The crystal phase transformation of Ag₂WO₄ through loading onto g-C₃N₄ sheets with enhanced visible-light photocatalytic activity

Abstract

A g-C₃N₄/Ag₂WO₄ composite photocatalyst was synthesized via a simple co-precipitation method at room temperature and was thoroughly characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, UV-visible diffuse reflectance spectroscopy and photoluminescence spectroscopy. The characterization results indicated that upon depositing Ag₂WO₄ onto the surface of g-C₃N₄ its morphology changed from primary rod-like particles to globular nanoparticles and its phase changed from α-type to β-type Ag₂WO₄. When the composite was used as the catalyst in the photodegradation of Rhodamine B, 40%-g-C₃N₄/Ag₂WO₄ exhibited the highest photocatalytic activity with its rate constant was about 10.8 times larger than that of pure g-C₃N₄ and 3.12 times larger than that of bare Ag₂WO₄. The enhanced photocatalytic activity can be attributed to the complementary potentials of the conduction bands and valence bands of g-C₃N₄ and Ag₂WO₄, which could not only realize the effective separation of photoinduced electron and hole pairs, but also retained the catalysts high stability and photocatalytic performance even after five recycles. Furthermore, a possible photocatalytic mechanism for the degradation of RhB over the g-C₃N₄/Ag₂WO₄ composite was proposed according to the results of active species quenching experiments.

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1. Introduction

In recent years, semiconductor photocatalytic technology has revealed potential applications in converting solar energy to hydrogen and eliminating environmental contaminants.^1,2 Among the numerous photocatalytic materials, TiO₂ is one of the most widely studied photocatalysts due to its low price, non-toxicity, strong oxidation ability and good stability. However, the wide band gap of TiO₂ (3.2 eV) can be excited by UV light, which only accounts for about 4% of the solar energy. Thus, it makes TiO₂ difficult to be applied on large scale. Hence, it is very urgent to develop an efficient and stable photocatalyst with high activity under visible light.

Recently, various silver-based photocatalysts with unique physical and chemical properties, including AgX (X = Cl, Br, I),^3–5 Ag₂CO₃,^6,7 Ag₂O,⁸ Ag₃PO₄,^9,10 Ag₂CrO₄ (ref. 11) and AgVO₃,¹² have been proved to be efficient photocatalytic materials under visible light. As one of them, silver tungstate (Ag₂WO₄) also displays excellent photocatalytic capacity.^13,14 Generally, Ag₂WO₄ has three different crystal forms: α, β, and γ phases. α-Ag₂WO₄ is the thermodynamically stable phase, β-Ag₂WO₄ and γ-Ag₂WO₄ are relatively unstable.¹⁵ In regard their photocatalytic properties, although pure β-Ag₂WO₄ exhibited improved photocatalytic activity when compared with pristine α-Ag₂WO₄ due to the different crystallinities and PL emissions, β-Ag₂WO₄ is easily inactivated due to photocorrosion during illumination.¹⁶ Hence, how to keep β-Ag₂WO₄ stable is also the important issue. Nowadays, graphite-like carbon nitride (g-C₃N₄) has been explored as a promising candidate for hydrogen evolution and environmental purification under visible-light irradiation.^17–19 Although g-C₃N₄ possesses good chemical and thermal stability, the use of g-C₃N₄ in photocatalysis is limited by its high recombination of photogenerated electron–hole pairs. To solve this drawback, abundant strategies have been developed including doping,²⁰ deposition²¹ and sensitization.²² Constructing a heterostructure is also an effective way for decreasing the recombination rate of the photogenerated carriers. Using the combination of g-C₃N₄ and other appropriate semiconductors, including AgX/g-C₃N₄ (X = Br, I),²³ MoS₂/g-C₃N₄,^24,25 Ag₂CrO₄/g-C₃N₄ (ref. 26) and g-C₃N₄/Bi₂O₂CO₃,²⁷ the complementary potentials of the conduction band and valence band can realize the effective separation of photoinduced electron–hole pairs and thus, the photocatalytic activity would be anticipated to improve.^28,29

Therefore, in this paper, g-C₃N₄/Ag₂WO₄ composites were successfully synthesized via a facile co-precipitation method. Amazingly, Ag₂WO₄ deposition on the surface of g-C₃N₄ lead to a change in its morphology from primary rod-like particles to globular nanoparticles and its phase also changed from the α-type to β-type phase. When the g-C₃N₄/Ag₂WO₄ composite was employed as a photocatalyst, it exhibited an enhanced photocatalytic activity when compared with pure g-C₃N₄ and Ag₂WO₄, and its photocatalytic property remained stable even after five recycles. Furthermore, a possible photocatalytic mechanism was also proposed based on the obtained experimental results.

2. Experimental

2.1 Preparation of the g-C₃N₄/Ag₂WO₄ composite

All reagents except for melamine were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Melamine was supplied from Aladdin. All of the reagents were of analytical grade and used as received without any further purification.

g-C₃N₄ was prepared by heating melamine in a muffle furnace. 10 g of melamine was placed in a covered crucible and heated to 550 °C for 4 h in a muffle furnace at a heating rate of 2 °C min⁻¹.

The process used to prepare g-C₃N₄/Ag₂WO₄ is described as follows: in a typical procedure, 0.16 g of g-C₃N₄ was dispersed in 20 mL of an aqueous solution containing 0.28 g of Na₂WO₄·2H₂O by ultrasonication and then 20 mL of an aqueous solution containing 0.29 g of AgNO₃ was slowly added dropwise into the above solution with stirring for 4 h at room temperature. The mixture was filtered, washed three times with deionized water and ethanol, respectively and dried for 24 h at room temperature to give 40%-g-C₃N₄/Ag₂WO₄ (the weight percentage of g-C₃N₄ in g-C₃N₄/Ag₂WO₄ was 40%). Similarly, a series of composites were fabricated by changing the mass of g-C₃N₄ added and are denoted as 10%-g-C₃N₄/Ag₂WO₄, 20%-g-C₃N₄/Ag₂WO₄ and 60%-g-C₃N₄/Ag₂WO₄. For comparison, pure Ag₂WO₄ was prepared without the addition of g-C₃N₄. For reference, a physical mixture of Ag₂WO₄ and g-C₃N₄ was prepared with 40% (weight percentage) of g-C₃N₄ and was denoted as g-C₃N₄/Ag₂WO₄ (mixed).

2.2 Characterization

X-ray diffraction (XRD) measurements were carried out on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (40 kV, 40 mA) for phase identification. Fourier transform infrared spectroscopy (FTIR) was recorded on a Perkin Elmer spectrum 100 FTIR spectrometer using KBr discs. X-ray photoelectron spectroscopy (XPS) measurements were recorded using a Thermo Fisher Scientific Escalab 250. The morphology of the product was examined by Tecnai G² T12 transmission electron microscopy. The BET surface area was collected at 77 K using a Micromeritics Tristar 3020 analyzer; the sample was outgassed at 120 °C for 12 h prior to the measurement. The UV-vis diffuse reflectance spectra (DRS) were measured using a Perkin Elmer Lambda 750 UV-vis spectrometer. The photoluminescence spectra (PL) were obtained on a Perkin Elmer LS55 spectrometer with an excitation wavelength of 325 nm.

2.3 Evaluation of the photocatalytic activity and stability

The photocatalytic performance of the g-C₃N₄/Ag₂WO₄ composites was evaluated by degrading Rhodamine B (RhB) under visible light. Visible light was provided by a 300 W Xe lamp with a UV cut off filter (λ > 400 nm). 50 mg of the g-C₃N₄/Ag₂WO₄ composite was dispersed into 50 mL of a RhB solution (5 mg L⁻¹) under magnetic stirring at room temperature. Prior to the light irradiation, the dispersion was kept in the dark for 60 min under magnetic stirring to reach an adsorption–desorption equilibrium. Upon irradiation, the solutions were collected every 10 min, centrifuged to remove the catalyst and then analyzed according to the absorbance at 552 nm using a UV-vis spectrophotometer. The degradation efficiency of the solution was calculated according to the following formula:

Degradation efficiency = (C₀ − C)/C₀ × 100% (1)

C₀ is the initial concentration of RhB and C is the RhB concentration after light irradiation at different times.

Recycling experiments of the g-C₃N₄/Ag₂WO₄ composite and the pure Ag₂WO₄ reference were also investigated under visible light irradiation. After the photodegradation of RhB, the separated photocatalyst was washed several times with water and ethanol, dried at room temperature for 24 h, then applied for the degradation of a fresh 5 mg L⁻¹ aqueous solution of RhB under the same conditions for next cycle.

3. Results and discussion

3.1 The chemical structure and features of the g-C₃N₄/Ag₂WO₄ catalyst

To determine the crystal form of the as-prepared samples, the XRD patterns were investigated and shown in Fig. 1A. Pure Ag₂WO₄ displayed obvious diffraction peaks at 2θ = 16.7°, 30.2°, 31.4°, 33.0° and 45.4°, which were attributed to the (0 1 1), (0 0 2), (2 3 1), (4 0 0) and (4 0 2) diffraction planes of α-Ag₂WO₄ (JCPDS no. 34-0061).¹⁶ For the pure g-C₃N₄ sample, there were two diffraction peaks at 27.4° and 13.1°, corresponding to the graphite-like stacking and in-plane structural repeating motifs of the conjugated aromatic units of g-C₃N₄, which were indexed to the (0 0 2) and (1 0 0) planes of g-C₃N₄.³⁰ Amazingly, for the resultant g-C₃N₄/Ag₂WO₄ composites, the crystal planes of α-Ag₂WO₄ were not observed, however, peaks appearing at 18.4°, 30.0°, 32.2°, 37.4° and 44.6° were indexed to the (0 2 0), (0 2 2), (2 2 0), (0 4 0) and (0 4 2) planes of a β-Ag₂WO₄ phase (JCPDS no. 33-1195),¹⁵ which implied that the α-Ag₂WO₄ phase was transformed into the β-Ag₂WO₄ phase in the g-C₃N₄/Ag₂WO₄ composites due to the addition of g-C₃N₄. To support the evidence of phase transformation, the XRD patterns were monitor with the preparation time as shown in Fig. 1B. It was clearly found that β-Ag₂WO₄ was produced in a short reaction time of 1 h and the XRD typical diffraction peaks of α-Ag₂WO₄ gradually decreased with increasing reaction time and correspondingly, those ascribed to β-Ag₂WO₄ were increased. Finally, the α-Ag₂WO₄ was completely transformed into the β-Ag₂WO₄ phase in 4 h.

Fig. 1 (A) The XRD patterns obtained for (a) 10%-g-C₃N₄/Ag₂WO₄, (b) 20%-g-C₃N₄/Ag₂WO₄, (c) 40%-g-C₃N₄/Ag₂WO₄, (d) 60%-g-C₃N₄/Ag₂WO₄, (e) pure Ag₂WO₄ and (f) g-C₃N₄. (B) The XRD patterns obtained for 40%-g-C₃N₄/Ag₂WO₄ prepared with different reaction times.

Subsequently, the morphologies of pure Ag₂WO₄, g-C₃N₄ and 40%-g-C₃N₄/Ag₂WO₄ were investigated using TEM as shown in Fig. 2. Clearly, pure Ag₂WO₄ showed a rod-like morphology (Fig. 2A) in agreement with α-Ag₂WO₄.¹⁵ g-C₃N₄ exhibited a flat aggregated structure whose surface was smooth (Fig. 2B). As presented in Fig. 2C and D, after g-C₃N₄ and Ag₂WO₄ were composited, it was observed that small-sized nanoparticle-like Ag₂WO₄ were evenly loaded on the surface of the g-C₃N₄ and the possible mechanism for the formation of the nanoparticle-like Ag₂WO₄ was tentatively explained as follows: g-C₃N₄ exhibits a negative Z-potential,³¹ so Ag⁺ ions can be adsorbed on the surface of g-C₃N₄ through electrostatic interactions. After the WO₄²⁻ ions reacted with Ag⁺, the as-formed Ag₂WO₄ was deposited on the g-C₃N₄. To reduce the interface energy, Ag₂WO₄ exists as an nanoparticle form in the composite. Hence, the morphology of Ag₂WO₄ was greatly changed after introduction of the g-C₃N₄ support, which may be the reason that the α-Ag₂WO₄ phase was transformed to β-Ag₂WO₄ in the g-C₃N₄/Ag₂WO₄ composites.

Fig. 2 The TEM images of (A) pure Ag₂WO₄, (B) g-C₃N₄ and (C and D) the 40%-g-C₃N₄/Ag₂WO₄ composite.

Moreover, the surface areas of the as-prepared samples were determined. The surface areas of Ag₂WO₄, g-C₃N₄, 10%-g-C₃N₄/Ag₂WO₄, 20%-g-C₃N₄/Ag₂WO₄, 40%-g-C₃N₄/Ag₂WO₄ and 60%-g-C₃N₄/Ag₂WO₄ were 0.8, 4.7, 0.3, 0.5, 1.1 and 1.9 m² g⁻¹, respectively. Clearly, the small surface areas play a negligible role in the photocatalytic activity.

The FTIR spectra of pure Ag₂WO₄, g-C₃N₄ and the 40%-g-C₃N₄/Ag₂WO₄ composite are shown in Fig. 3. In the case of pristine g-C₃N₄, the peaks at 1247 cm⁻¹, 1325 cm⁻¹, 1408 cm⁻¹ and 1640 cm⁻¹ correspond to the stretching vibrations of C–N heterocyclic compounds, the peak at 808 cm⁻¹ was ascribed to the breathing mode of the triazine unit³² and the peak in the range of 3000–3500 cm⁻¹ was caused by N–H and O–H stretching vibrations.³³ When compared with bare g-C₃N₄, a new peak at 860 cm⁻¹ was observed in the 40%-g-C₃N₄/Ag₂WO₄ composite, attributed to the anti-symmetric stretching vibration of the O–W–O bond,³⁴ which further revealed that g-C₃N₄ was successfully coupled with Ag₂WO₄ in the composite.

Fig. 3 The FTIR patterns obtained for (a) Ag₂WO₄, (b) 40%-g-C₃N₄/Ag₂WO₄ and (c) g-C₃N₄.

In order to investigate the atomic species and the binding modes of the various atoms in the g-C₃N₄/Ag₂WO₄ composites, the XPS spectra of 40%-g-C₃N₄/Ag₂WO₄ was recorded and shown in Fig. 4. Obviously, the peaks of C, N, Ag, W and O in the survey spectra indicated that the sample incorporated g-C₃N₄ and Ag₂WO₄. The Ag 3d was located at 374.2 eV and 368.2 eV, which were attributed to the Ag(I) oxidation state (Fig. 4B).³⁵ The W 4f_5/2 and W 4f_7/2 peaks were located at 36.1 and 34.1 eV (Fig. 4C), corresponding to W⁶⁺.³⁶ The peaks at 531.1 and 532.3 eV were ascribed to the binding energies of O 1s (Fig. 4D). Besides, the C 1s spectrum exhibited two main peaks at 284.6 and 288.1 eV, the former was identified as carbon atoms in a pure carbonaceous environment, the latter was assigned to the sp² C atom bonded in N–C=N.³⁷ Fig. 4F presents the N 1s XPS spectrum, which was deconvoluted into three peaks with binding energies at 398.8, 400.3 and 401.1 eV, which were attributed to the sp²-bonded N atom to two carbon atoms (C–N=C), tertiary nitrogen (N–(C)₃) and amino functional groups (N–H) respectively.^38,39

Fig. 4 The XPS spectra of the 40%-g-C₃N₄/Ag₂WO₄ composite: (A) survey spectra, (B) Ag 3d, (C) W 4f, (D) O 1s, (E) C 1s and (F) N 1s.

Furthermore, the optical properties of the 40%-g-C₃N₄/Ag₂WO₄ composite were measured by UV-vis DRS. As shown in Fig. 5, the absorption band edges of pure Ag₂WO₄ and g-C₃N₄ were estimated to be 425 and 466 nm. After g-C₃N₄ was coupled with Ag₂WO₄, there was red-shift in the band edge position to 502 nm, suggesting that the light harvested was shifted to the visible region, which is helpful in regard the utilization of the sunlight. Additionally, the band gap energy (E_g) was obtained according to the empirical formula E_g = 1240/λ (λ represents the absorption edge). Thus, the band gap energies of g-C₃N₄ and Ag₂WO₄ were estimated to be about 2.66 eV and 2.92 eV, respectively.

Fig. 5 The UV-vis DRS patterns obtained for (a) Ag₂WO₄, (b) 40%-g-C₃N₄/Ag₂WO₄ and (c) g-C₃N₄.

PL spectroscopy has been widely used to investigate the mitigation, transfer and recombination process of photogenerated electron–hole pairs in a semiconductor. It is generally believed that a weaker PL intensity indicated the lower recombination probability of the photogenerated charge carriers, which results in higher photocatalytic activity. The PL spectra of pure g-C₃N₄ and the 40%-g-C₃N₄/Ag₂WO₄ composite at an excitation wavelength of 325 nm are shown in Fig. 6. The remarkable decrease in the PL intensity of the 40%-g-C₃N₄/Ag₂WO₄ implied that photogenerated electron–hole pairs were effectively separated in the g-C₃N₄/Ag₂WO₄ composite.

Fig. 6 The photoluminescence spectra of (a) pure g-C₃N₄ and (b) the 40%-g-C₃N₄/Ag₂WO₄ composite.

3.2 The photocatalytic activity and stability of the g-C₃N₄/Ag₂WO₄ catalyst

The photocatalytic activities of pristine Ag₂WO₄, g-C₃N₄, g-C₃N₄/Ag₂WO₄ (mixed) and the as-prepared g-C₃N₄/Ag₂WO₄ composites were evaluated using the photodegradation of RhB dye under visible light irradiation. As shown in Fig. 7A, the blank experiment indicated that RhB was stable under visible light irradiation. When g-C₃N₄ and Ag₂WO₄ were used alone as the photocatalyst, about 32% and 76% of the RhB was decomposed within 40 min. The g-C₃N₄/Ag₂WO₄ (mixed) showed a 90% degradation rate for RhB. Attractively, the g-C₃N₄/Ag₂WO₄ composites showed significantly enhanced photocatalytic activities under the same conditions when compared with the reference g-C₃N₄ and Ag₂WO₄ catalysts, and the highest photocatalytic activity was obtained using 40%-g-C₃N₄/Ag₂WO₄, which contained an appropriate ratio of the two composites. To have a better understanding of the reaction kinetics of the RhB degradation catalyzed by the various samples, Fig. 7B shows the relationships between ln(C₀/C) and irradiation time. From the linear relationships, the photocatalytic degradation curves obtained in this case all fit with first-order kinetics. Correspondingly, the 40%-g-C₃N₄/Ag₂WO₄ composite showed the highest reaction rate constant at 0.113 min⁻¹, which was about 10.8 times larger than that found for pristine g-C₃N₄ (0.0104 min⁻¹) and was 3.12 times larger than that found for bare Ag₂WO₄ (0.0362 min⁻¹). For the g-C₃N₄/Ag₂WO₄ (mixed) reference, the highest reaction rate constant obtained was 0.0585 min⁻¹.

Fig. 7 (A) The degradation rates (B) first-order kinetic plots, (C) rate constants and (D) recycling runs for the photodegradation of RhB: (a) blank catalyst, (b) pristine g-C₃N₄, (c) Ag₂WO₄, (d) 10%-g-C₃N₄/Ag₂WO₄, (e) 20%-g-C₃N₄/Ag₂WO₄, (f) 40%-g-C₃N₄/Ag₂WO₄, (g) 60%-g-C₃N₄/Ag₂WO₄ and (h) g-C₃N₄/Ag₂WO₄ (mixed).

The stability of the as-prepared catalyst is a critical issue for its practical applications. To test the stability of the g-C₃N₄/Ag₂WO₄ composites, five cycles of the photocatalytic process were carried out as shown in Fig. 7D. The results showed that the 40%-g-C₃N₄/Ag₂WO₄ composite possessed excellent stability and the degradation rate could still reach 90% even after five cycles. While the activity of bare Ag₂WO₄ catalyst gradually decreased upon recycling; at the 5th recycle, the photodegradation rate was only 45%. Moreover, when the photocatalytic reaction was completed after 5 cycles, the primary light yellow color of the Ag₂WO₄ catalyst became black, suggesting that Ag nanoparticles were formed due to the reduction of Ag⁺ by the photoinduced electrons,⁴⁰ which was further verified by the XRD characterization shown in Fig. 8A.

Fig. 8 (A) The XRD patterns obtained for pure Ag₂WO₄. (B) The XRD patterns and (C) FTIR spectra obtained for 40%-g-C₃N₄/Ag₂WO₄ before and after the photocatalytic degradation of RhB.

To further determine the catalyst structure after five photocatalytic cycles, the XRD patterns of pristine Ag₂WO₄ and the 40%-g-C₃N₄/Ag₂WO₄ composite before and after the degradation of RhB were investigated as shown in Fig. 8A and B. In the case of the pristine Ag₂WO₄ catalyst after five catalytic runs, a new peak at 38.1° attributed to Ag appeared and the Ag intensity increased upon recycling, which was a result of the reduction of Ag⁺ in Ag₂WO₄ by the photogenerated electrons. For the resultant 40%-g-C₃N₄/Ag₂WO₄ composite, there were almost negligible peaks of Ag at 38.1° in the recycled samples when compared with the spent bare Ag₂WO₄ catalyst. This phenomenon may be a result of the photogenerated electrons rapidly transferring between g-C₃N₄ and Ag₂WO₄ in the g-C₃N₄/Ag₂WO₄ composite and thus, the photocorrosion of Ag₂WO₄ was effectively inhibited, retaining the g-C₃N₄/Ag₂WO₄ composites excellent structural stability and photocatalytic activity. In addition, the FTIR spectrum of the g-C₃N₄/Ag₂WO₄ composite after photodegradation was almost unaltered as shown in Fig. 8C, further proving the stability of the g-C₃N₄/Ag₂WO₄ photocatalyst.

3.3 Detection of the photocatalytic mechanism

In the photodegradation process, some active species such as holes (h⁺), hydroxyl radicals (˙OH) and superoxide radicals (˙O₂⁻) are formed upon light irradiation. The active species generated during the photodegradation process over the 40%-g-C₃N₄/Ag₂WO₄ composite were identified using active species scavenging experiments, where ammonium oxalate (AO) was used as a h⁺ scavenger,⁴¹ p-benzoquinone (p-BQ) used as a ˙O₂⁻ scavenger⁴² and t-butanol (t-BuOH) used as an ˙OH quencher.⁴³ As shown in Fig. 9, after the addition of p-BQ into the reaction system, the degradation rate decreased moderately, suggesting that ˙O₂⁻ was an active species. For the addition of AO, the degradation efficiency of RhB exhibited a significant reduction, indicating that h⁺ were a key active species in this system. However, in the presence of t-BuOH, the effect was not obvious, showing that ˙OH was not the main active species. Hence, ˙O₂⁻ and h⁺ were the main active species during the degradation process over the g-C₃N₄/Ag₂WO₄ composite.

Fig. 9 The influence of active species scavengers on the photodegradation of RhB under visible light irradiation upon the addition of: (a) AO (1 mM), (b) p-BQ (1 mM), (c) t-BuOH (5 mM) and (d) no scavenger.

To clearly understand the photocatalytic process over the g-C₃N₄/Ag₂WO₄ composite, the positions of the conduction band (CB) and valance band (VB) of Ag₂WO₄ and g-C₃N₄ were calculated using the following empirical equations:^44,45

E_VB = X − E_e + 0.5E_g (2)

E_CB = E_VB − E_g (3)

where E_VB is the VB potential, E_CB is the CB potential, X is the absolute electronegativity of the semiconductor, E_e is the energy of free electrons on hydrogen scale (4.5 eV) and E_g is the band gap. The X values of Ag₂WO₄ and g-C₃N₄ are 6 and 4.73, respectively.^46,47 Thus, the calculated CB and VB positions of g-C₃N₄ were −1.1 eV and 1.56 eV, respectively. In the case of Ag₂WO₄, the CB and VB positions were located at +0.04 eV and 2.96 eV, respectively. Hence, the photoinduced electrons (e⁻) on the CB of g-C₃N₄ can react with O₂ to form ˙O₂⁻ radicals because the CB position of g-C₃N₄ was more negative than the O₂/˙O₂⁻ potential (−0.33 eV vs. NHE).⁴⁸ However, its VB potential was lower than the standard redox potential of ˙OH/H₂O (2.68 eV vs. NHE),^49,50 indicating that the photoinduced holes (h⁺) in the VB of g-C₃N₄ do not oxidize the adsorbed H₂O to form ˙OH. On the contrary, for Ag₂WO₄, its CB potential was more positive than the of O₂/˙O₂⁻ potential, implying it does not form ˙O₂⁻ by e⁻ with O₂, whereas, its VB potential was higher than the standard redox potential of ˙OH/H₂O, which means that the h⁺ of VB can oxidize the adsorbed H₂O to form ˙OH. Finally, based on the band gap structures of the g-C₃N₄/Ag₂WO₄ composite and the effects observed from the scavenger experiments, a possible pathway for the photodegradation of RhB over the g-C₃N₄/Ag₂WO₄ composite was proposed and is shown in Fig. 10. Under the visible-light irradiation, g-C₃N₄ and Ag₂WO₄ were simultaneously excited to form electron–hole pairs. Because the CB and VB of Ag₂WO₄ lie below those of g-C₃N₄, the e⁻ in the CB of Ag₂WO₄ can transfer and recombine with the h⁺ in the VB of g-C₃N₄. As such, the some of the remaining h⁺ left in the VB of Ag₂WO₄ can directly oxidize the organic pollutant, the remaining h⁺ will reacted with water to form ˙OH radicals. At the same time, the photoinduced electrons on the CB of g-C₃N₄ will react with O₂ to generate highly active ˙O₂⁻. Therefore, a Z-scheme mechanism was established, the photogenerated electron–hole pairs were effectively separated by the above pathway. The main active species, such as ˙O₂⁻ and h⁺, which were generated in the photocatalysis process are responsible for the degradation of RhB.

Fig. 10 A possible pathway for the photodegradation of RhB over the g-C₃N₄/Ag₂WO₄ composite.

4. Conclusions

In summary, a stable and novel g-C₃N₄/Ag₂WO₄ composite photocatalyst has been synthesized successfully via a facile co-precipitation method at room temperature. Due to the introduction of g-C₃N₄, the morphology of the Ag₂WO₄ changed from primary rod-like particles to globular nanoparticles, which resulted in its phase change from the α-type to β-type. The g-C₃N₄/Ag₂WO₄ composite exhibited an enhanced photocatalytic activity for the degradation of RhB when compared with the reference samples of bare g-C₃N₄, pristine Ag₂WO₄ and its physical mixture g-C₃N₄/Ag₂WO₄ (mixed). The improved photocatalytic activity was mainly attributed to the suitable band edge positions of Ag₂WO₄ and g-C₃N₄, which contributed to accelerating the separation of the photogenerated electron–hole pairs. Besides, the transformed globular nanoparticles of β-Ag₂WO₄ on the g-C₃N₄ support also helped the g-C₃N₄/Ag₂WO₄ composite retain its photocatalytic and structural properties even after five recycles. Hence, the as-prepared g-C₃N₄/Ag₂WO₄ composite photocatalyst will have great potential due to its effective pollutant purification applications.

Acknowledgements

We sincerely acknowledge the financial supports from the National Natural Science Foundation of China (21373069, 21673060), Fund for Research and Development of Science and Technology in Shenzhen (No. JCYJ20160427184531017, No. ZDSYS201603301417588) and the State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2015DX08).

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Footnote

† The authors contributed equally to this study.

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