Photocatalytic degradation of two different types of dyes by synthesized La/Bi₂WO₆

Abstract

A lanthanum doped Bi₂WO₆ composite (La/Bi₂WO₆) was prepared using a simple hydrothermal method. The synthesized samples were well characterized using XRD, SEM, TEM, UV-vis DRS, XPS, and BET analysis methods. The photocatalytic activities of La/Bi₂WO₆ on reactive brilliant red X-3B (X-3B) and rhodamine B (RhB), under simulated solar irradiation, were systematically investigated. As a result, the doped Bi₂WO₆ samples possessed a higher specific surface area than pure Bi₂WO₆, but still retained the crystalline phases, morphology and visible light absorption properties of pure Bi₂WO₆. The initial concentrations of dyes, the dosage of catalyst and H₂O₂ all displayed the same effects on the degradation of the two dyes. However, pH showed different effects on X-3B and RhB because X-3B is an anionic dye while RhB is a cationic dye. Moreover, among all the inorganic ions studied (Na⁺, K⁺, Ca²⁺, Mg²⁺, NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻), the addition of Cl⁻ significantly affected the adsorption and degradation of the two dyes; Cl⁻ inhibited the total removal efficiency of X-3B, but enhanced the removal efficiency of RhB. The effects of inorganic cations were attributed to the effect of Cl⁻ because the four metal ions were used as their chloride salts.

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1. Introduction

As one of the most promising technologies used to degrade organic pollutants in water, the photocatalytic oxidation process has received intense attention in many fields and has been researched widely.^1–5 Undoubtedly, among the various semiconductor photocatalytic materials, titanium dioxide (TiO₂) is a typically used photocatalyst due to its good chemical and biological stability, low cost, ease of availability and significant photocatalytic activity under ultraviolet irradiation.⁶ However, the main shortcoming of TiO₂ is that it only absorbs ultraviolet light at wavelengths no longer than 387.5 nm, which only accounts for about 4% of sunlight.^7–9 Therefore, it is of great interest to develop visible-light-driven photocatalysts and make full use of solar energy. One important way is to extend the photoresponse of TiO₂ into visible region, which has already been widely studied.^5,10,11 Another important way is to exploit new photocatalysts, which is more anticipated and pursued.^12–14

Bismuth-based photocatalysts have been investigated in recent years due to their visible-light-activity.^15–17 Bi₂WO₆, belongs to the aurivillius family of layered perovskites, and is reported to be capable of degrading dyes and endocrine disrupting chemicals, under irradiation with visible light or simulated solar light with a band gap of 2.7–2.8 eV.^18–20 Bi₂WO₆ has potential been a promising visible-light-driven photocatalyst. However, its application remains limited because of its high electron–hole recombination rate during photocatalytic process. To resolve this problem, one of the important methods is the adulteration of Bi₂WO₆ with metal or non-metal elements to increase the migration efficiency of the photogenerated electrons and decrease the recombination rate of electron–hole pairs. For instance, doping with B can affect the pore structure and volume with 0.5% B/Bi₂WO₆ displaying more mesopores with a higher total pore volume than pure Bi₂WO₆.¹² The Ag@AgCl/Bi₂WO₆ composite presents excellent photocatalytic activity due to the synergetic effect of the metal with Bi₂WO₆.^21,22 In recent years, doping photocatalysts with rare earth (RE) metals has proven to be an efficient method to improve the photocatalytic properties of photocatalysts because the f-orbitals of the lanthanide ions can form complexes with various Lewis bases and thus, concentrate the substrates onto the catalyst surface.^23–25 It has been reported that Bi₂WO₆ doped with rare earth metals can also enhance the photocatalytic activity when compared to an undoped material.^26–30

In this study, La/Bi₂WO₆ was prepared via a hydrothermal method. The structure, optical properties, specific surface area and morphology of La/Bi₂WO₆ were characterized using BET, XRD, XPS, UV-vis DRS, SEM and TEM methods. The photocatalytic properties of La/Bi₂WO₆ were investigated by employing the photodegradation of brilliant red X-3B (X-3B) and rhodamine B (RhB) in an aqueous phase as model pollutants. To the best of our knowledge, investigation of two different classes of dyes (anionic and cationic) was carried out for the first time. Most papers on the photodegradation of dyes have focused only on one dye or one class of dyes. Moreover, the amount of doped lanthanum, the initial concentration of dyes, dosage of catalysts, pH of the reaction solution, H₂O₂ and common inorganic ions (NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺) were also used to study their effect on photodegradation of the two dyes.

2. Experimental

2.1. Material and reagents

The analytical reagents Bi(NO₃)₃·5H₂O, Na₂WO₄·2H₂O, La(NO₃)₃·6H₂O, reactive red X-3B and rhodamine B were purchased from Sinopharm Chemical Reagent Co., Ltd., China (Fig. S1† gives the chemical structure of the two dyes). Cations of Na⁺, K⁺, Ca²⁺ and Mg²⁺ were used in their chloride salts and anions of NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻ were used as their sodium salts. All these salts, NaOH, HNO₃ and H₂O₂ (30 wt%) were of analytical grade and purchased from Chemical Technology Co., Ltd., Tianjin, China. All the reagents mentioned above were used as received.

2.2. Preparation and characterization of Bi₂WO₆ and La/Bi₂WO₆

2.2.1. Preparation of Bi₂WO₆ and La/Bi₂WO₆. The Bi₂WO₆ catalyst was prepared via a hydrothermal method using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O. The detailed preparation process refers to our previous study.¹⁹ The synthesis of La/Bi₂WO₆ is similar to Bi₂WO₆: a certain amount of La(NO₃)₃·6H₂O was dissolved in 20 mL of a solution of 0.1 mol L⁻¹ Bi(NO₃)₃·5H₂O, which was defined as solution A. 20 mL of a 0.05 mol L⁻¹ Na₂WO₄·2H₂O was added to solution A, which was stirred for 10 min and ultrasonicated for 20 min. The obtained solution was labeled as solution B. Dilute NaOH and HNO₃ solution was used to adjust the pH of solution B and then, the resulting solution B was transferred into a 50 mL Teflon-lined autoclave. The autoclave was sealed and heated in an oven at 140 °C for 20 h. After cooling it to room temperature, the solution was centrifuged at 3000 rpm. The precipitate was collected and washed several times with distilled water and absolute ethanol to remove any impurities before being dried at 120 °C for 4 h. The prepared materials with a molar percentage ratio of La³⁺ to Bi₂WO₆ were denoted as La/Bi₂WO (x), where x stands for the value of the molar percentage of La³⁺ to Bi₂WO₆.

2.2.2. Characterization. Crystallographic information of the prepared materials were characterized by X-ray diffraction (XRD, D8 Advance, Cu Kα radiation, λ = 1.5418 Å, Germany). The morphologies of the photocatalysts were characterized using a S-4800 type of field emission scanning electron microscope with energy dispersive spectrometer (EDS). The Brunauer–Emmett–Teller (BET) surface area measurements were performed on an automatic analyzer (ASAP2460, Micromeritics, USA). The nitrogen adsorption and desorption isotherms were measured at 77 K, after degassing the sample. The UV-vis diffused reflectance spectra (DRS) of the composites were measured on a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). The spectra were recorded in the range of 200–800 nm and the absorption spectra were referenced to BaSO₄. The surface chemical composition was determined on a Thermo ESCALAB 250XI X-ray photoelectron spectroscopy (XPS) using an Al Kα X-ray source.

2.3. Photocatalytic degradation of the two dyes

The photocatalytic degradation experiments were carried out in a photochemical reactor. Simulated sunlight irradiation was provided by a 500 W xenon lamp (Institute of Electric Light Source, Beijing, China), which was positioned in a cylindrical quartz cold trap. The system was cooled by circulating water and maintained at room temperature. Before irradiation, the suspension was magnetically stirred in the dark for 30 min or 60 min to ensure adsorption equilibrium of RhB or X-3B on the catalysts. Approximately 5 mL of the reaction solution was taken at given time intervals and centrifuged to remove the catalyst particles. The supernatant was analyzed on a Unico UV-2100 spectrophotometer. The dye concentration was measured at λ = 554 and 538 nm for RhB and X-3B, respectively. All the experiments were performed at least twice. The removal ratio (R) of the two dyes was determined as follows:

R = (1 − C/C₀) × 100% (1)

where C₀ is the initial concentration of dye and C is the concentration of dye at reaction time t (min).

Besides, there would be a vertical dotted line in the graph of R vs. time, which represents the boundary of dark and light irradiation.

3. Results and discussion

3.1. Characterization of the prepared catalysts

The typical 2θ values of Bi₂WO₆ and La/Bi₂WO₆ are consistent with those of russellite Bi₂WO₆ [JCPDS no. 39-0256] (its standard XRD pattern is shown at the bottom of Fig. 1). However, there are no diffraction peaks for another phase of La³⁺ due to the small dopant contents of the La/Bi₂WO₆ composites. The diffraction peak intensity of La/Bi₂WO₆ was slightly lower than that found for pure Bi₂WO₆. An increase in the amount of doped La³⁺, lowered the intensity of the peak. The crystallite size was calculated using the Scherrer equation (eqn (2)):³¹

L = Kλ/β cos θ (2)

where L, K, λ, β and θ are the crystallite size (nm), shape factor (0.89), wavelength of X-ray radiation source (CuKα = 0.15418 nm), full width at half maximum intensity (radians) and Bragg angle of diffraction, respectively. The calculated results are shown in Table 1 and the doped La³⁺ decreased the crystallite size of Bi₂WO₆.

Fig. 1 The XRD patterns obtained for Bi₂WO₆ and La/Bi₂WO₆.

Table 1 Parameters of the prepared catalysts

Samples	La% (experiment)	La% (EDS)	La% (XPS)	L (nm)	SBET (m^2 g^−1)
Bi2WO6	0	0	0	16.1	21.58
La/Bi2WO6 (0.01%)	0.01	0	0	15.8	22.20
La/Bi2WO6 (5%)	5	2.85	1.90	14.7	25.24

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It has been reported that lanthanide ions can enhance the surface area of a catalyst.^32,33 The results of this study are in accordance with those reported as shown in Table 1, which agree with the result that the particle size decreased with the three catalysts from Bi₂WO₆ to La/Bi₂WO₆ (0.01%) then to La/Bi₂WO₆ (5%). Fig. 2 shows the N₂ adsorption–desorption isotherms obtained for the catalysts. The isotherm was identified as type IV, which is characteristic of mesoporous materials.^34,35 The pore-size distribution obtained from the isotherms indicates a number of pores in the range of 10–50 nm (the inset of Fig. 2). Moreover, the higher the amount of doped lanthanide ions, the higher the pore volume of the 10–50 nm pores, which may be the reason why rare earth element lanthanide ions can enhance the surface area of catalysts.

Fig. 2 The nitrogen adsorption–desorption isotherms and pore size distribution of the catalysts.

The SEM and TEM images illustrate the morphology of the synthesized Bi₂WO₆ and La/Bi₂WO₆. As seen in Fig. 3 and 4, Bi₂WO₆ and La/Bi₂WO₆ are present as sphere-shaped crystal forms composed of layered nanoparticles. There is no possibility of sphere-shaped particles between the pure and doped catalysts, which are ca. 3 μm in diameter, but there was improved dispersion and homogeneity of the particles in the doped catalysts when compared to pure Bi₂WO₆. Nguyen-Phan showed that rare earth doped materials can inhibit the collapse of the structure³⁶ and the results of this study are in agreement with their results. The evidence for the penetration of La into the bismuth tungstate matrix or the existence in bismuth tungstate walls were confirmed by EDS as reported in Table 1. The atomic concentrations of lanthanum was 2.85% for La/Bi₂WO₆ (5%). Moreover, the XPS measurements also gave information on the amount of doped La.

Fig. 3 SEM images of Bi₂WO₆ (A–C), La/Bi₂WO₆ (0.01%) (D–F) and La/Bi₂WO₆ (5%) (G–I).

Fig. 4 TEM images of Bi₂WO₆ (A and B), La/Bi₂WO₆ (0.01%) (C and D) and La/Bi₂WO₆ (5%) (E and F).

It is well known that the electronic structural features of a semiconductor, affects the optical absorption and migration of the light-induced electrons and holes, which then determines its photocatalytic activity. Fig. 5 shows the UV-vis DRS of the catalysts of Bi₂WO₆ and La/Bi₂WO₆. Similar to pure B₂WO₆, the doped composites demonstrate high photo-absorption capacity in the range of UV light to visible light around 450 nm. The yellow color of the catalysts was in accordance with the absorption band edge at 450 nm. In addition, the UV-vis DRS spectra of the doped catalysts have a slight red-shift, suggesting their potential photocatalytic activity under visible light. To estimate the optical band gap, the plot of (αhν)² versus (hν) is shown in Fig. 5B.³⁷ The E_g values of La/Bi₂WO₆ are smaller than those found for Bi₂WO₆.

Fig. 5 The UV-vis DRS spectra of Bi₂WO₆ and La/Bi₂WO₆.

XPS analysis was used to investigate the chemical state and content of all the elements in the prepared materials. Fig. 6 shows the high resolution scanning XPS spectra of Bi 4f, W 4f, O 1s and La 3d at 158.9, 35.2, 530.1 and 834.6 eV, respectively. The XPS spectra of the samples were similar except for the peaks of La 3d. As shown in Fig. 6C, the La 3d XPS peak of La/Bi₂WO₆ (5%) at 834.6 eV was detected, whereas, the La 3d XPS peak of La/Bi₂WO₆ (0.01%) was not detected due to the low amount of doped La³⁺. The peak of 834.6 eV may indicate the existence of La₂O₃ in the La/Bi₂WO₆ sample.³⁸ After being doped with a rare earth metal, the W and Bi elements in La/Bi₂WO₆ display a slight shift, indicating a chemical interaction between Bi₂WO₆ and La₂O₃. However, the concentration of the rare earth metal oxides is too low to be detected by XRD analysis (Fig. 1). Besides, the XPS data indicated there was about 1.90% of La³⁺ doping in the La/Bi₂WO₆ (5%) composite. This number was different with the result reported by EDS (2.85%). This was thought to be reasonable because XPS and EDS are semi-quantitative analyses for low content elements.

Fig. 6 The XPS spectra of Bi₂WO₆ and La/Bi₂WO₆.

3.2. The photocatalytic degradation of RhB and X-3B using La/Bi₂WO₆

A set of X-3B and RhB degradation tests were carried out by varying the dopant content (from 0.005% to 20% of La³⁺) in La/Bi₂WO₆ to evaluate the photocatalytic activity of La/Bi₂WO₆ (Fig. 7). It was demonstrated that all the Bi₂WO₆ catalysts doped with La³⁺ increase the photodegradation percentage of dyes (in the case of pure Bi₂WO₆ there was 61% removal during X-3B degradation and 75% during RhB decomposition, whereas after La³⁺ doping 80% X-3B and 89% RhB degradation was observed). Moreover, it was observed that the Bi₂WO₆ catalyst containing 5% La³⁺ shows higher catalytic activity under solar radiation to X-3B and 0.01% La³⁺ to RhB, which would be the optimum dose of La³⁺ doping to form a nanocomposite of rare earth metal oxide and Bi₂WO₆. Another factor that can significantly influence the photocatalytic activity is the surface area of the catalysts. As mentioned above, the S_BET of La/Bi₂WO₆ was higher than that of pure Bi₂WO₆, which results in an improved adsorption ability to dyes when compared to pure Bi₂WO₆ as shown in Fig. 7. On the other hand, the rare earth metal doped composite catalysts display different activities to the different target pollutants. This may be due to X-3B being an anionic dye, whereas RhB is a cationic dye. Therefore, the surface area of the materials is an important aspect that affects the removal ability of a pollutant, but not the only one. The degree of crystallization and the types of contaminant are also important factors to be taken into account.

Fig. 7 The effect of the La³⁺ doped Bi₂WO₆ composite on the photodegradation and dark adsorption of X-3B and RhB.

The La-doped composite increased the removal efficiency of the two dyes when compared to pure Bi₂WO₆. The different doping amounts of lanthanum indicated the different degradation activities to the same dye. However, the same one doped catalyst also showed different degradation activities to the two different dyes. Moreover, the removal efficiency of total organic carbon (TOC) was up to 67.41% and 62.14% for X-3B and RhB, respectively at the optimum dose of La/Bi₂WO₆.

The recycling performance of the catalyst for X-3B and RhB was also investigated (Fig. S2†). The most difference in every run was the adsorption amount for the two dyes, whereas the photodegradation ability was almost the same during each run. The improved photocatalytic activity of La/Bi₂WO₆ to X-3B and RhB was mainly due to the enhanced S_BET of the doped materials. The results were similar to the conclusions from Liu's review article: (i) the quantum size effect and (ii) the unique textural properties (mesoporosity with larger BET surface areas and pore sizes).³⁹

3.3. The effect of different factors on the photodegradation of X-3B and RhB

With the exception of the different doping content of lanthanum discussed above, the pH of the photodegradation solution, dosage of catalyst, initial concentration of dyes, cocatalyst H₂O₂ and common inorganic ions in natural water, may be important factors that affect the photodegradation of dyes using La/Bi₂WO₆ (X-3B was photocatalyzed by La/Bi₂WO₆ (0.01%) and RhB by La/Bi₂WO₆ (5%)). The following sections depict the effects of these factors in detail.

3.3.1. The effects of pH. The photochemical degradation depends strongly on the initial pH of the reaction medium. Therefore, the effect of pH on the photocatalytic degradation of X-3B and RhB was studied at pH ranging from 3 to 11.

As shown in Fig. 8, the pH of photodegradation solution indicated significant effects on the different dyes. It is beneficial to perform the adsorption and removal of X-3B under basic conditions, whereas it is opposite for RhB owing to X-3B and RhB being different classes of dyes: anionic and cationic, respectively. However, many published papers on the effect of pH on the photodegradation of dyes support that lower pH values are propitious for the degradation of X-3B,^40–43 whereas higher pH values are favorable for the degradation of RhB.^34,44 Moreover, in this study, the concentration of X-3B or RhB over the photocatalyst depends on two factors: the adsorption and the photodegradation of the dyes, which is in accordance with Ma's report on RhB.⁴⁵ So, pH not only determines the chemical properties of the photocatalyst but also influences the adsorption behaviour of the pollutants. We propose that the difference between the results can be attributed to the different type of catalyst used and the different target pollutants chosen.

Fig. 8 The effects of the solution pH on the removal of dyes using La/Bi₂WO₆.

3.3.2. The effects of the initial concentrations of dyes. Under normal conditions, the different initial concentrations of the pollutant will result in different removal efficiency. As seen from Fig. 9A and B, it is clear that the degradation efficiency decreases upon increasing the concentration of X-3B and RhB from 5 to 30 mg L⁻¹. The pseudo first-order kinetic equation (eqn (3)) was used to calculated the reaction rate constant (k_ap) to explore the photocatalytic degradation of the two dyes using La/Bi₂WO₆.⁴⁶ The constant decreased with an increase in the concentration of the two dyes (Fig. 9C). The presumed reason is that a mass of irradiation light may be absorbed by the X-3B and RhB molecules in aqueous solution rather than the catalyst particles at high dye concentrations, which can reduce the efficiency of the catalytic reaction. Another possible reason is that the intermediate products formed upon the photocatalytic degradation of the dyes may compete with the dye molecules for the limited adsorption and catalytic sites on the surface of catalyst particles and thus, inhibit the degradation efficiency to a certain extent.⁴³

−ln(C/C₀) = k_apt (3)

where k_ap represents the apparent degradation rate constant, which was determined by plotting ln(C/C₀) versus reaction time t.

Fig. 9 The effects of the initial dye concentration on the removal of the dyes using La/Bi₂WO₆.

3.3.3. The effects of the dosage of catalyst. A series of 0.1, 0.5, 1.0, 1.5, 2.0 g L⁻¹ catalyst dosages were chosen to study the effects of dosage on the removal of the dyes using La/Bi₂WO₆. It can be seen from Fig. 10 that the removal rate increases with an increase in the La/Bi₂WO₆ dosage. However, it is interesting to find that the removal rate which was constant initially increased upon increasing the dosage of the catalyst and then decreased with a further increase in the catalyst dosage. The kickpoints were 1.0 g L⁻¹ and 1.5 g L⁻¹ for X-3B and RhB, respectively. When the catalyst was overdosed, an increase in the opacity of the suspension leads to a blockage of light penetration and thus, the shadowed catalyst particles become inert to generating radicals. In other words, the process was gradually transformed from an optically dilute system into an optically dense system because of the light deficiency as the dosage increased.⁴⁷

Fig. 10 The effects of dosage on the removal of the dyes using La/Bi₂WO₆.

3.3.4. The effects of H₂O₂. H₂O₂ is often taken as a cocatalyst due to its oxidizing properties and an ability to generate ˙OH under irradiation.^4,48,49 As seen from Fig. S3† and 11, under dark conditions H₂O₂ has no oxidation capacity to both dyes. Under irradiation with simulated solar light, the X-3B and RhB dyes were completely degraded by H₂O₂ + La/Bi₂WO₆ or H₂O₂ alone. The larger the H₂O₂ amount, the higher of removal efficiency of the dyes, which indicates the oxidation ability of H₂O₂. Under irradiation, H₂O₂ can provide ˙OH through photolysis as shown in eqn (4) ⁵⁰ and Fig. S4† demonstrates that H₂O₂ adsorbs UV light from 200–400 nm. Besides, as an electron scavenger, H₂O₂ can be oxidized to ˙OH with electrons as shown in eqn (5).⁵¹ So, the removal was accelerated with adequate H₂O₂.

H₂O₂ + e⁻ → ˙OH + HO⁻ (5)

Fig. 11 The effects of H₂O₂ on the removal of dyes using La/Bi₂WO₆.

However, the increase in the removal efficiency was not in a linear relationship with an increase in H₂O₂ irrespective of whether with H₂O₂ alone or H₂O₂ + La/Bi₂WO₆. Eqn (6) may explain this phenomenon, which means excessive ˙OH react with each other and H₂O₂ is produced again.⁵²

˙OH + ˙OH → H₂O₂ (6)

3.3.5. The effects of inorganic ions. NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ are naturally present in water. Besides, some inorganic ions belong to dye auxiliaries. For example, NaCl enhances the diffusion of the dye and its adsorption onto fibers and bicarbonate increases the dye bath pH.⁵³ Therefore, it is necessary to disclose the effect of these common inorganic ions on the dye's photodegradation and the results are shown in Fig. 12. As reported in many previous studies on anionic dyes,^53–57 Cl⁻ and SO₄²⁻ inhibited the removal of X-3B because of the adsorption on the surface of the catalyst and the weaker active radicals of Cl⁻˙ and SO₄²⁻˙. NO₃⁻ and HCO₃⁻ almost had no effect on the removal of X-3B. However, there exists a different trend for the effects of inorganic anions on the removal of RhB; NO₃⁻ and SO₄²⁻ show a negligible effect, Cl⁻ accelerates the removal of RhB and HCO₃⁻ inhibits adsorption while enhancing the degradation of RhB. As we know, NaHCO₃ makes the solution basic and this is why the removal efficiency was increased for X-3B due to improved adsorption and the contrary result was found for RhB.

Fig. 12 The effects of inorganic ions on the removal of dyes using La/Bi₂WO₆.

When compared with inorganic anions, the cations Na⁺, K⁺, Ca²⁺ and Mg²⁺ displayed more regular trends for X-3B and RhB. All the cations inhibit the removal of X-3B and promoted the removal of RhB as shown in Fig. 12C and D. In general, the four cations are all in the highest and stable oxidation state and cannot capture electrons or holes in solution. It is hypothesized that these metal ions do not show a significant impact on the photodegradation of dyes.⁵⁸ The results of this study can be attributed to the effect of Cl⁻ ions present in the solution. The metal ions used were as their chloride salts. As described above, Cl⁻ ions inhibit the removal of X-3B while enhancing the removal of RhB. CaCl₂ and MgCl₂ displayed higher promoting effects than NaCl and KCl at the same molar concentration of 0.1 mmol L⁻¹ used for the removal of RhB, which is expected since the concentration of Cl⁻ in the CaCl₂ and MgCl₂ solutions was twice of that in the NaCl and KCl solutions.

4. Conclusions

In summary, La/Bi₂WO₆ was successfully prepared using a hydrothermal method. When compared with pure Bi₂WO₆, La/Bi₂WO₆ preserved the crystal structure of russellite Bi₂WO₆ [JCPDS no. 39-0256] and showed improved dispersity and homogeneity of the sphere-shaped morphology. However, La/Bi₂WO₆ possessed a larger specific surface area and stronger optical absorption ability. The optimal doping amount of lanthanum to the photodegradation of X-3B and RhB was 0.01% and 5%, respectively, which demonstrated that the surface area was not the only key factor for the photocatalytic activity of the prepared materials. The initial concentration of dye and the dosage of catalyst all have an important effect on the photodegradation of dyes. Studies on the pH revealed different effects on the adsorption and photodegradation of X-3B and RhB using La/Bi₂WO₆. X-3B was more easily adsorbed and photodegradated by La/Bi₂WO₆ under basic conditions while RhB required acidic conditions. Besides, the common inorganic ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, NO₃⁻, HCO₃⁻, Cl⁻ and SO₄²⁻) present in water also had different effects on X-3B and RhB due to X-3B being an anionic dye while RhB is a cationic dye.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51408277, 21263005), Natural Science Foundation of Jiangxi Province (No. 20142BAB213019), China's Postdoctoral Science Fund (2015M582776XE, 2016T90967) and Program of Qingjiang Excellent Young Talents, JXUST.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17798k

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