Tuning the morphological structure, light absorption, and photocatalytic activity of Bi₂WO₆ and Bi₂WO₆-BiOCl through cerium doping

1 Introduction

As one of the simplest members of the Aurivillius oxide family, Bi₂WO₆ is composed of perovskite-like [WO₄]²⁻ layers sandwiched between bismuth oxide [Bi₂O₂]²⁺ layers, favoring the efficient separation of photogenerated electron–hole pairs and improving the photocatalytic activity because of the formed internal electric fields between the slabs (Zhang et al., 2014; Zhu et al., 2014). Recent studies have demonstrated that the photocatalytic performance of Bi₂WO₆ can be enhanced by compositing with other semiconductors, such as BiVO₄ (Ju et al., 2014), WO₃ (Peng et al., 2015), Fe₃O₄ (Xu et al., 2012), ZnWO₄ (Hojamberdiev et al., 2013), and CeVO₄ (Hojamberdiev et al., 2017a), BiOI (Hojamberdiev et al., 2017b), and reduced graphene oxide (Hojamberdiev et al., 2018).

Particularly, to improve its photocatalytic performance, Bi₂WO₆ was composited with BiOX (X = Cl, Br, and I), which are important V–VI–VII ternary semiconductor compounds that have a tetragonal matlockite structure, a layered structure characterized by [Bi₂O₂] slabs interleaved by double slabs of halogen atoms (Ye et al., 2014). For instance, a BiOCl–Bi₂WO₆ heterojunction with a chemically bonded interface was synthesized via a facile one-step solvothermal method and yielded a high photodegradation rate of rhodamine B under visible light irradiation due to the favorable band offsets across the Bi_I–O–Bi_II bonded interface, leading to efficient interfacial charge carrier transfer (Yang et al., 2013). In comparison with the BiOBr–Bi₂WO₆ nanosheet composite, the BiOBr–Bi₂WO₆ mesoporous nanosheet composite prepared by using a facile hydrothermal synthetic route with a controlled fraction of Ti(OⁱPr)₄ exhibited a much improved photocatalytic activity under visible light irradiation because of (i) the large specific surface area with hierarchical pore sizes and a highly exposed BiOBr (0 0 1) facet, (ii) the evidently enhanced charge separation efficiency, and (iii) high light harvesting efficiency (Li et al., 2013). The stable 30%Bi₂WO₆/BiOI heterojunction photocatalyst synthesized using a facile hydrothermal method based on the efficient chemical etching principle showed high visible-light-induced photocatalytic and antibacterial activities for the degradation of methylene blue and the killing of P. aeruginosa, E. coli, and S. aureus owing to the efficient separation of photogenerated electron–hole pairs on the formed p–n heterojunction (Xiang et al., 2016).

In recent years, doping with lanthanide ions, such as La³⁺ (Wang et al., 2016a), Sm³⁺/Sm²⁺ (Wang et al., 2016b), Eu³⁺ (Tian et al., 2012), Gd³⁺ (Tian et al., 2014), Ho³⁺ (Li et al., 2017a), Er³⁺ (Wang et al., 2015), and Yb³⁺ (Li et al., 2017a), has shown to be an efficient approach to improve the photocatalytic activity of Bi₂WO₆ because of their unique optical properties and 4f electron configuration.

As a common emerging water pollutant, salicylic acid (2-hydroxybenzoic acid) is widely used in the production of cosmetics, food, and pharmaceutical (a main precursor of aspirin) and dermatological products (Stamatis and Konstantinou, 2013). Due to its widespread use, salicylic acid has been consistently detected up to relatively high concentrations of 50 μg·L⁻¹ in surface water (Kosma et al., 2014) and 89.1 μg·L⁻¹ in the effluents of municipal wastewater treatment plants (Evgenidou et al., 2015). As an example, aspirin degrades in an aqueous medium into several toxic intermediates causing environmental pollution, which ultimately affects human health (Mukherjee et al., 2016; SCCNFP/0522/01, final, 2002). However, microbiological and/or conventional secondary physicochemical treatments (Sirés and Brillas, 2012) cannot completely remove salicylic acid. Therefore, effective advanced oxidation processes are needed to ensure the total elimination of salicylic acid in water.

Very recently, the immobilized photocatalytic structures of self-organized and well-aligned TiO₂ nanotubular films fabricated by galvanostatic anodization demonstrated a slightly higher efficiency for the degradation of salicylic acid compared with that of TiO₂-P25 at alkaline to acidic pH values due to the easy removal of the formed intermediates from the highly porous surface (Arfanis et al., 2017). Ma et al. (2017) synthesized magnetically separable F-N co-doped TiO₂ using a facile sol-gel method grafted on magnetically activated carbon decorated with carbonized chitosan and found that the degradation of salicylic acid under visible light irradiation was strongly dependent on the molar ratio of F:Ti. A 3.0 L solar flow plant with a Pt/air-diffusion (anode/cathode) cell, a solar photoreactor and a photocatalytic photoreactor filled with TiO₂-coated glass spheres was utilized to couple solar photoelectro-Fenton (SPEF) and solar heterogeneous photocatalysts (SPC) for the efficient mineralization of salicylic acid (Garza-Campos et al., 2016).

The present study aims at tuning the morphological structure, light absorption, and photocatalytic activity of Bi₂WO₆ and Bi₂WO₆-BiOCl through cerium doping. The adsorption and photocatalytic activities of pure and Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl powders were evaluated for the removal of salicylic acid, as an emerging water pollutant in the dark and under visible light irradiation. It was found that the cerium doping can effectively improve the photocatalytic activity, and the use of Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl powders can be extended to the degradation of other organic pollutants in water.

2 Experimental

3 Results and discussion

The crystal structures of the as-synthesized samples were characterized by powder X-ray diffraction. The XRD patterns of the samples doped with different contents of cerium ranging from 0 mol% to 10 mol% are shown in Fig. 1 with the International Center for Diffraction Data Powder Diffraction File (ICDD PDF) data. As shown in Fig. 1a, the diffraction lines in the XRD pattern of the sample hydrothermally synthesized using ethylene glycol as a solvent correspond to orthorhombic Bi₂WO₆ with a space group of Pca2₁ (ICDD PDF# 39–0256), confirming the phase purity of Bi₂WO₆. The diffraction lines in the XRD pattern of the sample hydrothermally synthesized using dilute HCl as a solvent are in good agreement with the orthorhombic phase of Bi₂WO₆ (ICDD PDF# 39–0256) and the tetragonal phase of BiOCl with a space group of P4/nmm (ICDD PDF# 06–0249), indicating the co-existence of the Bi₂WO₆ and BiOCl phases in the sample (Fig. 1b). Along with Bi₂WO₆, the BiOCl phase was also formed because of a rapid reaction of Bi³⁺ and the available Cl⁻ ions dissociated from HCl in the synthesis solution: Bi³⁺(aq) + Cl⁻(aq) + H₂O(l) → BiOCl(s) + 2H⁺(aq).

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Fig. 1

XRD patterns of (a) Bi₂WO₆ and (b) Bi₂WO₆-BiOCl powders doped with different contents of cerium ranging from 0 mol% to 10 mol%.

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Since the effective ionic radius of Ce³⁺ (r_ion = 1.01 Å for CN = 6) is very similar to that of Bi³⁺ (r_ion = 1.03 Å for CN = 6), there is no significant shift in the 2θ angle of the diffraction patterns, demonstrating that the Bi³⁺ ions were substitutionally replaced by the Ce³⁺ ions in the alternatively stacked [Bi₂O₂]²⁺ layers of the Bi₂WO₆ and BiOCl crystal structures. Notably, an increase in the Ce content up to 10 mol% neither segregated the Ce-rich phases nor changed the preferential orientations, implying that all doped cerium ions were fully accommodated into the Bi₂WO₆ and BiOCl lattices. However, an increase in the Ce content up to 4 mol% gradually decreased the content of the Bi₂WO₆ phase in the sample, while the BiOCl phase became more predominant due to the increase in the concentration of the Cl⁻ ions in the synthesis solution highly saturated with cerium trichloride. From 6 mol% to 10 mol% Ce, the Bi₂WO₆ phase completely vanished, leaving behind BiOCl as a single phase. The diffraction lines resulting from the {1 1 1} plane of the Bi₂WO₆ and BiOCl crystals are obviously stronger compared to other diffraction lines, confirming the dominance of the {1 1 1} plane in the synthesized crystals. It is thought that the Ce²⁺ and Ce⁴⁺ ions were unlikely formed under the current synthesis conditions applied in this study. The XRD results suggest that the selection of a solvent type (ethylene glycol or dilute HCl) and the content of cerium dopants are important for the synthesis of the phase-pure Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl powders.

The morphology and microstructure of the as-synthesized samples were examined by scanning electron microscopy, and the results are represented in Figs. 2 and 3. Fig. 2 shows the typical SEM images of the non-doped and Ce-doped Bi₂WO₆ powders. As depicted in Fig. 2a, non-doped Bi₂WO₆ powders exhibit hierarchical, not well-defined, structures with an average diameter of less than 3 μm. These hierarchical structures were assembled by a random aggregation of numerous nanosheets with a thickness of approximately 25 nm. Interestingly, with an increasing content of cerium dopant up to 6 mol%, these hierarchical structures gained their clear flower-like morphology constructed by parallelly and perpendicularly stacked nanosheets (Fig. 2b–f). A further increase in the content of cerium dopant to 8 mol% led to the formation of acicular crystals on the surface of these flower-like hierarchical structures (Fig. 2g), which were ultimately converted into the aggregated acicular nanocrystals at 10 mol% Ce (Fig. 2h). Apparently, the thickness of the Bi₂WO₆ nanosheets was gradually reduced with increasing content of cerium dopant. Here, ethylene glycol was used as a solvent as well as a complexing agent forming a chain structure on the surface of the crystallites via either covalent or hydrogen bonding. During the hydrothermal reaction, Bi₂WO₆ nuclei were formed first and served as seeds for the growth of Bi₂WO₆ nanosheets in the supersaturated solution. Due to its high intrinsic anisotropic nature, Bi₂WO₆ grew into two-dimensional nanosheets. Further, the nanosheets with a high anisotropic surface energy were self-assembled readily into flower-like hierarchical structures through an ethylene glycol-induced Ostwald ripening (Wang et al., 2014; Cui et al., 2016). For the non-doped Bi₂WO₆-BiOCl composite, irregularly shaped nanosheets with an average size of 350 nm, which did not form flower-like hierarchical structures, are shown in Fig. 3a. When the cerium was doped, the randomly distributed nanosheets of the non-doped composite were gradually transformed into mostly parallel intersected nanoplates with scalloped edges due to acid etching (Xu et al., 2018), resulting in the flower-like hierarchical structures with the size of approximately 3 μm (Fig. 3b–f), and the nanoplates became completely dominant in the range of 6–10 mol% Ce. Strikingly, at 8 and 10 mol% Ce, the distance between the stacked nanoplates was significantly shortened, and a greater number of enlarged nanoplates were parallelly stacked (compactly packed) to form spherical structures (Fig. 3g and h). On one hand, the morphological transformation can be interpreted by the gradual dominance of the BiOCl phase over the Bi₂WO₆ phase in the composite. On the other hand, as most cerium ions were substitutionally introduced into BiOCl and Bi₂WO₆ without a noticeable lattice distortion, the selective adsorption of excessive Cl⁻ ions on specific crystal facets possibly governed the crystal growth, leading to the morphological variation with increasing concentration of cerium trichloride in the synthesis solution (An et al., 2008). These flower-like hierarchical structures can absorb more photon energy due to multiple scattering, the photogenerated charge carriers can easily transfer to the surface/interface because of the nanoscale thickness of the nanosheets, and the mesopores of the synthesized samples can improve the transfer rate of the organic molecules, which contribute to the enhancement in the photocatalytic activity of the synthesized samples.

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Fig. 2

SEM images of Bi₂WO₆ powders doped with different contents of cerium: (a) 0 mol%, (b) 0.5 mol%, (c) 1 mol%, (d) 2 mol%, (e) 4 mol%, (f) 6 mol%, (g) 8 mol%, and (h) 10 mol%. The scale bar is 5 μm.

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Fig. 3

SEM images of Bi₂WO₆-BiOCl powders doped with different contents of cerium: (a) 0 mol%, (b) 0.5 mol%, (c) 1 mol%, (d) 2 mol%, (e) 4 mol%, (f) 6 mol%, (g) 8 mol%, and (h) 10 mol%. The scale bar is 1 μm.

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The surface chemical compositions and chemical states of the as-synthesized samples (non-doped and 2 mol% Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl) were investigated by X-ray photoelectron spectroscopy (XPS), and the Bi 4f, W 4f, O 1s, Cl 2p, and Ce 3d XPS core-level spectra of the samples are shown in Fig. 4. The XPS peaks centered at 164.6 and 159.3 eV can be assigned to the binding energies of Bi 4f_5/2 and Bi 4f_7/2, respectively, indicating that bismuth has an oxidation state of +3. The XPS peaks centered at 37.1 and 34.9 eV can be ascribed to the binding energies of W 4f_5/2 and W 4f_7/2, respectively, confirming that the tungsten has an oxidation state of +6. The O 1s binding energy of 530.8 eV can be attributed to the lattice oxygen in Bi₂WO₆ and BiOCl, and no other pronounced peaks characteristic of the chemisorbed or dissociated oxygen species are noted. In the XPS spectra of Cl 2p, the binding energies of Cl 2p_1/2 and Cl 2p_3/2 are observed at 199.4 and 197.8 eV, respectively, which is the characteristic of Cl⁻ in BiOCl. The Ce 3d XPS spectra can be deconvoluted into two pairs of spin-orbital bands: Ce³⁺ 3d_3/2 and Ce³⁺ 3d_5/2 at 906.7 and 888.6 eV, respectively and Ce⁴⁺ 3d_3/2 and Ce⁴⁺ 3d_5/2 at 903.1 and 885.3 eV, respectively (Francisco et al., 2001). To determine an exact content of cerium doped in the prepared photocatalysts, the X-ray fluorescence spectroscopy (XRF) analysis was performed. According to the XRF results, about 0.99, 2.01, 3.92, 5.93, 7.98, and 9.90% Ce were detected in the Bi₂WO₆ and Bi₂WO₆-BiOCl samples doped with 1, 2, 4, 6, 8, and 10 mol.% Ce, respectively. The obtained values are very close to the theoretical content of Ce, confirming the successful doping of cerium in the Bi₂WO₆ and Bi₂WO₆-BiOCl samples.

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Fig. 4

XPS core-level spectra of Bi 4f, W 4f, O 1s, Cl 2p, and Ce 3d of non-doped (a,c) and 2 mol% Ce-doped (b, d) Bi₂WO₆ and Bi₂WO₆-BiOCl powders.

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The UV–vis diffuse reflectance spectra were measured to evaluate the light absorption properties of the as-synthesized samples. As shown in Fig. 5, pure Bi₂WO₆ (Fig. 5a, black line) displays a typical absorption edge at approximately 460 nm, whereas pure Bi₂WO₆-BiOCl (Fig. 5b, black line) shows a slightly lower absorption edge at approximately 450 nm in the visible range owing to the incorporation of the BiOCl. For Ce-doped Bi₂WO₆, the absorption edge is significantly extended up to >700 nm and the intensity of the background absorption is markedly increased with the increasing content of cerium dopant in Bi₂WO₆ from 0 to 10 mol%, which is possibly due to the formation of defects. Interestingly, the Ce-doped Bi₂WO₆-BiOCl samples indicated a smaller redshift (up to 600 nm with 10 mol% Ce) of the absorption band compared to that of the Ce-doped Bi₂WO₆ samples. This finding is because most cerium ions were incorporated into the BiOCl lattice rather than in the Bi₂WO₆ lattice. The redshift in the absorption spectra can be ascribed to the charge transfer between the Bi₂WO₆ and BiOCl valence or conduction bands and the cerium ion 4f level. The corresponding band gap energies calculated from the Tauc plot of (αhν)^1/2 versus photon energy (hν) for the Bi₂WO₆ and Bi₂WO₆-BiOCl samples with increasing cerium dopant content follow the order: 2.71 and 2.77 eV for 0 mol% Ce > 2.44 and 2.50 eV for 0.5 mol% Ce > 2.23 and 2.26 eV for 1 mol% Ce > 2.00 and 2.22 eV for 2 mol% Ce > 1.93 and 2.21 eV for 4 mol% Ce > 1.80 and 2.17 eV for 6 mol% Ce > 1.67 and 2.13 eV for 8 mol% Ce > 1.37 and 2.11 eV for 10 mol% Ce. This sequence implies that the addition of cerium plays an important role in changing the optical properties, utilizing the visible light of solar energy, and enhancing the photocatalytic activity of Bi₂WO₆ and Bi₂WO₆-BiOCl.

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Fig. 5

UV–vis diffuse reflectance spectra (top) and Tauc plots (bottom) of Bi₂WO₆ (A) and Bi₂WO₆-BiOCl (B) powders doped with different contents of cerium: (a) 0 mol%, (b) 0.5 mol%, (c) 1 mol%, (d) 2 mol%, (e) 4 mol%, (f) 6 mol%, (g) 8 mol%, and (h) 10 mol%.

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Fig. 6 shows the zeta-potential values of non-doped and Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl powders as a function of the cerium dopant content. As shown, both samples indicate no point of zero charge and exhibit negative zeta-potential values at the neutral pH of deionized water, indicating that the as-synthesized samples have negatively charged surfaces. The surface charge of the as-synthesized samples mainly result from the edge surfaces and structural charge sites. The edge surface charge comes from proton adsorption by the hydroxyl groups, whereas the structural charge sites have permanent negative charges. Compared with Bi₂WO₆, the Bi₂WO₆-BiOCl powders possess a greater negative charge due to the presence of Cl⁻ ions on the surface of BiOCl, creating negatively charged monolayers, which are eventually replaced partly by OH⁻ because of a surface hydration. Adjacent hydroxyl groups interact with the particle surface during the formation of a hydrated layer, which may result in hydrogen bonding between neighboring hydroxyl groups. In both cases, cerium doping may possibly reduce the negative charge values that provide a less favorable environment for the adsorption of negatively charged hydroxyl groups on the surfaces through electrostatic interactions.

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Fig. 6

Zeta potential of (a) Bi₂WO₆ and (b) Bi₂WO₆-BiOCl powders at neutral pH as a function of cerium dopant content.

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The adsorption capacity and photocatalytic activity of non-doped and Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl samples were evaluated via the adsorption and photodegradation of salicylic acid in the dark and under visible light irradiation, respectively, and the results are shown in Fig. 7 and Table 1. The adsorption capacity of the synthesized samples for salicylic acid was directly proportional to the measured specific surface area. Specifically, the higher the specific surface area is, the greater the adsorption capacity is. With an increasing content of Ce dopants, the specific surface area and adsorption of salicylic acid gradually increase in the following order: 6.1 m²·g⁻¹ and 7.2% (0 mol% Ce) < 6.7 m²·g⁻¹ and 8.8% (0.5 mol%) < 7.3 m²·g⁻¹ and 11.6% (1 mol%) < 9.4 m²·g⁻¹ and 14.0% (2 mol%) < 11.6 m²·g⁻¹ and 17.6% (4 mol%) < 14.2 m²·g⁻¹ and 23.2% (6 mol%) < 15.8 m²·g⁻¹ and 25.6% (8 mol%) < 18.3 m²·g⁻¹ and 30.4% (10 mol%) for Bi₂WO₆ and 9.6 m²·g⁻¹ and 11.6% (0 mol%) < 11.4 m²·g⁻¹ and 14.0% (0.5 mol%) < 14.8 m²·g⁻¹ and 21.6% (1 mol%) < 18.7 m²·g⁻¹ and 27.2% (2 mol%) < 29.5 m²·g⁻¹ and 34.0% (4 mol%) < 36.9 m²·g⁻¹ and 35.6% (6 mol%) < 40.2 m²·g⁻¹ and 39.2% (8 mol%) < 48.6 m²·g⁻¹ and 43.6% (10 mol%) for Bi₂WO₆-BiOCl. In contrast, the photodegradation of salicylic acid over the synthesized samples did not follow this trend. The results indicate that cerium doping has not only tuned the morphological structure but also increased the specific surface area, improving the adsorption capacity of Bi₂WO₆ and Bi₂WO₆-BiOCl for salicylic acid.

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Fig. 7

Adsorption and photocatalytic activity for the removal of salicylic acid of (a) Bi₂WO₆ and (b) Bi₂WO₆-BiOCl powders doped with different contents of cerium ranging from 0 mol% to 10 mol%. (c) Photocatalytic activity for the degradation of salicylic acid over the Bi₂WO₆-BiOCl powders doped with 1 mol% Ce for four cycles.

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The adsorption affinities and preferential sites of salicylic acid molecules on non-doped and Ce-doped Bi₂WO₆, BiOCl, and Bi₂WO₆-BiOCl were computationally studied using molecular dynamics simulations (Hojamberdiev et al. (2017a,b)). The adsorption of salicylic acid molecules onto the surfaces is visually illustrated in Figs. 8 and 9. The adsorption energies of the most stable configuration of a salicylic acid molecule and a water molecule over mineral surfaces were exothermic due to the strong mineral–adsorbate interaction (Fig. 8). The simultaneous adsorption of the salicylic acid and water molecules on the Ce-doped Bi₂WO₆ mineral is stronger (more exothermic) than that on Bi₂WO₆ (Δ_ads = (E_ads(HSal + H₂O) − (E_ads(HSal)+E_ads(H₂O))_Bi2WO6-(E_ads(HSal + H₂O) − (E_ads(HSal)+E_ads(H₂O))_{Ce-doped Bi2WO6} = 10 kcal·mol⁻¹). However, the Δ_ads value is less (0.25 kcal·mol⁻¹) for the Ce-doped BiOCl surface than the BiOCl surface. The obtained results can be explained via an approximation of the formation of different electronic structures of the adsorbed layer of water molecules that interact with the mineral surface through the H-bonds network and close contact. The adsorbed salicylic acid molecules can be stabilized by interaction with the layer of water molecules and direct interaction with the mineral surface. The salicylic acid molecules can interact with the surface atoms using the carboxyl (O_C=O, O_COH, H_COOH) and hydroxyl (O_OH, H_OH) groups.

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Fig. 8

Visualization of adsorption of the salicylic acid and water molecules onto (1 1 1) cleavage surfaces of (a) Bi₂WO₆, (b) Ce-doped Bi₂WO₆, (c) BiOCl, and (d) Ce-doped BiOCl: bismuth, violet; tungsten, blue; oxygen, red; chlorine, light-green; and cerium, beige. Density isosurface: salicylic acid, red; water, green.

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Fig. 9

Structure model of adsorption of (a) water molecules, (b) salicylic acid molecules, and (c) water and salicylic acid molecules: bismuth, violet; tungsten blue; oxygen, red; chlorine, light-green; and cerium, beige. Isosurface: Connolly surface, blue.

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Fig. 9 shows the most stable molecular configurations with H bonds over the Bi₂WO₆, Ce-doped Bi₂WO₆, BiOCl, and Ce-doped BiOCl surfaces:

1. Salicylic acid adsorption model. Over the Bi₂WO₆ and Ce-doped Bi₂WO₆ surfaces, salicylic acid molecules have an intramolecular H bond (O_COH…H_COOH – 2.354 Å) and prefer to connect to the surface oxygen atoms directly (H_COOH…O—Bi – 2.736 and 2.739 Å). The BiOCl surface contacts the salicylic acid molecules via H_COOH…Cl – 2.932 Å, whereas the Ce-doped BiOCl surface has no H bonds except a typical intramolecular H bond (O_COH…H_COOH – 2.322 Å).

2. Water adsorption model. The layered structure of the adsorbed water molecules differs for the Bi₂WO₆ and Ce-doped Bi₂WO₆ surfaces. Over the Bi₂WO₆ surface, the water molecules form a monolayer with intermolecular H bonds (O_H2O…H_H2O – 2.443–2.669 Å) and interact with the surface (H_H2O…O—W – 2.774–2.902 Å, H_H2O…O—Bi – 2.851–2.983 Å). However, over the Ce-doped Bi₂WO₆ surface, the water molecules form separated agglomerates that are in contact with the surface (H_H2O…O—W – 2.989–2.905 Å), whereas the H bond provides the possibility for the salicylic acid molecules to enter the surface directly. However, over the Bi₂WO₆ and Ce-doped Bi₂WO₆ surfaces, the salicylic acid molecules have no direct interaction with the surface and connect to the layer of water molecules by the intermolecular H bonding. The results from the zeta-potential analysis of the Bi₂WO₆ and Ce-doped Bi₂WO₆ particles confirm the formation of a layer of water molecules on the mineral surface. Over the BiOCl and Ce-doped BiOCl surfaces, the water molecules are chained by H bonds (O_H2O…H_H2O – 2.443–2.669 Å) without a strong interaction with the surface (close contacts H…Cl – 3.1 Å). The layer of water molecules over the Ce-doped BiOCl surface exhibits much closer contact H…Cl and some periodical order depending on the presence of Ce atoms.

3. Salicylic acid and water adsorption model. The salicylic acid molecules prefer to bind to the surface oxygen atoms indirectly through the water molecules that are already in contact with the Bi₂WO₆ (H_H2O…O—W – 2.741–2.805 Å) and Ce-doped Bi₂WO₆ (H_H2O…O—Bi – 2.763, H_H2O…O—W – 2.835–2.954, and H_H2O…O—Ce – 2.854 Å). The BiOCl surface directly binds with the salicylic acid molecules via H bonding (H_COOH…Cl – 2.932 Å), the water molecules involved in the H-bond network with the salicylic acid molecules (O_OH…H₂O, O_C=O…H₂O, O_COOH…H₂O) and other water molecules. The Ce-doped BiOCl surface has demonstrated no direct bonding to the salicylic acid molecules, which involve the layer of water molecules located over the surface using close contacts Cl…H₂O – 3.133–3.148 Å. The field density shows separated spaces breaking near the randomly distributed Ce atoms (Fig. 8).

The results indicate that the Ce-doped Bi₂WO₆ surface exhibits a better interaction with the salicylic acid and water molecules, resulting in higher photocatalytic activity, as predicted by the molecular dynamics calculations. Interestingly, the Ce-doped BiOCl surface possesses no pronounced impact on the adsorption of salicylic acid molecules, but the positive effect of the combination of the high specific surface area and electronic interaction between the salicylic acid and layer of water molecules is observed for the Ce-doped Bi₂WO₆-BiOCl sample.

The photodegradation efficiency of salicylic acid over the synthesized samples was estimated based on the change in the intensity of absorption peaks of salicylic acid at 296 nm and the results of the total organic carbon (TOC) analysis. As shown in Fig. 7a and b, the direct photolysis of salicylic acid performed for comparison under identical experimental conditions in the absence of photocatalysis was negligible, indicating that salicylic acid is stable under visible light irradiation. In both cases, the photodegradation efficiency of salicylic acid over the synthesized samples at 5 h gradually reaches the maximum (91.6% for Bi₂WO₆ and 96.8% for Bi₂WO₆-BiOCl) at 1 mol% Ce.

A further increment in the Ce content up to 10 mol% gradually decreased the photodegradation efficiency of salicylic acid over the Bi₂WO₆ and Bi₂WO₆-BiOCl samples. Excess Ce species possibly covered the active sites or acted as a recombination center, reducing the separation efficiency of the charge carriers. Therefore, 1 mol% was found to be the optimum concentration of Ce doping in Bi₂WO₆ and Bi₂WO₆-BiOCl, promoting the effective separation and transfer of photogenerated charge carriers, resulting in high photocatalytic performance. Compared to Bi₂WO₆, the Bi₂WO₆-BiOCl samples exhibited higher photocatalytic activity owing to the formation of a p–n heterojunction in the composite. The experimentally obtained data were fitted in the pseudo first-order kinetic equation ln(C/C₀) = –kt, and the apparent rate constants of the photodegradation of salicylic acid are given in Table 1. Among the samples, the 1 mol% Ce-doped Bi₂WO₆-BiOCl sample showed the highest apparent rate constant (k = −0.360 min⁻¹), which is more than three times higher than that of non-doped Bi₂WO₆ (k = −0.105 min⁻¹), due to the synergistic effect of cerium doping and the formed p–n heterojunction. For future practical applications, it is important to investigate the reusability and stability of the synthesized materials during a photocatalytic reaction. In this context, the 1 mol% Ce-doped Bi₂WO₆-BiOCl sample was subjected to four successive photocatalytic experiments by adding a fresh aqueous solution containing salicylic acid in each run under visible light irradiation. As shown in Fig. 7c, the photodegradation efficiency of salicylic acid over the sample slightly decreased due to the loss of photocatalyst particles during the separation after each cycle, suggesting that the synthesized composite has good stability and can be consecutively used in water purification processes.

Based on the above results and previous reports, a possible mechanism for the enhanced photocatalytic activity of the Bi₂WO₆-BiOCl composite for the degradation of salicylic acid is schematically illustrated in Fig. 10. When p-type BiOCl and n-type Bi₂WO₆ are in contact, a p–n heterojunction forms, resulting in the formation of an internal electric field at the interface of the p–n heterojunction after equilibrium. The formed heterojunction in turn accelerates the separation of photogenerated electron-hole pairs (Chen et al., 2018). According to the calculated data reported previously (Yang et al., 2013), in the Bi₂WO₆-BiOCl heterojunction, the electrons in the conduction band of BiOCl transfer to the conduction band of Bi₂WO₆ because the conduction band of the latter is more positively positioned than that of the former. Simultaneously, additional electrons are excited to the conduction band of Bi₂WO₆ under visible light irradiation. The transferred and excited electrons can reduce O₂ to ^•O₂⁻ and transform into ^•OH. Meanwhile, the holes in the valence band of Bi₂WO₆ transfer to the valence band of BiOCl because the valence band of BiOCl is more negatively located than that of Bi₂WO₆ and oxidize H₂O and OH⁻ to ^•OH. Along with the holes, the generated ^•OH by both electrons and holes will be involved in the degradation of salicylic acid under visible light (Hojamberdiev et al., 2017a,b; Li et al. 2017b; Zhu et al., 2015).

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Fig. 10

Schematic representation of the photodegradation mechanism of salicylic acid over the Bi₂WO₆-BiOCl powders doped with 1 mol% Ce under visible light irradiation.

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As the holes and electrons are separated in the valence band of BiOCl and the conduction band of Bi₂WO₆, respectively, the formed Bi₂WO₆-BiOCl heterojunction therefore possesses a lower recombination rate of photogenerated electrons and holes at the interface, contributing to the enhanced photocatalytic activity:

Although the formation of the intermediates is complex and not particularly investigated in the present work, it is thought that the photodegradation of salicylic acid over the Bi₂WO₆ and Bi₂WO₆-BiOCl samples follows the most likely reaction pathway, resulting in the formation of 1,2-dihydroxybenzene, 2,3-dihydroxybenzoic acid, hydroquinone, benzoquinone, muconic acid, maleic acid, oxalic acid, malic acid, malonic acid, and acetic acid as intermediates, which explains the opening of the aromatic ring (Mukherjee et al., 2016). Previously, a high degree of mineralization of salicylic acid over TiO₂/SiO₂ fibers was achieved through the homogeneous photochemical oxidation by adding an adequate amount of H₂O₂ due to the formation of ^•OH radicals (Adán et al., 2006). Arfanis et al. (2017) reported that TiO₂ nanotubular films showed a negligible variation in their photocatalytic activity towards the photodegradation of salicylic acid in a wide pH range of aqueous solutions, making them suitable for applications under different conditions. Lang et al. (1996) suggested that salicylic acid molecules can also act as sensitizers for photochemical oxidation by absorbing photons at higher wavelengths, which contributes to the photocatalytic activity.

As the Bi₂WO₆ amount in the composite gradually decreases with increasing Ce content, a decrease in the photocatalytic activity is observed, demonstrating the pivotal role of the formed heterojunction. The improvement in the photocatalytic activity of the samples doped with an appropriate amount of cerium stems from (i) the presence of a Ce³⁺/Ce⁴⁺ redox couple that can act as an electron scavenger and superoxide radicals (^•O₂⁻) producers, (ii) the formation of oxygen vacancies with the relatively high mobility of bulk oxygen species, and (iii) the different optical properties that result from the various electronic structures of Ce³⁺ with 4f¹5d⁰ and Ce⁴⁺ with 4f⁰5d⁰. During the reaction, the photogenerated electrons are trapped by Ce⁴⁺, and Ce⁴⁺ is reduced to Ce³⁺, which can be oxidized back to Ce⁴⁺ by the adsorbed oxygen in the system. The adsorbed oxygen reacts with Ce³⁺ and e⁻ to produce ^•O₂⁻. In addition to the efficient separation of electrons and holes, the Ce³⁺/Ce⁴⁺ redox couple also promotes the generation of ^•O₂⁻ that can also contribute to the improvement of photocatalytic activity along with the ^•OH and holes (Hojamberdiev et al. (2017a,b); Huang et al., 2014; Hojamberdiev et al., 2014):

The obtained results showed that the synthesized materials can be used for the purification of water contaminated with various pharmaceuticals and personal care products.

4 Conclusions

In summary, we have successfully synthesized non-doped and Ce-doped Bi₂WO₆ and Bi₂WO₆-BiOCl powders by a hydrothermal method. The Bi₂WO₆-BiOCl composite was formed when ethylene glycol was replaced by dilute HCl as a solvent in the hydrothermal system. The formation of flower-like morphologies was governed by a complexing agent (ethylene glycol) forming a chain-like structure on the crystallite surface and the selective adsorption of excessive Cl⁻ ions on the specific surface of crystallite, leading to a morphological variation. Cerium doping could extend the absorption edges of Bi₂WO₆ and Bi₂WO₆-BiOCl up to 460 and >700 nm, respectively. According to the molecular dynamics calculation results, Ce doping enhances the interaction of the Bi₂WO₆ surface with the salicylic acid molecules in Ce-doped Bi₂WO₆-BiOCl due to the improved interaction between the Bi₂WO₆ surface and the layer of water molecules through which the salicylic acid molecules are connected by hydrogen bonding. Among the samples, the 1 mol% Ce-doped Bi₂WO₆-BiOCl sample showed the highest apparent rate constant (k = −0.360 min⁻¹), which is more than three times higher than that of non-doped Bi₂WO₆ (k = -0.105 min⁻¹), due to the synergistic effect of cerium doping and the formed p–n heterojunction. More importantly, the synthesized composite possesses good photostability and can be reused during water treatment.

5 Declarations of interest.

None.