Ion exchange synthesis of an all tungsten based Z-scheme photocatalytic system with highly enhanced photocatalytic activity

Abstract

The first example of an all tungsten based Z-scheme photocatalyst (composed of WO₃·0.33H₂O and PbWO₄) has been synthesized via an ion exchange method. The composites exhibit much higher photocatalytic activities than those of individual hydrated tungsten trioxide and lead tungstate. The apparent photodegradation rate constants of RhB over the composites are more than 6 to 8 times higher than those of the two monomer components. After 30 min, the RhB can be removed almost completely by the composite, while only 30% RhB and 35% RhB can be, respectively, eliminated for the two single components. In addition, the composite photocatalyst displays excellent activity in the photodegradation of methyl orange (MO). The relationship between the two monomer components has been investigated using XRD, Raman, XPS, SEM, and TEM measurements. The hydrated tungsten trioxide is formed on the surface of lead tungstate via a complex process that includes ion exchange and crystal structure rearrangement. Ion exchange occurs at the end of the (110) facet of lead tungstate. The results of photocatalytic mechanism and photodeposition Pt experiments indicate that the excellent photocatalytic activity of the composite photocatalyst is due to the advantage of its Z-scheme configuration. This study suggests that highly effective Z-scheme photocatalysts can be built by ion exchange.

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Introduction

Photocatalysts with d⁰- and d¹⁰-related electronic configurations have attracted much interest. Many transition-metal oxide photocatalysts have been developed for water decomposition, such as Ti⁴⁺,^1,2 Zr⁴⁺,^3,4 Nb⁵⁺,^5,6 and Ta⁵⁺,^7,8 which has a d⁰ electronic configuration. Recently, RuO₂-loaded PbWO₄, the first example of tungsten oxide, was shown to exhibit photocatalytic activity for the overall splitting of H₂O into H₂ and O₂.^9,10 Lead tungstate is a typical d¹⁰s² (Pb²⁺)–d⁰ (W⁶⁺) electronic configuration photocatalyst.¹¹ Its large band dispersion leads to high mobility of its photo-generated electrons and holes. The Pb²⁺ (d¹⁰s²) in the oxide, with the advantage of orbital hybridization between its s and p orbitals with O 2p and W⁶⁺ (d⁰), induces a large dispersion in bands, as shown in Fig. S1.† However, lead tungstate itself does not show photocatalytic activity for water splitting. This may be due to the quick recombination of photo-generated electrons and holes. Usually, lead tungstate is investigated for applications in photoluminescence materials.^12–15 However, the photocatalytic properties of lead tungstate have been ignored. Few reports concern the photocatalytic properties of lead tungstate,^16–20 and studies on their modification have rarely been reported.⁹ This work will focus on the investigation of composite photocatalysts based on lead tungstate composites with tungsten trioxide.

Ion exchange is a convenient and effective method to construct high quality composite photocatalysts; it can enable the formation of closer contact between different components.^21–23 Such contact is conducive to the separation of photo-generated carriers between two components.²⁴ Generally, there are two kinds of photo-generated carrier separation processes in composite photocatalysts. One kind of separation process (type I) is the transfer of photo-generated electrons from a higher conduction band (CB) to a lower CB, while the photo-generated holes in the lower valence band (VB) transfer to a higher VB. This process will result in photo-generated electrons with lower reduction ability and photo-generated holes with lower oxidation ability, respectively.²⁵ Another type of separation process is the Z-scheme process (type II). In this type, photo-generated electrons with lower reduction ability in one component will combine with photo-generated holes with lower oxidation ability in another component, with the result that photo-generated carriers with higher redox abilities will be retained in the composite photocatalyst.^26,27 It is apparent that the Z-scheme process will be more beneficial to photocatalytic reactions. Many studies based on Z-scheme photocatalysts have been carried out. Arai et al. reported the synthesis of an efficient Z-scheme photocatalyst by mixing CuBi₂O₄ with WO₃.²⁶ Liu et al. synthesized a Z-scheme photocatalyst based on CaFe₂O₄/WO₃ with efficient visible light activity.²⁸ In addition, the interface between the components of a Z-scheme photocatalyst plays a key role in its photocatalytic activity. Therefore, the synthesis of Z-scheme photocatalysts with high quality interfaces is important.

Herein, a novel Z-scheme WO₃·0.33H₂O/PbWO₄ composite photocatalyst is synthesized via an ion-exchange method. To the best of our knowledge, this is the first report of a Z-scheme WO₃·0.33H₂O/PbWO₄ photocatalyst. This photocatalyst possesses a perfect contact interface, which ensures the excellent transport of photo-generated carriers between hydrated tungsten trioxide and lead tungstate. In this study, hydrated tungsten trioxide is formed on the surface of lead tungstate through the processes of ion exchange and crystal structure rearrangement. Ion exchange occurs at the end of the (110) facet of lead tungstate. The band structures of the two components were studied by UV-Vis diffuse reflectance spectrum (DRS) and valence band XPS spectrum (VB XPS) analysis. Combined with the results of photodeposition Pt and radical detection experiments, the photocatalytic mechanism of composite was confirmed to be the Z-scheme configuration.

Experimental section

Materials

Nitric acid (HNO₃, 65–68%, AR, Aladdin Inc.), sodium tungstate dihydrate (Na₂WO₄·2H₂O, AR, Sinopharm Chemical Reagent Co., Ltd.), lead nitrate (Pb(NO₃)₂, AR, Sinopharm Chemical Reagent Co., Ltd.), and ultra-pure water are used as acquired (18.2 MΩ cm, CSR-1-10, Beijing Aisitaike Technology Development Co., Ltd.).

Synthesis of WO₃·0.33H₂O/PbWO₄

Na₂WO₄·2H₂O (0.5 g, 1.516 mmol) was dissolved in 33 mL of ultra-pure water at room temperature (solution A). A mixture of different amounts of Pb(NO₃)₂ and 2.6 mL dilute nitric acid solution (V_HNO3:V_H2O was 1 to 5) was added to 15 mL of ultra-pure water (solution B). Solution B was slowly dropped into solution A and the mixture was then stirred at room temperature for two days. A solid was obtained by centrifugation and washed with water to remove residual nitrate. Finally, the washed solid was mixed with 33 mL ultra-pure water and underwent hydrothermal reaction for 24 h (140 °C).

In this study, short names are used for the as-prepared samples: hydrated tungsten trioxide is labeled as W, lead tungstate is PW, and the composites (synthesized with different amounts of Pb(NO₃)₂) are PWH. Details are shown in Table S1.†

Characterization of samples

The products were characterized using a powder X-ray diffractometer (Rigaku D/max-2000) with Cu Kα radiation at a scanning rate of 5° min⁻¹ in the 2θ range of 10°–90°. The morphology of the products was investigated by FESEM (Helios Nanolab 600i), TEM, HRTEM and SAED (carried out on an FEI Tecnai G2F30 instrument); UV-Vis diffuse reflectance spectra were obtained using a spectrophotometer (SHIMADZU UV-2550) and BaSO₄ was used as a standard. X-ray photoelectron spectroscopy (XPS) spectra were recorded on an HI5700ESCA instrument with Al Kα (1486.6 eV) monochromatic X-ray radiation, which enables the determination of the atomic percentages, atomic binding energies and VB positions of semiconductors. Raman spectra were acquired using a JY-HR800 instrument with a 458 nm laser PL was measured using an Edinburgh FLS 920 fluorescence spectrophotometer.

Photocatalytic reactions

In these experiments, a 300 W Xe lamp (Trusttech PLS-SXE 300, Beijing) was used as a light source. 10 mg L⁻¹ of RhB and MO aqueous solution were used as the target photodegradation solutions. In each experiment, a quartz beaker containing 100 mL RhB or MO aqueous solution and 50 mg photocatalyst was firstly placed into an ultrasonic bath to disperse the photocatalyst in the dark for 10 min, and then stirred for 40 min in the dark continuously in order to achieve adsorption–desorption equilibrium. The concentration of RhB or MO during the degradation process was monitored by colorimetry using a UV-Vis spectrometer (JH-721) every 10 min. The supernatant was obtained by centrifugation at 10 000 rpm for 5 min.

Photocatalytic cycle experiments were performed continuously throughout the entire photodegradation process. In every cycle, the composite was removed, washed, and dried. The supernatant was removed by centrifugation, and the specimen was returned to the beaker after dye concentration detection. New dye solution was then added before the next cycle.

Results and discussion

The X-ray diffraction (XRD) patterns of the as-prepared samples are shown in Fig. 1. For the PWHs (Fig. 1a), all the diffraction peaks are matched with the tetragonal stolzite phase of PbWO₄ (JCPDS Card no. 86-0843) and the orthorhombic phase of WO₃·0.33H₂O (JCPDS Card no. 87-1203). No impurity peaks are observed in the XRD patterns. The sharp and narrow peaks show that the samples have high crystallization. In the XRD patterns of the PWHs, it can be seen that the diffraction peaks of WO₃·0.33H₂O weaken with increasing amounts of Pb(NO₃)₂. It was found that tungstic acid precipitation could not be formed over a long period of time if only dilute nitric acid was added. Meanwhile, lead ion was able to combine with tungstate ion immediately. This phenomenon indicates that the core of the PWH compounds may be lead tungstate. WO₃·0.33H₂O is formed when an excess of lead source is added. This is due to the ion exchange between hydrogen and lead. When a small amount of lead source is added, instead of the ion exchange, the excess tungstate ion can adsorb on the surface of lead tungstate, and the adsorbed tungstate ion can continually react with hydrogen ion to generate tungstic acid after a sufficient period of time. When no lead source is added, the orthorhombic phase of WO₃·0.33H₂O (JCPDS Card no. 87-1203) can be obtained, as shown in Fig. 1b. For PW, the indexed diffraction peaks can be ascribed to PbWO₄ with tetragonal stolzite structure (JCPDS Card no. 86-0843), as shown in Fig. 1b. No impurity can be found in the XRD pattern, and the sharp peaks indicate a high degree of crystallinity. The mass percentages of PW and W in the PWH samples was calculated using the XRD data (see ESI† for methods), as shown in Table S2.†

Fig. 1 XRD patterns of PWH (a), W and PW (b).

Raman spectra were also used to investigate the components of PWH-5. Fig. 2 shows the Raman spectra of the as-prepared samples. The lead tungstate sample has five bands in the range of 1000–300 cm⁻¹. These peaks are located at 902.3 cm⁻¹, 763.6 cm⁻¹, 750 cm⁻¹, 354.5 cm⁻¹, and 325 cm⁻¹. According to previous research, the peaks can be assigned to the vibration modes ν₁(A_g), ν₃(B_g), ν₃(E_g), ν₂(B_g), ν₂(A_g), and A_g, respectively.²⁹ In the case of the PWH-5 sample, the five unique bands of lead tungstate can be seen. In addition to the mentioned five bands, there are two bands at around 700 cm⁻¹ and 800 cm⁻¹. Compared with the Raman spectrum of the W sample, it can be seen that the two additional bands of PWH-5 are similar to the ν(W–O–W) of the W sample. Therefore, the additional bands of PWH-5 belong to WO₃·0.33H₂O. The results of the Raman spectra are consistent with the XRD patterns, further indicating that the composite is composed of lead tungstate and hydrated tungsten trioxide.

Fig. 2 Raman spectra of PW, PWH-5, and W.

Fig. 3 shows the XPS spectra of the PW, PWH-5, and W samples. The element content on the surface of the sample was investigated and is shown in Table S3.† The content of lead in PWH-5 is less than that in PW, indicating that hydrated tungsten trioxide exists on the surface of lead tungstate in the PWH sample, which is consistent with the experimental phenomena. The surface properties of the as-prepared samples were also investigated. The O 1s at ∼530 eV and W 4f at ∼35 eV and 37 eV indicate the presence of W⁶⁺ and O²⁻.³⁰ In addition, the locations of O 1s and W 4f of the PWH-5 sample are close to those of the W sample, which may be due to the existence of hydrated tungsten trioxide on the surface of lead tungstate in the PWH sample. Furthermore, the Pb 4f binding energy of PWH-5 is larger than that of PW, which is induced by the hydrogen ion exchange with lead ions. The exchange of lead ion by hydrogen ion will lead to a decrease in the electron cloud density of the lead (near the exchanged lead ion).

Fig. 3 XPS spectra of PW (a–c), PWH-5 (d–f), and W (g–i).

After investigating the composition of the PWH samples, the relationship between the two components can be established. Fig. 4a shows SEM images of the PWH-5 sample, and Fig. S2† shows the other PWH samples and the PW sample. The average size of the PWH samples is around 1 μm. The PW sample displays an irregular shape (Fig S2†), and the morphologies of the PWH samples are octahedral, with some particles on the surface (Fig. 4a and S2†). These results suggest that dilute nitric acid plays an important role in the shape of lead tungstate. Previous studies have shown that different pH values can affect the morphology of lead tungstate.^31–33 Therefore, the effect of the dilute nitric acid amount on the morphology of lead tungstate was studied. Fig. S3† displays the SEM images of samples with different amounts of dilute nitric acid. When 0.3 mL dilute nitric acid was added (V_HNO3:V_H2O was 1 to 5), the lead tungstate had a rod-like appearance, and the morphology of lead tungstate was approximately octahedral after the addition of 0.65 mL dilute nitric acid. When the volume of dilute nitric acid was greater than 0.65 mL, octahedral lead tungstate was obtained. The change in geometrical shape is induced by the pH of the solution. This phenomenon is consistent with previous studies.

Fig. 4 (a) SEM image of PWH-5 (inset is the whole morphology of PWH-5 by TEM measurement), (b) the test area of TEM image and SAED pattern of PWH-5, (c) HRTEM image of PWH-5 (marked as c area in Fig. 5b; the insets are the Fourier transforms of the two components), (d) HRTEM image of PWH-5 (marked as d area in Fig. 5b; the insets are the Fourier transforms of the two components), (f) crystal structure of PWH-5 with a drawing of the (110) plane (the position of ion exchange), (e) the schematic diagram for the growth mechanism of PWH.

The structure of the composites was further investigated by TEM measurements. The inset of Fig. 4a shows the TEM image of the PWH-5 sample; the morphology of the PWH sample is square, indicating that the observation direction is along the c axis. Fig. 4b shows the detailed characterization of the parts which are marked by circles and the letters c and d (c and d correspond to Fig. 4c and d). The inset of Fig. 4b shows the SAED pattern of the inner part of the composite; the diffraction spots are indexed with the (110), (200), and (020) facets of lead tungstate, indicating that the core of the composite is lead tungstate. Fig. 4c and d display the HRTEM images of two sides of the square (marked with c and d). The measurement results of the two areas are the same due to the symmetry. The obvious interface between the two phases can be observed. The lattice spacing of the outer part is 0.368 nm, corresponding to the (200) facet of WO₃·0.33H₂O. For the inner part, the lattice spacing of 0.385 nm is in agreement with the (110) plane of lead tungstate. The Fourier transforms of the two different areas are shown in the insets of Fig. 4c and d. It is worth noting that the lattice spacing of WO₃·0.33H₂O is almost equal to that of lead tungstate. Therefore, the ion exchange may occur at this position. Fig. 6 shows the crystal structure of lead tungstate; it can be seen that the lead ion is exposed at the end of the (110) facet. Therefore, it can be concluded that it is beneficial for the ion exchange between hydrogen and lead to occur at the end of the lead tungstate (110) facet, which can be proved from Fig. 4c and d. Two conclusions can be obtained from the above results. First, the outer part of the PWH sample is WO₃·0.33H₂O and the core of PWH is PbWO₄. Second, the ion exchange occurs at the end of the (110) facet of lead tungstate, and a perfect contact interface is formed between the two parts. Based on the above analysis, the growth mechanism of PWH can be illustrated by Fig. 4f.

Photodegradation experiments were carried out to investigate the photocatalytic activity of the as-prepared samples, as shown in Fig. 5. Fig. 5a shows the photodegradation curves of RhB over the as-prepared samples under 300 W Xe lamp irradiation, and Fig. 5b shows the corresponding −ln(C/C₀) vs. time curves of the as-prepared samples. Compared with the PW sample and the W sample, the PWH samples displayed much higher photocatalytic activity. After 30 min, almost all of the RhB had been eliminated over the PWH samples, while only 30% RhB and 35% RhB were eliminated over the W sample and the PW sample, respectively. This indicates that the generation of hydrated tungsten trioxide on the lead tungstate surface effectively promotes the photocatalytic activity. The kinetics of RhB photodegradation on all the samples were investigated using the Langmuir–Hinshelwood model, −ln(C/C₀) = kt, where C₀ is the initial concentration of RhB, C is the concentration of RhB at a certain time, and k is the pseudo-first-order rate constant.³⁴ The plots of −ln(C/C₀) vs. illumination time (t) were all found to be linear. This suggests that the photodegradation reactions follow pseudo-first order kinetics, and the obtained apparent reaction rate constants k can be seen in Fig. 5b. The apparent reaction rate constant of the PWH samples is more than 6 to 8 times higher than those of the PW and W samples. The photocatalytic activity is related to such factors as the absorption of light, the position of the bands, and the utilization of photo-generated carriers.

Fig. 5 Photodegradation curves of RhB solution using the as-prepared samples under irradiation with a 300 W Xe lamp (a), the −ln(C/C₀) vs. time curves of the photodegradation of RhB (b). The apparent rate constant of RhB photodegradation for the PWH samples is more than 6–8 times higher than that for the PW and W samples.

Fig. 6a shows the UV-Vis diffuse reflectance spectra of the as-prepared samples. The absorption edge of pure lead tungstate is 313 nm, and that of hydrated tungsten trioxide is 361 nm. For the PWH samples, the absorption edges were 414 nm (PWH-1), 385 nm (PWH-2), 387 nm (PWH-3), 375 nm (PWH-4), and 374 nm (PWH-5). The absorption edges of the PWH samples are all close to that of hydrated tungsten trioxide, which is caused by the existence of hydrated tungsten trioxide on the surface of lead tungstate in the PWH sample. The photocatalytic activity of the PWH samples is higher than that of the W sample, suggesting that the increase of light absorption has no effect in improving the photocatalytic activity.

Fig. 6 UV-Vis diffuse reflectance spectra (DRS) of the as-prepared samples (a), photoluminescence (PL) spectra of the PW sample and PWH-5 (b).

Photoluminescence (PL) reflects the migration, transfer, and recombination of photo-generated carriers in a photocatalyst.³⁵ Generally, a lower PL intensity indicates a lower recombination of photo-generated carriers, which results in higher photocatalytic activity. Fig. 6b displays the PL spectra of the PWH-5 sample and lead tungstate. The lower PL intensity of the PWH-5 sample suggests the enhanced separation efficiency of the photo-generated carriers. This indicates that that the heterojunction structure formed in the PWH composite promotes effective electron–hole pair separation, which is the origin of the superior photodegradation activity of the PWH composite. Therefore, the utilization of photo-generated carriers plays the key role in the photocatalytic activity of the composite. Furthermore, the photo-generated carrier transport mechanism remained to be determined.

In order to determine the photo-generated carrier transport mechanism, the band structure of the two components were first investigated. Fig. 7a and b show the Tauc curves and VB XPS of the single components. According to the formula of Tauc,³⁶ the E_g of PW is 3.88 eV and the E_g of W is 3.15 eV; the VB potentials of PW and W are 1.77 eV and 2.95 eV respectively. Therefore, the conduction bands of PW and W are −2.11 eV and −0.2 eV respectively.

Fig. 7 Tauc curves of PW sample and W sample (a), VB XPS of PW sample and W sample (b).

In order to understand the photocatalytic mechanism, photodegradation experiments with different quenchers as well as different kinds of irradiation over the PWH-5, W, and PW samples were carried out. In these experiments, pumping nitrogen, DMSO, and sodium oxalate were used to quench superoxide radical (˙O₂⁻), hydroxyl radical (˙OH), and photo-generated holes (h⁺), respectively. Fig. 8a shows the results over the PWH composite. Adding DMSO or sodium oxalate greatly decreased the photocatalytic activity, indicating that ˙OH and h⁺ are the dominant active species. In addition, the main role of h⁺ is the generation of ˙OH, as seen from the result of the experiment with the addition of DMSO. Pumping nitrogen had nearly no effect on the photodegradation of RhB over the PWH composite, indicating that ˙O₂⁻ has little influence. The same experiments were carried out over the PW sample and the W sample. For the PW sample (Fig. 8b), ˙O₂⁻ (pumping nitrogen) and ˙OH (adding DMSO) had a major influence on the photodegradation of RhB, and h⁺ (adding sodium oxalate) had little effect on the photodegradation of RhB. This indicates that the photo-generated electron plays a major role, while h⁺ has a small role. Inspecting the W sample (Fig. 8c), pumping nitrogen hardly reduced the photodegradation activity. Adding DMSO showed a certain inhibition effect on the photodegradation activity. Adding sodium oxalate obviously inhibited the activity of the photodegradation of RhB. These results indicate that h⁺ plays the main role and the photo-generated electron plays a minor role in the case of the W sample. Based on these results, it can be known that the photo-generated electron is the main factor in the photodegradation of RhB for lead tungstate, while the h⁺ is the main factor in the photodegradation of RhB for hydrated tungsten trioxide and the composite sample, due to the fact that the h⁺ on lead tungsten has no effect on the photodegradation of RhB. The photocatalytic mechanism is not in conformity with type I (photo-generated carriers transfer from the band with higher redox abilities of one component to the band with lower redox abilities of another component). Furthermore, the degree of inhibition over the composite is larger than that of either single component, indicating that the composite has greater photo-generated carrier utilization efficiency, which can be also verified by the PL measurements (Fig. 6b) and higher photodegradation rate (Fig. 5b). This indicates that the photocatalytic process may comply with the Z-scheme configuration (type II).

Fig. 8 Photodegradation curves of RhB solution using different quenchers for PWH-5 (a), PW (b), W (c), and TEM images of the photodeposition of Pt on PWH-5 (d–f).

In order to further prove the Z-scheme photocatalytic process, a platinum photodeposition experiment was carried out to further understand the photocatalytic mechanism, as shown in Fig. 8d–f. The lattice spacing of a small particle of 0.225 nm corresponds to the platinum (111) facet. This indicates that the reduction of platinum by light irradiation occurs on the surface of lead tungstate, indicating that photo-generated electron stays on lead tungstate rather than on hydrated tungsten trioxide. In addition, the band positions of the two components are suitable for constructing the Z-scheme configuration. Therefore, this suggests that the photo-generated carrier transport mechanism is the Z-scheme (type II) configuration.

The photodegradation of organic dyes is usually accompanied by sensitization. Therefore, we also wished to examine whether sensitization occurred in the photocatalytic process. Fig. S4† exhibits the photodegradation of RhB over composite and monomer under visible light. Lead tungstate had no photocatalytic activity, indicating that no photosensitization effect exists, but this was not the case for hydrated tungsten trioxide and composite. However, the photosensitization effect is inconspicuous. According to the results, the Z-scheme is an effective method for the elimination of organic dyes. The photocatalytic mechanism schematic diagram can be depicted as shown in Fig. 9.

Fig. 9 Schematic for the photocatalytic degradation process of PWH.

After investigating the photocatalytic process of the photodegradation of RhB over PWH, we further studied the stability of the Z-scheme PWH; this is because stability is a desirable property for a photocatalyst. To study the stability of the PWH composite, a PWH-5 sample was employed to carry out the cycling RhB photodegradation test, as shown in Fig. 10. After eight cycles, there was no obvious decrease, indicating that the PWH composite has outstanding stability. In addition, the dark adsorption of every loop also shows no obvious change; this may be due to the fact that the number of adsorption sites remains the same after the photodegradation process. These results indicate that the perfect heterojunction exhibits excellent stability in the photocatalytic process.

Fig. 10 Recycled photodegradation of RhB under the irradiation of a 300 W Xe lamp over PWH-5.

Finally, the photocatalytic activity of PWH-5 toward another organic dye was also investigated. MO was chosen as the photodegradation target over PWH-5 under 300 W Xe lamp irradiation, as shown in Fig. S5a.† The PWH-5 sample exhibited excellent photocatalytic activity for the photodegradation of MO. After 60 minutes of irradiation, 82% of MO was eliminated. Photodegradation mechanism experiments were also carried out to investigate which active species plays the key role in the MO photodegradation process, as shown in Fig. S5b.† The results show that h⁺ is the key factor for the photodegradation of MO.

Based on the above investigations, it is suggested that ion exchange is the possible method for the construction of a Z-scheme system. Meanwhile, the Z-scheme system, which can retain higher redox abilities for photo-generated carriers, is a promising approach for the use of solar energy to eliminate environmental pollution.

Conclusions

In this work, a highly efficient Z-scheme composite photocatalyst WO₃·0.33H₂O/PbWO₄ was synthesized via an ion-exchange method. The HRTEM images display a perfect heterojunction between hydrated tungsten trioxide and lead tungstate, indicating that a perfect composite photocatalyst was constructed. Dye photodegradation experiments indicated that the Z-scheme composite exhibits considerably improved efficiency for the photodegradation of RhB in comparison with pure hydrated tungsten trioxide and lead tungstate. The excellent photocatalytic activity of the composite is due to the close integration of the heterogeneous structure and the highly desirable Z-scheme configuration. In the photodegradation of RhB, the reaction rate of the composites was more than 6 to 8 times higher than those of its single components, and the RhB was almost eliminated after 30 min. Moreover, the composite showed excellent stability. In addition, the Z-scheme photocatalyst also shows excellent activity in the photodegradation of MO.

Acknowledgements

This work is financially supported by projects of the Natural Science Foundation of China (21271055) and the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. A. 201410).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Table S1: the short names of the synthesized samples with different synthesis conditions. Table S2: XPS data of PW, PWH-5, and W. Fig. S1: band dispersion and density of states (DOS) for PbWO₄ (a), partial DOS for Pb atom, W atom, and O atom of PbWO₄ (b). Fig. S2: SEM images of PWH-1 sample (a), PWH-2 sample (b), PWH-3 sample (c), PWH-4 sample (d), and PW sample (e). Fig. S3: SEM images of PWH samples with different amount of dilute nitric acid (V_HNO3:V_H2O is 1 to 5), (a) 0.3 mL, (b) 0.65 mL, (c) 1.3 mL, (d) 3.9 mL. Fig. S4: photodegradation of RhB over composite and monomer under visible light. Fig. S5: photodegradation curves of different dye solutions using the PWH-5 sample under irradiation with a 300 W Xe lamp (a), photodegradation curves of MO solution using different quenchers over the PWH-5 sample under 300 W Xe lamp illumination (b). See DOI: 10.1039/c5ra06394a

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