Environmental hazards pose a great threat to human health and retard social development. The use of solar energy is a good method for dealing with the problems caused by hazardous materials [1-3]. Photocatalysts should: (1) respond to visible-light irradiation, which accounts for the majority of solar energy; (2) have high quantum efficiencies and stabilities; and (3) be produced from safe, cheap, and nontoxic raw materials. Currently used photocatalysts such as TiO₂ [4-8], ZnO [9, 10], ZnS [11], Zn₂GeO₄ [12], and BiPO₄ [13, 14] have excellent photocatalytic activities under ultraviolet (UV) irradiation. Although UV radiation, which has high energy, is a highly efficient light source and can excite most semiconductor photocatalysts, it forms only a small part of the solar energy spectrum and is harmful to human health. In recent years, photocatalysts with wide band gaps have been modified and improved by doping [15-17], using surface plasmon resonance (SPR) with noble metals [18, 19], and constructing heterojunction or solid-solution photocatalysts [20, 21]. However, because of the inherent properties of the materials, the improvements are too limited to make the photocatalysts suitable for practical applications. The development of high-efficiency visible-light-driven photocatalysts is therefore important. In recent years, visible-light-driven photocatalysts such as g-C₃N₄ [2, 22-25], CdS [26-28], Ag₃PO₄ [29-31], AgX (X = Cl, Br and I) [32], WO₃ [33, 34], Bi₂MoO₆ [35], Bi₂WO₆ [36, 37], and BiVO₄ [38] have been reported to be good photocatalysts. However, in most cases, photocatalysts that can be excited by visible light do not have high redox capabilities. For example, g-C₃N₄ and CdS, which have strong conduction band (CB) reduction capabilities, have weak valence band (VB) oxidation capabilities. Conversely, Ag₃PO₄, WO₃, and BiVO₄ have strong oxidation but weak reduction capabilities. The development of composite photocatalysts has therefore attracted much attention. Although type-Ⅱ composite photocatalysts can effectively boost the separation of charge carriers and enhance the photocatalytic activity, the redox capabilities of the individual components of the composite are impaired. The use of Z-scheme photocatalysts provides a feasible solution to this problem [39-43].

Ag₃PO₄, which is responsive to irradiation at wavelengths less than 550 nm but has enhanced light absorption at wavelengths greater than 550 nm because of the SPR effect of Ag nanoparticles, shows excellent photocatalytic activities in the degradation of organic contaminants and oxygen evolution reactions [1, 44-47]. Although the holes in the VB of Ag₃PO₄ have enough energy to degrade contaminants and generate hydroxyl radicals (∙OH), which are essential for photodegradation, the electrons in the CB cannot be used to produce superoxide anion radicals (∙O₂^-) and further degrade organic pollutants. The electrons in the CB are therefore easily trapped by Ag⁺, and this decreases the stability of the Ag₃PO₄ photocatalytic system. The photodegradation capability of Ag₃PO₄ is several tens of times higher than those of conventional semiconductors such as TiO₂ [2, 48], g-C₃N₄ [38, 49], Bi₂MoO₆ [50], BiVO₄ [51], and BiOCl [52, 53], therefore it is worth optimizing the photocatalytic activity and improving the physicochemical stability of Ag₃PO₄.

Two-dimensional nanomaterials are of interest because of their large specific surface areas, large numbers of reactive sites, and fast separation of charge carriers. They are also convenient for the preparation of composites with large-area heterojunctions [54-56]. Recently, Bi₂WO₆ nanosheets (NSs) have been prepared using a wet-chemical bottom-up route. These Bi₂WO₆ NSs have high activities in photodegradation and CO₂ reduction [57-59]. Bi₂WO₆ can respond to visible-light irradiation and the electrons in the CB are sufficiently negative to participate in photocatalytic reduction. Composites of Ag and Bi have been reported to be useful for preparing high-efficiency stable photocatalysts; this led us to prepare Ag₃PO₄/Bi₂WO₆ [32, 35, 38, 60].

In this work, the successful preparation of Ag₃PO₄/Bi₂WO₆ was confirmed using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy (DRS) showed that the composite absorbed light intensely, as in the case of Ag₃PO₄. Photoluminescence (PL) spectroscopy showed that the composite effectively hindered recombination of charge carriers. Ag₃PO₄/Bi₂WO₆ showed enhanced photocatalytic activity. The methylene blue (MB) degradation rate achieved using Ag₃PO₄/Bi₂WO₆ was 0.61 min^-1, which is 1.3 and 6.0 times higher than those achieved using Ag₃PO₄ (0.47 min^-1) and Bi₂WO₆ (0.10 min^-1), respectively. Five successive photodegradation experiments showed that Ag₃PO₄/Bi₂WO₆ had sufficient stability. Reactive species trapping experiments showed that the photocatalytic mechanism was Z-scheme rather than type Ⅱ.

AgNO₃, NH₃∙H₂O, HNO₃, KH₂PO₄, Na₂WO₄∙2H₂O, cetyltrimethylammonium bromide (CTAB), and Bi(NO₃)₃∙5H₂O were purchased from the Shanghai Chemical Reagent Co., Ltd. (China). L-ascorbic acid, isopropanol (IPA), triethanolamine (TEOA), and MB were purchased from the Sinopharm Chemical Reagent Corp. (China). Double-distilled water was used in the experiments. All the reagents were analytical grade and used without further purification.

Bi₂WO₆ NSs were prepared using a previously reported method [57]. Bi(NO₃)₃∙5H₂O (2 mmol), CTAB (0.05 g), and Na₂WO₄∙2H₂O (1 mmol) were mixed with deionized water (80 mL) under stirring for 30 min. The mixture was transferred to a 100 mL Teflon-lined autoclave and heated at 120 ℃ for 24 h. Bi₂WO₆ NSs were obtained by washing six times with deionized water and vacuum freeze-drying for 12 h. AgNO₃ (3.4 g) was dispersed in H₂O (340 mL) and then NH₃∙H₂O was added dropwise; the solution changed from clear to brownish black and then to clear again. The pH was adjusted to 7 by dropwise addition of HNO₃. Bi₂WO₆ NSs (0.15 g) were then added under constant stirring for 30 min. The Ag₃PO₄/Bi₂WO₆ composite was obtained by dropwise addition of KH₂PO₄ solution (0.2 mol/L, 35 mL). The Ag₃PO₄/Bi₂WO₆ composite was washed six times with deionized water and vacuum freeze-dried for 12 h. Bulk Ag₃PO₄ was obtained in the same way but without adding Bi₂WO₆ NSs.

XRD patterns were recorded using a Panalytical Empyrean diffractometer with Cu K_α radiation (λ = 1.5406 Å) as the X-ray source. The morphologies and elemental compositions of the prepared samples were investigated using field-emission scanning electron microscopy (FE-SEM; SU8020) with EDS (INCA x-act). XPS was performed using an ESCALAB 250 instrument with Al K_α radiation. UV-vis DRS was performed using a PerkinElmer Lambda 950 UV-vis spectrophotometer. PL spectra were recorded using an FLS920 instrument at an excitation wavelength of 325 nm.

The photocatalyst (30 mg) was mixed with MB solution (30 mL, 20 mg/L). Light-emitting diode (LED) arrays (active power 50 W, λ = 410 nm) were used as a solar model. The irradiation time intervals were set at 2 min and the process was repeated five times. After each light irradiation, the absorbance of the MB solution was determined using the clear liquid obtained after centrifuging the mixture. The dominant reactive species affecting MB photodegradation were identified using IPA, L-ascorbic acid, and TEOA as scavengers for ·OH, ·O₂^-, and holes, respectively. In a typical process [32, 61, 62], either L-ascorbic acid (150 mg), IPA (20 mmol/L, 5.4 mL), or TEOA (10 mmol/L, 6.1 mL) was added to a mixture of the photocatalyst and MB solution. Radical-trapping experiments were performed using the same method as for the photodegradation experiments.

As shown in Fig. 1, Bi₂WO₆ NSs were prepared using a bottom-up route [57]. When Br^- ions are adsorbed on Bi₂WO₆ seed crystals, coulombic repulsion forces restrain bulk growth of Bi₂WO₆ crystals. This is the key factor in the generation of Bi₂WO₆ NSs. Ag₃PO₄/Bi₂WO₆ was prepared by in situ growth of Ag₃PO₄ on Bi₂WO₆ NS surfaces. More highly active crystal surfaces were exposed by using Ag–NH₃∙H₂O solution to regulate the Ag₃PO₄ growth rate.

In Fig. 2(a), the strong diffraction peaks at 2θ = 20.93°, 29.75°, 33.32°, 36.58°, 42.49°, 47.76°, 52.65°, 55.12°, 57.32°, 61.67°, 69.88°, 71.90°, and 73.97° match the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (420), (421), and (332) crystal planes of cubic Ag₃PO₄ (PDF #84-0510, P-43n, a = b = c = 6.011 Å). In Fig. 2(c), the strong diffraction peaks at 2θ = 28.38°, 32.87°, 47.21°, 55.96°, 58.58°, 68.80°, 75.99°, and 78.46° are assigned to the (131), (002), (202), (133), (262), (004), (391), and (064) crystal planes of orthorhombic Bi₂WO₆ (PDF #79-2381, Pca2₁, a = 5.43726 Å, b = 16.43018 Å, c = 5.45842 Å). All the strong diffraction peaks in Fig. 2(b) can be indexed to a mixture of cubic Ag₃PO₄ and Bi₂WO₆. However, the diffraction peaks from Bi₂WO₆ are weak because of its low content (about 5 wt% of the composite).

Fig. 3(a)–(c) show SEM images of Ag₃PO₄, Bi₂WO₆, and Ag₃PO₄/Bi₂WO₆. Ag₃PO₄ consists of particles with sides of length about 1 μm. The size of the Bi₂WO₆ NSs is several micrometres, and the Ag₃PO₄/Bi₂WO₆ composite retains the Ag₃PO₄ morphology. Fig. 3(d)–(f) show that all the elements in Ag₃PO₄, Bi₂WO₆, and the Ag₃PO₄/Bi₂WO₆ composite can be detected. The C is derived from the conductive adhesive, and no other impurities can be observed, implying that the samples are pure. The elemental mapping (Fig. 3(g)) shows a homogeneous distribution of Ag, P, O, Bi, and W in the prepared Ag₃PO₄/Bi₂WO₆ composite.

In the XPS spectrum in Fig. 4(a), peaks from O, Ag, Bi, W, and P elements can be clearly seen. In Fig. 4(b), the peaks at binding energies of 374.2 and 368.2 eV can be assigned to Ag 3d_3/2 and Ag 3d_3/2 orbitals, respectively [63]. According to a previous report, the binding energies for Ag⁰ are 375.32 and 369.32 eV. In this case, the peaks for Ag⁰ could not be obtained by peak fitting, indicating that the Ag⁰ content was low [64, 65]. In Fig. 4(c), the peaks at binding energies of 532.6 and 531.0 eV correspond to surface-absorbed hydroxyl oxygen and lattice oxygen [66]. In Fig. 4(d), the peak at a binding energy of 133.8 eV is associated with the P 2p orbital of P⁵⁺ [67, 68]. In Fig. 4(e), the peaks at binding energies of 164.6 and 159.6 eV correspond to the Bi 4f_5/2 and Bi 4f_7/2 orbitals, respectively, of Bi³⁺ [69, 70]. In Fig. 4(f), the peaks at binding energies of 42.4 and 40.3 eV are indexed to the W 4f_5/2 and W 4f_7/2 orbitals of W⁶⁺, respectively [66].

The insets in Fig. 5(a), of color yellow, white, and dark yellow (left to right), correspond to Ag₃PO₄, Bi₂WO₆, and the Ag₃PO₄/Bi₂WO₆ composite, respectively; the colors indicate the differences among the optical properties of these materials. Ag₃PO₄ and Bi₂WO₆ have absorption edges at 527 and 429 nm, respectively. The absorption edge of the Ag₃PO₄/Bi₂WO₆ composite is almost the same as that of Ag₃PO₄; this is attributed to the low Bi₂WO₆ content. Bi₂WO₆ shows almost no response to radiation of wavelength between 429 and 800 nm because the irradiation energy is lower than the excitation energy. Ag₃PO₄ shows a little absorption on irradiation at wavelengths between 527 and 800 nm, which can be ascribed to the SPR effect of Ag nanoparticles. The energy band gaps E_g in Fig. 5(b) were calculated using the following equation:

(1)

where n = 1 is for a direct semiconductor and n = 4 for an indirect semiconductor; A and hv stand for the absorbance and irradiation energy, respectively. According to a previous report, Ag₃PO₄ and Bi₂WO₆ are indirect semiconductors. The E_g values for Ag₃PO₄ and Bi₂WO₆ are 2.21 and 2.64 eV, respectively.

(2)

(3)

where X is 5.96 eV for Ag₃PO₄ and 4.86 eV for Bi₂WO₆ [64, 71], and E^e is a constant (4.5 eV). According to equations (1)–(3), E_CB and E_VB are 0.36 and 2.57 eV, respectively, for Ag₃PO₄, and -0.98 and 1.70 eV, respectively, for Bi₂WO_6.

PL spectroscopy can be used as an indicator of recombination of electron–hole pairs [32, 61]. Fig. 6 shows that the main emission peaks for Ag₃PO₄, Bi₂WO₆, and the Ag₃PO₄/Bi₂WO₆ composite are all centred at about 415 nm. The PL intensities of Ag₃PO₄ and Bi₂WO₆ are high, indicating large quantities of charge carriers, which would recombine in the photocatalyst interior rather than being transferred to the surface to participate in the photocatalytic reaction. This difficulty in achieving highly efficient separation of charge carriers in a single material is consistent with previous reports [72]. The PL intensity of the Ag₃PO₄/Bi₂WO₆ composite is lower than those of Ag₃PO₄ and Bi₂WO₆, indicating that fewer charge carriers recombine inside this photocatalyst than in the single materials. This shows that composite construction is an effective method for curbing the recombination of electron–hole pairs.

Fig. 7 shows the N₂ adsorption–desorption isotherms for Bi₂WO₆, Ag₃PO₄, and the Ag₃PO₄/Bi₂WO₆ composite. The Brunauer-Emmett-Teller surface area (A_BET) of Ag₃PO₄ is 2.8 m²/g. When Ag₃PO₄ is coupled with Bi₂WO₆, the surface area of the Ag₃PO₄/Bi₂WO₆ composite increases to 10.7 m²/g because of the large A_BET of Bi₂WO₆ (23.4 m²/g).

The photocatalytic activities were evaluated by degradation of MB solution (Fig. 8(a)). The photocatalytic activity of the Ag₃PO₄/Bi₂WO₆ composite was higher than those of Ag₃PO₄ and Bi₂WO₆, and about 97.5% degradation of the MB solution was achieved within 6 min. The linear relationships between lnC₀/C and t (Fig. 8(b)) show that the MB degradation curves are in accord with pseudo-first-order kinetics. The degradation rate achieved using Ag₃PO₄/Bi₂WO₆ was 0.61 min^-1, which is 1.3 and 6.0 times higher than those achieved using Ag₃PO₄ (0.47 min^-1) and Bi₂WO₆ (0.10 min^-1), respectively.

The stabilities of Bi₂MoO₆, Ag₂MoO₄, and Bi₂MoO₆@ Ag₂MoO₄ 2:1 were evaluated by performing five successive MB degradation experiments. Fig. 9 shows that Bi₂MoO₆@Ag₂MoO₄ 2:1 showed long-term stability, with almost no loss of activity after five cycles. This indicates that the heterojunction benefits not only the photocatalytic activity but also the stability of the photocatalyst.

The main reactive species that affect the degradation of MB using the Ag₃PO₄/Bi₂WO₆ composite were investigated using various scavengers. L-ascorbic acid, IPA, and TEOA were used as scavengers for ·O₂^-, ·OH, and holes, respectively. Fig. 10 shows that the degradation efficiency of the Ag₃PO₄/Bi₂WO₆ composite in the absence of scavengers was 97.5%. Addition of L-ascorbic acid, IPA, and TEOA during the photocatalytic experiments changed the degradation efficiency to 83.6%, 76.2%, and 29.2%, respectively. These results show that holes play a key role in photocatalytic degradation of MB, ·OH plays a secondary role, and ·O₂^- has little involvement.

Fig. 11 shows that the photocatalytic mechanism was probably a Z-scheme rather than a typical type-Ⅱ process. According to the results in Fig. 5(b), the E_g values for Ag₃PO₄ and Bi₂WO₆ are 2.21 and 2.68 eV, respectively. The electrons in the VBs of Ag₃PO₄ and Bi₂WO₆ can be excited to the CBs under visible-light irradiation. The E_CB and E_VB values for Ag₃PO₄ are 0.36 and 2.57 eV, respectively, and those for Bi₂WO₆ are -0.98 and 1.70 eV, respectively. If the photocatalytic mechanism for the Ag₃PO₄/Bi₂WO₆ system is type Ⅱ, the electrons in the CB of Bi₂WO₆ will be transferred to the CB of Ag₃PO₄ and holes in the VB of Ag₃PO₄ will be transferred to the VB of Bi₂WO₆ because of the clear electric potential difference. If charge carriers are transferred in this way, the reduction reaction will occur at the CB of Ag₃PO₄, and the oxidation reaction will occur at the VB of Bi₂WO₆. The potential of the Ag₃PO₄ CB is 0.36 eV, which is more positive than the potential at which O₂ is reduced to ·O₂^-, and the potential of the Bi₂WO₆ VB is 1.70 eV, which is more negative than the potential at which H₂O molecules are oxidized to ∙OH; this is not in agreement with the results of the reactive species trapping experiments for the Ag₃PO₄/Bi₂WO₆ composite, shown in Fig. 10. However, if the photocatalytic mechanism is a Z-scheme, the electrons in the CB of Ag₃PO₄ will recombine with holes in the VB of Bi₂WO₆. The oxidation reaction will occur at the Ag₃PO₄ VB and the reduction reaction will occur at the Bi₂WO₆ CB. The electric potential of holes in the Ag₃PO₄ VB is 2.57 eV, which can oxidize H₂O to ∙OH. The potential of the electrons in the Bi₂WO₆ CB is -0.98 eV, which can reduce O₂ to ∙O₂^-. These results are consistent with those shown in Fig. 10. Organic contaminants will be decomposed to H₂O, CO₂, or other small molecules by the synergistic effect of holes, ∙OH, and ∙O₂^-. The recombination of electrons in the CB of Ag₃PO₄ with holes in the VB of Bi₂WO₆ effectively promotes separation of charge carriers and curbs photocorrosion of Ag₃PO₄; this is consistent with the results shown in Figs. 8 and 9.

In this work, a Ag₃PO₄/Bi₂WO₆ composite was successfully prepared. Formation of a composite of Bi₂WO₆ and Ag₃PO₄ effectively curbed recombination of electrons and holes in the photocatalyst. The photocatalytic activity of the Ag₃PO₄/ Bi₂WO₆ composite was higher than those of Ag₃PO₄ and Bi₂WO_6. The composite had good stability, with almost no activity loss after five successive runs. Reactive species trapping experiments confirmed a Z-scheme photocatalytic mechanism and showed that holes, ∙OH, and ∙O₂^- played specific roles in the photodegradation process. This excellent Ag₃PO₄/Bi₂WO₆ photocatalyst is expected to have applications in the removal of organic contaminants.