Ag₃PO₄ nanocrystals and g-C₃N₄ quantum dots decorated Ag₂WO₄ nanorods: ternary nanoheterostructures for photocatalytic degradation of organic contaminants in water

Abstract

Visible-light-driven Ag₃PO₄/graphite-like carbon nitride/Ag₂WO₄ photocatalysts with different weight fractions of Ag₃PO₄ were synthesized. Ag₂WO₄ nanorods with a scale of 500 nm to 3 μm were prepared by using a hydrothermal reaction. Via a facile deposition–precipitation technique, graphite-like carbon nitride (g-C₃N₄) quantum dots and Ag₃PO₄ nanocrystals were then deposited onto the surface of Ag₂WO₄ nanorods sequentially. Under visible-light irradiation (λ > 420 nm), the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ nanorods degraded Rh B efficiently and displayed much higher photocatalytic activity than that of pure Ag₂WO₄ and the g-C₃N₄/Ag₂WO₄ composite, and the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ hybrid photocatalyst with 30 wt% of Ag₃PO₄ exhibited the highest photocatalytic activity. The quenching effects of different scavengers demonstrated that reactive h⁺ and ·O²⁻ played the major roles in Rh B degradation. It was elucidated that the excellent photocatalytic activity of Ag₃PO₄/g-C₃N₄/Ag₂WO₄ for the degradation of Rh B under visible light (λ > 420 nm) can be ascribed to the efficient separation of photogenerated electrons and holes through the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ heterostructure.

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1. Introduction

The development of visible-light-driven photocatalysts with excellent performance and good stability is a precondition for harvesting more sunlight and realizing efficient photocatalysis. In recent years, Ag-based photocatalysts, such as AgX (where X = Cl, Br, and I), Ag₂O, Ag₂CO₃, Ag₆Si₂O₇, Ag₂WO₄, Ag₃PO₄, and Ag₂Mo₂O₇, have been developed for photocatalysis applications.^1–8 Among those Ag-based photocatalysts, Ag₂WO₄ has been the most studied because of its weak crystal field and was verified to exhibit H₂ evolution and organic pollution degradation ability under UV light irradiation.⁹ However, the band gap of Ag₂WO₄ is theoretically calculated to be 3.55 eV, resulting in its sluggish reaction to visible light, like TiO₂.¹⁰ Thereupon, many efforts have been devoted to make Ag₂WO₄ sensitive to visible light, including its combination with sensitizers, i.e., AgI, Ag/AgCl, and Ag. Unfortunately, Ag₂WO₄ itself has not yet been modified to be sensitive to visible light.^11–13 However, some researchers found that Ag₃PO₄ shows strong oxidation power, which can achieve quantum efficiency of up to 90% under visible light for O₂ generation from water splitting.^14,15 However, as Ag₃PO₄ is light-sensitive and slightly soluble in aqueous solution, it will be photocorroded and decompose to weakly active Ag during the photodegradation, and, accordingly, the photocatalytic activity gradually deteriorates, which is the main hindrance for the practical application of Ag₃PO₄ as a recyclable and highly efficient photocatalyst. Recent reports have shown that forming complexes with special structures such as core–shell nanostructures, plasmonic structures, and heterostructures could not only effectively protect the Ag₃PO₄ crystals from dissolution in aqueous solutions but also enhance their stabilities and photocatalytic performance.^15,16 Therefore, modifying Ag₂WO₄ with Ag₃PO₄ could significantly accelerate the transfer of photogenerated charge carriers.

Nowadays, graphite-like carbon nitride (g-C₃N₄) has been studied as a promising candidate for hydrogen evolution and environment purification under visible-light irradiation.^17–19 Although g-C₃N₄ possesses good chemical and thermal stability, its use in photocatalysis is limited because of its high recombination of photogenerated electron–hole pairs.²⁰ To solve this problem, abundant strategies, including doping, deposition, and sensitization, were developed.^20–23 Constructing the heterostructure is also an effective approach for decreasing the recombination rate of photogenerated charges. However, most of the reported g-C₃N₄ nanostructures are composed of stacked two-dimensional nanosheets, and very few available examples concentrating on zero-dimensional quantum dots (QDs) exist, especially on QD-based heterostructures.^19,24 Given the enhanced light absorption and, more importantly, excellent electron conductivity of carbon materials, it is rationally speculated that decoration of Ag₂WO₄ with g-C₃N₄ QDs in an intimate integration fashion would significantly improve the photocatalytic performances of Ag₂WO₄.

Herein, we report a successful attempt at the preparation of Ag₃PO₄/g-C₃N₄/Ag₂WO₄ nanoheterostructures via a facile in situ precipitation synthetic strategy. The photocatalytic performance of the ternary composite was investigated by measuring the photodegradation of rhodamine B (Rh B) under visible-light irradiation (λ > 420 nm). Up to date, construction of Ag₃PO₄/g-C₃N₄/Ag₂WO₄ ternary heterostructures in combination with a systematic study on the photocatalysis mechanism has not yet been reported.

2. Experimental

2.1 Preparation of photocatalysts

2.1.1 Preparation of Ag₂WO₄ nanorods. All reagents for synthesis and analysis were commercially available and used without further treatments. The Ag₂WO₄ nanorods were synthesized through a facile hydrothermal method. In a typical procedure, a solution of Na₂WO₄ (0.10 mol L⁻¹, 10 mL H₂O) was stirred for 30 min, then 20 mL aqueous solution of AgNO₃ (0.40 mol L⁻¹, 5 mL H₂O) was dropped slowly into the above solution, which was stirred magnetically for another 30 min. The mixed solution was transferred into a 20 mL Teflon-lined steel autoclave, which was heated in an oven at 160 °C for 24 h. Then the system was allowed to cool to room temperature naturally. Lastly, the obtained samples were collected and washed with ethanol and distilled water several times then and dried at 70 °C for 2 h.

2.1.2 Preparation of g-C₃N₄ QDs. Bulk g-C₃N₄ was prepared by heating melamine for 4 h to 550 °C and keeping it at this temperature for another 4 h in air.²⁴ g-C₃N₄ QDs was prepared by using Yu's method:²⁴ First, 1 g of bulk g-C₃N₄ was treated in the mixture of concentrated sulfuric acid (H₂SO₄) (20 mL) and nitric acid (HNO₃) (20 mL) for ∼2 h at room temperature. The mixture was then diluted with deionized water (1 L) and washed several times. Second, 50 mg of the obtained solid was dispersed in 30 mL concentrated NH₃·H₂O, and then the mixed suspension was transferred into a 20 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 24 h. Upon cooling to room temperature, the precipitate was washed with water several times to remove the adsorbed NH₃ molecules. Third, 10 mg of the synthesized solid was dispersed in 100 mL of water, and then treated ultrasonically for ∼6 h. The as-obtained aqueous suspension was then centrifuged at ∼7000 rpm and dialyzed in a dialysis bag to remove large-sized nanoparticles.

2.1.3 Preparation of g-C₃N₄/Ag₂WO₄ photocatalyst. The as-prepared Ag₂WO₄ nanorods (300 mg) were mixed with 200 mL of deionized water by ultrasonication for 30 min. Then, 1.0 mL of a 5% polyethylene glycol (PEG) 2000 solution was added and the dispersion was stirred for another 10 min. For wrapping g-C₃N₄ QDs on Ag₂WO₄ nanorods, 30 mL of the g-C₃N₄ QDs solution (1 mg mL⁻¹) was added and the reaction temperature was kept at 70 °C for 60 min. The resulting suspension was filtered, washed with deionized water three times, and dried at 60 °C for 24 h in a vacuum oven. The theoretical wrapping amount of g-C₃N₄ QDs was 10 wt%.

2.1.4 Preparation of Ag₃PO₄/g-C₃N₄/Ag₂WO₄ photocatalyst. The Ag₃PO₄/g-C₃N₄/Ag₂WO₄ hybrid photocatalyst was synthesized through an in situ precipitation method at room temperature. In a typical process, g-C₃N₄/Ag₂WO₄ nanorods (300 mg) were dispersed in 100 mL of deionized water by ultrasound for 30 min, and then an emulsion of AgNO₃ + Na₂HPO₄ (with a molar ratio of Ag : P = 3 : 1) was dropwise added to the suspension under magnetic stirring. The pH value was adjusted to 3 by adding 1.0 M NaOH. The resulting suspension was stirred in the dark for another 30 min. Finally, the precipitate was washed with deionized water three times, collected by centrifugation, and then dried at 60 °C in the vacuum drying oven to obtain Ag₃PO₄/g-C₃N₄/Ag₂WO₄ (denoted as 10% Ag₃PO₄/g-C₃N₄/Ag₂WO₄, 20% Ag₃PO₄/g-C₃N₄/Ag₂WO₄, 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄, 40% Ag₃PO₄/g-C₃N₄/Ag₂WO₄, and 50% Ag₃PO₄/g-C₃N₄/Ag₂WO₄, respectively).

2.2 Characterization of photocatalysts

X-ray diffraction (XRD) was performed on a D/MAX 2500 V diffractometer (Rigaku, Japan) with monochromatized Cu K_α radiation, λ = 0.15418 nm, and the scanning range was from 10° to 70°. The morphologies and microstructures of the products were characterized by transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS, VG Scientific) using 300 W Al K_α radiation as the excitation source was applied to study the composition and chemical state of the elements. Fourier-transform infrared spectroscopy (FT-IR) spectra were obtained with an FT-IR spectrometer (America Perkin Elmer, Spectrum One) using the standard KBr disk method. UV-vis diffuse reflection spectra (DRS) of the samples were tested on a scan UV-vis spectrophotometer (UV-2550) equipped with an integrating sphere using BaSO₄ as the reference sample. The surface areas were measured by using the nitrogen adsorption Brunauer–Emmett–Teller (BET) method (BET/BJH Surface Area, 3H-2000PS1).

2.3 Photocatalytic activities study

The photocatalytic properties of the as-prepared samples were evaluated using Rh B as a model compound. In experiments, the Rh B solution (0.01 mmol L⁻¹, 100 mL) containing 0.02 g of photocatalyst were mixed in a Pyrex reaction glass. The reactivity experiments were conducted in air at room temperature. A 300 W Xe lamp (λ > 420 nm) with 100 mW cm⁻² illumination intensity was employed to provide visible-light irradiation. A 420 nm cutoff filter was inserted between the lamp and the sample to filter out UV light (λ < 420 nm). Prior to visible-light illumination, the suspension was strongly stirred in the dark for 40 min. Then the solution was exposed to visible-light irradiation under magnetic stirring. At given time intervals, 4 mL of the suspension was periodically collected and analyzed after centrifugation. The Rh B concentration was analyzed by using a UV-2550 spectrometer to record the intensity of the maximum band at 552 nm in the UV-vis absorption spectra.

2.4 Active species trapping experiments

For detecting the active species during photocatalytic reactivity, some sacrificial agents, such as 2-propanol (IPA), disodium ethylenediamine tetraacetic acid (EDTA-2Na), and 1,4-benzoquinone (BQ) were used as the hydroxyl radical (·OH) scavenger, hole (h⁺) scavenger, and superoxide radical (O₂˙⁻) scavenger, respectively. The method was similar to the former photocatalytic activity test with the addition of 1 mmol of quencher in the presence of Rh B.

3. Results and discussion

To determine the crystal form of as-prepared samples, the XRD patterns were recorded, as shown in Fig. 1. Pure Ag₂WO₄ shows obvious diffraction peaks at 2θ = 30.2°, 31.4°, 33.0°, and 45.4°, which are attributed to the (002), (231), (400), and (402) diffraction planes of α-Ag₂WO₄ (JCPDS no. 34-0061), respectively.²⁵ For the pure g-C₃N₄ sample, the diffraction peak at 27.4° can be clearly observed, which are indexed to (002) plane of g-C₃N₄.²⁶ From the XRD pattern of the g-C₃N₄/Ag₂WO₄ composite, all the crystal planes of α-Ag₂WO₄ can be detected. However, there are no any diffraction peaks of g-C₃N₄ that can be observed for the g-C₃N₄/Ag₂WO₄ sample, owing to small crystal size of g-C₃N₄ QDs.²⁶ For the pure Ag₃PO₄, the diffraction peaks can be indexed to the cubic structure of Ag₃PO₄ (JCPDS 06-0505). The diffraction peaks of Ag₃PO₄ are clearly observed in Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composites, and the intensities of the diffraction peaks of Ag₃PO₄ increase with increasing weight ratio of Ag₃PO₄ in the composites.

Fig. 1 XRD patterns of the as-prepared samples.

Herein, the successful loading of g-C₃N₄ QDs was illustrated by FT-IR spectra, as displayed in Fig. 2. The main characteristic peaks of g-C₃N₄ QDs are observed in both g-C₃N₄/Ag₂WO₄ and Ag₃PO₄/g-C₃N₄/Ag₂WO₄ spectra, showing the successful loading of g-C₃N₄ QDs. The absorption bands at 1700–800 cm⁻¹ are attributed to either C=N or C–N stretching vibrations.¹⁹ The broad range centered at 3200 cm⁻¹ can be ascribed to the N–H stretching vibration.¹⁹

Fig. 2 FT-IR spectra of the as-prepared samples.

XPS measurement was also employed to investigate the surface chemical states of the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite. Fig. 3a shows the XPS spectrum of Ag 3d; two different peaks centered at binding energies of 366.5 and 372.6 eV can be detected, and these are assigned to Ag 3d_5/2 and Ag 3d_3/2,²⁷ respectively. The XPS spectrum of W 4f shows two different peaks centered at 38.3 and 36.1 eV, which are ascribed to W 4f_5/2 and W 4f_7/2, respectively (Fig. 3b).²⁶ The P 2p peak is observed at ∼134.1 eV (Fig. 3c), corresponding to P⁵⁺.²⁷ As can be seen in the XPS spectrum of C 1s (Fig. 3d), the peak at 283.3 eV can be assigned to the sp²-bonded carbon in C–C, whereas the peak located at 287.0 eV reveals the formation of N–C=N.¹⁹ The XPS spectrum of N 1s can be deconvoluted into two peaks with binding energies at 397.5 and 400.0 eV, which are attributed to C–N=C and (N–(C)₃),¹⁹ respectively.

Fig. 3 XPS spectra of the as-obtained 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄: (a) Ag 3d, (b) W 4f, (c) P 3p, (d) C 1s, and (e) N 1s.

Fig. 4 shows scanning electron microscopy (SEM) images of pure Ag₂WO₄ and the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite. Fig. 4a shows a rod-like structure, and the average diameter of the nanorods was ∼100 nm. The Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite displays a similar morphology to that of pure Ag₂WO₄ (Fig. 4b), showing that loading Ag₃PO₄ nanocrystals and g-C₃N₄ QDs did not have any significant influence on the morphology of the Ag₂WO₄ nanorods. The as-prepared Ag₃PO₄ sample was composed of nanocrystals with average sizes of ∼20 nm (Fig. S1†). A TEM image of g-C₃N₄ QDs is shown in Fig. S2,† revealing that the prepared g-C₃N₄ QDs are monodispersed in the water solution and that each QD is uniform in size. Fig. 4c shows a TEM image of an individual Ag₃PO₄/g-C₃N₄/Ag₂WO₄ nanorod. It can be seen that Ag₃PO₄ nanocrystals and g-C₃N₄ QDs were loaded on the surface of the Ag₂WO₄ nanorod. A high-resolution TEM image of the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite reveals that the crystal lattice fringe patterns are ∼0.336 and ∼0.213 nm (Fig. 4d), which correspond to the (002) crystal plane of g-C₃N₄ and the (210) crystal plane of Ag₃PO₄, respectively. The lattice distance of 0.229 nm in the nanorod is correlated to the (421) crystal plane of Ag₂WO₄ (Fig. 4e).

Fig. 4 SEM images of the as-prepared samples: (a) Ag₂WO₄ and (b) 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄. (c) TEM image of 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄. (d and e) High-resolution TEM images of 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄.

The BET surface areas of pure Ag₂WO₄ and Ag₃PO₄/g-C₃N₄/Ag₂WO₄ were analyzed using an automated surface area analyzer (Fig. 5). The calculated BET surface area of Ag₃PO₄/g-C₃N₄/Ag₂WO₄ is 110.602 m² g⁻¹, which is larger than that of pure Ag₂WO₄ (20.817 m² g⁻¹). It can be concluded that the loading of Ag₃PO₄ and g-C₃N₄ QDs can facilitate more efficient contact of the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite with organic contaminants, which is beneficial to the improvement of photocatalytic performance.¹⁹

Fig. 5 N₂ adsorption–desorption isotherm curves of the as-prepared samples.

UV-vis DRS are shown in Fig. 6 to study the optical absorption properties of the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite. The band gap of anatase Ag₂WO₄ is ∼3.20 eV, so its absorption onset is located at 410 nm. The band gap for g-C₃N₄ QDs is 2.75 eV, corresponding to the absorption onset of 450 nm. In addition, the band gap of Ag₃PO₄ is calculated to be 2.13 eV, indicating an absorption onset at ∼580 nm. In comparison with pure Ag₂WO₄, the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite shows an absorption in the visible range that accompanies a red shift in the absorption edge resulting from the synergistic effect among Ag₂WO₄, Ag₃PO₄, and g-C₃N₄. The results reveal that the obtained Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite could be employed as a visible-light photocatalyst. The chronoamperometric I–t curves are shown in Fig. S3.† Compared with pure Ag₂WO₄, the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite exhibited enhanced photocurrent responses. The ternary nanoheterojunctions can facilitate photoelectron–hole separation and thus retard photoelectron–hole recombination in the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite.^28–30

Fig. 6 UV-vis diffuse reflection spectra of the as-obtained samples. (b) Plots of (αhν)² versus photon energy (hν) for the band gap energies of Ag₃PO₄, g-C₃N₄, and Ag₂WO₄.

In this study, Ag₃PO₄/g-C₃N₄/Ag₂WO₄ along with sole Ag₂WO₄ and g-C₃N₄/Ag₂WO₄ composite was tested for photocatalytic Rh B degradation under visible-light irradiation (Fig. 7a). The blank experiment demonstrated that Rh B was stable under visible-light irradiation for 80 min. The pure Ag₂WO₄ composite exhibited generally low photocatalytic performances, with only 40.0% Rh B degradation, after 80 min visible-light irradiation. The low performance is probably due to the fast recombination of photoinduced charges in the single Ag₂WO₄. Compared with the pure Ag₂WO₄ sample, the g-C₃N₄/Ag₂WO₄ composite exhibited enhanced photocatalytic efficiency (50% Rh B degradation). The photocatalytic activity of Ag₃PO₄/g-C₃N₄/Ag₂WO₄ further improved the degradation of Rh B under visible light (λ > 420 nm), which can be ascribed to the efficient separation of photogenerated electrons and holes through the ternary heterostructure. Among the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ nanocomposites, the highest activity was obtained with the 30% Ag₃PO₄ loading sample, with which almost 100% of Rh B was photodegraded after 80 min of visible-light irradiation. This is because hybridization of Ag₂WO₄ with Ag₃PO₄ can result in efficient separation of photogenerated charges, thus improving the photocatalytic performance. However, too many Ag₃PO₄ can even cover the surface of Ag₂WO₄, which can reduce the density of active sites, thus reducing the photocatalytic efficiency. Therefore, the suitable loading content of Ag₃PO₄ is a primary prerequisite for optimizing the photocatalytic reaction.

Fig. 7 (a) Photodegradation efficiencies of Rh B as a function of irradiation time for different photocatalysts. (b) Trapping experiment of active species during the photocatalytic degradation of Rh B over C₆₀CNTs/bismuth-based oxide nanocomposites under visible-light irradiation.

It is important to study the active species in the photocatalysis process. As shown in Fig. 7b, the photocatalytic efficiency of the 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite was greatly suppressed after the addition of BQ and EDTA-2Na, indicating that O₂˙⁻ and h⁺ were the main reactive species. In addition, a slight decrease in the photocatalytic activity was observed by the addition of IPA, suggesting that ·OH was not the main reactive species.

The stability of the 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄ photocatalyst was studied through performing recycle experiments, as shown in Fig. 8. After four photocatalytic runs, the photocatalytic efficiency did not display any significant decay. The stability of the photocatalyst was also studied by using XRD (Fig. 9a) and SEM analysis (Fig. 9b) of fresh and used 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄. It can be seen that the phase structure, as well as the morphology of the 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄, remains intact after four recycles, revealing the high stability of the 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄ photocatalyst.

Fig. 8 Cycling runs for the photocatalytic degradation of Rh B over the ternary nanocomposites under visible-light irradiation.

Fig. 9 (a) XRD patterns of the as-prepared 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄ sample before and after photocatalysis. (b) SEM image of the 30% Ag₃PO₄/g-C₃N₄/Ag₂WO₄ sample after photocatalysis.

Based on the results and discussion above, heterostructure formed in the g-C₃N₄/Ag/BiVO₄ photocatalyst played an important role in the efficient separation of photoinduced charges. The potentials of VB and CB of a semiconductor can be calculated according to the following empirical equations:

E_VB = X − E^e + 0.5E_g (1)

E_CB = E_VB − E_g (2)

where E_VB is the valence band edge potential, E_CB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV), E_g is the band gap energy of the semiconductor. According to the DRS characterization results (Fig. 6) and eqn (1) and (2), the valence band (VB) edges of g-C₃N₄ QDs, Ag₂WO₄, and Ag₃PO₄ were measured to be +1.585, 2.73, and 2.535 eV versus SHE, respectively, while the conduction band (CB) edges for g-C₃N₄ QDs, Ag₂WO₄, and Ag₃PO₄ were −1.165, −0.27, and 0.505 eV versus SHE, respectively.

Fig. 10 shows the proposed mechanism for the charge-carrier transfer process in the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ photocatalyst. When the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite is irradiated, electrons can be excited from CBs of g-C₃N₄ QDs and Ag₃PO₄. Because the CB position of g-C₃N₄ QDs was more negative than that of Ag₂WO₄ and Ag₃PO₄, the photogenerated electrons in the CB of g-C₃N₄ QDs can easily flow into the CB of Ag₂WO₄ or Ag₃PO₄, leading to the redistribution of electrons and holes so that the oxidation reaction is highly likely to take place and effectively prevent the electron–hole recombination process. Resulting in the improved photocatalytic performance of the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite photocatalyst. The holes in the VB of Ag₃PO₄ with strong oxidation power can directly degrade Rh B. In addition, the electrons on the g-C₃N₄ can react with O₂ to produce ·O²⁻, which can completely oxidize organic molecules to water and carbon dioxide.^31,32

Fig. 10 Schematic diagram of the separation and transfer of photogenerated charges in the Ag₃PO₄/g-C₃N₄/Ag₂WO₄ composite photocatalyst under visible-light irradiation.

4. Conclusions

In summary, novel Ag₃PO₄/g-C₃N₄/Ag₂WO₄ nanorods were successfully synthesized by using simple deposition–precipitation of Ag₃PO₄ nanocrystals and g-C₃N₄ QDs on Ag₂WO₄ nanorods. The as-prepared Ag₃PO₄/g-C₃N₄/Ag₂WO₄ nanostructures showed excellent photocatalytic performance on the decolorization of Rh B, which was superior to those of pure Ag₂WO₄, or g-C₃N₄/Ag₂WO₄ under visible-light irradiation (λ > 420 nm). The Ag₃PO₄ and g-C₃N₄ loading contributed to enhanced visible-light harvesting and reduced recombination of photogenerated electron–hole pairs owing to the synergistic effect of the heterojunction structure of Ag₃PO₄/g-C₃N₄/Ag₂WO₄, thereby leading to enhanced photocatalytic performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21407059, 51603086) and the Open Subject of the State Key Laboratory of Rare Earth Resource Utilization (RERU2017011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra09815h

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