Enhanced visible-light photocatalytic activity and photostability of Ag₃PO₄/Bi₂WO₆ heterostructures toward organic pollutant degradation and plasmonic Z-scheme mechanism

Abstract

Novel Ag₃PO₄/Bi₂WO₆ heterostructured materials with enhanced visible-light catalytic performance were successfully synthesized by assembly combined with a hydrothermal treatment. The microstructures, morphologies, and optical properties of the prepared samples were characterized by multiple techniques. The irregular Ag₃PO₄ nanospheres dispersed on the surface of Bi₂WO₆ nanoflakes, and their catalytic performances were evaluated via the degradation of organic pollutants including rhodamine B (RB), methylene blue (MB), crystal violet (CV), methyl orange (MO), and phenol (Phen) under visible-light irradiation. The resulting Ag₃PO₄/Bi₂WO₆ heterostructured materials displayed higher photocatalytic activity than that of either pure Bi₂WO₆ or Ag₃PO₄. The enhanced photocatalytic activity was due to the good formation of heterostructures, which could not only broaden the spectral response range to visible light but also effectively promoted the charge separation. Meanwhile, the reasonable photoreactive plasmonic Z-scheme mechanism was carefully investigated on the basic of the reactive species scavenging tests, photoelectrochemical experiments, and photoluminescence (PL) spectrum. In addition, the excellent photostability of Ag₃PO₄/Bi₂WO₆ was obtained, which Ag formed at the early photocatalytic reaction acted as the charge transmission-bridge to restrain the further photoreduction of Ag₃PO₄.

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1. Instruction

Recently, semiconductor photocatalysis has attracted more and more attention as a green, highly efficient technology for resolving the current energy and environmental problems.^1–3 Photocatalysts based on TiO₂, ZnO are inactive under visible light irradiation due to their wide bandgap energy and improper band position, which requires ultraviolet irradiation that only accounts for less than 5% of the solar energy.^4–6 And the main reason for their poor photocatalytic activity might be due to the strong recombination of photogenerated electron–hole (e⁻–h⁺) pairs, which is a widespread phenomenon among semiconductor photocatalysts. Therefore, it is still challenging to extend the photo-response range and improve the charge separation efficiency of the semiconductor photocatalysts for satisfying the requirements of applications.

Recently, Bi-based semiconductor materials, such as Bi₂WO₆, BiVO₄, Bi₂MO₆, Bi₂Mo₂O₉, Bi₂₄O₃₁Br₁₀, and BiOBr, have attracted widespread attention in photocatalytic application owing to the relatively narrow gap, nontoxicity, chemical inertness, stability, and sunlight utilization for wastewater treatment.^7–14 Among the Bi-based semiconductor materials, Bi₂WO₆, as a typical aurivillius oxide, is regarded one of the ideal photocatalyst materials because of its unique layered structure feature and relatively high visible photocatalytic activity. However, the photocatalytic activity of bare Bi₂WO₆ is limited by the high recombination rate of photogenerated e⁻–h⁺ pairs and the low absorption capacity of visible-light (less than 450 nm).^15,16 Therefore, how to improve separation efficiency of photoinduced e⁻–h⁺ pairs during the photocatalytic reactions becomes a key issue for developing the Bi₂WO₆-based photocatalysts.

More recently, among the reported Ag-based semiconductors, especially silver orthophosphate (Ag₃PO₄), which has a strong absorption (λ < 530 nm), has been considered as a promising photocatalyst with outstanding visible-light catalytic activity.^17,18 Meanwhile, Ag₃PO₄ possesses extremely oxidation power, which not only oxidizes H₂O to produce O₂ but also degrades organic pollutants under visible-light irradiation.^19–22 However, there are some flaws limit its extensive use as following: (1) Ag₃PO₄ is slightly soluble in water due to its larger solubility product constant (K_sp = 1.6 × 10⁻¹⁶)²³; (2) as the photosensitive material, it can be easily corroded by light and reduced to Ag⁰ in the photocatalytic process without electron acceptors;²⁴ (3) it suffers from the fast recombination of photogenerated charge-carries.²⁵

In the present study, Ag₃PO₄ is introduced on the basis of the following points: (1) Ag₃PO₄ is a narrow-semiconductor (E_g = 2.4 eV), and its introduction is beneficial to extend the light response of Bi₂WO₆ to a higher wavelength region and thereby decreasing its band gap; (2) two semiconductors possess the matching band structure, similar band gap, and the proper molar ratio, and build effective heterostructures, which promote the efficient separation of the e⁻–h⁺ pairs and therefore increasing the quantum yield; (3) part Ag₃PO₄ is reduced to metallic Ag during the initial photocatalytic reaction. It is precisely this part of the elemental silver can be used as a solid-state electron mediator to accelerate charge separation through Z-scheme system, which improve the charge separation, prevent the continuing reductive decomposition of Ag₃PO₄, and enhance its stability. Hence, the coupling of Bi₂WO₆ with Ag₃PO₄ to form heterostructures is considered to be one of the most promising methods to improve the visible-light-driven catalytic performance of photocatalysts (Bi₂WO₆ or Ag₃PO₄) because the heterostructures not only broaden the spectral response range to visible light but also promote the charge separation. This point is confirmed by the deposition of Ag₃PO₄ onto certain semiconductor materials. Please see the Table 1 (ESI†). Table 1 summarizes the detailed information on literature reported Ag₃PO₄ photocatalysts. Compared with the semiconductor materials, such as ZnO,²⁶ graphene²⁷ (G), InVO₄/BiVO₄,²⁸ and Bi₂MoO₆,²⁹ Bi₂WO₆ as support can well improve the photocatalytic activity of Ag₃PO_4.

Table 1 Textural parameters of various materials

Samples	SBET (m^2 g^−1)	Vp (cm^3 g^−1)	Dp (nm)
Ag3PO4	8.06	0.038	11.47
Ag3PO4/Bi2WO6-0.3	14.50	0.049	9.46
Ag3PO4/Bi2WO6-0.5	18.21	0.070	10.74
Bi2WO6	43.55	0.16	10.44

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Among a good deal of the heterojunction photocatalysts, the all-solid-state Z-scheme photocatalytic system possesses advantages over improving separation ratio of photo-induced e⁻–h⁺ pairs and enhancing the stability of photocatalyst. In general, the all-solid-state Z-scheme photocatalytic system consists of two different semiconductor materials and an electron mediator. Notably, metallic Ag as a solid-state electron mediator would contribute to the construction of Z-scheme system. Recently, Yuan et al. successfully prepared Ag₃PO₄/CuBi₂O₄ photocatalyst and formed Ag₃PO₄/Ag/CuBi₂O₄ photocatalyst during the photocatalytic degradation process, which can enhance photocatalytic activity and stability of photocatalyst through the formation of Z-scheme system.³⁰ Liu et al. also designed a novel visible-light-driven Ag/Ag₃PO₄/WO₃ Z-scheme heterostructures by a facile deposition–precipitation method followed by photoreducing Ag⁺ into metallic Ag. The Ag/Ag₃PO₄/WO₃ showed enhanced photocatalytic RhB efficiency, indicating the formed Z-scheme heterostructures could efficiently promote the separation and transfer of photogenerated e⁻–h⁺ pairs.³¹ So the combination of Bi₂WO₆ and Ag₃PO₄ to construct Z-scheme would be an efficient strategy.

To the best of our knowledge, there is less report on the achievement of Bi₂WO₆-based heterojunction by the introduction of Ag₃PO₄. For instance, Somchai Thongtem prepared Ag₃PO₄/Bi₂WO₆ toward RB (5 ppm) degradation efficiency can reach 100% after 80 min visible-light illumination.³² Because it is still not sufficient for practical application. It is necessary to further improve the photocatalytic activity of Ag₃PO₄/Bi₂WO₆. In addition, there is no corresponding research reports to discuss the Ag₃PO₄/Bi₂WO₆ for pollutants' removal based on the plasmonic Z-scheme mechanism. Therefore, it is meaningful to apply the SPR effect of Ag nanoparticles into the hybrid composite to design highly efficient visible-light-driven photocatalysts based on Z-scheme charge transfer mechanism.

Herein, we successfully fabricated Ag₃PO₄/Bi₂WO₆ heterostructures via the assembly combined with a hydrothermal technique. The Ag₃PO₄/Bi₂WO₆ heterostructures displayed enhanced photocatalytic performance toward organic pollutants (including rhodamine B (RB), phenol (Phen), methyl orange (MO), crystal violet (CV), and methylene blue (MB)) degradation under visible-light irradiation. Subsequently, microstructures, surface chemical states, specific surface areas, optical, and photoelectrochemical properties of Ag₃PO₄/Bi₂WO₆ heterojunction were systematically studied. Meanwhile, a possible visible-light-induced photocatalytic mechanism of Ag₃PO₄/Bi₂WO₆ heterostructures was proposed.

2. Experiment section

2.1 Preparation of photocatalysts

2.1.1 Preparation of Bi₂WO₆. All the reagents used in the experiment were of analytic grades, commercially purchased and used without further purification. In a typical process, 0.972 g Bi(NO₃)₃ and 0.329 g Na₂WO₄ were dissolved in glacial acetic acid (10 mL) and H₂O (10 mL), respectively. Then the Na₂WO₄ solution was dropwise added into Bi(NO₃)₃ solution. After being continuously stirred for 4 h, the mixture was transferred to Teflon-coated autoclave and held at 150 °C for 20 h followed by cooling at room temperature naturally. To remove any residue of by-products and reactants, the obtained Bi₂WO₆ was washed with deionized water several times and dried at 80 °C for 24 h.

2.1.2 Preparation of Ag₃PO₄/Bi₂WO₆. Ag₃PO₄/Bi₂WO₆ composites with the different molar ratio were synthesized as follows. 1.092 g AgNO₃ and 0.762 g Na₃PO₄ were dissolved in deionized water (10 mL) and stirred for 30 min, respectively. Then Na₃PO₄ solution was dropwise added into the AgNO₃ solution to form yellow sediments with stirring for 2 h. Subsequently, a certain amount of as-prepared Bi₂WO₆ was slowly added to the above mixture and stirred for 4 h followed by drying at 60 °C for 24 h. Ultimately, the obtained Ag₃PO₄/Bi₂WO₆ samples were collected by washing, filtration, and drying at 60 °C for 48 h. The series of photocatalysts prepared were labeled as Ag₃PO₄/Bi₂WO₆-x, where x presents the molar ratio of Bi₂WO₆ to Ag₃PO₄. Pure Ag₃PO₄ was acquired in the absence of Bi₂WO₆ through the same process.

2.2 Characterization of photocatalysts

The crystal structures were obtained on a Bruker-AXS (D8) X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) characterization was carried out on an ESCALAB 250Xi spectrometer equipped with Al Kα radiation at 300 W. N₂ adsorption–desorption isotherm analysis of samples were obtained at 77 K using Micromeritics 3H-2000PS2. The morphologies of synthesized samples were analyzed using a scanning electron microscope (SEM) (Hitachi S-4300) and transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) (JEM-2100F). The UV-visible diffuse reflectance spectra (UV-vis-DRS) were recorded using a UV-vis spectrophotometer (TU-1901) over the wavelength range of 200–800 nm using BaSO₄ as the reflectance standard material. Fourier transform infrared (FT-IR) spectra were recorded using an FT-IR spectrophotometer (PE Company, America). Photoluminescence spectra (PL) were obtained by a Hitachi F-7000 spectrofluorometer with an excitation wavelength of 360 nm, and all the samples were pressed into pellets in the sample holder.

2.3 Photocatalytic tests

Photocatalytic activities of the Ag₃PO₄/Bi₂WO₆ heterostructures were studied by monitoring the degradation behaviors of organic contaminants, including RB, Phen, MO, CV, and MB. The photocatalytic experiments were carried out in a hollow cylindrical photoreactor equipped with a water jacket. 400 W Xe lamp (λ > 410.0 nm; moreover, the inner sleeve was made of No. 11 glasses to filter out ultraviolet from the Xe lamp) was used as the visible-light source. The Xe lamp was positioned within the inner part of the photoreactor and cooling water was circulated through a pyrex jacket surrounding the lamp to keep room temperature. In a typical experimental procedure, 200 mg of catalyst was placed into 220 mL of dye (50 ppm) or Phen (25 ppm) solution via ultrasonication for 10 min, and magnetically stirred in the dark for 1 h to establish the adsorption–desorption equilibrium between the catalyst and organic contaminant. Then the suspension was exposed to visible light irradiation under magnetic stirring. 4 mL of the suspension was collected at a regular time interval and analyzed after centrifugation. The dye concentration was analyzed by UV-vis spectrophotometer (TU-1901) at the maximum absorption spectra. Changes of Phen concentrations were monitored by a Yilite P230II HPLC: C₁₈ column, UV detector (λ = 270 nm), methanol/water (50/50, v/v), and 1 mL min⁻¹.

2.4 Photoelectrochemical experiments

Electrochemical measurements were carried out in a traditional three-electrode system (CHI660E, China). Indium-tin oxide (ITO) glass electrode (1 cm²), saturated calomel electrode (SCE), and Pt sheet were used as the working electrode, reference electrode, and the counter electrode, respectively. The sample mixed with Nafion ionomer was dissolved ethanol aqueous solution to obtain 5 mg L⁻¹ suspension. And then the suspension was uniformly drop-coated onto the clean ITO electrode surface and dried in air. A Xe lamp was used as the light source, and aqueous Na₂SO₄ solution (0.01 mol L⁻¹) served as the electrolyte. All the experiments were performed at room temperature (about 25 °C). The photocurrent was measured under light illumination from a 400 W Xe lamp. The electrochemical impedance study was carried out over a frequency domain of 1 Hz to 100 kHz with a sinusoidal perturbation potential of 5 mV.

3. Results and discussion

3.1 Characteristics of microstructures of Ag₃PO₄/Bi₂WO₆

XRD is used to investigate the crystal structure of the samples. As displayed in Fig. 1, the distinct diffraction peaks of Bi₂WO₆ can be found 28.3°, 32.8°/32.9°, 47.1°, 56.0°, 58.5°, 68.8°, 75.9°, and 78.5°, which can be attributed to the (131), (200)/(002), (202), (133), (262), (400), (193), and (204) crystal planes of orthorhombic phase of Bi₂WO₆ (JCPDS 39-0256). While for pure Ag₃PO₄, the characteristic peaks of 21.7° (110), 29.7° (200), 33.3° (210), 36.5° (211), 47.8° (310), 52.6°(222), 54.9°(320), 57.2° (321), 61.8°(400), and 71.8°(421) are well indexed as the body-centered cubic phase of Ag₃PO₄ (JCPDS 06-0505). In case of Ag₃PO₄/Bi₂WO₆ composites, they show the coexistence of Bi₂WO₆ phase and Ag₃PO₄ phase. Furthermore, the peak intensity of Bi₂WO₆ increases with increasing the Bi₂WO₆ loading, while the peak intensity of Ag₃PO₄ lowers simultaneously. However, the peak position of Ag₃PO₄ does not significantly change, which indicates Bi₂WO₆ is not incorporated into Ag₃PO₄ lattice. In addition, no diffraction peaks of Ag or other impurities are found.

Fig. 1 XRD patterns of the pristine Bi₂WO₆, Ag₃PO₄, and Ag₃PO₄/Bi₂WO₆ composites.

X-ray photoelectron spectroscopy (XPS) as surface analytic technique is employed to detect the electronic structures of Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆ composites. Fig. 2a shows the high-resolution XPS spectra of Ag of the Ag₃PO₄ and Ag₃PO₄/Bi₂WO₆ composite catalysts. The typical peaks of Ag 3d at 368.0 eV (Ag 3d_5/2) and 374.0 eV (Ag 3d_3/2) are ascribed to Ag⁺ in Ag₃PO₄.³³ After the Bi₂WO₆ introduction, the binding energy of spin-obit Ag 3d is divided into two peaks at 368.3 and 374.3 eV (ref. 24 and 34), which is 0.3 eV higher than those of Ag₃PO₄. The characteristic peaks of Ag nanocrystals existing on the surface of Ag₃PO₄ and Ag₃PO₄/Bi₂WO₆ are not found, suggesting no Ag⁰ was formed during the preparation of the catalysts. Compared with pure Ag₃PO₄, the binding energy of P 2p of Ag₃PO₄/Bi₂WO₆ is turned from 132.8 eV to a higher value of 133.8 eV (Fig. 2b). As shown in Fig. 2c, the peaks located at 159.3 eV, 164.6 eV, and 159.9 eV, 165.2 eV correspond to the Bi 4f_7/2 and Bi 4f_5/2 of Bi₂WO₆ and Ag₃PO₄/Bi₂WO₆, respectively, implying the bismuth species in the composite is Bi³⁺ cations. The peaks for W 4f_7/2 (35.98 eV) and W 4f_5/2 (37.98 eV) can be attributed to a six-valent oxidation state for W⁶⁺ in Ag₃PO₄/Bi₂WO₆, meaning 0.2 and 0.1 eV deviation of 4f_7/2 and 4f_5/2 relative to the values in pure Bi₂WO₆ (Fig. 2d).

Fig. 2 The high-resolution XPS spectra of Ag 3d (a), P 2p (b), Bi 4f (c), and W 4f (d) for the Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆-0.3.

The O 1s high-resolution XPS spectra of samples are provided in Fig. S1 (ESI†). The O 1s XPS spectra of Bi₂WO₆ (Ag₃PO₄) show three individual peaks with the binding energies of 530.2 eV (530.2 eV, denoted as O 1s(1)), 531.6 eV (531.3 eV, denoted as O 1s(2)), and 533.1 eV (533.1 eV, denoted as O 1s(3)), which can be attributed to the lattice oxygen in Bi₂WO₆ (Ag₃PO₄), the external hydroxyl groups, and adsorbed oxygen species at the surface of the composite.^35–37 For Ag₃PO₄/Bi₂WO₆-0.3, the binding energies of O 1s orbits are 529.9 eV, 531.4 eV, and 533.1 eV, respectively. And they also display more or less lower energy shift compared with individual counterparts. The variations of the binding energies of Ag 3d, P 2p, Bi 4f, W 4f, and O 1s are attributed to chemical interactions between Bi₂WO₆ and Ag₃PO₄, which proves the formation of heterostructures and facilitates interfacial charge transfer, leading to the improved photocatalytic activity of Ag₃PO₄/Bi₂WO₆ nanocomposites. Beyond that, the tight chemical interactions can improve the structural stability of photocatalyst. The similar results were also reported by another group.^38,39

FT-IR spectra were carried out to investigate the presence of Ag₃PO₄ and Bi₂WO₆ in the Ag₃PO₄/Bi₂WO₆ composite. As shown in Fig. 3, the typical peaks located at 577.8 cm⁻¹, 731.6 cm⁻¹, and 1384 cm⁻¹ are attributed to the Bi–O, W–O, and W–O–W bridging stretching modes, respectively, which indicates the existence of Bi₂WO₆.^40,41 For pure Ag₃PO₄, the stronger peak at 543.1 cm⁻¹ is ascribed to bending vibration of O=P–O. The other absorption bands at 859.9 cm⁻¹ and 1076.4 cm⁻¹ are assigned to the symmetric and asymmetric stretching vibrations of P–O–P rings.⁴² After the Bi₂WO₆ introduction, the typical peaks corresponding Bi–O, W–O, and W–O–W stretching modes in Ag₃PO₄/Bi₂WO₆ composite disappear, shift to lower value, and become weak, respectively. However, compared with pure Ag₃PO₄, the wavenumbers of P–O–P symmetric and asymmetric stretching vibrations shift to lower values of 817.6 cm⁻¹ and 1017.5 cm⁻¹. Meanwhile, that of O=P–O changes from 543.1 cm⁻¹ to 559.3 cm⁻¹. In addition, the bands at 3400 cm⁻¹ and 1654 cm⁻¹ are related to the –OH stretching and vibration of adsorbed H₂O on the surface of photocatalysts. These results further verify the interaction between Bi₂WO₆ and Ag₃PO₄, meaning Bi₂WO₆ was successfully modified Ag₃PO₄.

Fig. 3 FTIR spectra of Bi₂WO₆, Ag₃PO₄, and Ag₃PO₄/Bi₂WO₆.

The typical morphologies of Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆ composites were characterized by SEM images. From Fig. 4a, Ag₃PO₄ shows irregular spherical morphology with a diameter of 100–180 nm. While Bi₂WO₆ exhibits a typical structure of nanoflakes consist of nanoparticles with the side length of 50–250 nm (Fig. 4b). Fig. 4c illustrates the typical SEM image of Ag₃PO₄/Bi₂WO₆-0.3, where irregular spherical Ag₃PO₄ nanoparticles disperse on the surface of Bi₂WO₆ nanoflakes. These tiny particles intertwine with each other, suggesting that Ag₃PO₄ nanoparticles can restrain the agglomeration of Bi₂WO₆ nanoflakes.

Fig. 4 SEM images of Ag₃PO₄ (a), Bi₂WO₆ (b), Ag₃PO₄/Bi₂WO₆-0.3 (c), and HRTEM (d) and TEM images (inset d) of Ag₃PO₄/Bi₂WO₆-0.3.

The morphological and microstructural details of Ag₃PO₄/Bi₂WO₆-0.3 are obtained by TEM and HRTEM technique. As displayed in the inset of Fig. 4d, the Ag₃PO₄ irregular spheres disperse over the surface of Bi₂WO₆ nanoflakes, which is coincided with the aforementioned SEM observations. From the HRTEM image of Ag₃PO₄/Bi₂WO₆-0.3 (Fig. 4d), the lattice distances for the Bi₂WO₆ (131) and Ag₃PO₄ (211) facets are measured to be 0.315 and 0.245 nm, respectively, which is in well accordance with the XRD results. Furthermore, to further prove the formation of heterostructure, the energy dispersive spectroscopy (EDS) elemental mapping of Ag₃PO₄/Bi₂WO₆-0.3 was performed. As shown in Fig. S2(b–f) (ESI†), Ag₃PO₄ and Bi₂WO₆ are relatively uniform dispersion and well connected, which is in accordance with that of SEM and TEM images. Therefore, according to the above results, it gives solid evidence for the formation of heterostructure between Bi₂WO₆ and Ag₃PO₄.

The BET specific surface area and the porous structure of the samples were analyzed by nitrogen adsorption–desorption isotherms, the results shown in Table 1. As shown in Fig. 5a, the N₂ isotherms of samples show characteristic type IV isotherms with H3 hysteresis loops, implying the presence of mesopores in the size of 2–50 nm. This result can be further proved by the pore size distribution analysis (Fig. 5b). And the specific surface area of pure Ag₃PO₄, Ag₃PO₄/Bi₂WO₆-0.3, Ag₃PO₄/Bi₂WO₆-0.5, and Bi₂WO₆ is 8.06, 14.50, 18.21, and 43.55 m² g⁻¹, respectively. The BJH absorption cumulative pore volume increases from 0.038 to 0.070 cm³ g⁻¹ after incorporation with Bi₂WO₆. The results imply the specific surface area and pore volume of Ag₃PO₄/Bi₂WO₆ slight enhance with increasing Bi₂WO₆ loading contrast with Ag₃PO₄. But the specific surface area and pore volume have no obvious change among these samples, which play a minor role in the enhanced photocatalytic activity of Ag₃PO₄/Bi₂WO₆ composite.

Fig. 5 N₂ adsorption–desorption isotherm curves (a) and pore size distribution (b) of the as-prepared samples.

3.2 Optical absorption properties

The optical absorbance properties and gap energies of the Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆ composites were determined using the UV-vis-DRS, and the results were displayed in Fig. 6. From Fig. 6a, the pure Bi₂WO₆ exhibits strong absorbance in the wavelengths shorter than 450 nm owing to the intrinsic band-gap transition,⁴³ and Ag₃PO₄ has the absorption edge at about 530 nm, which is consistent with the reported result.⁴⁴ Compared with Bi₂WO₆, the wavelength regions of the Ag₃PO₄/Bi₂WO₆ composite are extended towards the visible-light region. Moreover, the more obvious red shift phenomenon is observed with increasing the Ag₃PO₄ loading, which implies more visible light is absorbed by the composite, and produce more e⁻–h⁺ pairs, further improve the photocatalytic activity.

Fig. 6 UV-vis DRS (a) and the plots of (αhν)^1/2 versus photon energy for the band gap energies (b) of Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆ photocatalysts.

The band gap energies of the photocatalysts are calculated according to the following equation:

(αhν) = A (hν − E_g)^n/2 (1)

In this equation, A, α, h, hν, and E_g are constant, absorption coefficient, Planck constant, the energy of the incident photon, and band gap, respectively. And n is 1 and 4 for a direct and indirect band gap semiconductor, respectively. The value of n for Bi₂WO₆ and Ag₃PO₄ is 1 (ref. 20 and 45). By calculating, the band gaps of Bi₂WO₆ and Ag₃PO₄ are 2.90 and 2.45 eV, respectively. The band edge positions of photocatalysts can be determined by the empirical equations:

E_VB = X − E^e + 0.5 E_g (2)

E_CB = E_VB − E_g, (3)

where E_VB is the valence band edge potential, E_CB is the conduction band edge potential, X is the electronegativity of the semiconductor obtained from the geometric mean of the electronegativity for the component atoms (6.21 eV for Bi₂WO₆ and 5.965 eV for Ag₃PO₄), E^e is the energy of free electrons on the hydrogen scale (4.5 eV), E_g is the band gap energy of the semiconductor. Thus, E_VB and E_CB are calculated to be 3.34 eV and 0.44 eV for Bi₂WO₆ and 2.69 eV and 0.24 eV for Ag₃PO₄, respectively.

3.3 Photocatalytic tests

Based on the above results, the photocatalytic activities of prepared photocatalysts were evaluated by degradation of RB under visible-light irradiation. As shown in Fig. 7a, adsorption tests display all suspensions of photocatalyst and RB reach adsorption–desorption equilibrium after 1 h stirring in dark. Meanwhile, the visible-light photocatalytic activities of samples were investigated. The results show that: (1) without the catalyst, no RB is degraded under visible-light irradiation for 180 min, implying RB is relative stability under longtime irradiation; (2) in the presence of catalysts and light, the RB-degradation efficiency is greatly improved. Compared with pure Bi₂WO₆ and Ag₃PO₄, Ag₃PO₄/Bi₂WO₆ composites show higher photocatalytic activity. Furthermore, with increasing molar ratio of Ag₃PO₄/Bi₂WO₆ from 0 to 0.3, Ag₃PO₄/Bi₂WO₆ composites display enhanced photocatalytic activity and then decrease with increasing to 0.5. For example, RB-degradation efficiency of Bi₂WO₆, Ag₃PO₄, Ag₃PO₄/Bi₂WO₆-0.3, and Ag₃PO₄/Bi₂WO₆-0.5 can reach 43.2%, 51.4%, 97.5%, and 91.4% after 120 min visible-light illumination.

Fig. 7 Photocatalytic activities of different photocatalysts toward (a) RB, (d) Phen, and (f) different organic dyes under visible-light irradiation. (b) UV-vis absorption spectra of the RB solution separated from Ag₃PO₄/Bi₂WO₆-0.3 suspension under visible-light illumination. Kinetic fitting curves for photocatalytic degradation of (c) RB and (e) Phen over different photocatalysts under visible-light irradiation.

Fig. 7b shows the absorption spectra of RB (λ_max = 553 nm) under visible-light irradiating the photoactive Ag₃PO₄/Bi₂WO₆-0.3. The absorption peak intensity of RB located at 553 nm reduces rapidly with prolonged irradiation time, and it is hardly observed after 120 min visible-light illumination. The results are consistent with Fig. 7a. In addition, we find the obvious blue shift of the absorption peak at 553 nm, which corresponds to the de-ethylation process. Therefore, the cleavage of the whole conjugated chromophore structure of RB leads to rapidly reducing the absorption peak intensity of RB, which indicates intermediate products were formed and then degraded to small molecules or CO₂ during the RB degradation process.^15,46

The pseudo-first-order model based on the Langmuir–Hinshelwood (LH) kinetic model is mainly used to estimate the kinetics of the photocatalytic degradation of RB, as shown in the following equation:

−ln(C_t/C₀) = k_appt, (4)

where k is the apparent rate constant of degradation; C_t and C₀ are the initial and instantaneous concentration of RB at the time t₀ and t, respectively. Plots of −ln(C_t/C₀) vs. time for photocatalysts are shown in Fig. 7c. All these photocatalysts display good linear relation meeting a pseudo-first-order reaction, and the kinetic constants calculated are 0 min⁻¹ (no photocatalyst), 0.00515 min⁻¹ (Bi₂WO₆), 0.00646 min⁻¹ (Ag₃PO₄), 0.0295 min⁻¹ (Ag₃PO₄/Bi₂WO₆-0.3), and 0.0164 min⁻¹ (Ag₃PO₄/Bi₂WO₆-0.5), respectively. During all photocatalysts, Ag₃PO₄/Bi₂WO₆-0.3 shows the highest photocatalytic activity and decomposition rate, which are 5.7 and 4.6 times than that of Bi₂WO₆ and Ag₃PO₄, respectively. In short, Ag₃PO₄/Bi₂WO₆-0.3 exhibits the dramatical enhancement on photocatalytic activity of identical conditions than pure Bi₂WO₆ and Ag₃PO₄.

In order to eliminate the influence of dye sensitization, Phen as the colorless compound is selected as a model molecule to further study the photocatalytic performance of Ag₃PO₄/Bi₂WO₆ under visible-light irradiation. Because that Phen (λ_max = 270 nm) has no absorption and no photosensitization in visible-light region. Fig. 7d and e show the relative concentration (C_t/C₀) and −ln(C_t/C₀) vs. time curves of Phen in the presence of Bi₂WO₆, Ag₃PO₄, and Ag₃PO₄/Bi₂WO₆-0.3. As displayed in Fig. 7d, the direct photolysis of Phen is neglected during the whole visible-light irradiation, indicating Phen is also a stable pollutant. However, the degradation efficiency of Phen is about 7.1%, 63.5%, and 83.1% in the presence of Bi₂WO₆, Ag₃PO₄, and Ag₃PO₄/Bi₂WO₆-0.3, respectively. Their corresponding rate constants are obtained from Fig. 7e to be 0.00033 min⁻¹, 0.00634 min⁻¹, and 0.00999 min⁻¹, respectively. The results are consistent with those obtained for RB degradation. Meanwhile, the above results confirm that photocatalytic performance of Ag₃PO₄/Bi₂WO₆-0.3 is due to the excitation of the photocatalyst rather than the sensitization mechanism.

In order to test the extensive adaptability of the Ag₃PO₄/Bi₂WO₆ photocatalyst, anionic dye (MO) and cationic dyes (RB, MB, and CV) are also employed as target pollutants. As depicted in Fig. 7f, MO, RB, MB, and CV are effectively eliminated after visible-light irradiation, which implies that the Ag₃PO₄/Bi₂WO₆ photocatalyst is highly efficient in the visible-light degradation of pollutants, especially cationic dyes. The excellent photocatalytic activity of Ag₃PO₄/Bi₂WO₆ towards to organic pollutants is due to the good formation of heterostructures, which not only broaden the spectral response range to visible light but also effectively promote the charge separation. In addition, the excessive Ag₃PO₄ may act as a recombination center, and cover the active sites on the Bi₂WO₆ surface, leading to reducing the separation efficiency of the photogenerated charge carriers. It can be confirmed from the following PL results (Fig. S4, ESI†). That is why Ag₃PO₄/Bi₂WO₆-0.3 shows higher photocatalytic activity than Ag₃PO₄/Bi₂WO₆-0.5.

The photostability of a photocatalyst is essential for practical application. To research the reusability of photocatalysts, recycled photocatalytic degradation RB test was performed over Ag₃PO₄/Bi₂WO₆-0.3 composite. As shown in Fig. 8a, the photodegradation percentage of RB after visible light irradiation for 90 min reduced from the original 92.9–86.5%, 75.8%, 71.5%, 68.5% and 68.1% after six cycles. Although the degradation efficiency of RB in the reactive system went on 24.8%, the photodegradation percentage of RB was almost the same after the five and six cycles, indicating that the catalyst had a certain stability. The slight decline of photocatalytic efficiency is attributed to the inevitable loss of photocatalysts during the recycle runs. In addition, the color of RB-photocatalysts suspension had been changed from pink to black, indicating metallic Ag was formed on the surface of the catalyst during the photocatalytic process. This phenomenon is confirmed by the comparative XRD patterns of Ag₃PO₄/Bi₂WO₆ before and after the photocatalytic experiments in Fig. 8b. There is one weak peak located at 38.1° for Ag₃PO₄/Bi₂WO₆-0.3 after cycles, which can be classified as the characteristic peak of metallic silver.⁴⁷ This part Ag formed at the early photocatalytic reaction. Meanwhile, UV-vis/DRS, the high-resolution XPS spectra of Ag 3d, and SEM image of Ag₃PO₄/Bi₂WO₆-0.3 after reaction are also shown in Fig. S3 (ESI†). As displayed in Fig. S3(a),† Ag₃PO₄/Bi₂WO₆-0.3 displays enhanced photo-absorption in the visible-light region after the cycle degradation experiments. In particular, a broad prominent absorption in the visible-light region of 450–800 nm is observed, owing to the surface plasmon resonance (SPR) effect of Ag nanoparticles.⁴⁷ Fig. S3(b)† shows the Ag XPS spectra of Ag₃PO₄/Bi₂WO₆-0.3 after cycles. The peaks located at 368.8 and 374.9 eV are assigned to Ag⁰, and the typical peaks located at 367.8 and 373.8 eV are ascribed to Ag⁺. According to the XPS results, the content of Ag⁰ is about 30% after cycles. As shown in Fig. S3(c),† although the aggregation phenomenon becomes more obvious after the photocatalytic reaction, Ag₃PO₄/Bi₂WO₆-0.3 still maintain their morphologies. These results further prove that the photocorrosion resistance and stability of Ag₃PO₄ were improved by introducing Bi₂WO₆ to construct Ag₃PO₄-Ag-Bi₂WO₆ Z-scheme heterostructures.

Fig. 8 Cyclic photocatalytic degradation of RB over Ag₃PO₄/Bi₂WO₆-0.3 (a); XRD patterns of Ag₃PO₄/Bi₂WO₆-0.3 before and after the cycle degradation experiments (b).

3.4 Photocatalytic mechanism discussion

3.4.1 Free radical and hole scavenging experiments. According to the photocatalytic literature,^48,49 the movement of the charge carriers such as e⁻ and h⁺ is the crucial parameter to generate reactive species such as ˙O²⁻, ˙OH, and h⁺. Thus, active species generated during the photodegradation process of RB over the Ag₃PO₄/Bi₂WO₆-0.3 are identified by free radical and hole trapping experiments in the presence of various scavengers such as tert-butyl alcohol (t-BuOH, ˙OH scavenger, 1 mM), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, h⁺ scavenger, 1 mM), and benzoquinone(BQ, ˙O²⁻ scavenger, 1 mM).

Fig. 9 depicts the effects of various scavengers on the degradation of RB. The addition of t-BuOH can hardly inhibit RB degradation, which demonstrates that hydroxyl radicals ˙OH have little influence in the photocatalytic process. However, when the EDTA-2Na and BQ were added into the reaction system, the photocatalytic activities of Ag₃PO₄/Bi₂WO₆-0.3 are obviously decelerated. Furthermore, the photodegradation percentage of RB is reduced from the original 100–6.6% and 45.1%, respectively. The above results suggested that h⁺ and ˙O²⁻ are main active species during the degradation process.

Fig. 9 Effects of different scavengers on the photodegradation of RB over the Ag₃PO₄/Bi₂WO₆-0.3 composite.

3.4.2 Photoelectrochemical experiments. Photoelectrochemical experiments as a useful technique to monitor the generation of photoelectrons and holes as well as their transfer and efficient separation in the Ag₃PO₄/Bi₂WO₆ system were performed. As shown in Fig. 10, the transient photocurrent responses of Bi₂WO₆, Ag₃PO₄, and Ag₃PO₄/Bi₂WO₆-0.3 are recorded for several on–off cycles under visible-light irradiation at a bias of 1 V. It is found that the photocurrent responses of the tested sample working electrodes decrease to zero as soon as the lamp is turned off, and a rapid increase appears when the light is on. Meanwhile, photocurrent responses regain a reproducible value when the lamp is turned on again during on–off intermittent irradiation cycles. In addition, the Ag₃PO₄/Bi₂WO₆-0.3 exhibits the enhanced the photocurrent compared with Bi₂WO₆ and Ag₃PO₄. The sequence of the photocurrent is in accordance with the order of the photocatalytic activity of photocatalysts. This suggests a smaller recombination and a more efficient separation of photogenerated e⁻–h⁺ pairs occurs across the interface between Bi₂WO₆ and Ag₃PO₄ in the Ag₃PO₄/Bi₂WO₆ composite.

Fig. 10 Photocurrent responses of Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆-0.3 under visible light irradiation.

Electrochemical impedance spectroscopy (EIS) is also employed to investigate the charge transfer resistance and the separation of photogenerated e⁻–h⁺ pairs at solid/electrolyte interfaces in the photocatalyst.⁵⁰ Fig. 11 displays the EIS Nyquist plots of Bi₂WO₆, Ag₃PO₄, and Ag₃PO₄/Bi₂WO₆-0.3 under visible-light irradiation. It is clearly seen that the smallest arc radius of the EIS Nyquist plot of Ag₃PO₄/Bi₂WO₆-0.3 implies that it has the fastest interfacial electron transfer and more separation of photogenerated e⁻–h⁺ pairs when compared to those of Bi₂WO₆ and Ag₃PO₄. It is the reason for the Ag₃PO₄/Bi₂WO₆-0.3 composites exhibit the highest photocatalytic activity.

Fig. 11 EIS Nyquist plots of Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆-0.3.

Photoluminescence (PL) spectrum is also an effective tool to reveal the migration, transfer and recombination processes of the photogenerated e⁻–h⁺ pairs in semiconductors. Usually, a lower PL intensity indicates lower recombination rate of the charge carriers and higher photocatalytic activities of the photocatalysts. Fig. S4 (ESI†) shows the PL spectra of as-prepared Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆ composites with an excitation wavelength of 360 nm. As for the Bi₂WO₆-based materials, their PL intensities follow the order Bi₂WO₆ > Ag₃PO₄/Bi₂WO₆-0.5 > Ag₃PO₄/Bi₂WO₆-0.3 > Ag₃PO₄. Compared with pure Bi₂WO₆, the PL intensities of Ag₃PO₄/Bi₂WO₆ composites present an obvious decrease, implying a lower recombination feasibility of free charges in the Ag₃PO₄/Bi₂WO₆ heterostructures. The Ag₃PO₄/Bi₂WO₆ nanocomposite displays a higher PL intensity than Ag₃PO₄, implying that the migration pathways of the photoexcited e⁻–h⁺ in the Ag₃PO₄/Bi₂WO₆ nanocomposite is plasmonic Z-scheme theory (Scheme 1a) not as heterojunction energy-band theory (Scheme 1b).^51,52

Scheme 1 schematic diagram of the separation and transfer of photogenerated charges in the Ag₃PO₄/Bi₂WO₆ heterostructures based on the plasmonic Z-scheme theory (a) and heterojunction energy-band theory (b) under visible light irradiation.

From the Scheme 1a, under visible-light irradiation, both Bi₂WO₆ and Ag₃PO₄ can be excited to produce e⁻ and h⁺ simultaneously owing to their visible-light response. On the one hand, the part of Ag₃PO₄ can be photoreduced to Ag⁰ during the initial photocatalytic process. Ag nanoparticles (NPs) can absorb visible light and induce e⁻–h⁺ pairs on account of the dipolar character and the SPR effect of the Ag NPs. The photogenerated electrons in the CB of Ag₃PO₄ shift to the photogenerated holes produced by plasmonic absorption in the Ag NPs, which results in higher PL intensity. On other the hand, the plasmon hot electrons generated by the localized SPR oscillations of Ag NPs can capture the dissolved O₂ in water to form ˙O²⁻ (ref. 53). Meanwhile, the plasmon hot electrons directly transfer from the Ag NPs to the CB of Bi₂WO₆. While the photogenerated holes are still in the VB of Ag₃PO₄ and Bi₂WO₆. Therefore, the photo-generated charge carriers are efficiently separated in space, which retards the photocorrosion of Ag₃PO₄. With the assistance of h⁺ and ˙O²⁻ the two main active species, the organic pollutant is effectively degraded in aqueous solution. But from the heterojunction energy-band theory described in Scheme 1b, the VB potential (2.68 eV vs. NHE) of Ag₃PO₄ is more negative than that of Bi₂WO₆ (3.08 eV vs. NHE), which leads to the migration of the h⁺ from Bi₂WO₆ to Ag₃PO₄. Since the CB potential (0.24 eV vs. NHE) of Ag₃PO₄ is more negative than that of Bi₂WO₆ (0.44 eV vs. NHE), photo-induced electrons from Ag₃PO₄ migrate to the CB of Bi₂WO₆ and then transfer to Ag⁰. Therefore, the photo-generated electrons cannot reduce O₂ to produce ˙O²⁻, due to the CB potential of Bi₂WO₆ being more positive than the redox potential of ˙O²⁻ formation (O₂/˙O²⁻ = −0.33 eV, NHE).⁵¹ Thus this phenomenon cannot explain the stronger effect of ˙O²⁻ on the degradation of RB. In conclusion, the plasmonic Z-scheme theory for the photocatalysis of Ag₃PO₄/Bi₂WO₆ is much more reasonable.

4. Conclusions

Novel Ag₃PO₄/Bi₂WO₆ heterostructured materials were successfully synthesized by assembling Ag₃PO₄ irregular nanospheres on the surface of Bi₂WO₆ nanoflakes. Ag₃PO₄/Bi₂WO₆-0.3 showed obviously superior visible-light catalytic activity toward degradation of organic pollutants. Free radical and hole scavenging experiments suggested h⁺ and ˙O²⁻ are two main active species through the degradation process. In brief, the enhanced photocatalytic activity was due to the good formation of heterostructures, which could not only broaden the spectral response range to visible light but also effectively promoted the charge separation. These results were solidly confirmed by photocurrent responses, electrochemical impedance spectroscopy, and photoluminescence spectrum. In addition, Ag₃PO₄/Bi₂WO₆-0.3 exhibited relative higher photostability toward RB degradation, which was explained by the reasonable photoreactive mechanism. This work provides the potential application of Bi₂WO₆-based heterostructures as efficient visible light responsive catalysts for environmental remediation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by The Fundamental Research Funds in Heilongjiang Provincial Universities (135109204).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. DOI: 10.1039/c8ra01477a

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