Electronic band structure and visible-light photocatalytic activity of Bi₂WO₆: elucidating the effect of lutetium doping

Abstract

Bismuth tungstate and 5% Lu-doped bismuth tungstate photocatalysts were synthesized by coprecipitation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectra (DRS), and Brunauer–Emmett–Teller (BET) surface area. The effect of lutetium doping on electronic structure was investigated using density functional theory calculations (DFT). The change of morphological and optical band gap was conditioned by lutetium doping. Under visible light irradiation, the as-prepared sheet-like Lu-Bi₂WO₆ sample exhibits the highest visible-light-responsive photocatalytic performance than pure Bi₂WO₆ for the degradation of Methylene Blue (MB). The photocatalytic mechanism was explained on the basis of electrochemical impedance spectroscopy (EIS), photoluminescence (PL) spectra, active trapping measurements and optoelectronic properties.

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1. Introduction

These past decades, water pollution has surprisingly increased and become one of the most serious environmental issues which constantly threaten human health and sustainability. Photocatalytic semiconductor materials have been extensively studied due to their remarkable properties for pollution remediation and hydrogen production from water splitting using solar energy.^1–4 Classical photocatalysts such as TiO₂ have been widely used for the photodegradation of toxic organic molecule^5,6 under UV irradiation. However, the main disadvantage of TiO₂ is that it can be only excited by ultraviolet light due to its wide band gap, thus limiting its utilization under the solar spectrum which represents only 3 to 4% of UV contribution.⁷ Accordingly, visible light active photocatalysts have been engineered and studied as a potential solution to the worldwide energy shortage and for counteracting environmental degradation.^8,9

Bismuth based photocatalysts has been given an extensive attention due to its unique band layered structures, photoanode properties and relatively high photostability.^10–13 Bismuth tungstate Bi₂WO₆ (BWO) possesses an orthorhombic structure with a narrow bad gap energy of ∼2.7 eV, important intrinsic physical and chemical properties^14,15 and excellent visible light driven photocatalytic activity.^16–19 Nevertheless, its utilization efficiency on visible light is still too low. In addition, the fast recombination rate of photoinduced electrons and holes also limits its practical application. To overcome the above drawback and further improve photocatalytic activity of Bi₂WO₆ under visible light irradiation, many attempts have been made to date to improve the photocatalytic efficiency of BWO such as metal deposition^20–22 ion doping,^23–25 heterostructure forming^26,27 and self-doping.²⁸ Clearly, the doping is considered the easiest method to expand the visible absorption, depress the recombination of the induced electron–hole pairs and change semiconductor electrical properties by increasing electron or hole densities, which can adjust the composition of and promote the separation of photoinduced electron–hole pairs.^29,30 Recently, we have reported the photoluminescence under monochromatic UV excitation of the Bi_2−xLu_xWO₆ with 0 ≤ x ≤ 1: the emissions seem to be strongly related to the formation of the monoclinic structure induced by the substitution of bismuth by lutetium, in the range 0.1 ≤ x ≤ 1. The emissions have been decomposed into two types: the classical emission of tungstate groups WO₆⁶⁻ with two components due to charge transfers “W5d → O2p” in the case of octahedral coordination, and a specific emission (narrow band at 1.25 eV) strongly related to the presence of lutetium.³¹

Herein, we report in this present study the effect of lutetium doping on the BWO photocatalytic activity interacting with methylene blue (MB) under visible light irradiation. We used density functional theory (DFT) calculation to investigate the effect of lutetium ions on the band structures and optoelectronic properties. The photocatalytic mechanism was suggested on the basis of electrochemical impedance spectroscopy (EIS) and active scavenger species.

2. Experimental

2.1. Elaboration

As reported earlier,²⁵ a typical synthesis procedure to elaborate the lutetium-doped Bi₂WO₆ samples can be described briefly as follows: a mixture of required amounts of bismuth nitrate Bi(NO₃)₃·5H₂O (Alfa Aesar 99.9%) and lutetium nitrate Lu(NO₃)₃·6H₂O (Alfa Aesar 99.9%) were dissolved in 50 mL of nitric acid (1 M) under vigorous stirring. Then, 50 mL of (NH₄)₁₀(W₁₂O₄₁)·6H₂O solution (Alfa Aesar 99.9%) was added drop by drop to the Bi/Lu solution with few drops of Ethylene Glycol (EG) as surfactant. The mixture was stirred for 2 hours. Afterwards, ammonium hydroxide (NH₄OH) was added in order to adjust the pH to 5.5. The solution was heated in a bath water at 80 °C, filtered and washed several times by distilled water and ethanol. Finally the powders were heated at 600 °C.

2.2. Characterizations

X-Ray diffraction. The X-ray diffraction (XRD) patterns were collected using an EMPYREAN PANALYTICAL diffractometer operating at 45 kV/35 mA, using CuKα radiation with Ni filter, and working in continuous mode with a step size of 0.013°.

UV-vis diffuse reflectance spectra. The UV diffuse reflectance spectra were measured, in the range 200–800 nm, using an UV-vis spectrophotometer equipped with an integrating sphere (PerkinElmer Lambda 1050). The reference sample used was a BaSO₄ coated standard pattern.

Microstructural characterization. Scanning electron microscopy (SEM) analyses were used to observe the morphology and the local composition of the polycrystalline material. The determination of chemical compositions was performed using Energy Dispersive Spectroscopy (EDS). Preliminary images were obtained with a SUPRA 40 VP COLONNE GEMINI ZEISS using a maximum voltage of 20 kV.

Specific surface area BET and pore size distribution. The Brunauer–Emmett–Teller (BET) specific surface area (S_BET) was determined by the nitrogen adsorption and desorption isotherm, pore size distribution and specific surface area were measured using an AUTOSORB-1 surface area and pore size analyzer at 77 K.

Electrochemical measurements. The electrochemical impedance spectroscopy (EIS) was performed in open circuit potential (OCP) with 10 mV as amplitude of the superimposed AC signal, and the applied frequency ranged from 100 KHz to 1 Hz. This was carried on three electrodes cell connected to Voltalab PGZ301 potentiostat/galvanostat, with pilot integration controlled by VoltaMaster 4. A platinum electrode was used as a counter electrode and Ag/AgCl (0.1 KCl) used as reference electrode. For the preparation of the working electrodes, 1 mg of sample was mixed by 0.1 mL of polyethylene glycol (PEG) to form a slurry which was then dip-coated onto fluorine thin oxide glass (FTO) (1 cm²). The working electrodes were dried in the oven at 100 °C and sintered at 300 °C for 2 hours. The electrolyte was a 0.5 M Na₂SO₄ aqueous solution.

Photoluminescence. The equipment used to perform the measurements of photoluminescence (PL) under UV excitation was a spectrometer Horiba Jobin-Yvon HR800 LabRam.

2.3. Photocatalytic experiments

The photocatalytic activities of the prepared powders were evaluated by the degradation of methylene blue (MB) in an aqueous medium. Commercial Phillips lamp (50 W lamp) were used as a visible light source. Each powder (mass of 250 g) was suspended in 250 mL of MB solution (10 mg L⁻¹). Before the photodegradation experiment, the solution was vigorously stirred for 1 hour to get the solid–liquid adsorption–desorption equilibrium. The temperature of suspension was maintained at 25 ± 1 °C. Then, the light was turned on and the absorbance spectrum of MB was measured with the same time intervals of 10 min. At these given time intervals, 5 mL aliquots were collected from the suspension and filtered, the characteristic absorption peak of MB at 663 nm was chosen to monitor the photocatalytic degradation process. The concentration of MB was determined as a function of irradiation time using a UV-Visible spectrophotometer (UV-2300). The photocatalytic efficiencies were determined from the variation of the concentrations resulting from these intensities:where C_o and C are the concentrations of the solution before and after visible irradiation.

3. Results and discussions

3.1. X-ray diffraction analyses

The diffraction profiles of both BWO and Lu-doped BWO, represented in Fig. 1, correspond to the pure orthorhombic russelite phase Bi₂WO₆ (JCPDS card no. 73-1126). The Bragg peaks of Lu-doped BWO present full widths at half maximum (FWHM) slightly larger than the ones of non-doped BWO, indicating a crystallization poorer than the crystallization of non-doped BWO, implying an improvement in the formation of nanosized particles as the fraction of Lu increases. Distinct diffraction peaks of samples can be attributed to the (1 3 1), (2 0 0)/(0 0 2), (2 0 2), (1 3 3), (2 6 2), and (4 0 0) crystal planes of orthorhombic Bi₂WO₆, respectively. Moreover, no other crystalline phase can be detected, thus indicating that the Lu doping did not affect the crystalline structure of BWO.

Fig. 1 XRD patterns of the BWO and Lu-BWO samples.

3.2. Band and electronic structure

The calculations were carried out at the Density Functional Theory (DFT) level with the spin–orbit coupling (SOC) using the Wien2k computer code, which is an implementation of full-potential method of linearized augmented plane waves.^32,33 The generalized-gradient approximation (GGA) of Perdew, Burke, and Ernzerh (PBE) was used for the exchange–correlation functional.³⁴ Several convergence parameter tests were carried out, which were found to be sufficient to achieve more accurate results. The convergence parameter R_MT × K_MAX is set to 7.6 (R_MT is the smallest of all atomics sphere radii and K_MAX is the cutoff of the interstitial plane wave). The energy cutoff was set to 7.0 Ry, which splits the core and valence states, while the angular momentum expansion inside the MT sphere was selected up to l_max = 11. The plane wave cut off value for the charge density and potential is selected to be G_max = 12.8 (Ry)^1/2. For structure relaxation and density of states calculations, the Brillouin zone was sampled using 300 k-points. The muffin tin radii for Bi, W, Lu and O atoms were chosen to be 2.190, 1.79, 1.83 and 1.52 au, respectively. For energy convergence, the self-consistent calculations were performed iteratively until the difference between two successive calculations is less than 10⁻⁴ Ry.

The presence of Lu atom was modeled by replacing one Bi atom in a 2 × 1 × 1 Bi₂WO₆ super-cell (Fig. 2), thus forming the Lu-doped Bi₂WO₆ compound. Finally, the structure geometries of these systems are optimized to an energy minimum.

Fig. 2 The 2 × 1 × 1 BWO and Lu-BWO super-cell.

The first-principle calculation showed that the calculated equilibrium lattice constants of pure Bi₂WO₆ are found to be a = 5.4814 Å, b = 16.556 Å, c = 5.497 Å, which are theoretically in reasonable agreement with XRD analyses. By partial substituting of one Bi atom with Lu atom, the lattice constant decreased to a = 5.474 Å, b = 16.536 Å and c = 5.491 Å. Indeed, the ionic radii of Lu³⁺ (0.86 Å) smaller than that of Bi³⁺ (1.03 Å) has led to the reducing of the crystal structure volume.

It is well known that the doping can give rise to a considerable modification in the electronic nature of semiconductors, including the band gap. Therefore, it is desirable to carefully analyze the electronic structures of pure and Lu-doped Bi₂WO₆. To the best of our knowledge, the band-structure calculation based on the DFT method has been investigated only for pure Bi₂WO₆ not for Lu-doped Bi₂WO₆.

The band structure for pure Bi₂WO₆ and Lu-doped Bi₂WO₆ materials, along the G–F–Q–Z–G path in the first Brillouin zone, are shown in Fig. 1. From the first view, the Fermi level locates between the conduction band minimum (CBM) and the valence band maximum (VBM) for both pure Bi₂WO₆ and 5% Lu-doped Bi₂WO₆. It indicates that these structures exhibit semi-conductive character. Likewise, Lu dopant does not introduce any undesirable deep impurity levels in the band gap, where they can act as a recombination center.

Using the UV-Vis reflectance spectra (DRS), we have estimated the experimental optical band gap: the resulting calculated values were 2.81 and 2.67 eV for BWO and 5% Lu-BWO respectively (Fig. S1†). In the present work, the spin–orbit DFT calculations have yielded a band gap of 1.96 eV for the non-doped BWO (Fig. 3a), which is underestimated as compared to the experimental measurement. The difference of about ∼0.9 eV observed between the theoretical and experimental band gaps values can be attributed to the well-known shortcoming of the DFT (exchange–correlation function) to describe accurately the excited states. However, this shortcoming does not represent a serious problem for the qualitative comparison between results of pure and doped structures. The calculated band gap for the 5% Lu-doped Bi₂WO₆ is 1.91 eV, which is slightly smaller than that of the non-doped structure. Therefore, a red-shift of absorption edge is also expected for the 5% Lu-doped Bi₂WO₆, which supports our reported DRS experimental measurements.

Fig. 3 Calculated band structures of (a) Bi₂WO₆ and (b) Lu doped Bi₂WO₆. The binding energy (E = 0) is referenced to the valence band maximum. Where, G = (0, 0, 0), F = (0, 1/2, 0), Q = (0, 1/2, 1/2), Z = (0, 0, 1/2).

As well known from the band structure theory, the mobility of charge carriers is proportional to the curvatures of conduction and valence bands, and inversely proportional to the effective masses of charge carriers.³⁵ As shown in Fig. 3, the valence bands of pure and Lu-doped BWO are characterized by curvatures much flatter than the curvatures of the conduction bands. Consequently, the effective masses of the holes of valence bands are larger than the masses of the electrons of conduction bands, and the mobility of electrons is larger than the mobility of holes.

Furthermore, the curvature of the conduction band obtained for the pure BWO structure appears as being larger than the curvature of the conduction bands of the Lu-doped compound. This means that the electron mobility is larger in the case of pure BWO compared to the electron mobility in Lu-doped samples. Consequently, in Lu-BWO, the separation and migration of photogenerated carriers should be relatively easier. In contrast, the curvature of the valence band becomes weaker after Lu doping, indicating a decrease in the holes mobility.

To deeply analyze the states involved in the structure electronic modifications, the partial densities of states (PDOS) of the pure and 5% Lu-doped Bi₂WO₆ were also calculated as shown in Fig. 4. The overall characteristics of the density of states of pure Bi₂WO₆, calculated in this work are consistent with the previous works.^36–38 From Fig. 4a, the total and partial densities of states (DOS) show that the upper valence band (VB) is dominated by O2p orbitals with considerable contribution of Bi6s, and the lower band is dominated by Bi6s, Bi6p and W5d states. The heavy holes masses in these structures could be due to the sharp form of the O2p density at the valence band maximum. On the other hand, the conduction band is formed by W5d, Bi6p and O2p states. For 5% Lu-doped Bi₂WO₆, the densities of states shown in Fig. 4b reveal that the major Lu4f states locate in the valence band and the Lu5d states locate in the conduction band. The density of state in the range of the valence band increases due to Lu4f involvement. We should note that the localization of Lu4f, close to the top of the valence band, and its strong hybridization with the O2p, Bi6s and Bi6p states lead to falsifying the band dispersion, which narrows the band gap.

Fig. 4 Densities of states of: (a) pure Bi₂WO₆ and (b) 5% Lu-doped Bi₂WO₆.

3.3. Samples morphologies and effects of Lu doping surface area and pore size distribution

The morphologies of the synthesized were studied using scanning electron microscopy. Fig. 5a shows that BWO consists of agglomerated round particles, whereas the doped bismuth tungstate consists of agglomerates of nanoplatelets or nanosheets with different orientations (Fig. 5b). The nanosheets are characterized by planar faces with dimensions L ranging from 100 to 400 nm and thicknesses e of 10 to about 40 nm. The assemble morphologies of the doped sample are significantly different from the ones observed in pure BWO. This can be explained by the anisotropic growth of the nanosheets conditioned by lutetium ions and NH₄OH, which could promote the nanosheets self-assembly and the rate of the nucleation and precipitation.²⁵ The EDS analysis of the Lu-BWO sample in Fig. S2† shows the presence of energy dispersive spectrum of the Bi, W and Lu atoms in agreement with the experimental nominal composition.

Fig. 5 (a and b) SEM micrographs of the BWO and Lu-BWO samples, (c) TEM image of the BWO with an inset image of the [010] zone axis pattern, (d) TEM and HRTEM of the doped sample Lu-BWO.

The compositions of the BWO and 5% Lu-BWO samples were confirmed by TEM/HRTEM. In Fig. 5c, the BWO nanoparticles are well separated from each other and have a diameter from 5 to 10 nm. The SAED pattern in the inset Fig. 5c reveals the single-crystal nature and the corresponding crystal zone axis is parallel to the [010] direction. Fig. 5d shows the nanosheet aspect of the lutetium doped sample; the typical lattice fringe of 0.271 nm corresponds to (002) lattice plane of the Lu-doped BWO sample.

Fig. 6 shows the nitrogen adsorption desorption isotherm for the as prepared samples; reliable analyses of surface area and porosity are obtained. The adsorption isotherms for all these samples can be categorized as a type IV isotherm. In addition, the hysteresis loop resembled H3 in IUPAC classification, which did not exhibit any limiting adsorption at high relative pressure: this loop type is observed in very disordered mesoporous pore networks, silt-like pores and lamellar pore structure.^39,40 The specific surface area for BWO is 4.68 m² g⁻¹, whereas the specific surface area for Lu-BWO is 5.48 m² g⁻¹. It has been seen that the doping effect consists in increasing slightly the specific surface area and modifying the morphology. A pore-size-distribution curve was calculated from the desorption branch of nitrogen isotherm by the BJH method. The main pore size is centralized at 3 nm for both samples which shows the mesoporous aspect of the as-prepared BWO samples. In the Lu-doped sample, additional pore diameter centralized at 6.8 nm was observed.

Fig. 6 Nitrogen adsorption–desorption isotherms and pore size distribution curves of BWO and Lu-BWO samples.

3.4. Photocatalytic activity

The degradation of MB in aqueous solutions was used to evaluate the photocatalytic activities of the samples. All the tests were established in the dark for 60 minutes to ensure the solid–liquid adsorption–desorption equilibrium; then, the visible light was turned on. Fig. 7 depicts the temporal evolution of absorption spectrum and illustrates the decrease of the MB spectral intensity at 663 nm for the BWO and 5% Lu doped samples.

Fig. 7 Temporal evolution of the photocatalytic degradation of MB solution over BWO (Right) and Lu-BWO (Left).

It is well confirmed that surface defects serve as charge carrier traps as well as adsorption sites where the charge transfers to adsorbed species can prevent the e–h recombination, whereas bulk defects only act as charge carrier traps where e–h recombines.⁴¹

After illumination, it is clearly seen that the color of the MB aqueous solution faded and the absorption bands intensities decreased with irradiation time. We should note that the control tests confirmed that MB was not degraded under light irradiation in the absence of catalyst (photolysis) implying a stable structure of the MB under visible light without BWO and Lu-BWO photocatalysts. The Lu-BWO exhibited significantly higher photocatalytic efficiency (99.3%) than the pure pristine BWO (79.8%). Overall, the higher photocatalytic activity of the Lu doped sample might be associated to:

(i) The strong modification of the morphology (formation of nanosheets) associated with an increasing surface area, leading to an increase of MB adsorption;

(ii) Formation of additional point defects due to Lu doping, e.g. formation of vacancies acting as electron trap centers and participating to visible light absorption: these defects could limit the recombination phenomenon and allow extending the charge carriers lifetimes, which is confirmed by the present DFT and DRS analyses;

(iii) DFT calculations show an easier migration of photogenerated carriers.

From the adsorption–desorption isotherm and pore diameter distribution, it can be deducted that the specific surface area and porosity should play a minor role in the enhanced photoactivity of the synthesized Lu-BWO samples.

Analyses of the MB photodegradation kinetics over different photocatalysts were performed. The photocatalytic degradation of MB follows pseudo-first-order kinetics process, which is confirmed by the linear relationship of ln(C₀/C) against time (Langmuir–Hinshelwood model):⁴²

−ln([C]_t/[C₀]) = k_obst

where k_obs is the observed first-order rate constant and [C]_t and [C₀] are the concentrations of MB at any time t and at time after adsorption process in dark nature, respectively. The experimental rate constant of the doped bismuth tungstate 2.8 × 10⁻² min⁻¹ is about three times higher than the pure bismuth tungstate 10⁻² min⁻¹ (Fig. 8). This sharp increase in rate constant observed for the doped compound could be mainly due to the difference in crystal morphologies, coupled with a change in the charge carrier mobilities. It therefore appears that the “nanosheet morphology” induced by Lu doping, allow a better photocatalytic efficiency, insofar as the same Lu doping allow a limitation of the recombination of these charge carriers.

Fig. 8 Pseudo-first-order kinetics of the photocatalysts.

3.5. Photocatalytic mechanism

To better understand this photocatalytic process, the electrochemical impedance spectroscopy (EIS) under visible light is performed (Fig. 9a and b). The best fits to experimental data were achieved using the equivalent circuit shown in Nyquist diagrams of Fig. 9b. The EIS analyses represent a simplified version of general transmission line models used to describe the overall charge transport processes. The equivalent circuit is composed of the ohmic resistance R_s and two components of R in parallel with a constant phase element (CPE). The (R₁//CPE₁) component in the intermediate frequency region is ascribed to the charge transfer processes at the working electrode/electrolyte interface, whereas the (R_d//CPE_d) component at the lowest frequencies is associated with the diffusion of ions in the electrolyte. Overall, the Nyquist arc radius of the Lu-BWO is smaller than that the one of the non-doped BWO implying an increased conductivity.

Fig. 9 Electrochemical impedance spectroscopy (EIS) under visible light of BWO and Lu-BWO, (a) Bode plots (f in logarithmic scale); (b) Nyquist representations and (c) the proposed equivalent circuit.

The lifetime of the involved electrons can be calculated using the classical equation:⁴³

t = 1/2πf_max

where f is the frequency determined from the maximum of the phase component extracted from the Bode diagram in Fig. 9a. The electrons lifetimes are 5.52 ms and 6.98 ms for BWO and Lu-BWO respectively. The higher electron lifetime of the doped sample suggests an increase of the separation and migration of electron–hole pairs. These findings are in accordance with the PL results presented in Fig. 10, Lu-BWO displays a lower intensity implying that charge carriers can be effectively restrained by Lu doping.

Fig. 10 Photoluminescence spectra (PL) of the synthesized materials.

To confirm the main active species in the photocatalytic process, we performed in situ trapping experiments of free radicals (˙O₂⁻ and ˙OH) and holes for the degradation of MB. These typical active species in photocatalytic reaction can be investigated using different types of active-species scavengers. In this study ethylene diamine tetra-acetic acid EDTA-2Na, L-ascorbic acid and isopropanol (IPA) were added to the reaction solutions as hole, (˙O₂⁻) and (˙OH) scavengers, respectively. A certain amount of scavengers (4 mmol L⁻¹) were added into the MB solution prior to addition of catalysts. We used the same MB degradation process as in the Section 3.4.

The photocatalytic activity of the Lu-BWO sample decreased to 18%, 27% and 61% in the presence of EDTA-2Na, L-ascorbic acid and IPA, which suggests that holes and (˙O₂⁻) superoxide would be the main active species in the photodegradation process of MB over the Lu-doped bismuth tungstate (Fig. 11). The standard redox potential of ˙O₂⁻/O₂ is about −0.28 to −0.33 eV and is lower than the reduction potential of the Lu-BWO, so the released charge carriers can be easily transferred to the O₂ molecules on the catalysts surface producing superoxide radicals ˙O₂⁻ which are the major reactive species.¹³ The hydroxyl radical quenching can also affect the degradation of MB with an inhibition efficiency of 38.3%.

Fig. 11 Photocatalytic degradation of MB in the presence of EDTA, IPA and acid ascorbic as active trapping species.

These findings suggest a synergic effect of MB degradation with a major role of holes in the photocatalytic process. Liu et al. have also reported recently that superoxide radicals, direct holes and hydroxyl radicals are the main active species responsible for the photocatalytic degradation of MB over Ag₃PO₄@g-C₃N₄ hybrid core@shell composite.⁴⁴

Fig. 12 illustrates the schematic diagram of the photodegradation process over Lu-BWO samples, when the photocatalyst is irradiated: electrons in the valence band (VB) jump to the conduction band (CB), leading to the formation of photo-generated e–h pairs. The migration of these charge carriers at the solid surfaces initiates a series of reactions with adsorbed MB molecules, to produce strong oxidant species such as superoxide ions (˙O₂⁻) and hydroxyl radicals (˙OH). Pollutants can be also degraded by interacting with valence band holes.

Fig. 12 Proposed photocatalytic mechanism for the degradation of MB over Lu-BWO samples.

3.6. Photostability of Lu-BWO sample

The photostability and reusability of the Lu-BWO compound was evaluated through five runs of the photocatalytic experiments using the same conditions as described in Section 2.3. After five recycling experiments during 120 minutes (Fig. 13), no apparent deactivation of the Lu-BWO photocatalysts was observed. Slight decrease in the samples efficiency was noted: this clearly suggests that Lu-BWO materials are stable during the photodegradation process.

Fig. 13 Cycling runs for the photodegradation of MB over Lu-BWO materials under visible light irradiation.

4. Conclusion

In summary, pure and Lu-doped Bi₂WO₆ samples were successfully synthesized by coprecipitation method and were fully characterized. The Lu-BWO nanosheets exhibit a higher photocatalytic activity under visible light irradiation. Lutetium doping promotes a strong modification in morphologies, favoring MB adsorption and easy migration of charge carriers to the surfaces of nanosheets under UV irradiation. This Lu doping allows the separation of photoexcited e–h pairs and lengthens the lifetime of photoinduced charge carriers. These findings are in agreement with the DFT calculations suggesting that the electrons mobility of Bi₂WO₆ is increased after Lu doping: this could allow an easier separation and migration of photogenerated carriers. Based on the results of active species trapping tests, holes and superoxide radicals play a crucial role in the photodegradation of MB over Lu-doped BWO and a photocatalytic mechanism was proposed. EIS measurements and PL analyses confirmed that Lu doping can improve efficiently the separation and restrain the possible recombination of the photoexcited charge carriers. These stable and reusable catalysts may have a potential application in pollutants photocatalysis.

Acknowledgements

A part of this work was financially supported by Materials and environment Laboratory (Morocco-Agadir). The authors would like to acknowledge the assistance of the CNRST-UATRS and MASciR foundation for providing facilities for TEM and BET/BJH analyses.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22669h

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