Uncovering the structural stabilities of the functional bismuth containing oxides: a case study of α-Bi₂O₃ nanoparticles in aqueous solutions

Abstract

The majority of bismuth containing oxides have been taken as important functional materials for applications including photocatalytic water splitting and highly conductive solid electrolytes, which however suffer from property degradation owing to the presence of intermediates. This work initiated a case study on α-Bi₂O₃ nanoparticles with an aim to uncover the origin of property degradation when bismuth oxides that contain traces of secondary phase α-Bi₂O₃ were exposed in either air or aqueous solutions. First, α-Bi₂O₃ nanoparticles with spherical and rod-like morphologies were prepared by developing solution chemistries. It was found that α-Bi₂O₃ transformed partially to Bi₂O₂CO₃ when stored in air for 6 months. Such transformation was further accelerated when α-Bi₂O₃ nanoparticles were exposed to water or methylene blue (MB) solution, probably because α-Bi₂O₃ reacts with the species HCO₃⁻ as formed by dissolution of CO₂ from air. Bi₂O₃ surfaces had an unusual tendency to adsorb MB molecules, which in turn prevented the reaction between α-Bi₂O₃ and HCO₃⁻. Consequently, MB solution became clear even without light irradiation. The results reported in this work may help to fully comprehend the property degradation of bismuth containing oxides when used in aqueous solutions.

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1. Introduction

As an important multi-functional material, bismuth containing oxides have shown many potential applications in photovoltaic cells, nonlinear optical glasses, fuel cells, oxygen sensors, and catalysis for industrial selective oxidation reactions.^1–4 These broad applications are based on its excellent properties like adjustable band gap, high ion conductivity, and rich polymorphism.⁵

Depending on the preparation methods, bismuth containing oxides may have traces of unreacted secondary phase of Bi₂O₃, which however may contribute greatly to the total properties in an uncontrolled way because of the polymorphs with distinct nature. For instance, Bi₂O₃ owns six polymorphs^6,7 including the monoclinic (α), tetragonal (β), bcc (γ), fact (δ), orthorhombic (ε), and triclinic (ω). Among these polymorphs, α-phase is thermodynamically stable and has a direct energy band gap of 2.8 eV.⁸ Electrons in the valence band of α-Bi₂O₃ could be excited by visible light for activating useful properties. For these reasons, preliminary efforts have been devoted to exploring the possible uses of α-Bi₂O₃ for solar energy conversion and environmental purification. For example, Zhang et al. prepared Bi₂O₃ by sonochemical route to degrade the methyl orange under visible light.⁸ Eberl et al.⁹ prepared Bi₂O₃ from a series of starting material using three different methods and concluded that photocatalytic activities in 4-chlorophenol degradation at λ ≥ 420 nm are strongly dependent on the sample preparation conditions. Recently, Bi₂O₃ was also reported to act as the substrates or photo sensitizers. Hameed et al.¹⁰ found that ZnO–Bi₂O₃ composites have the promoted activities under UV-vis irradiation in degradation of methylene blue, methyl orange, and phenol. In spite of these great efforts, it still remains uncertain as to the property degradation in the case when Bi₂O₃ appears as the secondary phase of bismuth containing oxides for applications in water or other aqueous solutions.

Therefore, it is highly necessary to investigate the durability of Bi₂O₃ in water, because (1) it may help to uncover the structural stabilities of bismuth containing oxides for applications in solar energy conversion and environmental purification where water (or CO₂) exists popularly, (2) it allows the comprehension of the surface compositions that activate the chemical reactions of bismuth compounds. It is well established that bismuth containing oxides are not stable in air, and thus their surfaces are usually covered by layers of secondary phases, which have contributed greatly to the activities for chemical reactions.^11,12

In this work, we prepared α-Bi₂O₃ with different morphologies by two solution chemistries and investigated their stabilities when exposed to water or methylene blue solution upon long-term storage in air. It was found that α-Bi₂O₃ nanoparticles easily reacted with CO₂ and formed Bi₂O₂CO₃, and that the formation process of Bi₂O₂CO₃ in water was different from that in MB solution because of the involvement of species HCO₃⁻ when dissolution of CO₂ from air. The reaction-adsorption model of α-Bi₂O₃ with HCO₃⁻ in MB solution was proposed and experimentally evidenced.

2. Experimental

2.1 Sample preparation

Bi₂O₃ nanoparticles were prepared by two approaches. Method I: Bi (NO₃)₃·5H₂O (A.R.), NaOH (A.R.), and HNO₃ (65%, A.R.) were used as the starting materials. The synthetic processes can be described as follows: Firstly, 1 mL HNO₃ was added into 19 mL H₂O, into which 3.123 g Bi(NO₃)₃·5H₂O was dissolved to get a clear solution. Then, an aqueous solution of NaOH with the given molar ratio was added into the above clear solution with continuous stirring. The as-obtained suspension was transferred to a Teflon-lined stainless steel autoclave with a filling capacity of about 80%, sealed, and reacted at 200 °C for 12 h. The autoclave was then cooled naturally to room temperature. The product was harvested by pressure filtration, washed fully with distilled water, and dried at 80 °C for 12 h. The sample thus obtained was named as A_as.

Method II: Bi₂O₃ nanoparticles were prepared from the precursor of BiOCl nanosheets. BiOCl was produced following the procedure reported in reference¹³ which used a mechanochemical processing method including milling, heat-treatment, and subsequent washings. Namely, BiCl₃, Na₂CO₃ and NaCl were milled in an agate mortar for 15 min with the fixed molar ratios of 1 : 1.5 : 5. The reactant mixture was heated in air at 325 °C for 3 h and was then washed several times and dried at 80 °C for 12 h to get BiOCl precursor. To obtain the Bi₂O₃ nanoparticles, 0.5 g BiOCl precursor was dispersed into 10 mL ethanol to form a white suspension solution, into which 4 mL NaOH (2 mol L⁻¹) was added into the above solution with continuous stirring. After stirring for 3 h and sonicating for 1 h at room temperature, the suspension color changed from white to yellow. The residual precipitate was washed with distilled water and ethanol several times, and dried at 80 °C for 12 h to get the sample B_as.

2.2 Stability test of Bi₂O₃ nanoparticles

The stabilities of Bi₂O₃ nanoparticles were measured by exposing to water or dye solution. 150 mg Bi₂O₃ nanoparticles (A_as or B_as) were first put in 150 mL H₂O or 5 × 10⁻⁵ mol L⁻¹methylene blue (MB) solution, while continuously stirring at ambient conditions for 1 h, 2 h, 4 h, 8 h, and 16 h, respectively. When exposing to the dye solution, a black bag was used to shade the external light. Residual precipitations were harvested by pressure filtration directly and dried at room temperature for further characterization. To reveal the influence of organic dye on the stability of Bi₂O₃ nanoparticles, 3 mL of the reacted MB suspensions were extracted and centrifuged at a rate of 9000 rpm for 15 min. The residual solutions were taken for UV-vis absorption spectrum measurements. Similar experimental procedure was also applied when exposing Bi₂O₃ nanoparticles to water. The samples were denoted as A_wat–nh when the residue is a consequence of exposing A_as to water for n h, or A_dye−nh when the residue is a consequence of exposing A_as to dye solution for n h. To justify the role of CO₂ in the reaction, the stabilities of samples A_as and B_as were also exposed to water for 16 h under oxygen conditions in a glove box, which led to the samples A_oxy-16h and B_oxy-16h, respectively. The stabilities of the samples A_as and B_as were also investigated after storage in air for a long-period of time like six months, which led to the samples A_stor and B_stor.

2.3 Sample characterization

Phase compositions of the samples were identified by powder X-ray diffraction on a Miniflex II diffractometer using Cu-Kα radiation at room temperature. The morphologies of the samples were studied by a field-emission scanning electron microscopy (FE-SEM) using a JEOL JSM-6700 apparatus, and transition electron microscopy (TEM) on a JEOL JEM 2010 instrument under an acceleration voltage of 200 kV. Samples for TEM observations were prepared by making dispersion of the samples in ethanol and putting drops of them on a carbon-coated copper grid. The elemental composition of the sample was determined using Ultima inductively coupled plasma spectrometry and Vario MICRO elemental analyzer. The infrared optical properties were measured on a Perkin-Elmer IR spectrophotometer at a resolution of 4 cm⁻¹ using a KBr pellet technique.

Valence states of the bismuth ions for the samples were determined by X-ray photoelectron spectroscopy (XPS). XPS measurements for powder samples fixed on double-sided tapes were performed on an ESCA-LAB MKII X-ray photoelectron spectrometer from VG Co. with Al Kα radiation. The base pressure was 10⁻⁷ Pa and the C 1s signal was used to correct the charge effects. The UV-vis absorption spectra of MB solution were measured on a Perkin Elmer UV WinLab Lambda 35 spectrophotometer.

3. Results and discussion

3.1 Stability of Bi₂O₃

XRD patterns of the as-prepared samples A_as and B_as are illustrated in Fig. 1. The strong diffraction lines indicate the high crystallinity.¹⁴ Careful diffraction data analysis showed that both samples crystallized in a monoclinic structure, since all diffraction peaks matched well the standard data for α-Bi₂O₃ (JCPDS, No. 71-2274). The diffraction lines for other polymorphs of Bi₂O₃ were hardly seen, which demonstrates single phase nature. However, when the as-prepared samples were stored in air for 6 months under ambient conditions, XRD patterns changed a lot. For sample A_stor, an extra weak diffraction line was detected at two theta of 30.3°. Comparatively, sample B_stor exhibited several relatively strong peaks at two theta of 12.9, 24.0, 30.3, and 32.7° in addition to the characteristic diffraction peaks for α-Bi₂O₃. These new peaks belonged to the planes (101), (013), and (110) for Bi₂O₂CO₃ (JCPDS, No. 41-1488). The occurrence of the secondary phases for both A_stor and B_stor was also confirmed by SEM and elemental analysis (see Table S1†). For sample A_as, as illustrated in Fig. 2, the nearly spherical particles with dimension of several microns are predominant. Alternatively, sample B_as is mainly composed of rod-like particles with diameters of several hundreds of nanometres. It is also noted that the morphology of sample A_stor is nearly the same as sample A_as, although some particle–particle attachments appeared at particle corners. Instead, sample B_stor showed a remarkably different morphology from B_as. Rod-like particles with smooth surfaces could hardly be seen, except for many plate-like particles. These plate-like particles could be ascribed to Bi₂O₂CO₃. These observations indicate that Bi₂O₃ nanoparticles are not stable at ambient conditions and would react with CO₂ to form Bi₂O₂CO₃ in air.

Fig. 1 XRD patterns of the as-prepared samples A_as, B_as, A_stor, and B_stor. The standard diffraction data for α-Bi₂O₃, B₂O₂CO₃, and (BiO)₄CO₃(OH)₂ are also shown for comparison.

Fig. 2 SEM images of samples A_as, B_as, A_stor, and B_stor.

Comparatively, transformation from Bi₂O₃ to Bi₂O₂CO₃ in water is relatively easier due to the existence of CO₂/H₂CO₃. Fig. 3 shows the XRD patterns for the samples after exposing A_as and B_as to water. It is seen that after exposing A_as to water for 1 h, the characteristic diffraction line (110) at two-theta of 32.7° for Bi₂O₂CO₃ was observed for A_wat-1h. Prolonging the exposure time led to the strengthened diffraction intensity for Bi₂O₂CO₃. A similar phenomenon was observed when exposing B_as to water for less than 4 h (Fig. 3b). When prolonging the exposure time to 8 h, Bi₂O₂CO₃ became the dominant phase. Further prolonging the exposure time to 16 h, the diffraction peaks of parent phase α-Bi₂O₃ in B_wat-16h became extremely weak, and sample B_wat-16h showed a white color that is distinct from the yellow color for sample B_as. It should be noted that at two-theta of about 18°, a weak diffraction peak for Teflon (JCPDS, No. 47-2217) was detected when samples A_as and B_as were exposed to water for more than 8 h. Alternatively, when samples A_as and B_as were exposed to water under a flow oxygen, both samples A_oxy-16h and B_oxy-16h contained much less Bi₂O₂CO₃ than that without flowing oxygen (i.e., A_water-16h and B_water-16h). The diffraction peaks belonging to (BiO)₄CO₃(OH)₂ were detected. The formation of (BiO)₄CO₃(OH)₂ should be related to the low CO₂ pressure under oxygen atmosphere.¹⁵

Fig. 3 XRD patterns of the samples obtained by exposing (a) A_as and (b) B_as to water in dark for given period of time. Vertical bars represent the standard diffraction data for α-Bi₂O₃, B₂O₂CO₃, and (BiO)₄CO₃(OH)₂.

Morphologies of the water exposed samples were investigated by TEM. As shown in Fig. 4, A_wat-16h and B_wat-16h were mainly plate-like structures with dimensions larger than 200 nm. High-resolution TEM (HRTEM) images of the samples A_wat-16h and B_wat-16h (Fig. 4) revealed the clear fringes, which indicates a single-crystalline nature.¹⁶ These fringes were separated by a spacing of 0.273 nm, consistent with the inter-planar spacing of plane (110) for Bi₂O₂CO₃. From these TEM images, it is hard to find the existence of Bi₂O₃ except for a few core-shell nanostructures in B_wat-16h (inset of Fig. 4). The nanoparticles or nanorods were coated by an amorphous layer of 20 nm in thickness. We believed that the inner nanoparticles or nanorods are the residual Bi₂O₃, while outer amorphous layer should be Bi₂O₂CO₃. Such core-shell nanostructures were not found in TEM images for A_wat-16h.

Fig. 4 TEM (left) and corresponding HRTEM (right) images for the samples A_wat-16h and B_wat-16h.

The results presented above show that α-Bi₂O₃ is not stable in aqueous solutions. What would happen when Bi₂O₃ nanoparticles were exposed to MB solution? XRD was used to monitor the processes when A_as and B_as were exposed to MB solution. As indicated in Fig. 5, dual-phase structures as well as the Teflon impurities were observed after exposing to MB solution, as that observed when exposing to water. Nevertheless, different from the yellow color for A_wat-16h and white color for B_wat-16h, the sample color for both A_dye-16h and B_dye-16h was blue. Sample color change could be closely related to the surface reaction between α-Bi₂O₃ and CO₂/HCO₃⁻. To examine the reaction rate of α-Bi₂O₃ with CO₂/H₂CO₃ when exposing to water and MB solution, the intensity ratios of diffraction line (110) for Bi₂O₂CO₃ to line (120) for Bi₂O₃ were calculated. Fig. 6 shows the variations of intensity ratio with the exposure time. It is seen that, (1) B_as exhibited a larger reaction rate than A_as in either water or MB solution, which might be the consequence of the relatively small particle dimension and special rod-like morphology; (2) the intensity ratio for A_as when exposing to water was slightly larger than that when exposed to MB solution; and (3) the intensity ratio kept nearly constant when exposing A_as and B_as to MB solution for a long period of time from 4 to 8 h. These facts imply that the reaction process of α-Bi₂O₃ with CO₂/HCO₃⁻ when exposed to MB solution could be different from that when exposed to water.

Fig. 5 XRD patterns of the samples when exposing (a) A_as and (b) B_as to MB solution in dark for given period of time.

Fig. 6 Exposure time dependence of intensity ratios of line (110) for Bi₂O₂CO₃ to line (120) for α-Bi₂O₃.

3.2 Reaction process and unusual adsorption of Bi₂O₃ nanoparticles in MB solution

When α-Bi₂O₃ nanoparticles were exposed to MB solution (5 × 10⁻⁵ mol L⁻¹), α-Bi₂O₃ reacted with CO₂/H₂CO₃⁻ to form Bi₂O₂CO₃. At the meanwhile, MB solution became clear. When the concentration of MB solution decreased from 5 × 10⁻⁵ to 1 × 10⁻⁵ mol L⁻¹, the solution became nearly colorless (S1†).

As is well known, MB is a common cationic dye of broad applications in the fields of chemistry and biology.^17,18 In this work, the concentration variation of MB can act as a probe to give the information about the reactions occurred in MB solution. UV-visible absorption spectra of MB solution were used to monitor the fading of MB solution which may in turn help to understand the transformation process of Bi₂O₃ to Bi₂O₂CO₃. Fig. 7 shows the time-dependent absorption spectra of MB solutions at the presence of A_as and B_as, while the normalized absorbance intensity changes of MB solution with time were also presented. After stirring for 16 h, the normalized absorbance C/C₀ decreased by 5% in the presence of A_as, much less than 30% for that contains B_as. It is interesting that C/C₀ does not decrease monotonously with prolonging the exposure (or stirring) time. For example, there was one abnormal data point at 4 h for A_as and two data points at 4 and 8 h for B_as. This unusual adsorption of MB molecules on surfaces of Bi₂O₃ nanoparticles is also reproducible, as indicated when changing the stirring rate (S2†).

Fig. 7 Time-dependent absorption spectra of MB solution in the presence of samples (a) A_as and (b) B_as, and (c) variations of the normalized absorbance intensity with the exposure time.

Many factors such as degradation, pH-induced structure conversion,¹⁹ as well as MB adsorption on surfaces of nanoparticles could affect the characteristic absorption intensity for MB molecules. To figure out the reasons for the varied characteristic absorption of MB molecules with exposure time in Fig. 7c, IR spectra of the exposed samples were measured. IR spectra of the typical exposed samples as well as A_as and B_as are compared. As indicated in Fig. 8, a set of bands were observed below 600 cm⁻¹, which are characteristic for Bi–O bonds.²⁰ After exposing to water and MB solution for 16 h, new bands appeared at 1460, 1400, and 850 cm⁻¹, which could be assigned to CO₃²⁻ vibrations in Bi₂O₂CO₃.²¹ The multiple bands appeared at 1240, 1215, and 1150 cm⁻¹, which can be assigned to CF₂ stretching vibrations of polytetrafluoroethylene (Teflon)²² as originated from the Teflon layers of the magnetic stirrers, as indicated by XRD. Relatively strong vibrations of CO₃²⁻ for B_wat-16h and B_dye-16h were also observed, which is due to the enriched layers of Bi₂O₂CO₃, consistent with XRD analysis. Compared to A_wat-16h and B_wat-16h, no additional bands were detected for A_dye-16h and B_dye-16h. It should be emphasized that most of the strong vibrations for MB molecules were overlapped with those of CO₃²⁻. Therefore, from IR spectra, it is still difficult to make sure which factor is responsible for the intensity variations of MB characteristic absorption.

Fig. 8 IR spectra of samples A_as, B_as, A_wat-16h, B_wat-16h, A_dye-16h, and B_dye-16h.

Several implications, however, show that the dye does not undergo decomposition. Firstly, during the adsorption process, the absorption bands of MB did not shift, and no new peaks appeared in the absorption spectra (Fig. 7a and b). Therefore, the variation of absorption intensity is not due to the catalytic degradation or pH-induced structure conversion of MB. As is well known, during MB photodegradation, N-demethylated analogues would contribute to the hypsochromic effect of the absorption band with the maximum at about 660 nm. Correspondingly, the possible colorless intermediates would be revealed by the new strong peaks in UV regions.^23,24 On the other hand, pH-induced structure conversion of MB would also result in the shift in the absorption band.¹⁹ During the exposure to water and MB solution, the pH of the solution was kept in the range of 6 to 9. Secondly, XPS data (S3†) of B_as demonstrate that Bi³⁺ ions are predominant, while in B_dye-16h, bismuth ions are in mixed valences of +3 and +5. Bi⁵⁺ came from Bi₂O₂CO₃ which would change its color from white to black under light irradiation. This color change is different from the photochromic effect of BiOX (X = Cl, I)²⁵ because it is irreversible. According to the basic principle of redox reaction in inorganic chemistry, oxidizing MB to inorganic molecules such as CO₂ and SO₄²⁻ would lead to the reduction of Bi ions, which is apparently inconsistent with the experimental observations by XPS. Thus, the variation of Bi valence is not the result of MB degradation.

Further, a simple method was designed to verify MB adsorption on surfaces of nanoparticles using UV-visible absorption spectrum. Firstly, A_as was allowed to stay in MB solution (1 × 10⁻⁵ mol L⁻¹) for 9 h, which was then dissolved in diluted hydrochloric acid to get a clear solution with pH = 1 (named as Solution-1). For comparison, pure MB solution (1 × 10⁻⁵ mol L⁻¹) was also adjusted to pH = 1 by diluted hydrochloric acid to get a solution (named as Solution-2). Fig. 9 shows UV-visible absorption spectra of Solution-1 and Solution-2. In the region of 420–800 nm, both solutions exhibited the similar absorptions except for the maximum positions. Namely, the maximum absorption of Solution-2 shifted from 600 nm for pure MB to 564 nm for Solution-1, which indicates the occurrence of pH-induced structure conversion in Solution-1.¹⁹ Namely, organic dye is further confirmed to be adsorbed on surfaces of Bi₂O₂CO₃. It can be thus concluded that MB adsorption on surfaces of nanoparticles affected the characteristic absorption intensity for MB molecules at the presence of Bi₂O₂CO₃ (Fig. 7).

Fig. 9 UV-visible spectra of (a) Solution-1 and (b) Solution-2.

Having the adsorption of MB molecules on nanoparticle surfaces in mind, it becomes clear as to the time-dependent variations of intensity ratio of diffraction peaks of Bi₂O₂CO₃ to Bi₂O₃ in Fig. 6. When exposing to water or MB solution, the formation reaction of Bi₂O₂CO₃ on surfaces of α-Bi₂O₃ particles may proceed in two steps: (1) the diffusion of CO₂/H₂CO₃⁻ towards the surfaces of α-Bi₂O₃, and (2) α-Bi₂O₃ reacted with CO₂/H₂CO₃⁻. As indicated in Fig. 6, when exposing to water, the supply of CO₂/H₂CO₃⁻ to the surface of Bi₂O₃ is sufficient. Transformation of Bi₂O₃ to Bi₂O₂CO₃ happened until Bi₂O₃ surface was densely covered by Bi₂O₂CO₃ to totally transform into Bi₂O₂CO₃. Alternatively, when exposed to MB solution, the intensity ratio of diffraction peaks of Bi₂O₂CO₃ to α-Bi₂O₃ kept almost the constant when the exposure time lasted from 4 to 8 h, which means that formation reaction of Bi₂O₂CO₃ was temporarily stopped. Because the reaction rate for A_as is much lower than that for B_as, the temporary reaction stop phenomena observed for both samples could not be ascribed to the insufficient HCO₃⁻. On the contrary, MB molecules would compete with HCO₃⁻ to be adsorbed on the surfaces of nanoparticles. When the surfaces of Bi₂O₃ nanoparticles were covered by MB molecules or Bi₂O₂CO₃, reaction of Bi₂O₃ with HCO₃⁻ to form Bi₂O₂CO₃ would be prevented. Then, prolonged period of time is needed to establish a new adsorption balance between HCO₃⁻ and MB molecules on nanoparticles. Such a re-adsorption process could only be achieved by diffusion of HCO₃⁻ and MB molecules or by detaching Bi₂O₂CO₃.

To clearly describe these steps for understanding the unusual MB absorption, a reaction-adsorption model was proposed, as illustrated in Scheme 1. As is well known, MB in solution is weakly basic.²⁶ Therefore, dissolved CO₂ in MB solution would transform to HCO₃⁻.²⁷ Similar to what is reported elsewhere,²⁸ surfaces of Bi₂O₃ nanoparticles prepared by wet chemical routes would be covered by numbers of unsaturated sites of Bi³⁺ and O²⁻ ions as well as hydroxyl sites. Once HCO₃⁻ ions are adsorbed on the surface of Bi₂O₃ as illustrated in Scheme 1a, these species would react with Bi₂O₃ to form amorphous Bi₂O₂CO₃ layer (Fig. 4b). This amorphous layer may accumulate and crystallize with prolonging the reaction time. After exposing to MB solution for about 4 h, surfaces of Bi₂O₃ nanoparticles would be entirely covered by Bi₂O₂CO₃.²⁹ The shear stress from stirring would tear Bi₂O₂CO₃ from the nanoparticles, and the etching of Bi₂O₂CO₃ would give the newly formed Bi₂O₃ surfaces. To reduce the surface energy, the newly formed Bi₂O₃ surfaces would adsorb MB molecules notably (Scheme 1b), which may result in the reduction of MB concentration (Fig. 7c). Then, CO₂ molecules from air dissolve into the solution to form HCO₃⁻, and the newly formed HCO₃⁻ would exchange with MB molecules at surfaces of Bi₂O₃ (Scheme 1b). As indicated in literature,³⁰ a surface structure-change might be possible during the exchange between MB and HCO₃⁻. The replaced MB molecules would re-dissolve into the solution and thus the MB concentration would increase (Scheme 1c). After several recycles, Bi₂O₃ was transformed to Bi₂O₂CO₃ completely (Scheme 1d). Therefore, according to the description of Scheme 1 for the adsorption-reaction-etching process occurred on surfaces of α-Bi₂O₃ nanoparticles, the concentration of MB molecules in aqueous solutions would vary in a fluctuation form, isomorphic to the experiment data in Fig. 7c.

Scheme 1 Reaction- adsorption model proposed for α-Bi₂O₃ with HCO₃⁻ in MB solution.

4. Conclusions

The stabilities of α-Bi₂O₃ in air and aqueous solution were systematically studied. It is demonstrated that at ambient conditions, α-Bi₂O₃ transformed partially into Bi₂O₂CO₃ after storage in air for 6 months. This transformation process was accelerated in aqueous solutions, as indicated by the fact that Bi₂O₂CO₃ appeared when α-Bi₂O₃ nanoparticles of spherical and rod-like morphologies were exposed to water for 2 h and that the content of Bi₂O₂CO₃ increased with prolonging the exposure time. When exposed to MB solution, transformation from α-Bi₂O₃ to Bi₂O₂CO₃ became complicated in comparison with that in water: there existed an unusual MB adsorption on surfaces of Bi₂O₃ nanoparticles. The reaction of α-Bi₂O₃ with HCO₃⁻ would be prevented when exposing Bi₂O₃ nanoparticles in MB solution for a period of time from 4 to 8 h. Meanwhile, the concentration of MB molecules in solution would be fluctuated with exposure time. As a result, MB solution became clear at the absence of light irradiation.

Acknowledgements

This work was financially supported by NSFC under the contract (no. 20773132, 20771101, 20831004), National Basic Research Program of China (no. 2009BAE89B01, 2009CB939801, 2007CB613301), and Directional program of CAS (no. KJCXZ-YW-M10).

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Footnote

† Electronic supplementary information (ESI) available: Compositional analysis results, photos of MB solution in the presence of B_as with stirring time, and time-dependent concentration variation of MB solution in the presence of Bi₂O₃ at different stirring rates. See DOI: 10.1039/c0nj00410c

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