Phase-controlled synthesis of bismuth oxide polymorphs for photocatalytic applications

Abstract

In this study, bismuth oxide polymorphs (α and β phases) were synthesized using a post-calcination method using Bi₂O₂CO₃ nanoflakes as an intermediate. These samples were characterized by various spectroscopic techniques such as powder X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, UV-visible absorption, photoluminescence studies and microscopic analysis (scanning and transmission electron microscopy). The mechanism of the formation of β-Bi₂O₃ nanoparticles and the α-Bi₂O₃ coral-like morphology has been proposed. Furthermore, the photocatalytic performance of the bismuth oxide polymorphs was investigated via the degradation of organic dyes (rhodamine B, methylene blue, acid orange 7 and methyl orange) under simulated solar illumination. The results show that the β-Bi₂O₃ nanoparticles possess superior photocatalytic activity compared to the α-Bi₂O₃ coral-like morphology for the degradation of these dyes due to a higher photon adsorption (band gap of 2.2 eV for β-Bi₂O₃ and 2.56 eV for α-Bi₂O₃). However, the photocatalytic activity of these bismuth oxide polymorphs for the degradation of organic dyes followed the order of Rh B > AO 7 > MB > MO under identical experimental conditions. A plausible mechanism has been proposed for the photocatalytic activity.

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Introduction

Over the past few decades, semiconductors (e.g., TiO₂, ZnO, Fe₂O₃, WO₃, SnO₂, CdS and ZnS) have been studied as photocatalytic materials for energy and environmental applications.^1–4 Because these materials have appropriate electronic band structures (absorption edges), they are found to be useful for the conversion of solar energy into clean energy (hydrogen production from water splitting) and degradation of pollutants (dyes, pesticides, pharmaceutical products and gaseous toxins) under light irradiation.^5–8 Often, TiO₂ is used as a remarkable photocatalyst for energy conversion and environmental remediation due to cost effectiveness, non-toxicity and chemical stability.^9,10 However, TiO₂ polymorphic phases (anatase, rutile or brookite) possess wide band gaps (≤3–3.2 eV), a disadvantage for utilizing natural sunlight or household indoor illumination (only 4% of UV light).^11–13 Doping or combination with other semiconductors has been found to improve the visible light activity of TiO₂.^11,14,15 Meanwhile, several other materials have been reported as efficient photocatalysts for the degradation of environmental contaminants under natural sunlight/visible-light.^16–23 Among them, bismuth containing materials with narrow band gap and positive conduction band potential have shown higher photocatalytic efficiency.^13,19–24 Bismuth oxide (Bi₂O₃) is found to be a good photocatalyst and a good photoanode in a dye sensitized solar cell attributed to a wide band gap of 2.0 to 3.96 eV.^10,25–35 Also, it possesses thermoelectric, ferroelectric and multiferroic functionality.^36,37 Also, the photoluminescence and photoconductivity properties are useful in optoelectronic devices.³⁷ Bi₂O₃ has been employed for various applications, for example as a solid support catalyst^38,39 in sensors,^40,41 and as an anode material in lithium-ion batteries,⁴² supercapacitors⁴³ and solid oxide fuel cells.⁴⁴ Bi₂O₃ has seven polymorph structures, namely: α (monoclinic), β (tetragonal), γ (body-centered cubic), δ (face-centered cubic), ε (orthorhombic), ω (triclinic) and hexagonal phase.⁴⁵ However, the stable (α) and metastable phases (β and γ) exhibit remarkable photocatalytic properties for environmental remediation under visible-light. Various studies report on the synthesis of Bi₂O₃: oxidative metal vapour transport deposition,⁴⁶ hydrothermal synthesis,⁴⁷ microwave-assisted method,⁴⁸ precursor-induced (based) synthesis,^{10,25,26,49,50} gel combustion method³⁵ and other methods.^51–53 Among these, precursor based synthesis is a more appropriate method for achieving Bi₂O₃ polymorphs, due to phase control by calcination temperature.^10,54,55 Different Bi₂O₃ polymorphs were accomplished by using Bi₂O₂CO₃ and bismuth oxalate as precursors.^{10,25,26,49,50,56} Until now, very few reports are available for synthesizing α and β- Bi₂O₃ polymorphs using Bi₂O₂CO₃ as a precursor. In this study, we synthesized β-Bi₂O₃ (tetragonal) and α-Bi₂O₃ (monoclinic) polymorphs by post-calcination method using Bi₂O₂CO₃ nanoflakes as a novel precursor. Bi₂O₂CO₃ nanoflakes obtained by a modified experimental procedure showed good efficiency towards the degradation of organic dyes.⁵⁷

Experimental

Materials

All chemicals were of analytical grades and used without further purification. Bismuth(III) nitrate pentahydrate (ACS reagent, 98%), urea (AR Grade, 99%), ethylene glycol (≥99%), rhodamine B (Rh B; 95%), methylene blue (MB), acid orange 7 (AO7: 85%), methyl orange (MO; 85%) and terephthalic acid were purchased from Sigma-Aldrich.

Synthesis of bismuth oxide polymorphs (α and β phases)

A typical synthetic procedure involved the dissolution of 4.85 g of bismuth(III) nitrate pentahydrate in 43 mL of ethylene glycol which was then mixed with 33 mL of urea solution (6.05 mol dm⁻³) in a 100 mL beaker. Then, the reaction mixture was kept at 110 °C for 18 hours in a vacuum oven. After the completion of the reaction, the white powder (Bi₂O₂CO₃) was collected and washed with distilled water several times and then dried at 80 °C for 4 hours. The white powder was then heated at 350 °C and 400 °C for 2 hours in air at the heating rate of 10 °C to obtain β-Bi₂O₃ (as bright yellow color) and α-Bi₂O₃ (as straw yellow color) powders, respectively.

Characterization

The crystal structures and phase identification of the as-synthesized samples were determined by powder X-ray diffraction (XRD) using a Rigaku D/max-2500 diffractometer with a Ni-filtered Cu Kα radiation source (40 kV, 30 mA, λ = 1.5406 Å). The microstructural characteristic bands were recorded using a Fourier transform infrared (FTIR) spectrometer on a KBr pellet with a Nicolet iS50. Laser-Raman spectra of the samples were measured by a SENTERRA Raman microscope. The thermogravimetric analysis (TGA) of Bi₂O₂CO₃ nanoflake decomposition was performed by a model Mettle-Toledo 851^e and 822^e in the temperature range of 100–600 °C (10 °C min⁻¹) under a dynamic air atmosphere. The morphological features of these samples were investigated with a field emission scanning electron microscope (FESEM model JEOL, JSM-6700F) and transmission electron microscope (TEM model FEI Tecnai G2 F20). The elemental composition of these samples was determined by energy dispersive X-ray spectroscopy (EDS, Oxford Inc.). The surface potential (zeta potential) analysis was performed by using a Zetasizer Nano Zs/ZEN3600, Malvern instrument. UV-visible absorption spectra (diffuse reflectance mode; DRS) of these samples were recorded on an ultraviolet-visible (UV-vis) spectrophotometer (T90+, PG instruments, UK) with an integrating sphere using BaSO₄ as a standard. The photoluminescence (PL) spectra of the samples were performed by using the Shimadzu (RF5301PC) spectrofluorometer at an excitation wavelength of 325 nm under ambient conditions. The time resolved PL experiments were performed by pulse laser using a NanoLED, Horiba Scientific Fluoromax 4CP spectrophotometer at a 1 MHz repetition rate with a band pass of 2 nm. The arrival time of photon counts was set as 200 ns and the emission decay was measured. The lifetime was calculated by fitting the data using DAS (data analysis software).

Photoelectrochemical measurements

The electrochemical measurements were performed in a three electrode electrochemical cell consisting of a working electrode, a Pt wire counter electrode and an Ag/AgCl/NaCl (3 mol L⁻¹) reference electrode. A pH 12 NaOH solution containing 0.1 mol L⁻¹ KNO₃ was used as the electrolyte. The working electrodes were prepared by using a spray coating technique. For this purpose, 500 μL of 2-propanol solutions containing 20 g L⁻¹ of bismuth oxide samples (α and β-Bi₂O₃) was sprayed on top of fluoride-doped tin oxide (FTO) conductive glasses heated to 50 °C. The solution was forced through the nozzle at a flow speed of 500 μL s⁻¹ using pressurized air at 2.5 bar. A solar simulator (LOT-Quantum Design GmbH) with a 300 W Xe lamp and an AM 1.5-global filter was employed as the light source. The open circuit potential (OCP) was measured at various light intensities by changing the output power of the Xenon lamp. Furthermore, the photocurrent performances were studied with an AUTOLAB12/FRA2 in a two-electrode system; as-synthesized samples were coated on FTO as a working electrode and platinum (Pt) was used as the counter as well as a reference electrode with polymer gel being used as a solid electrolyte under a xenon illumination source (150 W with standard 85 mW cm⁻²).

Evaluation of photocatalytic activity

The photocatalytic activities of the bismuth oxide samples (α and β-Bi₂O₃) were investigated via the degradation of organic dyes (Rh B, MB, AO7 and MO) in aqueous solutions. The experiments were performed under simulated solar illumination (150 W Tungsten halogen lamp) at natural pH under ambient atmospheric conditions as follows: 50 mg of sample was dispersed in a 125 mL borosilicate glass bottle which contained 100 mL of organic dye solution (5 × 10⁻⁵ mol L⁻¹). Prior to illumination, the suspension was magnetically stirred in the dark for 30 min to achieve adsorption/desorption equilibrium between the sample and organic dye molecules. During illumination, 4 mL of solution was collected at regular time intervals from the suspension mixtures and the photocatalyst was filtered by using a PVDF syringe filter (0.45 μm). The photocatalytic activity (degradation efficiency) of the bismuth oxide samples was calculated by measuring the concentration of organic dyes as a function of irradiation time using a UV-visible spectrophotometer. Then, the photocatalytic degradation rate constants (k) were calculated using the first-order rate equation (−ln(C/C₀) = kt).

Hydroxyl radical analysis

The detection of hydroxyl radicals during the photocatalytic process was carried out using a photoluminescence technique. For this experiment, 0.1 g of sample (β-Bi₂O₃) was dispersed in 100 mL of 5 × 10⁻⁴ M terephthalic acid in basic solution (2 × 10⁻³ M of NaOH). Before irradiation, the mixture was magnetically stirred in the dark for 60 minutes and then exposed to solar illumination. At regular time intervals, 5 mL of solution was taken from the mixture and the catalyst was filtered using a PVDF syringe filter (0.45 μm). Then, the PL intensity of filtrate solution was measured using a fluorescence spectrophotometer at an excitation wavelength of 315 nm.

Results and discussion

Formation of α and β bismuth oxide

The formation of Bi₂O₂CO₃ nanoflakes is shown in Scheme 1. The formation of Bi₂O₂CO₃ nanocrystals is shown in reactions (1)–(4).

Bi(NO₃)₃·5H₂O + OHCH₂CH₂OH → Bi(OCH₂CH₂OH)₃ + 5 H₂O + 3HNO₃ (1)

CO(NH₂)₂ + H₂O → 2NH₃ + CO₂ (2)

NH₃ + H₂O → NH₄⁺ + OH⁻ (3)

2Bi(OCH₂CH₂OH)₃ + CO₂+ 12OH⁻ → Bi₂O₂CO₃ + 6(HOCH₂CH₂OH) + 3H₂O + 3O₂ (4)

The crystal structure and phase purity of the as-synthesized intermediate Bi₂O₂CO₃ were examined by powder X-ray diffraction analysis. All diffraction lines can be indexed to the tetragonal phase with unit cell lattice parameters: a = 3.89 Å and c = 13.63 Å for Bi₂O₂CO₃, which are in good agreement with standard JCPDS card no. 41-1888 (Fig. 1a). Although, the sharp lines at 30.2 (2θ) for (013) indicate that the as-synthesized sample is a single crystal with a crystalline size of about 32 nm. The FTIR spectrum (Fig. 1b) of the Bi₂O₂CO₃ sample shows the symmetric vibrations (ν₁) at 1066 cm⁻¹ with a shoulder band at 1460 cm⁻¹ and a sharp band at 1388 cm⁻¹ for anti-symmetric vibration (ν₃) of carbonate ions (CO₃²⁻). Moreover, the outer-bending (ν₂) and inner-plane deformation (ν₄) for carbonate ions were at 845 cm⁻¹ and 690 cm⁻¹, respectively. Furthermore, the symmetric vibrations (ν₁) and inner-plane deformation (ν₄) of carbonate ions are combined vibration bands (ν₁ + ν₄) observed at 1754 cm⁻¹ and 1734 cm⁻¹, respectively.^57–59 However, additional bands at 2424 cm⁻¹ for carbonate ions and 3396 cm⁻¹ for O–H groups of water molecules are also observed on the Bi₂O₂CO₃ surface.^13,57 Besides, the Raman bands at 1064 cm⁻¹, 1358 cm⁻¹ and 664 cm⁻¹ for carbonate ion vibrations (ν₁, ν₃ and ν₄) were observed (Fig. 1c), whereas the stretching vibration band at 508 cm⁻¹ for Bi=O with additional bands at 160 cm⁻¹ and 360 cm⁻¹ are attributed to external vibrations during Raman analysis of the Bi₂O₂CO₃ sample.⁵⁹ The surface morphology of the as-synthesized Bi₂O₂CO₃ sample was studied via FESEM. As shown in Fig. 1d, the FESEM image for the as-synthesized sample had a flake like morphology with a uniform size of about 100–200 nm and tiny thickness of several nanometer length. However, the Bi₂O₂CO₃ morphology of the nanoflakes is quite different from that of Bi₂O₂CO₃ nanoplatelets due to the initial concentrations of bismuth nitrate pentahydrate and concerned reaction conditions.⁵⁸ Thus, the as-synthesized sample indicates Bi₂O₂CO₃ nanoflakes as a surface morphology. Furthermore, the EDS analysis of the Bi₂O₂CO₃ nanoflakes found the presence of Bi, C, and O elements (Fig. S1. ESI†). So, it could clearly indicate that the Bi₂O₂CO₃ nanoflakes are highly pure. The TGA analysis clearly shows that the Bi₂O₂CO₃ nanoflakes decompose in the temperature range of 300–400 °C to generate Bi₂O₃ (Fig. 2). The calculated mass loss ≤9% is almost close to the theoretical value of ∼8.9% when Bi₂O₂CO₃ nanoflakes are converted to Bi₂O₃ during decomposition. Hence, the Bi₂O₂CO₃ nanoflakes were calcined at 350 °C and 400 °C for 2 h to obtain β and α-Bi₂O₃, respectively.

Scheme 1 Schematic illustration of Bi₂O₂CO₃ nanoflakes and the formation of Bi₂O₃ polymorphs.

Fig. 1 (a) Powder XRD pattern, (b) FTIR spectrum, (c) Raman spectrum and (d) FESEM image of Bi₂O₂CO₃.

Fig. 2 TGA results of Bi₂O₂CO₃.

Spectroscopic studies of α and β bismuth oxide

The crystallographic phases and polymorphic structures of the as-synthesized Bi₂O₃ samples were investigated by X-ray diffraction analysis (Fig. 3). The Bi₂O₂CO₃ nanoflakes calcined at 350 °C for 2 h displayed the diffraction lines of (210), (201), (211), (002), (220), (212), (222), (400), (411), (203), (421), (402), (440), (610) and (224) planes at 2θ = 25.7°, 27.9°, 30.3°, 31.7°, 32.7°, 41.3°, 46.2°, 46.9°, 51.3°, 54.2°, 55.5°, 57.7°, 68.5, 74.5° and 75.9°, which can be indexed to a tetragonal phase of β-Bi₂O₃ with space group P4̄2₁c corresponding to JCPDS card no. 27-0050. While the Bi₂O₂CO₃ nanoflake sample calcined at 400 °C exhibits diffraction lines of (102), (002), (120), (012), (200), (212), (112), (222), (122), (041), (104), (321), (241), (314), (232), (252) and (161) plane at 2θ = 25.6°, 26.8°, 27.3°, 27.9°, 33.1°, 34.9°, 37.5°, 39.9°, 42.2°, 46.2°, 48.5°, 52.2°, 54.6°, 57.7°, 62.3°, 66.7° and 74.3°, which are in good agreement with JCPDS card no. 41-1449 for α-Bi₂O₃ as a monoclinic phase with space group P2₁/c. The calculated crystalline size and unit cell lattice parameters of these samples are provided in Table ST1 (ESI†). The crystalline size of α-Bi₂O₃ changed when compared with β-Bi₂O₃ due to a phase transition during calcination at high temperature (400 °C). The FTIR spectrum of the β-Bi₂O₃ and α-Bi₂O₃ samples is shown in Fig. 4a. Crystalline water molecules are chemisorbed and/or physisorbed on both the samples which show weak vibration bands at 3434–3442 cm⁻¹ and 1640 cm⁻¹.⁶⁰ Anti-symmetric vibration (ν₃) and inner-plane deformation (ν₄) for carbonate ions are present in these samples at 1388 cm⁻¹ with 850 cm⁻¹ and 636 cm⁻¹, respectively. Thus, the synthesized β and α-Bi₂O₃ samples have water molecules and carbonate ions present on the surface during phase transformation from Bi₂O₂CO₃ nanoflakes. A similar observation was reported in a previous study.¹⁰ Moreover, the phase lucidity of the as-synthesized Bi₂O₃ polymorphs was further confirmed by Raman analysis (Fig. 4b). The vibrational bands at 90, 128, 148, 160, 232, 314 and 466 cm⁻¹ for the β-Bi₂O₃ sample are attributed to a tetragonal phase. Similarly, the vibrational bands at 84, 92, 102, 120, 138, 152, 182, 210, 282, 314, 412, 448 and 530 cm⁻¹ for α-Bi₂O₃ possess a monoclinic phase with space group P21/c.^37,61 Among them, the band at 120 cm⁻¹ could be ascribed to the bismuth vibrational mode and Bi–O scattering at 138 and 152 cm⁻¹ for α-Bi₂O₃, whereas all others bands are attributed to anharmonicity of the oxygen vibrational mode observed in α-Bi₂O₃ during Raman analysis.³⁷ Hence, the as-synthesized samples exhibit a highly crystalline Bi₂O₃ polymorph phase (β and α-Bi₂O₃).

Fig. 3 Powder XRD patterns of Bi₂O₃ samples.

Fig. 4 (a) FTIR spectrum and (b) Raman spectrum of (i) β-Bi₂O₃ and (ii) α-Bi₂O₃ samples.

Microscopic studies of α and β bismuth oxide

The surface morphology of the as-synthesized bismuth oxide polymorphs was characterized by FESEM and TEM. Fig. 5a and c show the morphologies of β and α-Bi₂O₃ samples in low magnification FESEM images, respectively. At a higher magnification, the image of β-Bi₂O₃ shows an irregular size in the range of 200–300 nm length (Fig. 5b), whereas α-Bi₂O₃ exhibits a coral-like morphology with a length of several micrometers (Fig. 5d). The changes in crystalline size, shape and morphology of the Bi₂O₃ polymorphs during the calcinations may proceed via an Ostwald ripening process.^10,23

Fig. 5 (a and b) TEM low and high magnification FESEM images of β-Bi₂O₃ and (c and d) α-Bi₂O₃ samples.

EDS analysis of β-Bi₂O₃ shows the presence of Bi and O elements only, which confirms the formation of pure Bi₂O₃ during calcination (Fig. S2, ESI†). TEM images are also consistent with β-Bi₂O₃ as a nanoparticle and α-Bi₂O₃ shows a coral-like morphology (Fig. 6a and b). HRTEM images with corresponding FFT (inset) for β and α-Bi₂O₃ reveal that the samples are well crystalline in nature (Fig. 6c and d). The β-Bi₂O₃ nanoparticle has a lattice fringe of d = 0.295 nm for the (211) plane as a tetragonal phase (Fig. 6c), whereas α-Bi₂O₃ has a lattice fringe of d = 0.332 nm, ascribed to the (111) plane for the monoclinic phase (Fig. 6d).¹⁰ Then, the surface potential of the as-synthesized Bi₂O₃ samples was studied by using a zeta (ζ) potential analyzer. As shown in Fig. S3 (ESI†), the β-Bi₂O₃ and α-Bi₂O₃ particles show negative potentials of −13.1 and −12.3 mV in a natural solution (pH = 7), respectively.

Fig. 6 (a and b) TEM images and (c and d) HRTEM images of β-Bi₂O₃ and α-Bi₂O₃, respectively (inset corresponding FFT).

Optical properties of α and β bismuth oxide

The optical absorption characteristics of the as-synthesized Bi₂O₃ samples were studied by using a UV-vis spectrophotometer (DRS). The absorption spectra are given in Fig. 7a, and the β-Bi₂O₃ nanoparticles (green line) show a strong absorption in the visible light region when compared to α-Bi₂O₃ (red line). The optical band gaps (indirect) of these samples were calculated using eqn (5) (inset Fig. 7a),where α is the absorption coefficient, C is a constant, hν is the photon energy and E^bulk_g is the band gap. The indirect band gaps were found to be 2.2 eV for β-Bi₂O₃ and 2.56 eV for α-Bi₂O₃. However, the indirect band gap of α-Bi₂O₃ has increased when compared with β-Bi₂O₃ due to reduced crystalline size and change in polymorphic phase (tetragonal into monoclinic). The E_VB and E_CB positions of the as-synthesized Bi₂O₃ samples can be calculated by using eqn (6) and (7) and the results are given in Table ST2 (ESI†) (where X is the absolute electronegativity of Bi₂O₃ which is 6.23 eV and E_e is the energy of free electrons on the hydrogen scale which is about 4.5 eV).

E_VB = X − E_e + 0.5E_g (6)

E_CB = E_VB − E_g (7)

The insights of photo illuminated open circuit potential (OCP) for the bismuth oxide samples (β- and α-Bi₂O₃) are shown in Fig. 7b and c, respectively. As the light intensity increases, the potential of the semiconductor electrodes (equal to the Fermi level) is shifted to more negative values vs. NHE (Normal Hydrogen Electrode). The data were fitted using exponential decay functions and the asymptotic values (when the power of the light source is approaching infinity) were determined to be −0.01 and 0.01 V vs. NHE at pH 12 for these samples. Considering that the irradiation intensity is sufficiently intense to completely remove the band bending at the surface and the materials do not have fast carrier recombination rates, the values can serve as estimations of the flat band potentials of the semiconductors.⁶² To convert the data to pH 0, the Nernst equation (eqn (8)) was used,

V_fb(pH = 0) = V_fb(pH = 0) + 12 × 0.059 V (8)

According to this correction, the flat band potentials of 0.70 and 0.72 V vs. RHE (Reversible Hydrogen Electrode) are observed for α and β-Bi₂O₃, respectively showing n-type behaviour (electrons near to the conduction band). The PL emission studies of the as-synthesized Bi₂O₃ samples were carried out with an excitation wavelength of 325 nm (Fig. 7d). The PL spectrum shows a strong emission peak at 469 nm owing to photogenerated carrier recombination.¹⁰ The PL intensity of the β-Bi₂O₃ nanoparticles was distinctly lower when compared with α-Bi₂O₃ due to a greater extent of photogenerated carrier separation. Lifetime measurements of the photogenerated carriers in the as-synthesized Bi₂O₃ samples were carried out at an excitation wavelength of 325 nm and emission wavelength of 470 nm. It can be clearly observed that β-Bi₂O₃ nanoparticles have a longer lifetime (0.49 ns) that of the α-Bi₂O₃ sample (0.38 ns) (Fig. S4, ESI†). The photocurrent studies reveal that β-Bi₂O₃ nanoparticles show a higher efficiency (Fig. S5, ESI†).

Fig. 7 (a) DRS absorption (inset-Tauc plot), (b and c) photo illuminated OCP data (inset of corresponding photograph for the working electrode) and (d) PL emission spectrum of Bi₂O₃ samples.

Photocatalytic performances of organic dye degradation and mechanism

The photocatalytic performances of the as-synthesized bismuth oxide polymorphs were studied for the degradation of organic dyes (Rh B, MB, AO 7 and MO) under simulated solar illumination. Fig. S6a and S7a (ESI†) show the UV-vis absorption spectra of Rh B in an aqueous solution using β-Bi₂O₃ and α-Bi₂O₃. The characteristic absorption peak at 553 nm corresponding to Rh B is found to decrease during illumination. After 240 minutes, β-Bi₂O₃ nanoparticles exhibit 92% degradation of Rh B, higher than that observed with α-Bi₂O₃ (69%). Similarly, the other dyes such as MB (65% for β-Bi₂O₃ and 54% for α-Bi₂O₃), AO 7 (83% for β-Bi₂O₃ and 66% for α-Bi₂O₃) and MO (63% for β-Bi₂O₃ and 32% for α-Bi₂O₃) also show degradation under identical conditions (Fig. S6 and S7, ESI†). Furthermore, the results of the degradation profile are compared with illumination time on bismuth oxide samples (Fig. 8a and b). Then, the photocatalytic degradation kinetics of these dyes are calculated (Fig. 8c and d). The photocatalytic degradation of these dyes shows pseudo-first order kinetics and the rate constant values are provided in Table 1. Here, the photocatalytic degradation kinetics was dissimilar due to the molecular structure of the organic dye molecules and nature of the semiconductor under illumination.

Fig. 8 (a and b) Photocatalytic degradation curve and (c and d) kinetic plot of β-Bi₂O₃ and α-Bi₂O₃, respectively (indicating that the error bars show a standard deviation for the analyses).

Table 1 Comparison of the rate constant (k) and relative coefficients (R²) of the degradation of these dyes over the as-synthesized Bi₂O₃ samples under simulated solar illumination

Samples	Rh B		MB		AO 7		MO
	Rate constant (min^−1)	R ^2 value	Rate constant (min^−1)	R ^2 value	Rate constant (min^−1)	R ^2 value	Rate constant (min^−1)	R ^2 value
β-Bi2O3	0.0071	0.9861	0.0041	0.9928	0.0059	0.9996	0.0036	0.9908
α-Bi2O3	0.0035	0.9895	0.0029	0.9975	0.0027	0.9958	0.0013	0.9979

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Furthermore, to understand the photocatalytic processes of the as-synthesized Bi₂O₃ samples, their band structures were studied and a mechanism is proposed (Scheme 2). The calculated VB and CB values of β-Bi₂O₃ nanoparticles are located at 2.83 and 0.61 eV (vs. NHE), and at 3.01 and 0.45 eV (vs. NHE) for the α-Bi₂O₃ sample. However, for both cases, the CB values are more positive than that of O₂/O₂˙⁻ (−0.28 eV vs. NHE), and the CB values are more positive than that of OH˙/H₂O (2.8 eV vs. NHE). Thus, during illumination, photoreduction can occur at the CB (O₂ into O₂˙⁻) through the photogenerated electron and the photooxidation ensues at the VB (H₂O into OH˙) by the photogenerated holes.

Scheme 2 Schematic illustration of the band structure and mechanism for the photocatalytic performances of dyes over as-synthesized Bi₂O₃ samples under simulated solar illumination.

The OH˙ radical generation was confirmed using terephthalic acid as a probe. The PL emission spectrum shows emission at 425 nm when the excitation wavelength is at 315 nm, which is characteristic for 2-hydroxy terephthalic acid (Fig. 9). Hence, the involvement of OH˙ radical and O₂˙⁻ reactive species in the photocatalytic activity for the dye degradation process could be confirmed (reactions (9)–(11)).

Bi₂O₃ + hν → Bi₂O₃ (e_CB⁻) + Bi₂O₃ (h_VB⁺) (9)

O₂ + Bi₂O₃ (e_CB⁻) → Bi₂O₃ + (O₂˙⁻) (10)

H₂O + Bi₂O₃ (h_VB⁺) → Bi₂O₃ + (OH˙ + H⁺) (11)

On the other hand, Rh B and AO 7 dye molecules contain carboxyl (–COOH) and hydroxyl (–OH) groups. However, these functional groups (dyes) are connected to the surface sites of the as-synthesized bismuth oxide samples (through the surface OH). Thus, they strongly adsorb on the Bi₂O₃ surface, which leads to photosensitization of the dye to dye* under illumination.^63,64 Moreover, the calculated conduction band positions of β-Bi₂O₃ at 0.61 eV (vs. NHE) and 0.45 eV (vs. NHE) for the α-Bi₂O₃ sample have a higher energy level offset of Rh B/Bi₂O₃(≥1.87 eV) at the interface which is thermodynamically favorable, whereas in the case of AO 7/Bi₂O₃ the calculated energy level offset at the interface is ≥2.31 eV (conduction band position of Rh B in E_o = −1.42 eV (vs. NHE) and E_o = −1.86 eV (vs. NHE) for AO 7).^57,64,65 This creates an electron displacement between the dye* and Bi₂O₃, and electron recombination between the dye and Bi₂O₃ which helps towards the photodegradation of Rh B with a higher efficacy compared to AO 7. Furthermore, the photosensitization mechanisms of Rh B and AO 7 dye degradation are expressed for the following reactions (12)–(16):

Dye (Rh B and AO 7) → Dye* (12)

Dye* + Bi₂O₃ → Dye⁺ + Bi₂O₃ (e_CB⁻) (13)

Bi₂O₃ (e_CB⁻) + O₂ → O₂˙⁻ + Bi₂O₃ (14)

Dye⁺ + H₂O → OH˙ + H⁺ + Dye (15)

(OH˙ + H⁺) + (O₂˙⁻) + Dye → Degradation products (16)

Dye (MB and MO) + (OH˙ + H⁺) + (O₂˙⁻) → Degradation products (17)

Fig. 9 PL emission spectrum of terephthalic acid in a basic solution (with β-Bi₂O₃ nanoparticles under illumination).

Whereas, MB and MO do not adsorb on the Bi₂O₃ surface (no functional group), which illustrates that no photosensitization occurs under illumination (reactions (9)–(11) and (17)) (Fig. S8, ESI†). However, the conduction band position of MB for E_o = −3.60 eV (vs. Vacuum) and MO for E_o = −2.07 eV (vs. NHE) possesses thermodynamically unfavorable electron displacement between the dye and Bi₂O₃ samples (only oxidation–reduction at VB and CB of semiconductors). In addition, the MB dye (Ar–N–Ar bonds) shows a higher photocatalytic degradation when compared with MO (contains N=N bond) dye due to a smaller size. Then, the MB dye molecule consists of sulfur (=S⁺–aromatic) links which enhanced degradation owing to sulfonyl groups placed in the p-benzene ring.⁶⁶ Therefore, for the photocatalytic efficiency of the as-synthesized Bi₂O₃ samples we expressed the following order of degradation rate: Rh B > AO 7 > MB > MO under simulated illumination. Furthermore, β-Bi₂O₃ nanoparticles have a higher photocatalytic activity when compared with the α-Bi₂O₃ sample which was due to various facts such as band gap, longer recombination lifetime, crystalline size and surface potential etc.

Conclusions

In conclusion, we have successfully synthesized Bi₂O₃ polymorph structures (β-Bi₂O₃ nanoparticles and α-Bi₂O₃ coral-like morphology) using a post-calcination method using Bi₂O₂CO₃ nanoflakes as an intermediate. The microscopic (SEM, TEM and EDS) and spectroscopic (powder XRD, FTIR and Raman analysis) data confirmed that the synthesized Bi₂O₃ polymorphs are in pure phases and highly crystalline in nature. The enhanced photocatalytic activity for the degradation of organic dyes (Rh B, MB, AO 7 and MO) using β-Bi₂O₃ nanoparticles is attributed to large photon adsorption (band gap 2.2 eV) when compared with α-Bi₂O₃ (band gap 2.56 eV). Furthermore, the superior degradation of Rh B and AO 7 dyes is demonstrated due to photosensitization during illumination.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research described herein was financially supported by the Department of Science and Technology, India under the Water Technology Initiative scheme (DST/TM/WTI/2k16/258(G)).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S8: EDS spectra of Bi₂O₂CO₃ nanoflakes and β-Bi₂O₃ nanoparticles, zeta potential, time-resolved fluorescence spectrum, photocurrent response results of Bi₂O₃ polymorphs, and photodegradation profiles for organic dyes solutions. Tables T1 and T2: Unit cell parameters, and calculated E_VB and E_CB from E_g values. See DOI: 10.1039/c8qm00221e

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