Single step hydrothermal synthesis of beyerite, CaBi₂O₂(CO₃)₂ for the fabrication of UV-visible light photocatalyst BiOI/CaBi₂O₂(CO₃)₂

Abstract

The mineral beyerite (CaBi₂O₂(CO₃)₂), a member of the “sillen” family possessing a layer sequence, (Bi₂O₂)²⁺–CO₃²⁻–Ca²⁺–CO₃²⁻–(Bi₂O₂)²⁺ has successfully been synthesized under hydrothermal conditions in a single step for the first time. Uniform rectangular plates of beyerite crystallites with a surface area of 33 m² g⁻¹ were obtained by heating a solution containing bismuth nitrate and calcium carbonate in ethylene glycol along with the addition of a saturated solution of sodium carbonate at 140 °C. The sample was characterized by powder X-ray diffraction combined with Rietveld refinement, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectra and diffuse reflectance measurements. The band gap estimated for beyerite was found to be 3.97 eV and was utilized subsequently to form heterostructure with BiOI (band gap of 1.76 eV), another member belonging to the sillen family. The formation of the composite BiOI/CaBi₂O₂(CO₃)₂ has been confirmed by a variety of techniques and its photocatalytic properties as a UV-visible photocatalyst has been demonstrated for the degradation of aqueous solutions of rhodamine B and phenol. Photocatalytic experiments in presence of scavengers suggested the possible mechanism occurring through holes and peroxide radicals rather than the hydroxyl radicals.

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1. Introduction

Semiconductor photocatalysis has emerged as one of the most investigated research topics over the past decade specially from the point of view of environmental remediation by decomposing harmful organics and to generate hydrogen as an alternate fuel from water by efficiently making use of the solar energy.¹ Although the focus of research has been centered on TiO₂ and its modifications, a rapid surge in the identification of materials for the decomposition of toxic organic pollutants and/or for water splitting has resulted in several semiconducting photocatalysts such as BiVO₄,² NaBiO₃,³ AgSbO₃,⁴ Ag₃PO₄,⁵ CaCu₃Ti₄O₁₂,⁶ Ag₃SbS₃,⁷ and nitrogen incorporated CsCa₂Ta₃O₁₀,⁸ etc. The materials containing Bi³⁺ cation offer an attractive choice by providing nontoxic, photostable semiconductors with reduced band gap because of the overlap of its 6s band with the 2p band of the anionic oxygen. Apart from BiVO₄,² other bismuth containing compounds such as Bi₂MoO₆,⁹ Bi₂WO₆,¹⁰ Bi₂CuO₄,¹¹ Bi₂O₂CO₃ (ref. 12) and BiOX (X = Cl, Br, I)^13–15 have been investigated extensively. Most of these materials have also been made into different heterostructures in order to improve the photocatalytic efficiency specially under visible light irradiation. In this regard (Bi₂O₂CO₃), bismuth subcarbonate, naturally occurring as mineral bismutite, has been found to be an important compound. Bi₂O₂CO₃ belongs to the family of sillen phases and is an intergrowth of [Bi₂O₂]²⁺ layers and (CO₃)²⁻ layers formed in such a way that the plane of the (CO₃)²⁻ group is orthogonal to the plane of Bi–O layers.¹⁶ Bismuth subcarbonate has long been the material of investigation for medical purpose because of its antibacterial activity.¹⁷ More importantly, because of its band gap of 3.38 eV, it has been investigated as a potential photocatalyst under UV light irradiation for the degradation of many organic dyes.¹² Subsequently, several research groups have attempted to enhance the photocatalytic efficiency through different synthetic methods to achieve nanotubes,¹⁷ plates,¹² sponge,¹⁸ nanosheets,¹⁹ nanoscale crystallites,²⁰ and crystallites with flower like morphologies.²¹ Recently, single crystal nanoplates of Bi₂O₂CO₃ have also been engineered with enhanced photocatalytic activity.²² Apart from synthetic modifications, many studies have focused on forming photocatalytic heterostructures such as Bi₂O₂CO₃/Bi₂WO₆,²³ BiVO₄/Bi₂O₂CO₃,²⁴ Fe₃O₄/Bi₂O₂CO₃,²⁵ Bi₂S₃/Bi₂O₂CO₃,²⁶ Bi₂O₂CO₃/Bi₃NbO₇,²⁷ BiOI/Bi₂O₂CO₃,²⁸ Bi₂O₂CO₃/BiOCl,²⁹ Ag₂O/Bi₂O₂CO₃,³⁰ g-C₃N₄/Bi₂O₂CO₃,³¹ graphene–Bi₂O₂CO₃ composites,³² and the ternary composite of Bi₂O₃/Bi₂O₂CO₃/Sr₆Bi₂O₉.³³

While a vast literature has developed for bismutite (Bi₂O₂CO₃), the other bismuth carbonate containing calcium, known as mineral beyerite, CaBi₂O₂(CO₃)₂ has been relatively less explored except for a brief study of luminescence.³⁴ Beyerite has the [Bi₂O₂]²⁺ layers along with the additional Ca²⁺ ions and the layer sequence is (Bi₂O₂)²⁺–CO₃²⁻–Ca²⁺–CO₃²⁻–(Bi₂O₂)²⁺ (Fig. 1). One probable reason might be the difference in the synthetic conditions of Bi₂O₂CO₃ and CaBi₂O₂(CO₃)₂. The former could easily be precipitated simply by treating Bi³⁺ containing solution (e.g. Bi(NO₃)₃·5H₂O) with a solution of carbonates preferably followed by a hydrothermal treatment around 120–180 °C for 12–24 h.²⁰ On the other hand, the beyerite was reported to have synthesized around 300 bar, at 220 °C starting from CaCO₃ and Bi₂O₂CO₃. We successfully attempted and synthesized CaBi₂O₂(CO₃)₂ for the first time under relatively milder hydrothermal conditions in a single step by treating directly the bismuth nitrate solution along with calcium carbonate and a saturated solution of sodium carbonate around 140 °C for 72 h. The as synthesized beyerite was characterized using a variety of techniques such as powder X-ray diffraction (PXRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and UV-visible diffuse reflectance spectroscopy measurements. The details regarding the successful synthesis and the outcome from different characterizations are described in the present work. Since the results from the preliminary characterization suggested beyerite to be a semiconductor with a wide band gap of 3.97 eV, we further considered the construction of a heterostructure in order to increase its capability as a photocatalyst. Towards this purpose, BiOI¹⁵ with a reported band gap of 1.76 eV has been chosen to append with beyerite and the heterostructure BiOI/CaBi₂O₂(CO₃)₂ has been synthesized, characterized and studied as a photocatalyst for the degradation of aqueous solutions of Rh B and phenol under UV-visible light.

Fig. 1 The layered structure of beyerite, CaBi₂O₂(CO₃)₂.

2. Experimental section

2.1. Synthesis of CaBi₂O₂(CO₃)₂

All of the reagents employed in this study were of analytical grade and were used without further purification. For the synthesis of CaBi₂O₂(CO₃)₂, 0.485 g (1 mmol) of Bi(NO₃)₃·5H₂O (Thomas baker 99%) was dissolved in 10 ml ethylene glycol (Merck) and to this solution 0.05 g (0.5 mmol) of solid CaCO₃ (Thomas baker 99.5%) was added under vigorous stirring, followed by the addition of 5 ml saturated solution of sodium carbonate and 15 ml of distilled water. The resulting precursor suspension of dense milky nature was transferred to a 60 ml Teflon-lined stainless-steel autoclave and heated at 140 °C for 72 h. The hydrothermal vessel was cooled down to room temperature. The solid product was collected by normal filtration followed by repeated water washings and was finally dried at room temperature. The formation of calcium bismuth carbonate has been investigated (Table 1) by varying the synthetic conditions along with the use of various carbonate sources such as ammonium carbonate, urea and citric acid and the findings are discussed in detail in the Results and discussion section.

Table 1 Experimental conditions for the synthesis of CaBi₂O₂(CO₃)₂

Sample	Reagent^a	Temperature/duration	pH
a 1 mmol of Bi(NO3)3·5H2O and 0.5 mmol of CaCO3 were used.
CBCS1	Saturated sodium carbonate solution (5 ml) + ethylene glycol (10 ml)	120 °C/2 days	12.10
CBCS2		140 °C/2 days
CBCS3		140 °C/3 days
CBCS4		170 °C/3 days
CBCA1	Saturated ammonium carbonate solution (5 ml) + ethylene glycol (10 ml)	140 °C/3 days	9.35
CBCA2		170 °C/3 days
CBCC1	Saturated sodium carbonate solution (5 ml) + citric acid (10 ml)	140 °C/3 days	7.25
CBCC2		170 °C/3 days
CBCU1	Saturated urea solution (5 ml) + ethylene glycol (10 ml)	140 °C/3 days	2.60
CBCU2		170 °C/3 days

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2.2. Synthesis of BiOI/CaBi₂O₂(CO₃)₂ heterostructure

First, BiOI synthesis³⁵ was carried out by the addition of 7.2 ml of acetic acid to a solution obtained by dissolving bismuth nitrate (5 g) in 1 : 1 nitric acid. Separately 1.75 g of potassium iodide and 2.5 g sodium acetate were dissolved in 100 ml of water. Subsequently, the bismuth nitrate solution was added to the potassium iodide and sodium acetate solution. The whole mixture was kept stirring for nearly 3 h and the brick red precipitate was filtered and dried in air and used to synthesize the heterostructure with beyerite after verification by PXRD measurement.

The preparation of 10/90 (in weight percentage) BiOI/CaBi₂O₂(CO₃)₂ was carried out by dispersing 0.90 g of CaBi₂O₂(CO₃)₂ in 20 ml of deionized water by sonication, after its formation corroborated by various characterizations. To this suspension was added 0.1 g of BiOI and sonication was continued for another thirty minutes. The pale pink colored solid was collected by normal filtration and dried at room temperature. The reason for selecting the targeted composite with a 10/90 weight percentage was based on the initial screening experiments using ratios 1/99 and 5/95. The PXRD patterns of the composites registered the presence of BiOI only for the 10/90 composition.

2.3. Characterization

PXRD patterns were recorded using high resolution PANanalytical diffractometer, equipped with pixel detector employing Cu K_α radiation (λ = 1.5418 Å) obtained with a scan rate of 200 s per step and step size of 0.013° at 298 K. LeBail method is used to obtain the cell dimensions from the PXRD patterns and the Rietveld refinement of the PXRD patterns were carried out using GSAS + EXPGUI program.³⁶ The structure diagram were generated by DIAMOND, version 3.³⁷ Raman spectra were recorded using a Renishaw spectrometer via microscope system equipped with an Ar⁺ laser (λ = 785 nm). FTIR spectra were collected using a Perkin Elmer 2000 spectrometer using KBr pellet technique. UV-vis diffuse reflectance data were obtained over the spectral range 200–800 nm using Perkin-Elmer Lambda 35 scanning double beam spectrometer equipped with a 50 mm integrating sphere. BaSO₄ was used as a reference. The data were transformed into absorbance with the Kubelka–Munk function for the estimation of the band gap. The SEM micrographs of the samples were recorded using a FESEM Hitachi S-3700M and TEM was obtained using FEI Technai G² 20 electron microscope operating at 200 kV. Thermogravimetric experiments were performed using Netzsch STA 449F3 instrument with heating rate of 10 °C min⁻¹ over a range of 50 °C to 1000 °C. Surface area was measured by nitrogen adsorption–desorption technique with Quantachrome, Autosorb-1C TCD (Model: ASIC-X-TCD6) analyzer.

2.4. Photocatalytic experiments

Photocatalytic studies were carried out using a 450 W Xenon arc lamp (Oriel, Newport, USA) with a water filter to cut down IR radiation. Irradiation was carried out over an external pyrex container of volume 250 ml and water circulation was carried out to prevent any decomposition by thermal effect.⁴ Light fluxes were established using ferrioxalate actinometer³⁸ and the incident photon rate for UV-visible irradiation (250 < λ < 800 nm) experiments was I₀ = 1 × 10⁻⁷ einsteins s⁻¹. The dyes and the organic compounds were dissolved in doubly distilled water. All the experiments were carried out at room temperature. A typical experiment of degradation was carried out as follows: the catalyst (0.1 g) was taken along with 50 ml of an aqueous solution of Rh B and phenol with an initial concentration of 1 × 10⁻⁵ mol l⁻¹ and 1 × 10⁻⁴ mol l⁻¹ respectively in the pyrex container with constant stirring. Prior to irradiation, the suspension of the catalyst and dye solution was stirred in dark for thirty minutes to sixty minutes, so as to reach the equilibrium adsorption. After light irradiation, five milliliter aliquots were pipetted out periodically from the reaction mixture. The solutions were centrifuged, and the concentration of the solutions was determined by measuring the maximum absorbance (λ_max) at 552 and 268 nm for Rh B and phenol solutions respectively. To differentiate between the photocatalysis and the photolysis of the dye molecules, experiments were carried out in the presence and in the absence of catalyst BiOI/CaBi₂O₂(CO₃)₂ under UV-visible light irradiation. The rate of photocatalytic degradations were considered only after considering the extent of photolysis of the dye solutions. For comparison, photocatalytic degradation using single phase CaBi₂O₂(CO₃)₂ and BiOI as catalysts were also investigated under similar experimental conditions.

3. Results and discussion

3.1. Synthesis, crystal structure and physical properties of beyerite

The formation of the beyerite (CaBi₂O₂(CO₃)₂) phase under different hydrothermal treatments (Table 1) was closely monitored by recording the PXRD patterns. Initial experiments, carried out at temperatures between 120 °C and 170 °C for a duration of 2–3 days, clearly indicated a monophasic formation at 140 °C for three days. The products obtained after the reactions at 120 °C and at 140 °C for two days (CBCS1 and CBCS2) consisted of both beyerite and bismutite. When the reaction was extended for another day at 140 °C a pure beyerite product (CBCS3) was obtained as endorsed by its PXRD pattern (Fig. 2). Subsequent increase in the reaction temperature to 170 °C for three days also resulted in a monophasic beyerite (CBCS4). The PXRD patterns of the single phase products (CBCS3 and CBCS4) could readily be indexed in orthorhombic symmetry (S.G Immm) with the refined lattice parameters of a = 3.76283(5) Å; b = 3.75493(8) Å; c = 21.6611(5) Å.¹⁶ We explored the hydrothermal reactions for the formation of beyerite using ammonium carbonate reactant instead of sodium carbonate in the similar temperature range. Beyerite sample obtained by using ammonium carbonate solution at 140 °C and 170 °C for three days (CBCA1 and CBCA2) essentially resulted in homogeneous phases (Fig. 3). On the other hand, the usage of urea as a carbonate source at 140 °C (CBCU1) and at 170 °C for three days (CBCU2) resulted in a mixture of bismutite (Bi₂O₂CO₃) and beyerite (CaBi₂O₂(CO₃)₂). The use of citric acid together with sodium carbonate at 140 °C for three days (CBCC1), again resulted in a mixture consisting more of bismutite and less of beyerite (Fig. 3e). This particular reaction when repeated at a higher temperature (170 °C) for the same duration did not result in any solid product formation. Based on these experiments, it is clear that under basic conditions formation of beyerite is followed after the formation of bismutite. Carrying out reactions by modifying the experimental conditions such as the types of reactants, their concentration, reaction temperature, and the type of solvent might assist in understanding the detailed mechanism of beyerite formation. In addition such experiments might result in beyerite with interesting morphologies with varying surface related properties and investigation is currently underway along this direction.

Fig. 2 PXRD patterns of (a) CBCS1, (b) CBCS2, (c) CBCS3 and (d) CBCS4 * shows reflections corresponding to Bi₂O₂(CO₃).

Fig. 3 PXRD patterns of (a) CBCA1, (b) CBCA2, (c) CBCU1, (d) CBCU2 and (e) CBCC1 * shows reflections corresponding to Bi₂O₂(CO₃).

The morphologies of the different beyerite samples were further investigated by SEM and TEM techniques. The SEM images of CBCS3 and CBCS4 indicated the existence of beyerite as uniform rectangular plate like crystallites in the range 50–150 nm with the appearance of thinner plates in the latter (Fig. 4). The respective TEM images confirmed both the formation of rectangular plates with a decrease in thickness with increase in synthesis temperature from 140 °C (CBCS3) to 170 °C (CBCS4) (Fig. 4). Irregular thicker plates were obtained for CBCA1 (Fig. 4c). A similar feature has been noted for Bi₂O₂CO₃ formation using ammonium carbonate solution.¹⁸ BET surface area for the sample synthesized at 140 °C (CBCS3) was 33 m² g⁻¹. TGA measurement of the as synthesized CaBi₂O₂(CO₃)₂ conducted till 1000 °C has been presented in Fig. S1 (ESI†). The sample did not show any loss in weight till 300 °C. The weight loss is occurring in two stages starting around 400 °C and extending up to 800 °C. It has been ascribed to the decomposition of CaBi₂O₂(CO₃)₂ corresponding to the loss of two molecules of CO₂. The observed weight loss of 18 wt% from 400 to 800 °C roughly matched to the calculated value.

Fig. 4 SEM images of samples (a) CBCS3, (c) CBCS4 and (e) CBCA1. The respective TEM images are shown in (b), (d) and (f).

The only structural investigation so far on beyerite in the literature has been the single crystal X-ray diffraction studies carried out on a naturally occurring beyerite mineral.¹⁶ So in the present study, we carried out the structural confirmation of the synthesized bulk polycrystalline material of CaBi₂O₂(CO₃)₂ by carrying out the Rietveld refinement of the PXRD data obtained for the sample CBCS3 (Fig. 5). The single crystal structural study confirmed a centrosymmetric orthorhombic space group (Immm) in order to get a sensible positioning of the two carbonate groups in the unit cell (Fig. 1). For the powder refinement, therefore we started with a structural model comprising of the positional parameters arrived on the basis of single crystal structural study. The refinement converged successfully and the resulting positional and thermal parameters are listed in Table 2. The crystallographic parameters and the details of the Rietveld refinement results are listed in Table S1 (ESI†). The structure of beyerite consists of two cationic sites respectively for bismuth and calcium. The polyhedron around the bismuth atom is square antiprism with Bi–O1 bond lengths of 2.134 and 2.435 Å and with a longer Bi–O3 bond length of 2.758 Å. The calcium atom also has a regular eight coordination formed by O2 with a distance of 2.387 Å (Table 3). The layering is formed by Bi–O layers and Ca–O layers together with each one of the carbonate layer alternately pointing in both up and down directions along the c axis (Fig. 1). In this aspect the structure is different from that of the simple bismuth oxycarbonate, Bi₂O₂(CO₃) (bismutite), wherein all the carbonate groups have their apex pointing towards only one direction along the c axis, thereby resulting in a polar non-centrosymmetric space group (S.G Imm2). Both bismutite and beyerite belong to the sillen family of materials.

Fig. 5 Rietveld refinement PXRD pattern of CaBi₂O₂(CO₃)₂, experimental data; green line, calculated profile; pink line below, difference profile; vertical bars, Bragg positions.

Table 2 Positional, occupancies and thermal parameters of CaBi₂O₂(CO₃)₂

Atoms	Wyck	x/a	y/b	z/c	SOF	U(iso) Å^2
Bi1	4i	0	0	−0.30904(4)	1	0.0263(5)
Ca1	2c	0	0	0.5	1	0.030(1)
C1	4i	0.5	0.5	0.4236(7)	1	0.009(5)
O1	4j	0.5	0	0.2626(5)	1	0.032(2)
O2	8l	0.5	0.191(1)	0.4408(2)	1	0.037(2)
O3	4i	0.5	0.5	0.3431(5)	1	0.049(4)

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Table 3 Selected bond distances (in Å) for CaBi₂O₂(CO₃)₂

Atoms	Interatomic distances (Å)
Bi1–O1	2.13390(5) × 2
Bi1–O1	2.43512(2) × 2
Bi1–O3	2.75838(3) × 4
Ca1–O2	2.38652(3) × 8
C1–O2	1.21880(2) × 2
C1–O3	1.74400(4)

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The infrared spectrum of the sample was recorded in the spectral region between 400 and 4000 cm⁻¹ (Fig. 6a). The peaks at 1469 cm⁻¹ and at 1072 cm⁻¹ could be attributed respectively to the antisymmetric and symmetric stretching of CO₃²⁻ group. The bending modes of triangular CO₃ group gave rise to the band at 865 cm⁻¹. These observed features are in good accordance with the reported spectra for (BiO)₄(OH)₂CO₃ and Bi₂O₂CO₃.³⁹ The Raman spectrum of CaBi₂O₂(CO₃)₂ is presented in Fig. 6b. Bands observed at lower wavenumber (120–210 cm⁻¹) might arise from the lattice vibrations. Strong band positioned at 1070 cm⁻¹ was attributed to the symmetric stretching vibration (ν₁) for carbonate groups, while the peak at 687 cm⁻¹ to the CO₃²⁻ in plane bending (ν₄).³⁹

Fig. 6 (a) FTIR and (b) Raman spectrum of CBCS3.

Fig. 7 shows the UV-visible diffuse reflectance spectra of various samples of CaBi₂O₂(CO₃)₂, wherein the absorption edges were found to be in the UV region, well below 380 nm. It is clear that the optical behavior of all samples synthesized under different conditions seemed to be almost similar with small variations in the positions of their absorption edges. The band gap energies calculated by plotting the Kubelka–Munk function with photon energy (eV) were found to be 3.97 eV for both CBCS3 and CBCA1 and 3.92 eV for CBCS4 (Fig. 7). Application of the equation; αhν = A(hν − E_g)^n/2 (where α, ν, A and E_g are the absorption coefficient, frequency of light, proportionality constant and band gap respectively) in the present case, the plot of (αhν)² vs. hν using the value of 3.97 eV, resulted in nearly two for n, suggesting beyerite to be an indirect semiconductor.⁴⁰ The band gap energies are higher as compared to the value of 3.34 eV for Bi₂O₂CO_3. The possible reason could be the additional layer of calcium carbonate present in the beyerite. The valence band formation using O 2p and Bi 6s might be similar to that of bismutite,⁴¹ while the lifting up of the conduction band might occur because of the contribution of Ca 4s states. The optical behavior of beyerite and its wide band gap suggests the compound to be a potential photocatalyst under UV light irradiation. So we further considered the option of fabricating heterostructured photocatalyst with BiOI in order to check the improvement in terms of the extended visible light absorption and enhanced photocatalytic behavior.

Fig. 7 Diffuse reflectance spectra (DRS) of (a) CBCS3, (b) CBCS4 and (c) CBCA1, inset show the respective plots of Kubelka–Munk function vs. photon energy.

3.2. Synthesis, structure and physical properties of BiOI/CaBi₂O₂(CO₃)₂

BiOI, a p-type semiconductor belonging to the sillen family is an established bismuth containing material identified for the successful formation of heterostructures photocatalyst with Bi₂S₃,⁴² ZnSn(OH)₆,⁴³ and Bi₂O₂CO₃.⁴⁴ The composite formation of beyerite with BiOI has been attempted using a weight percentage of 10/90 as pointed out earlier. The PXRD patterns of the obtained products (Fig. 8a), showed clearly the diffraction peaks corresponding to the beyerite and BiOI (PDF# 850863). SEM image showed the existence of plates similar to those observed for pure beyerite that were not uniform anymore. The possibility might be the inclusion of BiOI that has resulted in bringing the irregularity seen in the SEM image of thickness of about 50 nm (Fig. 8b). The corresponding TEM image confirmed the square thinner plates in the nm scale (Fig. 8c). A surface area of 98 m² g⁻¹ has been noted which is higher than the individual surface areas of beyerite. The infrared spectrum of the composite was dominant by the features of beyerite such as the appearance of peaks at 1468 cm⁻¹ and 860 cm⁻¹ corresponding to the carbonate antisymmetrical stretching and bending modes (Fig. 8d). The stretching vibration of Bi–O observed around 489 cm⁻¹ in the pure BiOI⁴⁵ that is distinctly different from beyerite has probably been shifted to lower wave numbers and has not been noticed in the composite.²⁸ The Raman spectra of the composite clearly showed the bands of both beyerite and BiOI (inset in Fig. 8d). Specially the peak at 150 cm⁻¹ corresponding to the internal Bi–I stretching of BiOI is seen, thus confirming that the composite is a mixture of CaBi₂O₂(CO₃)₂ and BiOI.

Fig. 8 (a) PXRD, (b) SEM image, (c) TEM image and (d) FTIR (inset shows Raman spectra) of BiOI/CaBi₂O₂(CO₃)₂.

Fig. 9a shows the UV-visible diffuse reflectance spectra of the composite along with that of the pristine beyerite and BiOI for comparison. The absorption spectra of the BiOI/CaBi₂O₂(CO₃)₂ exhibit the mixed absorption properties of both the components with its absorption edge clearly shifted towards visible region (near 400 nm) along with the broad enhanced visible absorption in the region 400–670 nm. The band gap determined from the plot of the Kubelka–Munk function with photon energy (eV) (Fig. 9a) was found to be 3.18 eV as compared to the observed values of 3.97 eV and 1.83 eV for beyerite and BiOI respectively.

Fig. 9 (a) UV-visible diffuse reflectance absorption spectra (inset shows plot of Kubelka–Munk function vs. photon energy), (b), (c) temporal changes in absorbance spectra of Rh B dye (10⁻⁵ M) and phenol (10⁻⁴ M), under UV-visible irradiation (inset shows the plot of concentration vs. time (min)) respectively and (d) recycling experiments of the BiOI/CaBi₂O₂(CO₃)₂.

3.3. Photocatalytic properties of BiOI/CaBi₂O₂(CO₃)₂

The photocatalytic activities of the composite BiOI/CaBi₂O₂(CO₃)₂ sample along with the pure BiOI and CaBi₂O₂(CO₃)₂ were evaluated for the degradation of Rh B under UV-visible light (λ < 800 nm). Fig. 9b shows the change in absorption spectra of Rh B solution in the presence of the catalysts. The peak intensity of Rh B at 560 nm decreased with increasing irradiated time and disappeared completely after 2 h. The inset in Fig. 9b shows the variation in C/C₀, where C is the concentration (mg l⁻¹) of Rh B at time t and C₀ is the initial concentration (mg l⁻¹) with time. While photolysis in the absence of any catalysts is negligible, the activity of pure beyerite has been considerably increased after the composite formation with BiOI. The degradation activity is almost comparable or even slightly better than that of pure BiOI itself under similar experimental conditions. The catalyst was further checked for photocatalytic cyclic efficiency and we found that the activity of the catalyst remain unchanged for the decomposition of Rh B up to four cycles under UV-visible light irradiation. The rate of decomposition started showing a decrease during the fifth cycle (Fig. 9d). After five cycles of decomposition the catalyst was characterized by PXRD and was similar to the synthesized catalyst before photocatalysis (Fig. S2†). The photocatalytic oxidation of dyes occurs through reactive species such as h⁺, ˙OH or ˙O₂⁻, that were formed after the light absorption and electron–hole formation by the photocatalyst. In order to understand the nature of the reactive species, the Rh B dye degradation has been carried out in the presence of different scavengers. For example, benzoquinone (BQ), isopropyl alcohol (IPA) and ammonium oxalate (AO) were used to study the respective influence of ˙O₂⁻, ˙OH and h⁺.²⁸ The concentrations of the added BQ, IPA and AO scavengers were 0.1 mmol l⁻¹, 1.0 mmol l⁻¹ and 1.0 mmol l⁻¹ respectively. The photodegradation of Rh B seemed to remain almost unaffected after the addition of IPA, suggesting that ˙OH radicals were not playing a significant role (Fig. 10a). While the rate of photodegradation showed a marginal decrease for the addition of BQ and AO, indicating that the likely species involved were h⁺ and ˙O₂⁻. The generation of the active ˙OH⁻ radicals were also followed by using terephthalic acid as a probe and estimating the amount of 2-hydroxyterephthalic acid formed from its photoluminescent (PL) emission around 426 nm (for the excitation wavelength of 312 nm). The PL intensity again confirmed that ˙OH radicals are probably not the reactive species and may be neglected (Fig. 10b). It is therefore expected that only h⁺ and ˙O₂⁻ are mainly responsible for the photodegradation process.

Fig. 10 (a) Effects of different scavengers on the degradation of Rh B dye in the presence of BiOI/CaBi₂O₂(CO₃)₂ and (b) rate of ˙OH radical formation based on PL spectral changes over BiOI/CaBi₂O₂(CO₃)₂ using a solution of 3 × 10⁻⁴ M terephthalic acid (excitation at 312 nm) with irradiation time.

It is well known that BiOI is a p-type semiconductor and expecting beyerite CaBi₂O₂(CO₃)₂ to be a n-type semiconductor similar to that of Bi₂O₂CO₃, a possible reaction mechanism may be derived (Fig. 11). Based on the optical absorption behaviour of the heterostructure, it is clear that BiOI/CaBi₂O₂(CO₃)₂ could be excited by the irradiation of UV-visible (λ < 800 nm) light. The electrons formed from BiOI after light irradiation move towards the conduction band of CaBi₂O₂(CO₃)₂ because of the effect of inner electron field. These electrons react with the adsorbed O₂ to form the ˙O₂⁻ species. The holes on the other hand move in to the valence band of BiOI, leading to the oxidation of Rh B and the mentioned steps are shown schematically in Fig. 11. The formation of heterostructure with photocatalytic properties is therefore made possible as compared to the pristine CaBi₂O₂(CO₃)₂ with limited photocatalytic efficiency. The efficiency of the photocatalyst was probed further by carrying out the decomposition of phenol which has its absorption maximum only in the UV region (268 nm). The absence of visible absorption in the phenol unlike that seen in the case of Rh B helps to evaluate the photocatalytic nature by excluding the decomposition via photosensitization process. Photolysis of phenolic solution has been found to be negligible and the composite degraded phenol in almost 5 h under UV-visible irradiation (Fig. 9c). The independent decomposition of phenol by BiOI/CaBi₂O₂(CO₃)₂ despite being moderate, confirms the catalyst's photocatalytic ability. Efficient degradation activity has also been noted when Rh B solution was irradiated with visible light (400 < λ < 800 nm) in the presence of BiOI/CaBi₂O₂(CO₃)₂ (Fig. S3†). Here again, the decrease in the intensity of Rh B was found to occur nearly in 3.5 h duration which is better than that of pure BiOI, and beyerite. Additional experiments are required in order to evaluate the efficiency of the photocatalyst under visible light irradiation.

Fig. 11 Schematic diagram of depicting the possible reaction mechanism over BiOI/CaBi₂O₂(CO₃)₂ under UV-visible light irradiation.

4. Conclusions

The calcium bismuth oxy carbonate known as the mineral beyerite (CaBi₂O₂(CO₃)₂) has been successfully synthesized by a simple hydrothermal process. The structure of this material has been confirmed by carrying out the Rietveld refinement of the PXRD measurements. The morphology of the crystallites under varying synthesis conditions as monitored from their SEM and TEM pictures together with the observed band gap of 3.97 eV from its optical characterization showed beyerite to be a promising wide gap semiconducting material to develop photocatalysts and an ideal candidate for constructing new heterostructures. The formation of heterostructures has been shown by using a p-type semiconductor BiOI (E_g = 1.83 eV) and the composite corresponding to 10/90 of weight ratio of BiOI/CaBi₂O₂(CO₃)₂ has been characterized by several techniques. The efficiency of the photocatalyst has been shown by carrying out successful photodegradation of aqueous solutions of Rh B and phenol under UV-visible irradiation. Photodegradation experiments carried out to study the influence of ˙O₂⁻, ˙OH and h⁺ using respective quenching reagents such as benzoquinone (BQ), isopropyl alcohol (IPA) and ammonium oxalate (AO) suggested the likelihood of peroxide and hole being responsible species for the process as compared to hydroxyl radicals.

Acknowledgements

This work was supported by the DST (SR/S1/PC-07/2011), DST (SB/S1/PC-08/2012), DU-DST Purse Grant phase-II, DST – Nanomission Government of India, and University of Delhi under the “Scheme to Strengthen R&D Doctoral Research Program”. We thank the USIC for the use of facilities. We also gratefully acknowledge Prof. R. Nagarajan, Department of Chemistry, University of Delhi for constant support. VM and MP thank CSIR for SPMF and SRF fellowship respectively.

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Footnote

† Electronic supplementary information (ESI) available: Table of crystallographic parameters for the Rietveld refinements of beyerite, TGA plot, PXRD patterns of the BiOI/CaBi₂O₂(CO₃)₂ before and after the photocatalysis of Rh B and temporal changes in absorbance spectra of Rh B dye (10⁻⁵ M) under visible irradiation. See DOI: 10.1039/c6ra06723a

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