Novel preparation of stable and highly photocatalytic Z-scheme Cs₃PW₁₂O₄₀/Ag₃PO₄ photocatalysts for the photocatalytic degradation of organic contaminants in water Open Access

In recent years, environmental pollution has posed a serious threat to human health. Various types of dye wastewater produced by factories have become a severe challenge. Identifying new pollution treatment technologies with high efficiency, low energy consumption, and wide application range is an urgent task (Lai et al. 2010; Dong et al. 2016). Among the technologies, semiconductor photocatalysis has been widely studied because of its use of solar energy as the driving energy, and it offers the advantages of good redox capacity, low cost, environmental friendliness, and high efficiency. The treatment of dye wastewater by semiconductor photocatalysis is undoubtedly one of the most ideal and environmentally friendly technological methods (Chen et al. 2018; Wei et al. 2018). As a traditional wide band-gap semiconductor material, TiO₂ has a band gap of 3.2 eV and can only absorb ultraviolet light at less than 387 nm, which limits its further development. In addition, the high recombination rate of photogenerated electrons and holes hinders the treatment of highly concentrated industrial wastewater. Therefore, the development of new forms of visible light-driven photocatalysis for expanding the response range to a much wider wavelength or promoting the effective separation of photoinduced charge carriers is considered to be a promising method of degrading pollutant in wastewater (Xu et al. 2021).

Among the developed visible light-driven photocatalysts, silver photocatalysts have been widely studied. The d10 electronic structure of Ag can adjust different energy band structures and ensure a narrow band gap. Among the many silver photocatalysts, Ag₃PO₄ as a typical body-centered cubic structure is the most popular because of its high quantization efficiency (Liu et al. 2012), strong photooxidation, and high-efficiency photocatalytic degradation of organic pollutants. At the same time, Ag-Ag bands in Ag₃PO₄ are highly dispersed and have no localization state and thus can rapidly transfer small electrons to the surface and prevent carrier recombinations (Li & Mao 2012). However, Ag₃PO₄ nanoparticles have instability problems in practical application. If sacrificial reagents are ignored, then electrons will be absorbed by Ag ions released from the Ag₃PO₄ lattice, thus hindering their further development (Wang et al. 2012). At the same time, the top of the conduction band (CB) of Ag₃PO₄ is lower than the reduction potential of H₂O, thus limiting H₂O to capture light-induced electrons for H₂ production. It is an effective strategy of finding a semiconductor with a suitable energy band structure comprising Ag₃PO₄. Liu et al. (2016) synthesized Ag₃PO₄@g-C₃N₄ core-shell composites and it was found that the appropriate matching energy level between them can promote the effective separation and transfer of photogenerated electron holes at the interface. A methylene blue of 97% can be degraded in 30 min, enabling the photocatalytic performance to be greatly improved. In addition, the core-shell structure of g-C₃N₄ can protect Ag₃PO₄ from dissolution during photocatalytic reaction, enabling the stability to further improve. Thus far, assembling Ag₃PO₄ with other semiconductors to form a heterojunction has been proven to be an effective strategy of improving photocatalytic performance (Chen et al. 2015; Zhang et al. 2019).

As a solid acid catalyst, heteropoly acids (HPAs) have many applications. Recently, HPAs have been applied in catalysis and wastewater purification owing to their diverse structure, strong oxidation, and environmental friendliness (Luo et al. 2012; Ruiz et al. 2012). However, HPA has certain solubility defects, a small surface area, and a low utilization rate for its active center. In general, insoluble polyoxometalates (POMs) are deposited with cations with a large radius, including K⁺, Cs⁺, and NH₄⁺; similarly, HPAs are immobilized on appropriate carriers (Shi et al. 2006; He et al. 2020). Among the various POMs, HPAs with an appropriate Cs content have wide mesoporous surface areas because of the accumulation of primary particles. Therefore, the oxygenates formed by Cs⁺ deposition are often used as solid acid catalysts. The cesium-based POM Cs₃PW₁₂O₄₀, which is insoluble in water, is a good candidate for liquid-phase catalytic reactions. Tahmasebi et al. (2019) synthesized Cs₃PW₁₂O₄₀/WO₃ composites via thermal decomposition. The good removal efficiency of RhB by the catalyst is mainly attributable to the separation efficiency and strong interaction at the interface of the two semiconductors. Considering the higher CB potential of Ag₃PO₄ compared with the hydrogen potential, as well as the strong coordination ability of the surface oxygen atoms of polyanions, a matching of the energy bands of Cs₃PW₁₂O₄₀ and Ag₃PO₄ seems to be an effective approach of improving photocatalytic activity.

Herein, a series of CsPW/Ag₃PO₄ photocatalyst was successfully prepared via a simple in-situ precipitation method. The photocatalytic activities of the as-obtained CsPW/Ag₃PO₄ photocatalyst were estimated by RhB removal under the illumination of visible light.The improved visible light photocatalytic activity can be attributed to the synergistic effect of the heterojunction between CsPW and Ag₃PO₄. The Z-scheme structure can effectively improve the separation efficiency of the carriers in Ag₃PO₄, reduce the occurrence of photocorrosion, and improve the stability of Ag₃PO₄. In addition, the possible active substances in the process of photocatalytic degradation were determined, and the possible mechanism of RhB photodegradation was analyzed.

Silver nitrate (AgNO₃, 99.8%), trisodium phosphate dodecahydrate (Na₃PO₄·12H₂O, 98.0%), cesium carbonate (Cs₂CO₃, 99.0%), Phosphotungstic acid (H₃PW₁₂O₄₀·xH₂O, 99.0%, abbreviated as HPW), anhydrous ethanol, benzoquinone (BQ, 99.6%), isopropanol (IPA, 99.0%), triethanolamine (C₆H₁₅NO₃, 99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B (RhB, 99.0%), Tetracycline (C₂₂H₂₄N₂O₈·xH₂O,98.0%), Phenol (C₆H₆O,99.0%) were purchased from Aladdin Reagent Co. Ltd; all water used in the experiments was ultra-pure water. All chemicals were analytically pure and could be used without further purification.

Here, the CsPW samples were prepared via typical synthesis. First, 3.129 g of Cs₂CO₃ was dissolved in 200 mL of ultrapure water. Then, the H₃PW₁₂O₄₀ · H₂O (19.584 g dissolved in 200 mL ultrapure water) aqueous solution was added to the solution drop by drop under magnetic stirring to gradually generate white turbidity. The dropping was continued until no new white precipitation could be generated. Subsequently, the mixture was stirred for 10 h and allowed to stand for 2 h. The product was collected by centrifugation and washed repeatedly with distilled water. The AgNO₃ solution was used to ensure that the supernatant would not produce precipitation. The product was freeze-dried for 4 h and then placed in a muffle furnace for calcination at 300 °C for 2 h. Finally, the product was collected and ground into powder using an agate mortar for further use.

The CsPW/Ag₃PO₄ composite was prepared via a simple precipitation method. First, similar to a typical synthesis, 0.098 g of the as-prepared spherical CsPW was dispersed in 80 mL of ultrapure water via ultrasound. Then, 0.510 g of AgNO₃ was added to the suspension and stirred for 30 min. Then, Na₃PO₄·12H₂O aqueous solution (0.419 g dissolved in 20 mL ultrapure water) was added to the mixture slowly and drop by drop. The prepared suspension was further stirred in the dark for 2 h until the sediment could be obtained by centrifugation. Finally, the product was washed several times with ultrapure water and absolute ethanol, dried in an oven at 60 °C, and ground for further use. On the basis of the mass ratios of CsPW and Ag₃PO₄, other CsPW/Ag₃PO₄ composite photocatalysts with loading values of 1.5, 5, and 7% were prepared. For comparison, pure Ag₃PO₄ was obtained under the same conditions without CsPW.

X-ray diffractometer (XRD, Bruker D8 advance) was used to analyze the crystal structure and crystallinity of the photocatalyst. A field emission scanning electron microscope (SEM, Hitachi SU8100) and a transmission electron microscope (TEM, Tecnai G2 F20) were used to characterize the surface morphologies and structures of the samples. X-ray photoelectron spectrometer (XPS, Thermo Escalab 250) was used to analyze the chemical states and elemental compositions of the samples. The UV-visible absorption characteristics of the photocatalyst were measured by UV-Vis (TU-1901). The surface area of the catalyst was measured using a Brunauer-Emmett-Teller surface area analyzer (BET, Micromeritics ASAP 2050). The pore size distribution of the sample was obtained via the Barrett-Joyner-Halenda desorption method. Electrochemical impedance spectroscopy (EIS) was implemented using an electrochemical workstation (CHI660E) in a standard three-electrode system. The Pt electrode and saturated calomel electrode were selected as the counter-electrode and reference electrode. The working electrode was prepared by coating the catalysts on a fluorine-doped tin oxide (FTO) glass substrate, and 0.2 M of the Na₂SO₄ aqueous solution was used as the electrolyte.

The photocatalytic activity of the CsPW/Ag₃PO₄ composite photocatalyst was evaluated via the catalytic degradation of RhB under visible light irradiation. The light source setup was composed of a 350 W xenon lamp (CEL-300, Beijing Zhongjiao Jinyuan Technology Co., Ltd) and a 420 nm cut-off filter. The 10 mg photocatalyst was weighed and then poured into the 100 mL RhB solution (10 mg/L). Prior to illumination, the suspension was placed in a dark environment and stirred magnetically for 60 min to ensure the adsorption-desorption balance of RhB on the photocatalyst surface (Figure S1). During illumination, 4 mL of the suspensions from the sample were collected at 15 min intervals and then removed via centrifugation. The changes in the maximum value of the absorption band at 554 nm were recorded and analyzed using a UV-Vis spectrophotometer. The degradation percentage was recorded as C/C₀, where C is the RhB concentration of each irradiation time interval at 554 nm as recorded by the UV-Vis spectrophotometer, and C₀ is the initial concentration of RhB.

The degradation in the phenol and tetracycline experiment is consistent with that in the RhB experiment. The only difference is that the UV-Vis spectrophotometer was used at the wavelengths of 365 and 270 nm to measure the absorbance.

The crystal structure of the prepared photocatalyst was analyzed by XRD. As shown in Figure 1, the pure-phase Ag₃PO₄ has obvious diffraction peaks at 29.73°, 33.37°, and 36.68°, which is consistent with the standard spectrum of the Ag₃PO₄ cubic structure (PDF06-0505), and no other impurity peaks are observed. The diffraction peak of CsPW mainly appears in the range of 15° to 40° and 45° to 55°, indicating that the structure of phosphotungstic acid is the Keggin type, which belongs to tungsten oxide cesium phosphate (JCPDS.50-1857). The diffraction peak of CsPW appears in the characteristic peak of Ag₃PO₄ at 26.12°, and the diffraction peak of CsPW gradually increases with the gradual rise of CsPW load. This trend indicates that the introduction of CsPW will not change the crystal structure of the Ag₃PO₄. Furthermore, the peak shape is stable and does not appear with other impurity peaks, indicating that the purity of the photocatalyst is high. The above findings also prove that CsPW is successfully loaded on the surface of Ag₃PO₄.

The micromorphology of the sample was observed via SEM. Figure 2(a) shows the SEM observations of the prepared pure-phase Ag₃PO₄, which is closely stacked by irregular small polyhedra with a size of 200–300 nm. The morphology of the pure CsPW sample is spherical with a rough surface, as shown in Figure 2(b). The SEM image of 3% CsPW/Ag₃PO₄ composite photocatalyst is shown in Figure 2(c). The CsPW spherical particles are dispersed and adhered onto the outer surface of Ag₃PO₄. The good combination of the two composites is conducive to the transmission of electrons in the photocatalytic process for improving photocatalytic performance. More detailed structure of 3% CsPW/Ag₃PO₄ was detected by TEM and HRTEM. Figure 2(e) shows that most CsPW nanoparticles are decorated on the Ag₃PO₄ surface, which is consistent with the SEM observation. Figure 2(f) shows the HRTEM image of the 3% CsPW/Ag₃PO₄. The lattice fringes with a spacing of 0.268 nm correspond to the (210) plane of Ag₃PO₄ (Zhao et al. 2015). The results are consistent with the XRD findings. Figure S2 shows the EDS elemental mapping of the composite photocatalyst. The composite photocatalyst is composed of Ag, P, O, Cs, and W elements. All of the elements are well distributed in the composite photocatalyst, which further proves that CsPW can be successfully combined with Ag₃PO₄.

The molecular structure and functional groups of the composite samples were analyzed by FT-IR. Figure 3 shows the infrared spectra of CsPW, Ag₃PO₄, and a variety of CsPW/Ag₃PO₄ composite photocatalysts with different composite ratios. The infrared spectra confirm the existence of CsPW and Ag₃PO₄ in the prepared complex. Three main absorption regions can be clearly observed from the spectrum of Ag₃PO₄. The 557 cm⁻¹ corresponds to the in-plane bending vibration of the O = P-O group (Liu et al. 2018); the absorption peak at 1,387 cm⁻¹ originates from the stretching vibration and harmonic of the P = O double bond; and the absorption peaks at 1,659 cm⁻¹ can be attributed to the tensile vibration and bending vibration of the H-O bond of the surface with adsorbed H₂O (Anwer & Park 2018). The characteristic peak absorption bands of CsPW at 985 and 1,081 cm⁻¹ correspond to the terminal vibration of W = O bond in the center and the asymmetric tensile vibration of P-O bond (Liu et al. 2018). The absorption peak at 888 cm⁻¹ is related to W-Ob-W (Ob represents the oxygen atom between two different W₃O₁₃ groups) (Xu et al. 2012). The absorption peak at 825 cm⁻¹ can be attributed to the stretching vibration peak of the W–Oc bond, and the peak at 557 cm⁻¹ is related to the vibration of O-P-O (Yue et al. 2019). As for the infrared spectrum of the CsPW/Ag₃PO₄ composite photocatalyst with varying composite ratios, the characteristic peak has both CsPW and Ag₃PO₄, indicating the successful preparation of the CsPW/Ag₃PO₄ composite photocatalyst. Owing to the strong interaction of the composite catalyst, the peak intensity has a slight red shift.

The chemical composition and surface valence of the composites were further studied by XPS analysis. Figure S3(a) shows the high-resolution full-scan spectrum of the 3% CsPW/Ag₃PO₄ composite. The findings demonstrate that the sample mainly contains Ag, P, O, Cs, and W elements. Figure S3(b) shows the high-resolution 3d spectrum of Ag. The two main peaks at 366.86 and 372.88 eV are consistent with those of Ag 3d_5/2 and Ag 3d_3/2, respectively. The two double peaks indicate the existence of the Ag⁺ state of Ag₃PO₄ (Chai et al. 2014). With the compounding of CsPW, the peak shape moves to the direction of high-binding energy, increasing to 368.17 and 374.17 eV. It may be attributed to the CsPW enhancing the electron transfer by lowering the external electrons of the 3% CsPW/Ag₃PO₄, thus increasing the binding energy and improving the photocatalytic performance. Figure S3(c) shows the high-resolution 2p spectrum of P. The peak position appears at 132.93 eV, indicating the existence of a P⁵⁺ oxidation state. The two characteristic peaks at 530.90 and 533.21 eV in Figure S3(d) represent the binding energy of O 1 s, which can be attributed to O in the Ag₃PO₄ and W-O bond in CsPW (Yu et al. 2020). As shown by the high-resolution W4f spectrum in Figure S3(e), the binding energy peaks at 36.48 and 38.61 eV correspond to W4f_7/2 and W 4f_5/2, respectively. The W4f peak can be further divided into W4f_7/2 (36.08 eV) and the binding energy of W4f_7/2, which corresponds to the W⁶⁺ state in CsPW and K₃PW₁₂O₄₀ (Sasca et al. 2011). The XPS analysis further proves that CsPW and Ag₃PO₄ can coexist.

The specific surface area and pore structure of the material were analyzed using a nitrogen adsorption-specific surface area tester. The results are shown in Figure 5(a)–(c). In accordance with the IUPAC classification, a hysteresis loop can be easily observed at the high relative-pressure (P/P₀) region in the 3% CsPW/Ag₃PO₄. This finding indicates that the 3% CsPW/Ag₃PO₄ accordance with the type IV isotherm. The calculated specific surface area of the 3% CsPW/Ag₃PO₄ is 23.17 m²/g, which is larger than that of Ag₃PO₄ (3.12 m²/g). The difference can be explained by the participation of CsPW (86.05 m²/g) with a large specific surface area. The introduction of CsPW obviously affects pore size and pore volume. Figure 5(d) shows the pore size distribution of the 3% CsPW/Ag₃PO₄, indicating the presence of mesoporous. The large specific surface area is conducive to the contact between the catalyst and pollutants, and it can provide more active sites for achieving better photocatalytic performance.

The 3% CsPW/Ag₃PO₄ composite photocatalyst was used to explore the general applicability of the prepared composite photocatalyst in relation to the refractory pollutants. In this manner, the degradation of phenol and tetracycline solution could be assessed. The degradation results are shown in Figure S4. Under visible light irradiation, the degradation efficiency of the 3% CsPW/Ag₃PO₄ material for tetracycline is 95.17%. Phenol contains a refractory benzene ring structure. The degradation efficiency is 89.49% under visible light irradiation for 270 min. It is also proved that the 3% CsPW/Ag₃PO₄ composite photocatalyst has a photocatalytic degradation ability for different refractory pollutants.

The problems associated with photocatalysts in practical applications should be considered. Here, a catalyst circulation experiment in four consecutive cycles was conducted to explore the stability and reusability of the 3% CsPW/Ag₃PO₄. The results are shown in Figure S5(a). The degradation activity of the 3% CsPW/Ag₃PO₄ composite is not significantly reduced, and only slight changes can be observed. The removal efficiency of RhB in the photocatalytic process is 94.82%, which may be explained by the slight loss of the catalyst during recovery. By contrast, the degradation activity of Ag₃PO₄ manifests an obvious loss, and the photocatalytic activity is decreased from 49.78% to 33.67%.

The pure-phase Ag₃PO₄ and 3% CsPW/Ag₃PO₄ composite photocatalysts after the photocatalytic reaction were characterized by XRD (Figure S5(b)). The characteristic peak of Ag appears at 38.25° for both the composite and pure-phase Ag₃PO₄, and it corresponds to the (111) crystal plane of Ag (Peng et al. 2014). It shows that a certain amount of photocorrosion has likely occurred in the process of recycling. However, the diffraction peaks of the pure Ag₃PO₄ are significantly higher than 3% CsPW/Ag₃PO₄ composite materials, which confirms that the pure Ag₃PO₄ is more easily decomposed to Ag than 3% CsPW/Ag₃PO₄, resulting in the decline of the photocatalytic activity.

The active substances in the photocatalytic reaction were further verified by the addition of IPA, followed by BQ and TEOA. As shown in Figure 7(a), the added IPA has a negligible effect on the photocatalytic degradation of the 3% CsPW/Ag₃PO₄. This finding also indicates that the •OH radical has no effect on the photocatalytic activity. Meanwhile, the added BQ and TEOA have significantly inhibited the photocatalytic activity. Therefore, under visible light irradiation, the photocatalytic degradation of RhB by the 3% CsPW/Ag₃PO₄ composite is mainly controlled by h⁺ and ⁠. The order of the active species of photocatalytic materials in the degradation process is ⁠.

The electron-hole pair transfer and charge transfer characteristics of the photocatalysts were studied via EIS. A smaller radius implies more efficient charge transfer and faster interfacial charge transfer, as can be seen from Figure 7(b). The 3% CsPW/Ag₃PO₄ composite has a smaller arc radius than the pure Ag₃PO₄, indicating a reduced resistance. This finding indicates that the 3% CsPW/Ag₃PO₄ composite heterostructure can inhibit the recombination process of electrons and holes, thus promoting the migration of charge carriers in the system.

Figure S6 shows the Mott-Schottky diagram of CsPW. It can be seen that CsPW is a typical n-type semiconductor with a positive slope. For the n-type semiconductors, the CB position is 0.20 V higher than the flat-band potential. The tangent potential indicates that the E_f of CsPW is −0.6 eV. Therefore, the CB of CsPW is −0.4 eV. According to the abovementioned UV diffuse reflection test results and the formula Eg = E_VB-E_CB, the calculated valence band (VB) of CsPW is 2.51 eV.

Combined with the UV-Vis spectrum, the band gap of Ag₃PO₄ is 2.32 eV. According to the formula, the E_CB and E_VB values of Ag₃PO₄ are 0.54 and 2.86 eV, respectively. Furthermore, on the basis of the traditional type II photogenerated electron transfer mechanism (Figure 8(a)), CsPW and Ag₃PO₄ are simultaneously excited to produce photogenerated electrons and hole pairs under visible light irradiation. The CB and VB of Ag₃PO₄ are lower than those of CsPW. Therefore, photogenerated electrons are expected to transfer from the CB of CsPW to the CB of Ag₃PO₄, and holes are expected to transfer from the VB of Ag₃PO₄ to the VB of CsPW for redox reaction. However, the CB potential of Ag₃PO₄ is greater than the redox potential of (−0.33 eV vs. NHE). Thus, the e⁻ on the CB of Ag₃PO₄ cannot combine with O₂ to form ⁠. The VB potential of CsPW is lower than the H₂O/•OH redox potential (2.72 eV vs. NHE). Therefore, the holes of CsPW cannot produce •OH, which is inconsistent with the active species capture experiment.

Consequently, a new Z-scheme photodegradation mechanism was proposed in this study (Figure 8(b)). First, under the excitation of visible light and owing to the good energy band matching relationship between CsPW and Ag₃PO₄, photoinduced metal silver nanoparticles were used as a bridge to accelerate the charge transfer. Then, the photogenerated electrons in Ag₃PO₄ CB were transferred to the VB of CsPW to improve the effective separation of electrons. The results indicate the accumulation of electrons in the CB of CsPW and holes in the VB of Ag₃PO₄. The CB of CsPW has a smaller redox potential (−0.33 eV vs. NHE) than ⁠. Therefore, a reduction reaction has occurred by capturing oxygen molecules to form groups. The VB (2.86 eV) of Ag₃PO₄ are more positive than the redox potential of •OH/OH⁻ (2.38 eV to NHE). Thus, a small part of h⁺ on the VB can react with OH⁻ and H₂O to form •OH, which can partially or completely mineralize the organic pollutants. The remaining h⁺ is directly involved in the oxidation of RhB. In summary, the photocatalyst using the electron transfer path in the Z-scheme mechanism can effectively inhibit the recombination of electron-hole pairs and promote the separation of charge carriers, thus greatly improving the photocatalytic activity and structural stability.

In conclusion, a series of novel CsPW/Ag₃PO₄ photocatalysts with different mass ratios were prepared by means of a simple chemical co-precipitation method. The construction of the Z-scheme mechanism heterostructure can realize the significant separation of photogenerated electrons and holes. The UV-Vis characterization shows that the composites have a wider visible light absorption range. Among the photocatalysts with different mass ratios, the 3% CsPW/Ag₃PO₄ composite photocatalyst has the highest photocatalytic activity for the degradation of RhB. Furthermore, the RhB removal rate of 96.98% can be achieved given the same illumination time. The rate constant is 4.26 times that of pure Ag₃PO₄. The free radical capture experiments showed that h⁺ and play a major role in RhB degradation. In addition, the composite also has excellent cycle stability and recyclability. According to the experimental results, the prepared photocatalyst can be further developed for the practical treatment of refractory pollutants.

This research was supported by Hunan Province Strategic New Major Project (2019GK4041) and Changsha Science and Technology Plan Project (kq1907095).

The authors declare no competing interests.

All relevant data are included in the paper or its Supplementary Information.