference. Similar results are reported in the W L₃-edge XANES spectra of Bi₂WO₆ (octahedral) and Bi₁₄WO₂₄ (tetrahedral) by Wind and coworkers²⁹ and the W L₃-edge XANES spectra of other W-related oxides by Yamazoe and coworkers.⁴⁴ It is reasonable to consider that the differences of the local environments around Ta in the Ta₂O₅ (reference) and our Bi–Ta oxides result from a distortion or oxygen vacancies in the crystal structure induced by Ta addition.

The characteristics of the structural orders around Bi and Ta in the samples are also inferred from the comparison of the Fourier transforms of the EXAFS signals. In Fig. 5, Bi L₃ and Ta L₃ signals of Bi and Ta in the α-Bi₂O₃ reference, α-Bi₂O₃ precursor, and β-Bi₂O₃:Ta (5%) are compared. The main peak at ∼1.7 Å and double peaks at ∼3 Å shown in Fig. 5b correspond to the first coordination shell made of oxygen atoms and Bi neighbors.⁴⁵ Additionally, these peaks could be explained by the theoretical radial distribution function calculated from the optimized crystal structures. Fig. 5c shows the Ta L₃-edge EXAFS spectra of the Ta₂O₅ reference, Ta₂O₅ precursor, and β-Bi₂O₃:Ta (5%). The difference between the environments of Ta atoms in the Ta₂O₅ and our β-Bi₂O₃:Ta (5%) is suggested by the Fourier transforms shown in Fig. 5d. The enhancement of the main peak (∼1.5 Å) can be related to the distortions in distances and angles between the Ta–O bonds in the TaO₆ octahedra.⁴⁶ Furthermore, this distortion is also suggested by the shift of the main peak (∼1.5 Å) and appearance of the second peak at ∼2 Å.

Fig. 5 (a) Comparison of Bi L₃ EXAFS spectra of α-Bi₂O₃ (reference), α-Bi₂O₃ (precursor), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%) and Bi₃TaO₇. (b) Corresponding Fourier transforms. (c) Comparison of Ta L₃ EXAFS spectra of Ta₂O₅ (reference), Ta₂O₅ (precursor), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%) and Bi₃TaO₇. (d) Their corresponding Fourier transforms.

The structural orders in the δ-Bi₂O₃:Ta (10%) and Bi₃TaO₇ and their crystal systems were also studied using both Bi L₃ and Ta L₃ edge EXAFS spectra. The EXAFS spectra are shown in Fig. 5a and c and their Fourier transforms are shown in Fig. 5b and d. To perform the theoretical simulation, we assumed that when a Ta atom substitutes Bi sites in the cubic lattice in such a way, that the Bi–O distance decreases, the O atoms move close to Bi. This substitutability of Bi and Ta leads to tunability of the bandgap of these cubic-phase B–Ta oxides, as is discussed later. One might initially contend that Ta⁵⁺ and Bi³⁺ cannot substitute each other in the crystal lattice because the charge in the oxide cannot be balanced. However, the charge has been reasonably explained to be balanced by the change in the occupancy of O sites in previous studies.⁵ Therefore, it can be concluded that by mixing Bi₂O₃ and Ta₂O₅ at a molar ratio of Ta/Bi = 0.1, 0.33, the resulting Bi–Ta oxides stabilized and formed a substitutional solid solution with the cubic crystal phase.

Electronic structures and optical properties

The densities of states (DOS) and the charge density maps of β-Bi₂O₃(201) and δ-Bi₂O₃(111) slabs were computed using the HSE06 (Heyd–Scuseria–Ernzerhof) exchange–correlation functional, and they are shown in Fig. 6a. The VBM is dominated by O 2p states, whereas the CBM is mostly composed of O 3s and Bi 6p for β-Bi₂O₃ and Bi 6p for δ-Bi₂O₃. It is worth mentioning that the narrower bandgaps of β-Bi₂O₃ and δ-Bi₂O₃ were explained only by Bi and O, without contribution from the partial density of states of Ta. To study the qualitative nature of chemical bonding in the (201) slab of β-Bi₂O₃ and the (111) slab of δ-Bi₂O₃, the electron density distribution is presented in Fig. 6b. Comparing these two model surfaces, we observe more depletion of charge on Bi in (111) δ-Bi₂O₃ and more σ character of bonds on the surface of δ-Bi₂O₃, while β-Bi₂O₃ exhibits mixed bonds. However, there is a lack of strong hybridization (observed also in DOS in Fig. 6a) between Bi and O states in both surfaces; thus, fewer covalent bonds between atoms will exist, which could suppress the charge carrier transport. Moreover, we computed the effective mass tensors of the photogenerated holes () and electrons () at the band edges of the (201) slab of β-Bi₂O₃ and the (111) slab of δ-Bi₂O₃ using their electronic band structure, which were obtained from the PBE exchange-correlation functional; these results are listed in Table 2. The values are substantially larger than the benchmarking 0.5 m₀ for good charge carrier transport,^47–49 even compared to those based on the Bi–Ti–O system reported in our previous studies.^50,51 Large values of both electron mass and hole effective mass for both the (201) slab of β-Bi₂O₃ and (111) slab of δ-Bi₂O₃ predict poor charge separation capability of the materials and the resultant poor photocatalytic properties. The optimized structures and the local potential in the z-direction are shown in Fig. S5 (ESI†).

Fig. 6 (a) The total and partial electronic density of states (DOS) of a (201) slab of β-Bi₂O₃ and a (111) slab of δ-Bi₂O₃. The geometry optimization and electronic structure calculations were performed employing the HSE06 (Heyd–Scuseria–Ernzerhof) exchange–correlation functional along with spin–orbit coupling (SOC) corrections, as implemented in VASP (Vienna Ab initio Simulation Package). (b) Charge density maps obtained for the VBM and CBM electronic states of the (201) β-Bi₂O₃ and (111) δ-Bi₂O₃ slabs. The isosurface value is set at 6.7 × 10⁻³ au.

Table 2 Summary of electronic, dielectric, and optical properties of (201) oriented β-Bi₂O₃ and (111) oriented δ-Bi₂O₃ slabs

	(201) oriented β-Bi2O3	(111) oriented δ-Bi2O3
Electronic dielectric constant ε∞ along: xx (yy)	6.0 (4.8)	5.0 (7.3)
Macroscopic dielectric constant εr along: xx (yy)	98 (93)	100 (119)
Effective electron mass /me	1.2	3.2
Effective hole mass /me	20	13
VBM (vs. vacuum in eV)	−5.59	−5.36
CBM (vs. vacuum in eV)	−3.46	−2.68
Bandgap (eV)	2.13	2.68

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To determine the absorption characteristics of Bi-based samples, DR-UV-vis spectroscopy was used and compared with the simulated absorption coefficient as a function of wavelength. The results are shown in Fig. 7. The Kubelka–Munk functions of the representative ones (Fig. 7a) show that for the pure α-Bi₂O₃ precursor, the adsorption edge was 445 nm in the near UV region, whereas that of β-Bi₂O₃:Ta (5%) was 550 nm. The absorption edges of the δ-Bi₂O₃:Ta (10%) and Bi₃TaO₇ were 530 and 440 nm, respectively. The bandgap energy calculation yields values of 2.8, 2.2, 2.3, and 2.8 eV for α-Bi₂O₃, β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇, respectively. The visible-light responsiveness of β-Bi₂O₃ and δ-Bi₂O₃ were simulated on our slab models whose structures were optimized without adding Ta. The simulated absorption spectra are shown in Fig. 7b, and their absorption edges nearly correspond to those of the experimental spectra. In general, the degree of absorption coefficient is considered low for these Bi₂O₃, compared to other general compound-based semiconductors.⁴⁷

Fig. 7 (a) The DR-UV-vis spectra of α-Bi₂O₃ (precursor), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇. (b) Simulated UV-visible optical absorption spectra of (201) oriented β-Bi₂O₃ and (111) oriented δ-Bi₂O₃ slabs.

Because it remained possible that the visible-light responsiveness (2.2 eV bandgap) is derived from the existence of defects especially caused by added Ta in the β-Bi₂O₃:Ta (5%), photoluminescence spectroscopy was conducted. The results are shown in Fig. S6 (ESI†). For the α-Bi₂O₃ (precursor) shown in Fig. S6a (ESI†), the emission spectra triggered by 400 nm excitation light show a peak at 500–700 nm attributable to the existence of a defect level within the bandgap. However, the photoluminescence spectrum of our β-Bi₂O₃:Ta (5%) shown in Fig. S6b (ESI†) did not exhibit noticeable peaks other than a small peak at 560 nm attributed to near bandgap emission. This result does not contradict the statement that the photon absorption at band edges is mainly β-Bi₂O₃ bandgap excitation.

The band alignment of the CBM and the VBM positions of the synthesized samples was experimentally estimated by using PESA. The VBM potentials estimated by ionization potentials were obtained by the crossing point of the linear part of the curve with the background from PESA. The spectra of representative samples are shown in Fig. 8, and all other spectra of the Bi–Ta oxides are shown in Fig. S7 (ESI†). The VBMs do not shift significantly but remain constant at 6.4–6.6 eV vs. vacuum (i.e., 2.0–2.2 eV vs. SHE), as expected from the contribution of O 2p orbitals. The determined band positions are summarized in Fig. 9 together with the Ta/Bi ratio and the resulting crystal phase. The results suggest that the conduction band positions are largely altered while valence band positions are relatively consistent.

Fig. 8 Photoelectron spectra in air (PESA) of α-Bi₂O₃ (precursor), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇.

Fig. 9 The experimental band positions of α-Bi₂O₃ (precursor), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇. The bandgap (E_g) and valence band maximum (VBM) of each sample were determined by DR-UV-vis spectroscopy (Fig. 7a) and photoelectron spectroscopy in air (PESA) (Fig. 8), respectively.

To check the photoresponsive behavior of α-Bi₂O₃ (precursor) and β-Bi₂O₃:Ta (5%), LSV (linear sweep voltammetry) under chopped-light illumination was conducted for CoO_x/α-Bi₂O₃ (precursor)/Ti–Sn, and CoO_x/β-Bi₂O₃:Ta (5%)/Ti–Sn particle–transferred electrodes. The result is shown in Fig. 10. The CoO_x/α-Bi₂O₃ (precursor)/Ti–Sn electrode did not exhibit a pronounced response to the light, but the CoO_x/β-Bi₂O₃:Ta (5%)/Ti–Sn electrode showed a pronounced anodic photocurrent. Because the Xe-lamp was used with a cut-off filter as the light source (420 < λ < 650 nm), this spectrum indicates that this material can utilize visible light to produce photo-excited holes and electrons. The onset potential of the sacrificial reagent oxidation reaction was found to be 0.5 V vs. RHE under light illumination, despite the fact that it was 1.0 V vs. RHE under dark conditions. The anodic photocurrent of 0.1 mA cm⁻² was found to be 1.25 V vs. RHE.

Fig. 10 LSV at 10 mV s⁻¹ under light chops for CoO_x/α-Bi₂O₃ (precursor)/Ti–Sn and CoO_x/β-Bi₂O₃:Ta (5%)/Ti–Sn in 1 M potassium borate aqueous solution (pH 9). Sacrificial reagent: 0.1 M Na₂SO₃; RE: Hg/Hg₂Cl₂/saturated KCl; CE: Pt wire; light source: Xe-lamp (MAX-303, 300 W) with cut-off filter (L42), >420 nm; temperature: 25 °C.

To determine the dielectric properties of β-Bi₂O₃:Ta (5%) and δ-Bi₂O₃:Ta (10%), we computed their dielectric constants and effective masses. In a dielectric material, the static dielectric constant (ε_r) has contributions from two factors: the electron density (ε_∞) and the ion vibrations (ε_vib). The dielectric constants and effective masses are listed in Table 3, and the real and imaginary parts of the dielectric functions obtained using HSE06 and SOC are shown in Fig. S8 (ESI†). With these calculations, the charge carriers were found to be heavy, indicating poor photocatalytic properties.

Table 3 Summary of the computed properties for β-Bi₂O₃ and δ-Bi₂O₃

	(201) β-Bi2O3	(111) δ-Bi2O3
ε ∞ along x	6.0	5.0
ε ∞ along y	4.8	7.3
ε vib along x	98	100
ε vib along y	93	119
/me	1.2	3.2
/me	19.9	12.5

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Thermal redox properties

To investigate the difference of reducibility of our Bi-based oxides and consider an application for them as a material that is accompanied by thermal redox, TPR measurements were performed. The TPR profiles of the studied Bi–Ta oxides are shown in Fig. 11. A broad reduction peak of α-Bi₂O₃ (precursor) appeared at 700 °C. The atomic ratio of consumed H₂ to Bi contained in the sample (H₂/Bi ratio, Fig. 11) was calculated to be 1.58, and the total hydrogen consumption accounted for the complete reduction of the α-Bi₂O₃ (precursor) sample. The broadness of the peak can be attributed to the relatively large sizes of the α-Bi₂O₃ (precursor) particles, as seen in the SEM images (Fig. S3, ESI†). The peak was tailed to 400 °C, suggesting a faster reduction of the surface. TPR was also performed on the Ta₂O₅ (precursor), but substantial reduction was not observed throughout this temperature range up to 900 °C. β-Bi₂O₃:Ta (5%) showed reduction starting at around 400 °C, with a maximum at 640 °C, indicating facilitated reduction compared to α-Bi₂O₃ (precursor). Both the H₂/Bi and H₂/O ratios of 1.56 and 0.96 indicated the complete reduction of Bi atoms. δ-Bi₂O₃:Ta (10%) had a reduction peak starting at 300 °C, a markedly low temperature, with a maximum at 520 °C, again the lowest temperature among the samples investigated. These findings suggest that reduction of both bulk and the surface of δ-Bi₂O₃:Ta (10%) was easiest among the samples. This facilitated redox property might be explained by the high mobility of oxygen species related to the low occupancy of O sites due to its defective crystal structure. The consumption of hydrogen (H₂/Bi = 1.53 and H₂/O = 0.88) can be explained if it is assumed that the Ta atoms are not reduced, and that only Bi atoms are reduced, as expressed by the following reaction (note that δ-Bi₂O₃:Ta [10%] is expressed as Bi₂Ta_0.2O_3.5):

Bi₂Ta_0.2O_3.5 + 3H₂ → 2Bi + 3H₂O + 0.1Ta₂O₅ (1)

(H₂/Bi = 3/2 = 1.5, H₂/O = 3/3.5 = 0.857)

Bi₃TaO₇ showed a reduction peak at 640 °C, which appeared at a higher temperatures than that of δ-Bi₂O₃:Ta (10%); this may be because δ-Bi₂O₃:Ta (10%) has a less defective structure than Bi₃TaO₇. The peak was tailed to 300 °C, but the tailing was not as pronounced as that of δ-Bi₂O₃:Ta (10%). The stoichiometry of the consumption of hydrogen (H₂/Bi = 1.68 and H₂/O = 0.72) can be roughly explained by the following reaction:

Bi₃TaO₇ + 4.5H₂ → 3Bi + 4.5H₂O + 0.5Ta₂O₅ (2)

(H₂/Bi = 4.5/3 = 1.5, H₂/O = 4.5/7 = 0.643)

Among these oxides, δ-Bi₂O₃:Ta (10%) appeared to have a TPR peak at the lowest temperature, meaning the highest thermal redox properties. This material is expected to be utilized as a catalyst or its support for reactions that are accompanied by oxidation or reduction. Further investigations into the applications of this material as a thermal catalyst or its support are expected.

Fig. 11 TPR profiles of α-Bi₂O₃ (precursor), Ta₂O₅ (precursor), β-Bi₂O₃:Ta (5%), δ-Bi₂O₃:Ta (10%), and Bi₃TaO₇. The total hydrogen consumptions are expressed in moles of H₂ consumed per mole of metal atoms (H₂/Bi or H₂/Ta) and moles of H₂ consumed per mole of O atoms (H₂/O). The samples were pretreated in Ar (20 mL min⁻¹) at 400 °C before each measurement. The measurements were conducted on 20 mg of samples in 4% H₂ (in Ar, 20 mL min⁻¹) with a heating rate of 10 °C min⁻¹.

Conclusions

In summary, this investigation of the Ta-supplemented bismuth oxides, such as α-Bi₂O₃, β-Bi₂O₃, δ-Bi₂O₃ and Bi₃TaO₇, confirms that the crystal structures and optoelectronic properties of these oxides are altered according to the amount of added Ta (as Ta₂O₅). Heat treatment at 850 °C and cooling of Bi₂O₃ with different amounts of added Ta controllably led to the formation of α-Bi₂O₃, β-Bi₂O₃, and δ-Bi₂O₃. Among them, β-Bi₂O₃ was found to be a promising visible-light harvesting material, as it has the narrowest bandgap of 2.2 eV. DFT calculations using the HSE exchange–correlation functional and PESA measurements demonstrate that the conduction band has a large contribution from Bi orbitals in β-Bi₂O₃ and δ-Bi₂O₃. We confirmed that the presence of added Ta is not essential to alter the band structures of bismuth oxides by causing a defect level but, rather, by changing the crystal phase of bismuth oxides. However, the DFT calculations also suggested that β-Bi₂O₃ and δ-Bi₂O₃, which have bandgaps narrow enough to absorb visible light, are limited in their applicability as a photocatalyst due to their large effective masses and resultant low charge mobilities. In addition, different Bi₂O₃ polymorphs exhibited distinctive reducibility upon hydrogen treatment at high temperatures, showing that δ-Bi₂O₃ was reduced at the lowest temperature among the samples investigated. The wide range of methodologies adopted in this study and the knowledge gained into the semiconductor structure of a mixed oxide through these methodologies will allow us to identify and understand other novel candidate materials for applications such as photocatalysis and thermal catalysis.

Author contributions

All authors contributed to writing the manuscript. All authors approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Mohammed bin Salman Center for Future Science and Technology for Saudi-Japan Vision 2030 at the University of Tokyo (MbSC2030). LC and RA acknowledge Shaheen and HPC Core Labs team at KAUST.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc03718a

‡ These authors contributed equally.

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