Enhanced photocatalytic performance and morphology evolvement of PbWO₄ dendritic nanostructures through Eu³⁺ doping

Abstract

Eu³⁺ doped PbWO₄ dendritic nanostructures have been prepared by a facile hydrothermal technique and characterized by XRD, SEM, TEM, XPS and PL spectroscopy. The results indicate that the changing of the Eu³⁺ doping concentration in a reasonable range can adjust the morphology, and has no obvious effect on the phase structure of the obtained materials. Moreover, the Eu³⁺ doping can lead to the enhancement of photocatalytic activity of PbWO₄. Compared with pure PbWO₄ dendritic nanostructures, the PbWO₄:x%Eu³⁺ samples exhibit improved photocatalytic activity in the decomposition of Rhodamine B (RhB). The enhanced performance of the photocatalyst might be ascribed to efficient separation of electron and hole pairs after doping Eu³⁺ into the PbWO₄ lattice.

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Introduction

Photocatalysis is a green technology for environmental remediation, especially for degrading organic dyes or environmental pollutants. In recent years, the research into various nanocrystalline photocatalytic materials has mainly focused on TiO₂. Undoubtedly, TiO₂ is known as one of the most excellent photocatalysts. However, it has a wide band gap of 3.2 eV, which causes it to be so only in response to UV light.^1–3 Hence, exploiting new photocatalysts is more anticipated, among which tungstate is considered to be a promising candidate. Tungstate crystals have been investigated as photocatalysts because of their high average refractive index and low radiation damage, etc.^4–8 For example, Bi₂WO₆ has high photocatalytic activities for degradation of rhodamine B (RhB) under visible-light irradiation² and a WO₃/CdWO₄ photocatalyst exhibited enhanced photocatalytic efficiency for the degradation of different organic dyes under visible light irradiation.⁹ Reduced graphene oxide (RGO) hybridized CdWO₄ shows much higher photocatalytic activity than pure CdWO₄ for methylene blue degradation, because the RGO with a two-dimensional π–π conjugation structure can attract the photo-generated electrons and thus help to reduce the recombination rate of photo-generated charges.¹⁰

As an important member of the tungstate family, lead tungstate (PbWO₄) has been attracting increasing attention due to its optical applications in high-energy physics and scintillating crystal, etc.^11–16 To date, the controlled synthesis of PbWO₄ nano/microstructures has been investigated, and various morphologies such as nanowires, hollow spheres, hollow nanospindles, polyhedral have been reported by solid-state reaction method,¹⁷ solvothermal route,¹⁸ wet chemical route.¹⁹ PbWO₄ microcrystals with various morphologies (helixlike, chainlike, rodlike and crosslike structures) have been successfully synthesized by changing the pH value, temperature, and the addition of surfactant into the reaction solution.^20,21 Beside various morphologies, PbWO₄ is a popular multi-functional material possessing luminescence, photocatalysis and so on. Such as, PbWO₄ microspheres exhibited photocatalytic activity in dye degradation, the pH value and different hydrothermal temperature play key role in controlling the PbWO₄ morphology and photocatalytic activity.¹⁵ The photocatalytic activity of the erythrocyte-like Y-doped PbWO₄ mesocrystals is further investigated in terms of the degradation of the acid orange II under UV irradiation.²² However, it has been observed that the application of PbWO₄ in photocatalysis is usually restricted due to the high recombination rate of photoexcited electron–hole pairs which plays important role in photocatalytic reaction. In the course of developing photocatalysts, some techniques have been reported and adopted to improve the performance of photocatalysts, such as doping,^1,23 coupling plasmon resonance,²⁴ size and crystal facet of semiconductors and controlling morphology.^9,25 Among them, the doping is a simple and effective method that has been proved more effective for improving photocatalytic efficiency. It was also found that the doping could not only alter the phase structure and morphology, but also affect the property of the obtained samples. Recently, rare earth ions (RE³⁺) have proved to be the most promising doping ions because of the apparently enhancement of photogenerated electron transfer rate in the interfacial.²⁶ Due to the transitions of RE 4f electrons, the doping of Eu³⁺ could make the optical absorption of substrate material be increased, and separated the photo-generated electron–hole (e⁻–h⁺) pair as well.²⁷ Eu³⁺ is one of interesting luminescent rare earth ions due to its unique optical properties and its applications in a number of optical and biotechnological areas.

In this work, the aim was to design an enhanced PbWO₄ photocatalyst through the Eu³⁺ ion doping and investigate the effect of the Eu³⁺ ion doping concentration on the structure, luminescence properties, and their relationship with the photocatalytic properties. The Eu³⁺ ion doped PbWO₄ dendritic nanostructures were obtained by a simple hydrothermal method. The rhodamine B (RhB) was employed to appraise the photocatalytic activities of the PbWO₄:Eu³⁺ sample. The effect of Eu³⁺ ion doping concentration on the luminescent properties, morphology and photocatalytic efficiency was also investigated in detail.

Results and discussion

Fig. 1a shows the XRD patterns of Eu³⁺-doped PbWO₄ with different doping concentrations. All of the diffraction peaks of each sample can be identified as tetragonal structures with the same space groups of I4₁/a, and are in agreement with the standard data of bulk PbWO₄ (JCPDS card no. 19-0708). No peaks corresponding to any other phases or impurities were detected, indicating the high purity of these samples. However, when increasing the Eu³⁺ doping concentration from 0% to 9%, the (112) diffraction peaks of PbWO₄:x%Eu³⁺ in the range of 2θ = 26.5–28.5° are shifted slightly to the lower 2θ value, as shown in Fig. 1b. The ionic radii of Pb²⁺ is 1.19 Å, which is larger than that of Eu³⁺ ion (0.95 Å). Hence, it is reasonable to observe the peak shift towards lower 2θ value, and to some extent the results might imply that Pb²⁺ sites in PbWO₄ are partially occupied by the dopant Eu³⁺ ions. In PbWO₄:x%Eu³⁺ system, the imbalance of the electrical charges and the difference in the ionic radii between the host Pb²⁺ and the dopant Eu³⁺ ions lead to changes in the ion-to-ion distance and spatial arrangement. It means that Eu³⁺ ions enter the crystal lattice of PbWO₄ or introduce a high disorder into the lattice, which might be the reason for the different effects on the morphology and property of the obtained samples.

Fig. 1 (a) XRD patterns of PbWO₄:x%Eu³⁺ at different concentrations (x = 0, 1, 3, 5, and 9) and the standard data for bulk PbWO₄ (JCPDS card no. 19-0708); (b) diffraction peak positions of PbWO₄:x%Eu³⁺ in the range of 2θ = 26.5° to 28.5°.

It should be denoted that there are some impurity peaks in the XRD patterns when the Eu³⁺ doping concentration was further increased to 12% or above, which probably results from the formation of other tungstates or tungstate composites. The effect of Eu³⁺ doping with higher concentration on phase structure, composition, morphology, optical absorption property and photocatalytic activity of the obtained tungstates will be systematically investigated in future. Hence, in the present work, the morphology, optical absorption and photocatalytic performance of PbWO₄ with Eu³⁺ doping concentration of 0–9% were discussed and compared.

Fig. 2a shows SEM image of the as-prepared PbWO₄. It indicates that the sample consists of dendritic nanostructures which have many symmetric arms extending radially from the center. Further observations show that each arm is perpendicular to the center trunk, and those arms are in good symmetry and have the characteristic of self-similarity. Fig. 2b shows the morphology of PbWO₄ dendritic structures acquired in STEM-HAADF (high angle annular dark field) topology mode. The EDS elemental mappings (Fig. 2c–e) show that Pb, W and O elements are evenly distributed into the PbWO₄ dendritic structures.

Fig. 2 (a) SEM and (b) STEM-HAADF images of PbWO₄ dendritic structures; and EDS elemental mapping for (c) Pb, (d) W and (e) O of the dendritic crystal shown in (b).

It can be clearly seen that the obtained sample is composed of PbWO₄ dendritic structures, as shown in Fig. 3. The SAED pattern (Fig. 3b, inset) can be indexed to the (002), (112) and (110) planes of the PbWO₄ dendritic structures with the tetragonal phase structure, which is in agreement with the XRD result. The HRTEM image (Fig. 3c) of the top of the branch marked as an arrow in Fig. 3b displays single crystalline nature. The values of the interplanar spacings of PbWO₄ dendritic structures are 0.302, 0.193 and 0.325 nm, which are identical to the (002), (110) and (112) facet distances of the PbWO₄ dendritic structures, respectively.

Fig. 3 (a and b) TEM images of PbWO₄ dendritic structures (the inset is the SAED pattern recorded from the branched part), and (c) HRTEM image taken from the branch marked with arrow shown in (b).

The morphology of PbWO₄ was affected by doping of Eu³⁺ ions. Fig. 4 shows the SEM images of PbWO₄:x%Eu³⁺ with different doping concentrations. With increasing Eu³⁺ doping concentrations from 1% to 9%, the morphology change of PbWO₄ is very interesting. It can be seen that both PbWO₄:1%Eu³⁺ (Fig. 4a) and PbWO₄:3%Eu³⁺ (Fig. 4b) are composed of dendritic nanostructures with one trunk (long axis) and four branches (short axes), and the size of the latter is larger than that of the former. The trunk lengths of PbWO₄:1%Eu³⁺ and PbWO₄:3%Eu³⁺ are 9.5 μm and 15 μm respectively, while the lengths of branches for these two samples are 2.5 μm and 5 μm respectively. When gradually increasing doping concentrations to 5%, the sample is also composed of dendritic nanostructures with the trunk length of 12 μm (Fig. 4c), except that the four branches of the dendritic nanostructure became short and disappeared partially. When the doping concentration is increased to 9%, the four branches of the dendritic nanostructure disappeared completely, and the sample is composed of trunk-like structure with the length of 14 μm, as shown in Fig. 4d. It was found that the doping concentration of rare earth ions played a key role in the morphology control of PbWO₄. The morphology of PbWO₄:9%Eu³⁺ is very special. From the XRD pattern of PbWO₄:9%Eu³⁺ as shown in Fig. 1, it can be seen that the intensity of the plane diffraction peaks (200) at 2θ = 32.7°, (312) at 2θ = 55.3° is much stronger than those of the other samples. It indicates that the Eu³⁺ doping has little effect on the phase structure, but has evidently effect on the crystal growth direction and final morphology formation of PbWO₄, especially at high doping concentration of Eu³⁺ ions. The possible reason is partly attributed to the strong effect of the dopant ion on the crystal growth rate through surface charge modification upon increasing doping concentration. A schematic showing the morphology evolution process of PbWO₄:x%Eu³⁺ with different doping concentrations is illustrated in Fig. 4e.

Fig. 4 SEM images of PbWO₄:x%Eu³⁺ with different doping concentrations: (a) 1%, (b) 3%, (c) 5%, and (d) 9%; (e) schematic illustration of the morphology evolution process of PbWO₄:x%Eu³⁺ with the increase of Eu³⁺ doping concentration.

The existence of Eu³⁺ and the chemical state of each atom were investigated by X-ray photoelectron spectroscopy (XPS). The elements of Pb, W, O and Eu can be detected in the survey spectrum (Fig. 5a). Fig. 5b–d show the high-resolution XPS spectra of Pb 4f, W 4f and O 1s, respectively. The values of binding energy of Pb 4f_5/2, Pb 4f_7/2, W 4f_5/2, W 4f_7/2 and O 1s in PbWO₄:5%Eu³⁺ are 143.2, 138.3, 36.8, 34.7 and 530.1 eV, respectively. The XPS spectrum of Eu 3d was shown in Fig. 5d. The characteristic peak of Eu 3d region was displayed at 1136 eV, which is due to the binding energy of Eu 3d_5/2. The XPS results indicate that Eu doped PbWO₄ has been successfully obtained. The PbWO₄:5%Eu³⁺ was further characterized by EDS, and Fig. S1a† shows the STEM-HAADF image of PbWO₄:5%Eu³⁺ dendritic structures. From EDS elemental mappings and EDX spectrum (Fig. S1b–f, ESI†), it can be seen that the sample are composed of Pb, W, O and Eu elements and these elements are evenly distributed into the PbWO₄ dendritic structures.

Fig. 5 XPS analysis of PbWO₄:5%Eu³⁺: (a) survey, (b) Pb 4f, (c) W 4f, (d) O 1s, (e) Eu 3d.

The optical absorption performance relevant to the band gap of a semiconductor is considered to be a key factor in determining its photocatalytic activity. Fig. 6 shows the UV-vis diffuse reflectance spectra (DRS) of the PbWO₄:x%Eu³⁺ with different doping concentrations. According to the spectra, all the sample show intense absorption in UV (200–400) light region. However, PbWO₄:x%Eu³⁺ (x = 1, 3, 5, 9) samples display the visible-light response. Clearly, upon increasing Eu³⁺ doping concentrations from 1% to 9%, the absorption intensity of sample at 520 nm increases gradually, and their corresponding band gaps are 2.42 eV approximately.

Fig. 6 UV-vis DRS of PbWO₄:x%Eu³⁺ with different doping concentrations.

Based on the UV-vis DRS analysis, PbWO₄ have visible response through Eu³⁺ doping. Fig. 7 displays the photocatalytic degradation of RhB with the PbWO₄:x%Eu³⁺ under UV light irradiation. As a contrast, the curves showing the degradation of RhB by pure PbWO₄ and the blank experiment (without addition of any catalysts) were also presented. It can be seen from Fig. 7 that there is only little impact of RhB concentration in the blank experiment after 4 h, indicating that the photolysis of the RhB solution is negligible in this experimental conditions. It was found that the photodegradation rate of RhB by PbWO₄, PbWO₄:1%Eu³⁺, PbWO₄:3%Eu³⁺, PbWO₄:5%Eu³⁺ and PbWO₄:9%Eu³⁺ is 41.13%, 45.44%, 64.43%, 83.33%, and 90.85%, respectively. The photocatalytic activity of PbWO₄:9%Eu³⁺ is close to that of Y-doped PbWO₄ mesocrystals, and PbWO₄ microspheres^15,22 The pseudo-first-order kinetic curves of RhB photodegradation were plotted to quantitatively compare the degradation rate. The efficiency of RhB photodegradation by the photocatalysts was determined quantitatively using the following model: ln(C/C₀) = −kt, where C₀ and C are the concentrations of dye at time 0 and t, respectively, and k is the apparent rate constant (h⁻¹). The experiment data show that the k values of PbWO₄, PbWO₄:1%Eu³⁺, PbWO₄:3%Eu³⁺, PbWO₄:5%Eu³⁺ and PbWO₄:9%Eu³⁺ are 0.1586, 0.1733, 0.3082, 0.5175, and 0.6897 h⁻¹ respectively. Namely, the photocatalytic activity of PbWO₄:9%Eu³⁺ is 4.35 times than that of the pure PbWO₄. As mentioned above, the photodegradation rate of RhB by PbWO₄ with higher Eu³⁺ doping concentration was not given and compared because of the presence of impurity phase or other tungstate composites.

Fig. 7 Photodegradation curves of RhB over PbWO₄:x%Eu³⁺ (x = 0, 1, 3, 5, 9) samples.

To further investigate the mechanism of photocatalytic process over the PbWO₄:9%Eu³⁺ dendritic catalyst, a series of scavengers were developed to scavenge the relevant species and their concentration is about 2.0 mM. The results of the trapping experiments were shown in Fig. S2.† The quenchers used in this reaction system were potassium iodide (KI) for h⁺ and ˙OH, sodium oxalate (Na₂C₂O₄) for h⁺, benzoquinone (BQ) for ˙O₂⁻ and isopropanol (IPA) for ˙OH respectively.^28–32 It should be seen that IPA had little impact on the RhB solution, indicating that ˙OH on the surface of the PbWO₄:9%Eu³⁺ dendritic catalyst did not play a major role in the photodegradation system. However, the degradation efficiency decreased obviously after the addition of KI, BQ or Na₂C₂O₄. These results indicated that h⁺ and ˙O₂⁻ were produced and acted as dominating active species in the photodegradation process. In addition, Fig. S3† show that the removal rate of TOC with PbWO₄:x%Eu³⁺ (x = 0, 1, 3, 5, 9) samples on the degradation of RhB. After the degradation of RhB for 4 h, the removal rate of TOC with PbWO₄:x%Eu³⁺ (x = 0, 1, 3, 5, 9) reached 35.6%, 41.7%, 62.1%, 70.6% and 78.1%, respectively. Due to the degradation curve data were measured after the photocatalyst separation step by centrifugation, the removal rate of TOC was lower than photocatalytic degradation ratio (Fig. 7). The decrease in the carbon content indicates the degradation of the RhB dye into nontoxic compounds. It indicate that PbWO₄:x%Eu³⁺ sample have good photocatalytic activity.

It is well known that electron–hole separation is one of the key factors limiting the efficiency of the photocatalytic process.³³ With the Eu³⁺ ions doped into PbWO₄, and the excited electrons were trapped by the Eu³⁺, which effectively suppresses the electron–hole recombination. Furthermore, the XRD results suggested that Eu³⁺ with low concentration is incorporated into the lattice of PbWO₄ and occupied the sites of Pb²⁺, creating abundant oxygen vacancies and surface defects for electron trapping and pollutant adsorption, also accelerating the separation of electron–hole pairs and RhB photodegradation. Hence, it can greatly promote the separation of electrons and holes, and the photooxidation ability of holes (h⁺) will be enhanced. Meanwhile, the electrons are easier to react with oxygen molecules to form another active species superoxide radical anion (˙O₂⁻) for the degradation of RhB. All of the above processes can effectively improve the performance of the photocatalyst. And the result of the trapping experiments was also in agreement with the supposed photocatalytic mechanism.

The efficiency of charge carrier trapping and transfer and better understanding the fate of electron–hole pair in semiconductor particles could be investigated using PL emission spectrum.³⁴ Excited by the UV light of 280 nm, the emission spectra of PbWO₄ with different doping concentrations of Eu³⁺ were obtained, as shown in Fig. 8. It can be seen that pure PbWO₄ exhibited a blue-green emission band in the range of 400–680 nm with the emission peak centered at 485 nm. With increasing Eu³⁺ doping concentrations from 1% to 9%, the emission spectra not only contain the intrinsic blue-green emission of WO₄²⁻ groups but also include ⁵D₀ → ⁷F_J (J = 1, 2, 3) emission lines of Eu³⁺ ions. The doping of Eu³⁺ into PbWO₄ decreased the PL emission intensity of WO₄²⁻ groups gradually.

Fig. 8 PL emission spectra of PbWO₄:x%Eu³⁺ with different doping concentrations (x = 0, 1, 3, 5, 9).

According to the above result, there exist an opposite relationship between the photocatalytic activity and PL intensity of WO₄²⁻ groups for PbWO₄:x%Eu³⁺ photocatalysts. The higher PL intensity of WO₄²⁻ groups, the higher recombination rate of photo-generated charge carriers which bring about lower photocatalytic efficiency. In other words, a lower PL intensity shows a lower recombination rate of photo-generated electron–hole pairs, so more photo-generated electrons and holes can participate in the oxidation and reduction reactions, resulting in improvement of the photocatalytic performance.³⁵ This investigation strongly provides the evidence that the WO₄²⁻ groups in tungstate photocatalysts mainly play a key role in the process of photocatalysis and establishes the relationship between photocatalytic activity and PL intensity through Eu³⁺ as doping ion and fluorescent probe.

Experimental

Preparation of PbWO₄:x%Eu³⁺

All chemicals were analytical grade reagents and used directly without further purification. Pb(NO₃)₂ (99.99%) and Na₂WO₄ (≥99.0%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Eu(NO₃)₃ solutions were prepared by dissolving the corresponding oxides in diluted nitric acid. PbWO₄:x%Eu³⁺ (x = 0, 1, 3, 5, 9) dendritic nanostructures were prepared by hydrothermal technique. Typically, 10.0 mL of 0.2 mol L⁻¹ Na₂WO₄ solution was added to 10.0 mL of 0.2 mol L⁻¹ Pb(NO₃)₂ aqueous solution and the mixture was continuously stirred for 15 min. The obtained suspension was then transferred into a Teflon bottle held in a stainless steel autoclave, which was sealed and hydrothermally treated at 160 °C for 24 h. After the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with ethanol and distilled water twice, respectively, and dried at 80 °C for 24 h to obtain the undoped sample. Additionally, a series of Eu³⁺-doped PbWO₄ samples (1%, 3%, 5% and 9%) were prepared. The synthesis procedure was similar to that mentioned above except that 10.0 mL of 0.2 mol L⁻¹ Na₂WO₄ was added to the mixed solutions of Pb(NO₃)₂ and Eu(NO₃)₃ with a desired molar ratio, and the other conditions were kept unchanged.

Characterization

Phase structure of the obtained samples was characterized by a Bruker D8 Advance X-ray diffractometer (XRD) with Cu-Kα radiation (λ = 0.15406 nm). The accelerating voltage and emission current were 40 kV and 40 mA, respectively. The TEM image and selected area electron diffraction (SAED) pattern were obtained on a JEOL-2010 transmission electron microscope at an accelerating voltage of 200 kV. Photoluminescence (PL) spectrum was recorded using an FLS920P Edinburgh Analytical Instrument apparatus equipped with a 450 W xenon lamp as the excitation source. The diffuse reflection spectrum (DRS) was obtained using a UV-visible spectrophotometer with BaSO₄ as a reflectance standard. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific Escalab 250Xi instrument with Mg Kα radiation (1253.6 eV) at a scan step of 0.1 eV. Total organic carbon (TOC) analyses were conducted on a multi N/C 2100 (Analytik Jena AG, Germany) TOC analyzer.

Photocatalytic activity measurements

Photocatalytic activities of the as-prepared samples were evaluated by the degradation of rhodamine B in aqueous solution under simulated UV light (a 500 W mercury lamp) in XPA photochemical reactor. The detailed process was as follows: the PbWO₄:x%Eu³⁺ (x = 0, 1, 3, 5, 9) powders (0.05 g) were dispersed in an aqueous RhB solution (5 mg L⁻¹, 50 mL) and then transferred into a 80 mL cylindrical quartz reactor with constant magnetic stirring equipped with a water circulation facility. To reach an adsorption–desorption equilibrium before irradiation, the suspensions were first stirred for 30 min in dark condition. Then the light was switched on, and the solutions were kept stirring. At given time intervals, 5 mL of the suspension was sampled and centrifuged to remove the photocatalyst from the aqueous solution so as to obtain the supernatant for the analysis of RhB. The concentrations of RhB were monitored with a UV-vis spectrometer in terms of the absorbance at 553 nm during the photo-degradation process.

Conclusions

In summary, this work demonstrated a simple hydrothermal method of synthesizing undoped and Eu³⁺ doped PbWO₄ dendritic nanostructures. It has been found that the different doping concentrations of Eu³⁺ ions in the reaction system play a key role in controlling the morphology and photoluminescent property of the obtained samples. Eu³⁺ doped PbWO₄ had higher photocatalytic activity than pure PbWO₄ in the photocatalytic degradation of RhB. The enhanced photocatalytic performance of PbWO₄:x%Eu³⁺ was probably attributed to the increased electron–hole separation and surface defects.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (no. 51572303), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN009), and Innovation Scientists and Technicians Troop Construction Projects of Henan Province (2013259).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15045d

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