Insights into the impurities of Bi₂WO₆ synthesized using the hydrothermal method

Abstract

Bismuth tungstate (Bi₂WO₆) nanomaterials are widely used as visible-light driven photocatalysts. However, limited attention has been paid to the purity of prepared Bi₂WO₆ nanoparticles, which may affect the photocatalytic performance and hinder in-depth study of Bi₂WO₆. In this work, the impurities of Bi₂WO₆ formed during the hydrothermal process under a wide range of acid–base conditions (from 1.5 M HNO₃ to 0.5 M NaOH) were qualitatively analyzed and accurately quantified for the first time. After confirmation of Bi₂WO₆ stability, the impurities were dissolved using acid or base treatment, followed by measurements of the ion concentrations using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Furthermore, various characterization techniques including XRD, FE-SEM, TEM, UV-Vis DRS, XPS and FTIR were implemented to explore the change in morphology and optical properties of Bi₂WO₆ prepared in different acid–base environments, and to facilitate qualitative analysis of impurities. The hydrolytic properties of raw materials used for the synthesis of Bi₂WO₆ were also analyzed with UV-Vis transmittance observation. Following these analyses, the types and contents of impurities in Bi₂WO₆ prepared by the hydrothermal method under different acid–base conditions were determined. Results show that the primary impurity is WO₃·0.33H₂O (41.09%) for the precursor prepared in 1.5 M nitric acid solution. When the pH of the precursor was in the range of 0.97–7.01, the synthesized Bi₂WO₆ has relatively high purity, and the impure products were identified as BiONO₃. Bi₂O₃ began to appear when pH reached 9.01 and it reached 18.88% when pH was 12.98. The final product was Bi₂O₃ exclusively for the precursor conditioned in 0.5 M NaOH solution. In addition, the accuracy of the proposed quantitative method using ICP-MS was validated for several scenarios by weight difference experiments.

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1. Introduction

Many researchers consider photocatalysis to be a promising technology for sustainable energy production and environmental remediation. Photocatalysts such as TiO₂,¹ ZnO² and SnO₂ ³ have been extensively studied; however, such catalysts only respond to the ultraviolet light because of their wide band gaps. Meanwhile, ultraviolet light accounts for approximately 4%⁴ of the solar energy on the surface of the planet earth and 44% of sunlight is visible. As a result, visible-light responsive photocatalysts with a narrow band gap have attracted growing interest in research.

Bismuth tungstate (Bi₂WO₆) has recently gained considerable attention because of its physicochemical stability and efficient response to visible light.⁵ Bi₂WO₆ and its modified catalysts have been used for degradation of organic wastewater, conversion of carbon dioxide (CO₂), water splitting for hydrogen production, and removal of nitric oxide (NO) from atmosphere.^6–11 Since Kudo¹² first synthesize Bi₂WO₆ by conventional solid state reactions, many methods have emerged for its preparation including hydrothermal method,^13,14 microwave-¹⁵ and ultrasonic-assisted¹⁶ hydrothermal method, solvothermal procedure,¹⁷ sol–gel process,¹⁸ grinding and calcination,¹⁹ and co-precipitation.²⁰ Among these methods, the hydrothermal method is one of the most widely used because of its simplicity, safety, and high yield.^21,22

During the hydrothermal synthesis process, the final products may be affected by factors, such as hydrothermal reaction temperature and pH value of precursors. Zhang and Zhu²³ explored the impact of hydrothermal reaction temperature on the properties of Bi₂WO₆ nanocatalysts. They observed that the crystallinity and morphology of final products were affected by the temperature and time during hydrothermal process. Specifically, low temperature will lead to poor crystallinity, while high temperature will result in low photocatalytic activity. Additionally, it is well acknowledged that pH value also affects several physical properties of Bi₂WO₆ in a hydrothermal preparation. Huang et al.²⁴ produced Bi₂WO₆ nanomaterials with different morphologies by adjusting the pH level of the precursor solution. At pH 8, the fluffy microspheres-like Bi₂WO₆ exhibited the highest photocatalytic activity. Hu et al.²⁵ reported that the pH value of precursor solution affected the specific surface area and the porosity of their Bi₂WO₆ nanocatalysts. Lin et al.¹⁴ reported that pH value impacted on the particle sizes of the synthesized Bi₂WO₆ photocatalysts. Moreover, the pH value of precursor could affect the composition of final products. Some researchers prepared WO₃/Bi₂WO₆ composite in a strong acid environment.^26,27 Others discovered Bi_3.84W_0.16O_6.24 in Bi₂WO₆ nanomaterials prepared under alkali conditions.²⁸ Therefore, it is likely that the acidity and/or alkalinity of preparation conditions may have a strong impact on the final products. However, most researchers focused on properties like morphology and specific surface area; few attempted to determine the purity of Bi₂WO₆ nanomaterials prepared at different synthesis conditions.

It is therefore necessary to qualitatively and quantitatively analyze impurities to better understand the effects of synthesis conditions on the purity of Bi₂WO₆. The impure components in Bi₂WO₆ can be qualitatively analyzed by some characterization technologies such as XRD, SEM and FTIR.^29,30 Earlier studies have indicated that the impurities in Bi₂WO₆ prepared via hydrothermal method mainly resulted from the hydrolysis of raw materials.^26,31,32 Therefore, accurate and comprehensive understanding of the hydrolysis properties of Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O at different pH levels is also essential to qualitative analysis of impurities.

To the best of our knowledge, however, there is not relevant research on the quantification of impurities in Bi₂WO₆ nanomaterials. One feasible strategy is to take advantage of the difference in physio-chemical properties between impurities and Bi₂WO₆. As far as we know, the hydrolysate of the raw material for Bi₂WO₆ preparation can be dissolved in an acid or hot alkali solution,^33,34 while Bi₂WO₆ is relatively stable.^35,36 Hence it is reasonable to carry out quantitative analysis by dissolving impurities from the Bi₂WO₆ samples.

The objective of the current work is to study the impurities of prepared Bi₂WO₆ nanocatalysts. For this purpose, we first synthesized Bi₂WO₆ using hydrothermal method from 1.5 M nitric acid (HNO₃) to 0.5 M sodium hydroxide (NaOH) solutions. Then we identified the compounds of the impurities and other properties of the samples according to XRD, UV-Vis DRS, XPS FTIR, FE-SEM, TEM results and hydrolysis properties of raw materials for preparation of Bi₂WO₆. After that, the stability of Bi₂WO₆ was determined experimentally to ensure the feasibility of quantitative experiments. Later, impurities in the catalysts were dissolved into strong acid or hot alkaline solutions. Following that, the concentrations of key elements in the solutions were measured using ICP-MS, followed by conversion into mass weight of corresponding impurities. Finally, the accuracy of the quantitative method based on the ICP-MS technique was then verified by weight difference experiments. Overall, the present work is expected to shed light onto Bi₂WO₆-related studies and investigations using Bi(NO₃)₃·5H₂O and/or Na₂WO₄·2H₂O as raw materials.

2. Experimental

2.1 Materials

Reagents used in this research include Bi(NO₃)₃·5H₂O (≥99.0%), Na₂WO₄·2H₂O (≥99.5%), ethanol (≥99.7%), NaOH (≥96.0%) and concentrated HNO₃ (65–68.0%). They are all of analytical grade and were purchased from Sinopharm Chemical Reagent Co. These chemicals were used as received without further purification. Ultrapure water with a resistivity of 18.2 MΩ cm⁻¹ produced by Direct-Q 5 UV System (Merck Millipore Germany) was used in this study.

2.2 Synthesis of Bi₂WO₆ in different acid and alkali conditions

The Bi₂WO₆ nanomaterials were prepared using hydrothermal method. The molar ratio between bismuth and tungstate was maintained at 2 : 1. The preparation procedures of precursor suspension under strong acid or mild alkali condition differs slightly from those under dilute acid and base conditions. To prepare Bi₂WO₆ under strong acid condition, 1.0914 g of bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) and 0.3711 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O) were first weighed using an analytical balance (METTLER TOLEDO-ME104E/02). They were then transferred to two beakers containing 30 mL of nitric acid solution, the concentration of which ranged from 0.5 M to 1.5 M. These two solutions were magnetically stirred for 30 minutes separately, before they were mixed up for another 20 minutes. Nanomaterials synthesized under such strong acid conditions are denoted as 0.5MHNO, 1.00MHNO, 1.25MHNO, and 1.5MHNO; the numerical numbers stand for HNO₃ concentrate. As for mild base condition, the same amount of Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O were dissolved in 30 mL of 0.5 M NaOH solution, respectively. The subsequent procedures are the same as those under strong acid conditions and the corresponding nanomaterials are denoted as 0.5MNAOH.

For dilute acid and base conditions ranging from pH 0.97 to pH 12.98, 1.4552 g of Bi(NO₃)₃·5H₂O and 0.4948 g of Na₂WO₄·2H₂O were dissolved in 25 mL of 0.3 M HNO₃ solution and in 55 mL of ultrapure water, respectively. These two solutions were then magnetically stirred for 30 minutes separately, before they were mixed up for another 20 minutes. The pH of the precursor, which was monitored by a pH meter (S470-K, METTLER TOLEDO), was then adjusted to 0.97, 2.99, 4.99, 7.01, 9.01, 10.00, 11.05, 12.01 and 12.98 with NaOH and/or HNO₃. The corresponding nanomaterials synthesized under these conditions are defined as 0.97BWO, 2.99BWO, 4.99BWO, 7.01BWO, 9.01BWO, 10.00BWO, 11.05BWO, 12.01BWO, and 12.98BWO, respectively.

The precursor suspension was then transferred to a 100 mL PTFE autoclave, and heated at 180 °C for 24 h. After naturally cooling down to room temperature, the solid was collected by centrifuge, followed by washing 3 times with ultrapure water and ethanol, respectively. The nanomaterials collected were then dried at 60 °C for 12 h; the resultant nanomaterials were stored for subsequent use.

2.3 Sample characterization

Crystal phase of the prepared photocatalysts was analyzed by the X-ray diffraction technique (XRD, Bruker AXS D8 Advance, Cu Kα, λ = 1.5406 Å, 40 kV, 40 mA) at 2θ angles from 5 to 80°,and a scan speed of 10° per min. Their optical properties were determined using UV-Vis diffused reflectance spectra (UV-Vis DRS, Hitachi U-3900H) over the wavelengths in the range of 275–800 nm. X-ray photoelectron spectroscopy (XPS) was performed on ESCALAB 250Xi using Al Kα radiation to determine the chemical valence and relative percentage of surface elements. The data of binding energy for all samples were calibrated with the C 1s signal at 284.8 eV. Field emission scanning electron microscope (FE-SEM, Hitachi-SU8220), which was operated at 10 kV acceleration voltage and 9800 nA emission current, was used to observe the morphology of the samples. Additionally, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) observations were carried out on a Tecnai G2 F20 instrument in the bright field. Furthermore, chemical bonds of samples were determined using Fourier Transform Infrared Spectroscopy (FTIR, Bruker Vertex 80v) over the range of 400–4000 cm⁻¹.

2.4 Observation of critical point of hydrolysis

The raw materials [Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O] used for the preparation of Bi₂WO₆ were subject to severe hydrolysis in solution, producing insoluble precipitates.^9,27,31 The hydrolysis reactions contribute largely to the impurities in the Bi₂WO₆ produced during hydrothermal process. Therefore, understanding the hydrolysis properties of Bi³⁺ and WO₄²⁻ ions at different pH levels is important to impurity analysis.

The critical pH value of hydrolysis is defined as the pH value at which ions begin to hydrolyze. The critical pH values for Bi³⁺ and WO₄²⁻ ions were determined using the transmittance pattern of the UV-Vis spectrophotometer (Hitachi U-3900H). The precipitate may affect the transmittance intensity of a solution. Therefore, transmittance observations can be used to identify the critical pH values. At the critical pH value, transmittance drops abruptly due to the presence of precipitates.

The procedure of UV-Vis transmittance observation was described as follows. Firstly, Bi(NO₃)₃ and Na₂WO₄ were completely dissolved in a strong acidic solution and ultrapure water, respectively. The obtained transparent Bi(NO₃)₃ and Na₂WO₄ solutions were then gradually regulated using NaOH and HNO₃ solutions to reach a series of pH values with a small interval. The transmittance of these Bi(NO₃)₃ and Na₂WO₄ solutions was recorded at each pH value.

2.5 Catalysts stability

A new approach based on acid and base treatment is proposed herein to quantify the impurities in the Bi₂WO₆ nanomaterials produced by hydrothermal method. In this method, it is critical to ensure Bi₂WO₆ stability in the corresponding acid and base environments. For this purpose, 0.97BWO nanomaterials were treated with acid and alkaline solutions, respectively, following the same procedure as described in ICP-MS analysis, which can be found in the next section. Crystal structure stability and morphology stability were analyzed by examining the XRD patterns and FE-SEM images of 0.97BWO before and after treatment with acid or base solvents. In addition, chemical stability was observed based on the change in the mass of dissolved Bi and W elements during a period of 7 days.

Nanomaterials prepared at strong acid (1.5MHNO) and base (12.98BWO) precursors were also analyzed to further confirm the stability of Bi₂WO₆. The XRD patterns and FE-SEM images of 1.5MHNO before and after alkali treatment was compared. The stability of 12.98BWO was confirmed by XRD results before and after soaking with acid.

2.6 Quantitative analysis of impurities

Upon confirmation of Bi₂WO₆ stability, Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Agilent 7900) was used to determine the soluble bismuth and tungstate ions dissolved in solutions. Data is used for the quantification of impurities in the Bi₂WO₆ nanomaterials synthesized under different acid and alkali conditions. The samples for ICP-MS analysis were prepared following these steps: for those prepared at pH value equal to or above 0.97, and the 0.5MNAOH nanomaterials, samples were soaked in pH 0.5 HNO₃ solution for one week to dissolve potential acid-soluble impurities. However, 1.5MHNO, 1.25MHNO, 1.00MHNO, 0.5MHNO and 0.97BWO samples were treated by boiling 0.5 M NaOH solution for 1 min to dissolve alkali-soluble impurities. The resultant suspension was separated by a centrifuge operating at 4500 rpm for 3 min; then the supernatant was then sampled and diluted for ICP-MS analysis.

The ICP-MS results were further verified by weight difference tests. A specific amount of 1.5MHNO, 0.97BWO and 12.98BWO samples were treated using base or acid solution. The treated catalysts were then cleaned in a sintered disc filter. The 1.5MHNO nanomaterials after alkaline treatment were washed using ultrapure water during filtration. However, the 0.97BWO, 12.98BWO samples soaked in acid were washed 5 times using pH 0.5 HNO₃ solution, followed by a thorough rinse with ultrapure water. This way, the hydrolysis of Bi³⁺ ions left on solids could be prevented, ensuring the accuracy of final analysis. After drying at 60 °C for 12 hours, the weight difference before and after acid or base treatment was recorded.

3. Results and discussion

3.1 XRD patterns of Bi₂WO₆ nanomaterials

Fig. 1 and 2 show the XRD patterns of the catalysts prepared under different acid and base conditions. All samples, except for 12.98BWO and 0.5MNAOH, exhibited diffraction peaks that match up with Bi₂WO₆ standard card (JCPDS No.: 39-0256). The peaks appear to be sharp, indicating that the primary compound of the samples is highly crystalline Bi₂WO₆. Fig. 1 also shows that the phase of WO₃·0.33H₂O (JCPDS No.: 35-0270) is present in the nanomaterials synthesized in strong acid environments including 1.5 M, 1.25 M, and 1.0 M HNO₃ solutions. The presence of WO₃·0.33H₂O is consistent with the results of SEM analysis in the following section. In such cases, the H⁺ ion concentrations in the precursor were so high that WO₄⁻ ions combined with H⁺ to produce insoluble precipitation of H₂WO₄. H₂WO₄ then dehydrated during the hydrothermal process to generate WO₃·0.33H₂O. However, it was difficult to observe the peaks of WO₃·0.33H₂O in the XRD spectrum of the 0.5MHNO samples. The samples of 0.5MHNO, 0.97BWO, 2.99BWO, and 4.99BWO showed only characteristic peaks of Bi₂WO₆, and the intensity of the strongest (131) peak at 28.299° changed significantly from about 1572 to about 10 631 as the acidity decreased. This finding might be attributed to the increase in crystallite size and enhanced crystallinity perfection.²⁸

Fig. 1 XRD patterns of the catalyst synthesized in acidic environment.

Fig. 2 XRD patterns of the catalyst synthesized in alkaline environments.

Fig. 2 shows the XRD patterns of catalysts synthesized in neutral and alkaline environments. For pH values ranging from 7.01 to 11.05, the XRD spectrums showed the phase of Bi₂WO₆ only. When the pH value increased to 12.01, however, a new crystal phase of Bi_3.84W_0.16O_6.24 (JCPDS No.: 43-0447), besides Bi₂WO₆, appeared in the prepared nanomaterials. Moreover, the intensity of the diffraction peak of Bi_3.84W_0.16O_6.24 was stronger than that of Bi₂WO₆. This may be attributed to the solubility of [WO₄]²⁻ in the precursor suspension, which is much greater than that of [Bi₂O₂]²⁺ in a strong alkaline environment. Therefore, [Bi₂O₂]²⁺ can precipitate rapidly, forming the new phase of Bi_3.84W_0.16O_6.24.^28,37 As the pH value reached 12.98, however, the phase of Bi₂WO₆ disappeared and only Bi_3.84W_0.16O_6.24 existed. In this paper, Bi_3.84W_0.16O_6.24 is regarded as another crystalline phase of Bi₂WO₆ rather than impurities because of its strong stability and catalytic ability. Synthesized Bi_3.84W_0.16O_6.24 could be converted to α-Bi₂O₃ (JCPDS No.: 76-1730) when the precursor was further conditioned by 0.5 M NaOH solution. The formation of α-Bi₂O₃ resulted from the high concentration of OH⁻ in the precursor, leading to the production of Bi(OH)₃. Bi(OH)₃ further transformed into α-Bi₂O₃ under hydrothermal reaction conditions.³⁸ Consequently, Bi³⁺ ions were unable to participate in the synthesis of Bi_3.84W_0.16O_6.24 and Bi₂WO₆ with WO₄²⁻.

According to the XRD patterns, WO₃·0.33H₂O, Bi_3.84W_0.16O_6.24, and α-Bi₂O₃ were observed sequentially with increasing precursor alkalinity in addition to Bi₂WO₆. The results not only demonstrated the successful synthesis of Bi₂WO₆, but also strongly proved that different substances could be produced under different acid–base conditions.

3.2 Optical properties of catalysts

The UV-Vis DRS was used to determine the photo-response range of nanomaterials synthesized under different acid and alkali conditions. It can be seen from Fig. 3 that all samples showed high photo-absorption at around 380 nm, and their absorption bands were approximately 450 nm. This indicates that all the synthesized samples are responsive to visible light regardless of the pH value of the precursor. In addition, the optical band gap of the as-prepared photocatalysts were calculated using Kubelka–Munk (KM) method,³⁹ and the results are included in the legend of Fig. 3.

Fig. 3 UV-Vis DRS spectra of photocatalysts synthesized in different acid and base conditions.

Fig. 3 shows that the band gaps of different catalysts, including 0.97BWO, 2.99BWO, 4.99BWO, 7.01BWO, 9.01BWO, 10.00BWO and 11.05BWO, varied slightly between 2.76–2.80 eV. This range is consistent with those in other studies on Bi₂WO₆.^40,41 The absorption edges of 1.5MHNO, 1MHNO and 0.5MHNO are 446 nm, 451 nm and 454 nm corresponding to band gaps of 2.78 eV, 2.75 eV and 2.73 eV, respectively. The band gap of 12.98BWO (which is Bi_3.84W_0.16O_6.24) is 2.82 eV, corresponding to previous report as well.⁴² The results in this section indicate that all samples have visible light response capabilities and that the band gap lies between 2.73 eV and 2.82 eV.

3.3 Morphology and structure of prepared nanomaterials

Morphologies of the samples were characterized by FE-SEM and TEM techniques. It can be seen from Fig. 4a that the 1.5MHNO sample appears like a flower with dewdrops. Specifically, the flower-like base is made up with large nanosheets, which are decorated by small “nanocrumbs”. The large nanosheets are composed of Bi₂WO₆ and the nanocrumbs are WO₃·0.33H₂O. The presence of these two substances is further confirmed with the XRD pattern of 1.5MHNO. As the acidity decreases, regular large nanosheets disappear, and are replaced by irregular flocs consisting of small nanosheets for precursor prepared in 0.5 M HNO₃ solution (Fig. 4b). Fig. 4c shows the morphology of Bi₂WO₆ prepared at pH 0.97. Numerous small nanosheets accumulate to form large wheat spike-like shapes (see the inset on the top right). Further accumulation of these spike-like structures resulted in 3D spheres, the diameters of which are about 4–7 μm.

Fig. 4 SEM images of samples synthesized at different acid and alkali environments: (a) 1.5MHNO; (b) 0.5MHNO; (c) 0.97BWO; (d) 4.99BWO; (e) 7.01BWO; (f) 9.01BWO; (g) 11.05BWO; (h) 12.01BWO; (i) 12.98BWO; (j) 0.5MNAOH.

The morphologies of the synthesized catalysts are similar to each other when the pH values of the precursors fall into the range of 4.99–7.01 (Fig. 4d and e). Both appear to be building blocks, formed by interspersion between small rectangular pieces. The side lengths of these blocks are approximately 0.6–1.0 μm. The lower the acidity is, the thicker the blocks are.

Fig. 4f and g illustrate that the shapes of samples 9.01BWO and 11.05BWO are like square bricks. These square bricks of Bi₂WO₆ are randomly plied up without obvious aggregation. As the pH value reaches 12.01, a new crystal phase of Bi_3.84W_0.16O_6.24 emerges. Two different morphologies can be clearly observed in Fig. 4h. The sheet shape is for the Bi₂WO₆ and the regular octahedron shape is for Bi_3.84W_0.16O_6.24. It also shows that the Bi_3.84W_0.16O_6.24 crystal is the dominate phase. This finding agrees with the XRD patterns of 12.01BWO samples presented in Fig. 2, which show two crystal phases.

Fig. 4i shows the morphology of 12.98BWO nanomaterials. It does not show the structure of the Bi₂WO₆ sheet, but regular octahedral crystals of Bi_3.84W_0.16O_6.24 and some irregular small crystals. According to the FTIR results in the following section and those in earlier studies,^31,38 these irregular crystals are likely Bi₂O₃. Finally, the synthesized crystals become irregular Bi₂O₃ spheres and rods with various sizes when the precursor is conditioned using 0.5 M NaOH solution (Fig. 4j). The transformation arises from the fact that a large amount of OH⁻ ions in the precursor directly combines with Bi³⁺ ions to form Bi(OH)₃, which is then converted into Bi₂O₃ after dehydration during hydrothermal process.⁴³

Fig. 5 presents the TEM images of 1.5MHNO and 12.01BWO samples. The overall TEM image of 1.5MHNO in Fig. 5a shows large nanosheets decorated by small “nanocrumbs”. The section inside the circle was further analyzed using HR-TEM (Fig. 5b), which reveals the lattice interplanar spacing (0.31 nm) of the large nanosheets corresponding to the (131) plane of Bi₂WO₆. The nanocrumbs sprinkled onto the large nanosheet have interplanar distances of 0.32 nm and 0.37 nm, corresponding to the crystal lattice spacing of the (220) and (200) planes of WO₃·0.33H₂O, respectively. These findings are consistent with the SEM image shown in Fig. 4a. Fig. 5c shows the TEM image of 12.01BWO. Both granular and flaky crystals can be clearly seen. These SEM and TEM images clearly show the change of morphology and transformation of dominate compounds with the variation from strong acid to mild base circumstances.

Fig. 5 TEM and HRTEM of samples: (a) TEM and (b) HRTEM of 1.5MHNO; (c) TEM of 12.01BWO.

3.4 XPS analyses

Fig. 6 shows the percentage and chemical valence states of surface elements determined by XPS. The high-resolution XPS spectra of O 1s are shown in Fig. 6a. Through Gaussian distribution, the O 1s peaks of samples could be deconvoluted into three peaks at 529.19–529.87 eV, 529.91–530.46 eV and 531.10–532.34 eV. They are assigned to lattice oxygen, hydroxyl groups and H₂O molecules adsorbed on the samples surface.¹¹ Generally, a high ratio of the peak area of hydroxyl groups to that of lattice oxygen indicates a high concentration of oxygen vacancies.⁴⁴ According to this rule, it can be speculated that the concentration of oxygen vacancy depends upon the acidity of precursor environment. The 7.01BWO samples have the highest concentration of oxygen vacancy, while the 0.5MNAOH samples have the smallest one.

Fig. 6 High-resolution XPS spectrums of (a) O 1s, (b) Bi 4f and (c) W 4f of samples; (d) atomic percentage of surface elements on samples.

As shown in Fig. 6b, the spectrum of Bi 4f had two peaks located at around 158.74 eV and 164.05 eV, which correspond to Bi 4f_7/2 and 4f_5/2, respectively. Moreover, the difference between the two binding energies is about 5.31 eV, indicating the existence of Bi³⁺ in all samples.¹⁰ In Fig. 6c, the peaks around 34.96 and 37.10 eV are fitted into W 4f_7/2 and 4f_5/2, respectively. The peak distance of 2.14 eV indicates that W in these samples are mainly presented as W⁶⁺.⁴⁵ Moreover, the absence of W peaks in the 0.5MNAOH sample confirms that 0.5MNAOH samples are Bi₂O₃.

Fig. 6d shows the XPS atomic percentage of surface elements on different samples. The content of O element changes insignificantly with the variation in precursor acidity. When the acidity gradually increases, the W element gradually increases, whereas the Bi element gradually decreases. This finding is associated with gradual transformation of the main substances from Bi₂O₃ into Bi_3.84W_0.16O_6.24, into Bi₂WO₆, and into Bi₂WO₆/WO₃·0.33H₂O with increasing acidity.

3.5 FTIR analyses

Fig. 7 shows the FTIR spectra of the as-prepared samples. The absorption peaks mainly range from 400 to 1000 cm⁻¹. Specifically, the adsorption band at 441 cm⁻¹ and 578 cm⁻¹ are owing to the bending vibrations of Bi–O bond; the strong peaks at 727 cm⁻¹ and 820 cm⁻¹ are attributed to the W–O and W–O–W bond of Bi₂WO₆, respectively.^16,46 These peaks prove the successful synthesis of Bi₂WO₆ nanomaterials.

Fig. 7 The FTIR spectra of photocatalysts synthesized in different acid and base conditions.

The minor peaks at 1001 cm⁻¹ and 1608 cm⁻¹ of 1.5MHNO's FTIR spectrum are assigned to the W=O valence vibrations and δ(O–H) stretching vibrations of coordinated water, respectively. They are likely attributed to the crystal phase of WO₃·0.33H₂O.⁴⁷ Additionally, the weak peak at 1634 cm⁻¹ corresponds to the O–H bond of the adsorbed water molecules.⁴⁸ The peaks at 500 cm⁻¹ and 787 cm⁻¹ are likely originated from the vibrations of Bi–O and W–O bonds of Bi_3.84W_0.16O_6.24. Unlike the characteristic peaks in Bi₂WO₆, the intensity of the Bi–O peak is greater than that of W–O. This may result from the increase of Bi–O bond in Bi_3.84W_0.16O_6.24.

It is worth noting that the small peak at 1384 cm⁻¹ corresponds to the vibration of N–O bond.^46,48 It was ascribed to the hydrolysis products of Bi(NO₃)₃·5H₂O, such as BiONO₃ and Bi₂O₂(OH)NO₃.^31,49 This conclusion can also be verified by comparing the FTIR spectra of 0.97BWO before and after the acid treatment. After acid soaking, most of the impurities (BiONO₃ and Bi₂O₂(OH)NO₃) in 0.97BWO was dissolved, which corresponds to a sudden decrease in the peak at 1384 cm⁻¹ while others remained almost unchanged. This peak becomes weaker with the increasing alkalinity, and it disappears at pH 9.01. With the escalation of OH⁻ concentration in the precursor, BiONO₃ and Bi₂O₂(OH)NO₃ gradually transform into Bi(OH)₃, which could be further converted to Bi₂O₃ via hydrothermal reaction. Furthermore, the tiny peak at 1384 cm⁻¹ of 1.5MHNO is probably due to the hydrolysis of minor residual of Bi³⁺ ions, which forms BiONO₃ precipitates on nanocatalyst surface during washing with ultrapure water.

The nanomaterials synthesized at pH value greater than 7.01 has no obvious peaks between 3400 cm⁻¹ and 3800 cm⁻¹, which are the characteristic peaks of O–H.^50–52 It is believed that most Bi(OH)₃ was converted to Bi₂O₃, which is the main impurity at pH ≥ 9.01.

3.6 Stability analysis

The stability of the prepared Bi₂WO₆ is evaluated in terms of crystal structure, morphology, and chemical composition. Fig. 8a shows the XRD patterns of original 0.97BWO and 0.97BWO before and after treatment with acid or base solutions, which shows that the three XRD patterns are barely affected by the treatment. They are almost the same, and can match the Bi₂WO₆ standard card (JCPDS No.: 39-0256) well. It indicates that the crystal structure of as-prepared Bi₂WO₆ is not damaged during the acid or base treatment.

Fig. 8 (a) XRD of 0.97BWO and 0.97BWO after acid or alkali treatment; (b) XRD patterns and morphologies of 1.5MHNO before and after boiling alkali solution treatment; (c) XRD patterns of 12.98BWO before and after acid treatment; (d) mass of the Bi and W elements dissolved in acid or base solution, respectively, and the morphologies of 0.97BWO after acid or base treatment.

On the contrary, Fig. 8b clearly demonstrates that XRD pattern of 1.5MHNO changes after alkali treatment. The disappearance of the characteristic peaks of WO₃·0.33H₂O impurity indicates that WO₃·0.33H₂O in 1.5MHNO can be dissolved in a boiling alkaline solution, but Bi₂WO₆ is stable. The disappearance of the “nanocrumbs” scattered on the flower-like Bi₂WO₆ in the inset SEM images supports this conclusion. For Bi_3.84W_0.16O_6.24 (Fig. 8c), XRD spectrum remains the same before and after acid leaching, indicating that Bi_3.84W_0.16O_6.24 is relatively stable.

The chemical stability of Bi₂WO₆ is verified by ICP-MS data. Fig. 8d shows the variation of Bi and W elements leached from 1 g of 0.97BWO sample over 7 days. As soaking time increases, the mass of dissolved Bi element in the HNO₃ solution at pH 0.5 first increased and then leveled off. Similar trend is observed for the change of W element dissolved in 0.5 M NaOH solution. The constant quantity of the leaching elements after 3 days strongly proves the chemical stability of Bi₂WO₆. Meanwhile, undamaged microspheres of 0.97BWO in the inset SEM images of Fig. 8d indicates morphological stability.

In summary, Bi₂WO₆ and Bi_3.84W_0.16O_6.24 are stable enough to resist dissolution during acid and base treatments. Therefore, the Bi and W elements measured by ICP-MS in the following leaching experiment are believed to be derived from soluble impurities rather than from Bi₂WO₆ and Bi_3.84W_0.16O_6.24.

3.7 Determination of impurities

The impurities in Bi₂WO₆ prepared under a wide range of acid–base conditions were qualitatively analyzed according to the characterization results and hydrolysis properties of the raw materials. The mass percentage of impurities was calculated using ICP-MS results, and the accuracy was verified by several weight difference experiments.

3.7.1 Hydrolysis properties. The knowledge on the hydrolysis reactions of Bi³⁺ and WO₄²⁻ ions is critical to the analysis of possible impurities in the prepared nanomaterials. Fig. 9 shows the transmittances of Bi(NO₃)₃ solution and Na₂WO₄ solution at different pH values. According to Fig. 9a, it is observed that the critical pH is 0.67 for the hydrolysis of Bi³⁺ ion. When the pH value is less than 0.67, Bi³⁺ ion is present in a free state; otherwise, Bi³⁺ ion is subject to hydrolysis to form such precipitates as BiONO₃ and Bi₂O₂(OH)NO₃.³¹ Fig. 9b shows that the critical pH value of hydrolysis of WO₄²⁻ is 1.13. When pH is greater than 1.13, WO₄²⁻ ion is at a free state in solution; otherwise, WO₄²⁻ begins to hydrolyze and forms H₂WO₄ precipitate.⁵³ In summary, there is not a pH value at which the precursor of the hydrothermal reaction is a clear solution at room temperature. And both ions are hydrolyzed at pH value between 0.67–1.13.

Fig. 9 Transmittance of (a) Bi(NO₃)₃ solution and (b) Na₂WO₄ solution at different pH values.

It is believed that the impurities in the Bi₂WO₆ nanomaterials prepared using hydrothermal reaction are primarily hydrolysis products and derivatives of Bi³⁺ and WO₄²⁻ ions. The primary impurities include WO₃·0.33H₂O, BiONO₃, Bi₂O₂(OH)NO₃, and Bi₂O₃. Eqn (1)–(6) describe the formation of such impurities. When the pH of the precursor is lower than 1.13, WO₄²⁻ ions will combine with H⁺ ions to form H₂WO₄, which is transformed to WO₃·0.33H₂O in the subsequent hydrothermal reaction (eqn (1) and (2)). Bi³⁺ ions are subject to hydrolysis to form BiONO₃ and Bi₂O₂(OH)NO₃ precipitates when the pH is greater than 0.67 (eqn (3) and (4)). Since the degree of hydrolysis in the second step (eqn (4)) is much lower than that in the first step (eqn (3)), the hydrolysis products of Bi³⁺ ions are mainly BiONO₃ instead of Bi₂O₂(OH)NO₃. In a relatively strong base solution, impurities could progressively turn to Bi₂O₃, which is transformed from Bi(OH)₃ (eqn (5) and (6)).

WO₄²⁻ + 2H⁺ → H₂WO₄ (1)

H₂WO₄ → WO₃·0.33H₂O + 0.67H₂O (2)

Bi(NO₃)₃ + H₂O ↔ BiONO₃ + 2HNO₃ (3)

2BiONO₃ + H₂O ↔ Bi₂O₂(OH)NO₃ + HNO₃ (4)

Bi(NO₃)₃ + 3OH⁻ → Bi(OH)₃ + 3NO₃⁻ (5)

2Bi(OH)₃ → Bi₂O₃ + 3H₂O (6)

3.7.2 Qualitative analysis. The XRD patterns in Fig. 1 show that WO₃·0.33H₂O is the primary impurity in 1.5MHNO, 1.25MHNO, 1.00MHNO samples. Although the XRD pattern of 0.5MHNO does not show the phase of WO₃·0.33H₂O, the WO₄²⁻ ions were observed using ICP-MS after the 0.5MHNO sample was treated in boiling NaOH solution. It indicates the existence of WO₃·0.33H₂O in the 0.5MHNO sample. The presence of BiONO₃ is unlikely, because Bi³⁺ ions do not hydrolyze in the pH value lower than 0.67 according to Fig. 9.

Fig. 9 indicates both Bi³⁺ and WO₄²⁻ ions can be hydrolyzed at pH 0.97. Their major hydrolysis products, WO₃·0.33H₂O and BiONO₃, could appear in the catalysts. The presence of WO₃·0.33H₂O is proven by ICP-MS measurements. The peak at 1384 cm⁻¹ in the FTIR spectra confirms the presence of BiONO₃.

For the pH values between 2.99 and 7.01, the hydrolysis of WO₄²⁻ ions becomes negligible. BiONO₃, which is the predominant product of Bi³⁺ hydrolysis, constitutes to the impurity in the samples. The FTIR spectra of samples prepared within this pH range have a clear peak at 1384 cm⁻¹, indicating the existence of BiONO₃. As indicated by the FTIR results the hydrolysate BiONO₃ disappears when synthesizing catalyst under alkaline (pH ≥ 9.01) conditions. The presence of Bi(OH)₃ seems unlikely because there is not obvious vibration characteristic peak of O–H. Earlier studies indicate that impurities are very likely Bi₂O₃,^38,54–56 which could be converted from Bi(OH)₃ through hydrothermal reactions.

The Bi³⁺ ions were identified by ICP-MS after the 12.98BWO sample was soaked in acid solution, indicating the presence of acid soluble impurities. The aforementioned FTIR analysis proves that the acid-soluble impurity in 12.98BWO is likely Bi₂O₃. It is believed that the impurity is δ-Bi₂O₃ (ref. 57) because the positions of strong diffraction peaks of δ-Bi₂O₃ in XRD pattern (JCPDS No.: 27-0052 or JCPDS No.: 52-1007) are almost overlapping with those of the primary crystal phase of Bi_3.84W_0.16O_6.24.

Fig. 10 shows a concise diagram based on the preceding analyses presented to describe the transformation of samples and their impurities with the change of acid and alkali environments. Both the main compounds and the impurities in the samples undergo transformation. The main compounds begin to convert from Bi₂WO₆ to Bi_3.84W_0.16O_6.24 at precursor pH of around 12. Further increase in alkalinity leads to the production of Bi₂O₃. The impurities then gradually shift from WO₃·0.33H₂O to BiONO₃ around pH 1, and to Bi₂O₃ at near pH 7.

Fig. 10 Diagram of the transformation of the samples and their impurities with the change of acid and alkali environment.

3.7.3 Quantitative analysis. Summarizes the mass ratio of corresponding impurities quantified using ICP-MS technique. The impurity of WO₃·0.33H₂O in the synthesized catalysts decreased from 41.09% to 1.99% as the acidity weakens from 1.5 M HNO₃ to pH 0.97. It is well known that high acidity favors the hydrolysis of WO₄²⁻ in the precursor. The higher the degree of WO₄²⁻ hydrolysis, the greater the content of WO₃·0.33H₂O. When the pH value of the precursor solution is between 0.97 and 10.00, however, the content of impurity, which contains Bi³⁺ ions, in the synthesized catalyst does not change significantly. When the pH value increases to 11.05, the impurity content starts to increase exponentially, and all the hydrothermal reaction product was converted to Bi₂O₃ when the precursor prepared using 0.5 M NaOH solution. Sample 0.5MNAOH can be completely dissolved into the soaking solution of pH 0.5 HNO₃; therefore, it is deemed reasonable to conclude that no Bi₂WO₆ was produced in the sample. It is worth noting that the 0.97BWO sample contains both WO₃·0.33H₂O and BiONO₃, which confirms our previous assumption.

Weight difference experiments were conducted before and after purification to validate our identification analysis and the newly developed quantification method using ICP-MS. The results are tabulated in the last column of Table 1. ICP-MS provides similar results to the weight difference experiments. Most discrepancies are within 3.5%, except for the 12.98BWO samples, which is 11%. This indicates the analysis of impure compounds and quantitative determination of impurities using ICP-MS method are reliable.

Table 1 The mass fraction of impurities in various samples

Samples	Impurities	Mass weight (%) ICP-MS	Mass weight (%) weight difference experiment
1.5MHNO	WO3·0.33H2O	41.09	39.75
1.25MHNO	WO3·0.33H2O	14.27	—
1MHNO	WO3·0.33H2O	9.90	—
0.5MHNO	WO3·0.33H2O	5.99	—
0.97BWO	WO3·0.33H2O/BiONO3	1.99/3.58	—/3.63
2.99BWO	BiONO3	3.52	—
4.99BWO	BiONO3	2.75	—
7.01BWO	BiONO3	2.80	—
9.01BWO	Bi2O3	1.50	—
10.00BWO	Bi2O3	2.03	—
11.05BWO	Bi2O3	4.90	—
12.98BWO	Bi2O3	18.88	21.26
0.5MNAOH	Bi2O3	102.90	100.00

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4. Conclusions

In summary, Bi₂WO₆ nanomaterials synthesized by hydrothermal methods in different acid–base environments all contain impurities. The pathway of impurities conversion with continuously changing acidity or alkalinity was determined. Strong acidity can lead to the formation of WO₃·0.33H₂O as impurities. Impurities such as BiONO₃, Bi₂O₂(OH)NO₃ and Bi₂O₃ appeared as the alkalinity increases. The synthesized catalysts had a relatively high purity when the pH of precursor is between 1 and 10. In addition, the as-prepared Bi₂WO₆ and Bi_3.84W_0.16O_6.24 nanomaterials were stable in both acid and base environments. Furthermore, this study provides a simple and feasible method to quantitatively analyze the impurity of Bi₂WO₆, which was verified by weight difference experiments. This work is expected to shed light onto Bi₂WO₆-related studies and investigations using Bi(NO₃)₃·5H₂O and/or Na₂WO₄·2H₂O as raw materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by “the Fundamental Research Funds for the Central Universities” (No.: 2019XKQYMS59). The authors would like to acknowledge the technical support from Advanced Analysis and Computation Center within China University of Mining and Technology.

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