One-step hydrothermal synthesis of novel Ag₃VO₄/Ag₄V₂O₇ composites for enhancing visible-light photocatalytic performance

Abstract

Novel direct Z-scheme Ag₃VO₄/Ag₄V₂O₇ composites were synthesized by one-step hydrothermal method for the first time. The photocatalytic performance was evaluated by the degradation of methylene blue (MB) and phenol under visible light irradiation (λ ≥ 420 nm). The results showed that the as-synthesized heterojunctions can significantly enhance photocatalytic activity in comparison with pure Ag₃VO₄. The optimum photocatalytic efficiency of 3 : 1.5 mol% Ag/V sample for the degradation MB and phenol reached up to about 100% and 62.1%, respectively. In addition, 3 : 1.25 mol% Ag/V sample had an excellent visible light response range, which occurred around 652 nm. Based on the photocurrent and the radical-trapping experiments, the possible for the enhancing photocatalytic mechanism was found to be a direct Z-scheme heterojunction system, which not only can improve the photogenerated electron–hole pairs' separation but also exhibit strong oxidation and reduction ability.

---

1. Introduction

With the rapid development of industry, fossil fuels, such as coal, oil and gas, have been exploited and utilized excessively, which has caused serious environmental pollution and an energy crisis.^1,2 The exploitation of pollution-free techniques for environment remediation is an urgent task for sustainable development.^3,4 Semiconductor photocatalysis has been considered as a promising and environmentally friendly technology to degrade organic pollutants with solar energy, and has attracted tremendous attention.^5–8 As is known, traditional single-component photocatalyst has poor quantum efficiency and low photocatalytic performance because the photogenerated electrons and holes can easily recombine each other.⁹ In the past few decades, various strategies had been proposed to design and fabricate highly efficient photocatalysts.^10–14 Among them, the use of heterojunction-type photocatalyst, which can take full of advantages of each component, is an important strategy to overcome those drawbacks because it can efficiently improve the photogeneraterd electron–hole separation.^15–18 Unfortunately, this improvement on charge separation is at the cost of weakening the redox ability, because the photogeneraterd electrons and holes are concentrated on the band with lower redox potentials.¹⁹ This is not favourable for the photocatalytic process requiring high redox ability.^14,20

In recently, the construction of artificial Z-scheme photocatalytic system is an ideal and effective means because it not only can reduce the bulk electron–hole recombination, but also can preserve excellent redox ability.^21,22 Actually, it has the same band alignment with typical heterojunction, but exhibits an opposite direction of charge transfer. The photogeneraterd electrons on the semiconductor with lower conduction band potential will combine with the holes on another semiconductor with higher valence band potential, and the electrons and holes with stronger redox ability remain on two semiconductors. However, the majority of the synthesized artificial Z-scheme photocatalytic systems usually had noble metal (Ag, Au)^23–25 or redox mediators (Fe³⁺/Fe²⁺, IO³⁻/I⁻),^26,27 which led to lower optical absorption or existed a reversible reaction, respectively. And these results will bring about great difficulties to their practical application. Recently, the direct Z-scheme system attracts increasingly interest because the above problems can be solved well by this system.^28–30 The different charge carrier transfers path of typical heterojunction and Z-scheme heterojunction are depicted in Fig. 1.

Fig. 1 Scheme illustration of the different charge carrier transfer path of (a) typical heterojunction and (b) direct Z-scheme heterojunction.

As a kind of excellent inorganic antibacterial agent, Ag-based materials has been widely applied in many fields.^31–34 Meanwhile, they were deemed to important semiconductor photocatalysts.^35–40 Among the Ag-containing photocatalysts, the monoclinic scheelite Ag₃VO₄ has attracted much interest due to its special band structures and a narrow band gap (about 1.9–2.2 eV).^41,42 However, the activity of pure Ag₃VO₄ is still low and needs to be modified.^43,44 As one of the effective methods for enhancing photocatalytic performance for Ag₃VO₄, constructing heterojunction has been widely used.^15,45,46 Many researchers have used this method to improve the photocatalytic activity of Ag₃VO₄. Wang et al. have investigated the catalytic performance of the g-C₃N₄/Ag₃VO₄ heterostructure, finding that the coupled semiconductor exhibited higher photoactivity for triphenylmethane dye degradation than that of the single semiconductor under visible light irradiation.⁴⁷ Akbarzadeh et al. have successfully prepared the Ag₃VO₄/Ag₃PO₄/Ag heterojunction photocatalyst, which showed higher photocatalytic activity than Ag₃VO₄/Ag₃PO₄ and pure Ag₃VO₄.⁸ In addition, TiO₂/Ag₃VO₄,^1,48 Co₃O₄/Ag₃VO₄ ⁴⁵ and ZnFe₂O₄/Ag₃VO₄ ⁴⁶ have been developed with enhancing photocatalytic efficiency in visible light irradiation compared to the individual photocatalyst. As far as we know, the direct Z-scheme Ag₃VO₄-based heterojunction photocatalysts are not be reported. Based on the advantages of above-mentioned direct Z-scheme system, it is very necessary to construct direct Z-scheme heterojunction system. As another Ag-based narrow band gap semiconductor, the band gap of Ag₄V₂O₇ is about 2.5 eV, and the conduction band (CB) of it has much better reducibility than the CB of Ag₃VO₄ due to the more negative potential of the CB for Ag₄V₂O₇.^49,50 Therefore, it is very likely that constructing the direct Z-scheme Ag₃VO₄/Ag₄V₂O₇ system should be able to enhance photocatalytic performance remarkably.

Herein, we for the first time fabricated the direct Z-scheme Ag₃VO₄/Ag₄V₂O₇ heterojunction photocatalysts by one-step hydrothermal method. The photocatalytic performance was evaluated by the degradation of methylene blue (MB) and phenol under visible light irradiation (λ ≥ 420 nm). The results showed that the as-synthesized heterojunctions can significantly enhance photocatalytic activity in comparison with pure Ag₃VO₄. The optimum photocatalytic efficiency of 3 : 1.5 mol% Ag/V sample for the degradation MB and phenol reached up to about 100% and 62.1%, respectively. In addition, 3 : 1.25 mol% Ag/V sample had an excellent visible light response range, which occurred around 652 nm. Based on the photocurrent and the radical-trapping experiments, the possible for the enhancing photocatalytic mechanism was found to be a direct Z-scheme heterojunction system, which not only can improve the photogenerated electron–hole pairs' separation but also exhibit strong oxidation and reduction ability.

2. Experimental

2.1 Synthesis of Ag₃VO₄/Ag₄V₂O₇ composite photocatalysts

All the chemicals and solvents were of analytical reagent grade and were used without further purification. The Ag₃VO₄/Ag₄V₂O₇ composite photocatalysts were prepared via one-step hydrothermal method. The details were as follows: the molar ratio of silver to vanadium (Ag to V) used were 3 : 1.25, 3 : 1.5, 3 : 2, 3 : 3, respectively. 3 mmol AgNO₃ was dissolved in 30 mL of deionized water, corresponding molar NaVO₃ was dissolved in 30 mL of distilled water, which was heated at 60 °C with stirring for 10 min to form transparent solution. The as-prepared AgNO₃ solution was added dropwise to NaVO₃ solution under vigorous stirring for 30 min to obtain an orange-yellow slurry. Then, 4 mol L⁻¹ (M) NaOH was dropwise in the upper slurry to adjust the pH value of 11. After the mixture was stirred for about 10 min, the obtained precursor was transferred into a 100 mL Teflon-lined stainless autoclave at 140 °C for 8 h. Lastly, the products were cooled to room temperature naturally, and collected by centrifugation, washed with Milli-Q water several times to ensure that the residual impurities were removed, and dried at 80 °C for 10 h.

For comparison, pure Ag₃VO₄ was also prepared via the described method, but applying a molar ratio of 3 : 1 for Ag/V.

2.2 Characterization

The crystalline phases of pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ were determined by using X-ray diffraction (XRD) (D/MAX-RB, Rigaku, Japan). The diffraction patterns were recorded in the 20–70° range with a Cu Kα source (λ = 0.15405) running at 40 kV and 30 mA. The morphologies of the as-prepared samples were measured by using scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and the high-resolution transmission electron microscopy (HRTEM) images were obtained with a transmission electron microscope (F-20, FEI, USA) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (ESCALAB 250Xi) using Al Kα radiation. The UV-vis diffuse reflectance spectra (DRS) of the photocatalysts were recorded in air at room temperature in the wavelength range of 300–800 nm using a UV-vis spectrophotometer (U-3900H, Hitachi, Japan) equipped with an integrating sphere.

2.3 Photocatalytic experiments

The photocatalytic activities of the as-prepared samples were evaluated by the degradation of methylene blue (MB) with an initial dye concentration of 25 mg L⁻¹ at 664 nm and phenol with an initial concentration of 40 mg L⁻¹ at 270 nm under visible light irradiation (λ ≥ 420 nm, 400 W Xe lamp). In each experiment, 30 mg of the as-prepared samples were respectively dispersed in 30 mL of MB solution and 30 mL of phenol solution followed by stirring for 60 min in the dark to achieve adsorption–desorption equilibrium before light irradiation. During the irradiation, 3 mL suspensions from each reaction sample were collected at 30 min (60 min for phenol degradation) intervals and centrifuged to remove the photocatalyst particles. Finally, the centrifuged solution was checked absorbance spectrum using a UV-vis spectrophotometer (U-3900H, Hitachi, Japan) at 664 nm and 270 nm, respectively. The ratio (C/C₀) of the MB and phenol concentrations were adopted to evaluate the degradation efficiency, where C₀ was the initial concentration and C was the concentration at certain time.

2.4 Photocurrent measurements

The measurements of the photocurrent were carried out on an electrochemical workstation (5060F, RST, China) in a standard three-electrode system in which the samples, an Ag/AgCl electrode (saturated KCl) and a Pt wire were used as the working electrode, reference electrode and counter electrode, respectively. 0.5 M Na₂SO₄ aqueous solution was introduced as the electrolyte. A 100 W incandescent lamp with a 420 nm cut-off filter was used as the light source. For the preparation of the working electrode, 3 mg of the samples were dispersed in a certain amount of ethanol and Nafion solution homogeneously. The as-prepared samples were spread into a circle with a diameter of 6 mm on the bottom middle of an ITO glass substrate. The photocurrents of the photocatalysts when the light was switched on and off were measured at 0.8 V.

3. Result and discussion

3.1 XRD analysis

The crystallographic structure of as-prepared pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ composite photocatalysts were detected by XRD analysis. Fig. 2 exhibits the XRD diffraction patterns of the pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ composite photocatalysts. The diffraction peaks of Ag₃VO₄ can be well indexed into monoclinic Ag₃VO₄ (JCPDS card no. 43-0542). The main XRD peaks at 30.86°, 32.33°, 35.07°, 35.94°, 38.92° and 54.06° were well-matched with the crystal planes of (−1 2 1), (1 2 1), (3 0 1), (2 0 2), (0 2 2) and (3 3 1) of Ag₃VO₄. According to the ref. 49, it is difficult to prepare pure Ag₄V₂O₇, because Ag₄V₂O₇ often coexisted with Ag₃VO₄ in the synthesis. From Fig. 2, we can clearly found the diffraction peaks of Ag₄V₂O₇ in Ag₃VO₄/Ag₄V₂O₇ composites. The diffraction peaks of Ag₄V₂O₇ can be well indexed into orthorhombic Ag₄V₂O₇ (JCPDS card no. 77-0097). The main XRD peaks at 31.93°, 32.93°, 42.24°, 46.63° and 52.65° correspond to the crystal planes of (2 2 4), (0 4 0), (1 5 1), (1 1 7) and (1 3 7) of Ag₄V₂O₇, respectively. Therefore, the diffraction peaks of Ag₃VO₄/Ag₄V₂O₇ consisted of the characteristic peaks of Ag₃VO₄ (marked with ◆) and Ag₄V₂O₇ (marked with ●), which implied that the co-existence of Ag₃VO₄ and Ag₄V₂O₇ in the as-prepared Ag₃VO₄/Ag₄V₂O₇ composites. No signals for other crystalline phases were detected in the composite photocatalysts.

Fig. 2 XRD patterns of as-prepared pure Ag₃VO₄ and different molar ratio of Ag/V samples.

3.2 Morphology characterization

The morphology of pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ composites was observed by SEM. Fig. 3 shows the SEM images of (a) pure Ag₃VO₄, (b) 3 : 1.25 mol% Ag/V, (c) 3 : 1.5 mol% Ag/V, (d) 3 : 2 mol% Ag/V, and (e) 3 : 3 mol% Ag/V. As shown in Fig. 3a, pure Ag₃VO₄ presented irregular ragged particles and the size ranged from 0.5 μm to 3 μm. Similar results were also observed in the other samples (Fig. 3b–e). The results of SEM indicated that the presence of Ag₄V₂O₇ could not change the morphology of Ag₃VO₄ during the hydrothermal reaction.

Fig. 3 SEM images of (a) pure Ag₃VO₄, (b) 3 : 1.25 mol% Ag/V, (c) 3 : 1.5 mol% Ag/V, (d) 3 : 2 mol% Ag/V, and (e) 3 : 3 mol% Ag/V.

To further testify the existence of heterojunction between Ag₃VO₄ and Ag₄V₂O₇, we observed the composites by TEM and HRTEM. Fig. 4 shows the TEM images of (a) pure Ag₃VO₄ and (b) 3 : 1.5 mol% Ag/V and the HRTEM image of (c) 3 : 1.5 mol% Ag/V. As shown in Fig. 4a and b, the TEM images of samples revealed the irregular morphology, which were accordance with the SEM images. The interplanar spacings of 0.29 and 0.27 nm shown in Fig. 4c matched well with the (−1 2 1) and (0 4 0) lattice planes of the Ag₃VO₄ and Ag₄V₂O₇, respectively. Thus the results were corresponding with the XRD, and the formed heterojunction maybe ensure the better transfer of charge carriers.

Fig. 4 TEM images of (a) pure Ag₃VO₄ and (b) 3 : 1.5 mol% Ag/V and the HRTEM image of (c) 3 : 1.5 mol% Ag/V.

3.3 Chemical state analysis

The XPS analysis was used to further understand the chemical state of pure Ag₃VO₄ and 3 : 1.5 mol% Ag/V (Ag₃VO₄/Ag₄V₂O₇) composite photocatalysts. Before the analysis, all peaks of the other elements were calibrated according to the deviation between the C 1s peak and the standard signal of C 1s at 284.8 eV.

The survey scan spectra XPS spectra of 3 : 1.5 mol% Ag/V (Ag₃VO₄/Ag₄V₂O₇) display the characteristic peaks of Ag, V and O elements (Fig. 5a). Fig. 5b–d shows the high-resolution XPS spectra of Ag 3d, V 2p and O 1s for pure Ag₃VO₄ and 3 : 1.5 mol% Ag/V. The binding energy located at 368.24 eV and 374.21 eV were attributable to Ag 3d_5/2 and Ag 3d_3/2, respectively, which can be ascribed to Ag⁺ ions of pure Ag₃VO₄ (Fig. 5b).⁷ Besides, the binding energy of Ag 3d_5/2 and Ag 3d_3/2 for 3 : 1.5 mol% Ag/V were 368.19 eV and 374.16 eV, respectively, both of which had a shift of 0.05 eV to low energy region compared with the pure Ag₃VO₄.⁵¹ The result appeared to suggest the existence of covalent bonding between the oxygen and the silver ions in Ag₃VO₄ and Ag₄V₂O₇.⁵¹ No obvious characteristic diffraction peaks of Ag⁰ (located at 368.6 eV and 374.7 eV)⁵² and silver oxides (367.5 eV and 373.5 eV)⁵³ were detected in above composites, which well-matched with the XRD results.

Fig. 5 High resolution X-ray photoelectron spectroscopy (XPS) spectra: (a) overall spectra of 3 : 1.5 mol% Ag/V; (b) Ag 3d; (c) V 2p; (d) O 1s.

Fig. 5c shows that two peaks at 516.84 and 524.33 eV were separately attributed to V 2p_5/2 and V 2p_3/2 of V⁵⁺ ions of pure Ag₃VO₄.⁵⁴ In addition, the binding energy of V 2p_5/2 and V 2p_3/2 for 3 : 1.5 mol% Ag/V were 516.75 eV and 524.23 eV,⁵⁴ respectively, which had a shift of 0.09 eV and 0.10 eV compared with the pure Ag₃VO₄. The result indicated the existence of covalent bonding between the vanadium and the oxygen in the form of (VO₄)³⁻ and (V₂O₇)⁴⁻.⁵¹

The peaks for O 1s located at 530.07 eV corresponded to the O²⁻ anion (Fig. 5d). Compared with pure Ag₃VO₄, the signal of O 1s for 3 : 1.5 mol% Ag/V had a 0.05 eV shift to lower binding energy. The banding energy of O 1s could be attributed to the surface lattice oxygen and adsorbed oxygen species, respectively.⁴⁸

These results demonstrated the interaction between Ag₃VO₄ and Ag₄V₂O₇, confirming the existence of chemical bonds between Ag₃VO₄ and Ag₄V₂O₇ in the composites.

3.4 Optical properties

The optical properties of pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ composites were investigated using the UV-vis diffuse reflectance spectroscopy (Fig. 6). As illustrated in Fig. 6, all samples showed an absorbance in the visible light absorption ability. An obvious red shift of the optical absorption edge was observed for the composites. The onset of visible light absorption by 3 : 1.25 mol% Ag/V even occurred around 652 nm, which indicated that Ag₃VO₄/Ag₄V₂O₇ composites had an excellent visible light response range. The optical band gap for the semiconductor photocatalysts was estimated using the following equation:
where λ is the maximum wavelength of absorption by photocatalysts (nm) (illustrated by the dash line in Fig. 6), and E_g is the estimated band gap energy of the photocatalysts (eV). As shown in Fig. 6, the onset wavelength for pure Ag₃VO₄, 3 : 1.25 mol% Ag/V, 3 : 1.5 mol% Ag/V, 3 : 2 mol% Ag/V and 3 : 3 mol% Ag/V occurred around 613, 652, 642, 635 and 629 nm, respectively. On the basis of the above equation and the onset wavelength, the calculated band gaps of samples were 2.02, 1.90, 1.93, 1.95 and 1.97 eV, respectively. These results indicated that the Ag₃VO₄/Ag₄V₂O₇ composites could not only better absorption visible light but also broaden the light response range.

Fig. 6 UV-vis DRS spectra of pure Ag₃VO₄, 3 : 1.25 mol% Ag/V, 3 : 1.5 mol% Ag/V, 3 : 2 mol% Ag/V, 3 : 3 mol% Ag/V.

3.5 Photocatalytic properties

The photocatalytic performance of as-prepared pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ composites were evaluated by decomposing MB^2,55,56 under visible light irradiation (λ ≥ 420 nm) (Fig. 7). The adsorption ratio was recorded when adsorption–desorption equilibrium was achieved before irradiation. As shown in Fig. 7, the tiny changes of adsorption in dark indicated that the degradation of MB was mainly affected by visible light irradiation rather than surface adsorption.

Fig. 7 Photocatalytic activity of pure Ag₃VO₄ and Ag₃VO₄/Ag₄V₂O₇ composites for the degradation of MB under the visible light irradiation (λ ≥ 420 nm).

From Fig. 7, it can be seen that the Ag₃VO₄/Ag₄V₂O₇ composites was obviously higher than that of the pure Ag₃VO₄ samples, and the best activity was obtained for 3 : 1.5 mol% Ag/V. At reaching 30 min, the photodegradation rate of MB reached up to 87.1% for 3 : 1.5 mol% Ag/V. After 150 min of irradiation, it almost reached up to 100% which was about 1.79 times than pure Ag₃VO₄ (55.76%), while the 3 : 1.25 mol%, 3 : 2 mol% and 3 : 3 mol% samples reached rates of up to 90.9%, 84.0% and 60.7%, respectively. This observation implied that the heterojunction formed between Ag₃VO₄ and Ag₄V₂O₇ played an important role in improving the photocatalytic activity.

Moreover, Fig. 7 also demonstrates that the degradation rate of MB using the Ag₃VO₄/Ag₄V₂O₇ composites initially increased along with the increase of the vanadium content in the order of 3 : 1.25 mol% and 3 : 1.5 mol% at the beginning of irradiation. Whereas the degradation ratio of MB decreased when the mole ratio of Ag/V were 3 : 2 and 3 : 3. The plausible reason for the result in this finding can be explained by the following aspects. With the increase of the amount of vanadium, the proportion of Ag₃VO₄ decreased along with increasing Ag₄V₂O₇. Excess Ag₄V₂O₇ might act as recombination centers of the photogenerated electron–hole pairs. In addition, extra Ag₄V₂O₇ could lead to the reduction in active center sites of photocatalysts.

Furthermore, circulating runs in the photocatalytic degradation of MB were performed under visible light irradiation to check the stability of the best photocatalyst (3 : 1.5 mol% Ag/V). As shown in Fig. 8, the photocatalyst did not display any significant loss of photocatalytic performance. This result implied that the Ag₃VO₄/Ag₄V₂O₇ photocatalysts are not photo-corroded during the photodegradation of the pollutant molecules, which is particularly important for their application.

Fig. 8 Circulating runs in the photocatalytic degradation of MB in the presence of the 3 : 1.5 mol% Ag/V sample under visible light irradiation.

In addition to MB, phenol, a typical colorless contaminant, was also chosen to further evaluate the photocatalytic performance of the samples. There was tiny adsorption of phenol after 60 min of stirring in the dark and the photodegradation rate of phenol for pure Ag₃VO₄ and 3 : 1.5 mol% Ag/V are shown in Fig. 9.

Fig. 9 Degradation ratio of phenol using pure Ag₃VO₄ and 3 : 1.5 mol% Ag/V under visible light irradiation.

The results indicated that phenol could be hardly degraded under visible light irradiation without photocatalysts. After 300 min of irradiation, the degradation rate of phenol for 3 : 1.5 mol% Ag/V (62.1%) was 1.83 times than pure Ag₃VO₄ (33.9%). Fig. 10a and b show the temporal absorption spectral changes of phenol in photodegradation for pure Ag₃VO₄ and 3 : 1.5 mol% Ag/V under visible light irradiation. Although an obvious absorption peak at 270 nm could be observed after irradiation for 300 min, the absorption peak at 270 nm was much declined for 3 : 1.5 mol% Ag/V as shown in Fig. 10b, compared to pure Ag₃VO₄. These findings further confirmed the enhanced photocatalytic activity of the Ag₃VO₄/Ag₄V₂O₇ composites.

Fig. 10 Time-dependent UV-vis absorption spectra of phenol in the presence of various photocatalysts: (a) pure Ag₃VO₄ and (b) 3 : 1.5 mol% Ag/V.

3.6 Photocurrent measurements

On the basis of the experimental results above, it is believed that the formed heterojunctions play a key hole in reducing the recombination rate of electrons and holes. To better confirm this assumption, we investigated its photocurrent responses in an electrolyte under visible light, which may indirectly correlate with the generation and transfer of the photoinduced charge carriers in the photocatalytic process. Fig. 11 shows the photocurrent response of pure Ag₃VO₄ and the 3 : 1.5 mol% Ag/V sample when the light was switched on and off. Remarkably, the current abruptly increased and decreased when the light source was switched on and off. The 3 : 1.5 mol% Ag/V sample exhibited an obviously enhanced photocurrent response compared with pure Ag₃VO₄. This implies that more efficient separation of the photogenerated electron–hole pairs and fast transfer of photoinduced charge carriers occurred in the 3 : 1.5 mol% Ag/V photocatalyst, which could be attributed to close interfacial connections and the synergetic effect existing between Ag₃VO₄ and Ag₄V₂O₇. Meanwhile, the results are well corresponded to their photocatalytic performance.

Fig. 11 Photocurrent responses of pure Ag₃VO₄ and the 3 : 1.5 mol% Ag/V sample under visible light irradiation.

3.7 Photocatalytic mechanism

To further explore the effect of these active species during the photocatalytic degradation process, radical-trapping experiments with different scavengers were performance with the 3 : 1.5 mol% Ag/V as shown in Fig. 12. In this way, active species including holes (h⁺), superoxide radicals (˙O₂⁻) and hydroxyl radicals (˙OH) with effective oxidation and reduction potentials could be determined. In this study, 1 mM benzoquinone (BQ, a scavenger of ˙O₂⁻), 1 mM sodium oxalate (Na₂C₂O₄, a scavenger of h⁺) and 1 mM isopropanol (IPA, a scavenger of ˙OH) were adopted. Fig. 12 shows the variation of MB degradation with different scavenger added. As shown in Fig. 12, reaction rate constant k was decreased with the addition of IPA. Actually, the degradation efficiency of MB changed very slightly after 150 min of irradiation from the inset in Fig. 12, compared to no scavenger. This result indicated that ˙OH radicals were not the dominant active species in the photocatalytic reaction system of Ag₃VO₄/Ag₄V₂O₇. After addition of Na₂C₂O₄, the degradation efficiency of MB was remarkably depressed, which suggested that h⁺ was an important active species in the MB photocatalytic degradation. In addition, when BQ was added into the reaction system, the degradation efficiency of MB was almost completely depressed, whose apparent reaction rate constant k was decelerated obviously from 2.53 × 10⁻² to 2.85 × 10⁻³ min⁻¹. Considering the above results, h⁺ and ˙O₂⁻ were main active species in the degradation process, and ˙O₂⁻ was acted as the dominant active species responsible for MB degradation under visible light irradiation.

Fig. 12 Effects of different scavengers on the degradation of MB in the presence of 3 : 1.5 mol% Ag/V composite under visible light irradiation (λ ≥ 420 nm).

A possible mechanism for the MB photodegradation of the enhanced activity of the Ag₃VO₄/Ag₄V₂O₇ composites under visible-light irradiation could be proposed. The conduction band (CB) and valence band (VB) potentials of a semiconductor can be calculated by the following equation:⁵⁷

E_VB = E_CB + E_g (2)

where χ is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. E_e is the energy of free electrons in the hydrogen scale (about 4.5 eV), E_g is the band gap energy of the semiconductor. The χ value of Ag₃VO₄ is 5.64 eV ⁵⁸ and the band gap energy of Ag₃VO₄ is 2.02 eV. According to the above equations, the E_CB value of Ag₃VO₄ was 0.13 eV, and the homologous E_VB value was 2.15 eV. According to the ref. 49, the band gap energy, the E_CB value and the E_VB value for Ag₄V₂O₇ were 2.5, −2.03 and 0.47 eV, respectively.

Thus, the photogenerated electrons on the CB of Ag₄V₂O₇ can react with O₂ to form ˙O₂⁻ radicals due to the position of CB (−2.03 eV vs. NHE) of Ag₄V₂O₇ is more negative than the potential of O₂/˙O₂⁻ (−0.33 eV vs. NHE).⁴⁹ Whereas, the VB potential (0.47 eV vs. NHE) of Ag₄V₂O₇ is lower than the standard redox potential of OH⁻, H₂O/˙OH (2.38 eV vs. NHE), implying that the photogenerated holes in the VB of Ag₄V₂O₇ cannot oxidize the adsorbed H₂O or OH⁻ to generate ˙OH.⁴⁹ As for Ag₃VO₄, the photogenerated electrons on the CB of Ag₃VO₄ cannot react with O₂ to form ˙O₂⁻ radicals because the position of CB (0.13 eV vs. NHE) of Ag₃VO₄ is less negative than the potentials of O₂/˙O₂⁻ (−0.33 eV vs. NHE). The VB potential (2.15 eV vs. NHE) of Ag₃VO₄ is lower than the standard redox potential of OH⁻, H₂O/˙OH (2.38 eV vs. NHE), suggests that the photogenerated holes in the VB of Ag₃VO₄ also cannot oxidize the adsorbed H₂O or OH⁻ to generate ˙OH. The findings are corresponding to the results of the active species trapping experiments.

On the basis of the above experiment results, a novel direct Z-scheme mechanism for the enhanced photocatalytic performance of Ag₃VO₄/Ag₄V₂O₇ heterojunctions was proposed. As illustrated in Fig. 13, both Ag₃VO₄ and Ag₄V₂O₇ can be initiated by the visible-light to yield photogenerated electron–hole pairs. If the charge transfer path of photogenerated electron–hole pairs is like the typical heterojunction system, then the photogenerated electrons in the CB of Ag₃VO₄ will generate fewer ˙O₂⁻ radicals because of its low reducibility. Thus, the photogenerated electrons in the CB of Ag₃VO₄ tend to transfer and recombine with the photogenerated holes in the VB of Ag₄V₂O₇. In this way, the more photogenerated electrons accumulated in the CB of Ag₄V₂O₇ can reduce the adsorbed O₂ to form more ˙O₂⁻, which is a powerful oxidative specie can break down the chromophores of organic pollutants into small molecules, e.g. CO₂ and H₂O. Meanwhile, the photogenerated holes left behind in the VB of Ag₃VO₄ can directly oxidize organic pollutants. Therefore, it can draw a conclusion that the photocatalytic reaction of prepared Ag₃VO₄/Ag₄V₂O₇ heterojunctions followed a direct Z-scheme mechanism, which could improve the photogenerated electron–hole pairs' separation and transfer as well as show a strong oxidation and reduction ability for efficiency degradation of organic pollutants.

Fig. 13 The possible photocatalytic mechanism of Ag₃VO₄/Ag₄V₂O₇ heterojunction photocatalysts for degradation of organic pollutants under visible light irradiation.

4. Conclusions

In summary, we have firstly constructed the direct Z-scheme Ag₃VO₄/Ag₄V₂O₇ heterojunction photocatalysts by one-step hydrothermal method. Characterization tests indicated that the as-prepared photocatalysts had stable visible-light-driven photocatalytic performance and high efficiency. The Ag₃VO₄/Ag₄V₂O₇ composites had an excellent visible light response range, which occurred around 652 nm. Moreover, the molar ratio at 3 : 1.5 of the Ag/V sample performed optical photocatalytic properties for degradation of MB (∼100%) and phenol (∼62.1%). Hence, the enhancing photocatalytic activity was due to the formation of a direct Z-scheme heterojunction between the Ag₃VO₄ and Ag₄V₂O₇, which not only resulted in the electrons in the CB of Ag₄V₂O₇ exhibits high reducibility and the holes in the VB of Ag₃VO₄ shows high oxidizability, but also improve the photogenerated electron–hole pairs' separation.

Acknowledgements

We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 21271022).

References

1. J. X. Wang, H. Ruan, W. J. Li, D. Z. Li, Y. Hu, J. Chen, Y. Shao and Y. Zheng, J. Phys. Chem. C, 2012, 116, 13935–13943.
2. D. F. Xu, B. Cheng, S. W. Cao and J. G. Yu, Appl. Catal., B, 2015, 164, 380–388.
3. F. Z. Wang, W. J. Li, S. N. Gu, H. D. Li, H. L. Zhou and X. Wu, RSC Adv., 2015, 5, 89940–89950.
4. J. G. Hou, Z. Wang, C. Yang, W. L. Zhou, S. Q. Jiao and H. G. Zhu, J. Phys. Chem. C, 2013, 117, 5132–5141.
5. H. D. Li, W. J. Li, S. N. Gu, F. Z. Wang, H. L. Zhou, X. T. Liu and C. J. Ren, RSC Adv., 2016, 6, 48089–48098.
6. G. Wang, Y. Ren, G. J. Zhou, J. P. Wang, H. F. Cheng, Z. Y. Wang, J. Zhan, B. B. Huang and M. H. Jiang, Surf. Coat. Technol., 2013, 228, S283–S286.
7. D. P. Das, A. Samal, J. Das, A. Dash and H. Gupta, Photochem. Photobiol., 2014, 90, 57–65.
8. E. Akbarzadeh, S. R. Setayesh and M. R. Gholami, RSC Adv., 2016, 6, 14909–14915.
9. F. Le Formal, S. R. Pendlebury, M. Cornuz, S. D. Tilley, M. Grätzel and J. R. Durrant, J. Am. Chem. Soc., 2014, 136, 2564–2574.
10. S. G. Kumar and L. G. Devi, J. Phys. Chem. A, 2011, 115, 13211.
11. Y. Li, Y. Jiang, S. Peng and F. Jiang, J. Hazard. Mater., 2010, 182, 90.
12. X. Zhang and Q. Liu, Appl. Surf. Sci., 2008, 254, 4780.
13. D. E. Huang, S. Liao, S. Quan, L. Liu, Z. He, J. Wan and W. Zhou, J. Mater. Res., 2007, 22, 2389.
14. Z. Liu, X. Zhang, S. Nishimoto, M. Jin, D. A. Tryk, T. Murakami and A. Fujishima, Langmuir, 2007, 23, 10916.
15. H. L. Wang, L. S. Zhang, Z. G. Chen, J. Q. Hu, S. J. Li, Z. H. Wang, J. S. Liu and X. C. Wang, Chem. Soc. Rev., 2014, 43, 5234–5244.
16. M. Ge, Y. F. Li, L. Liu, Z. Zhou and W. Chen, J. Phys. Chem. C, 2011, 115, 5220–5225.
17. Y. Hou, F. Zuo, A. Dagg and P. Y. Feng, Nano Lett., 2012, 12, 6464–6473.
18. Y. J. Wang, Q. S. Wang, X. Y. Zhan, F. M. Wang, M. Safdar and J. He, Nanoscale, 2013, 5, 8326–8339.
19. P. Zhou, J. G. Yu and M. Jaroniec, Adv. Mater., 2014, 26, 4920–4935.
20. H. Y. Li, Y. J. Sun, B. Cai, S. Y. Gan, D. X. Han, L. Niu and T. S. Wu, Appl. Catal., B, 2015, 170, 206–214.
21. P. Zhou, J. G. Yu and M. Jaroniec, Adv. Mater., 2014, 26, 4920–4935.
22. H. J. Li, W. G. Tu, Y. Zhou and Z. G. Zou, Adv. Sci., 2016, 1500389.
23. X. F. Wang, S. F. Li, Y. Q. Ma, H. G. Yu and J. G. Yu, J. Phys. Chem. C, 2011, 115, 14648–14655.
24. L. Q. Ye, J. Y. Liu, C. Q. Gong, L. H. Tian, T. Y. Peng and L. Zan, ACS Catal., 2012, 2, 1677–1683.
25. Y. Sasaki, H. Kato and A. Kudo, J. Am. Chem. Soc., 2013, 135, 5441–5449.
26. K. Maeda, D. L. Lu and K. Domen, ACS Catal., 2013, 3, 1026–1033.
27. Y. Sasaki, A. Iwase, H. Kato and A. Kudo, J. Catal., 2008, 259, 133–137.
28. J. G. Yu, S. H. Wang, J. X. Low and W. Xiao, Phys. Chem. Chem. Phys., 2013, 15, 16883–16890.
29. T. Y. Wang, W. Quan, D. L. Jiang, L. L. Chen, D. Li, S. C. Meng and M. Chen, Chem. Eng. J., 2016, 300, 280–290.
30. X. Jia, M. Tahir, L. Pan, Z. F. Huang, X. W. Zhang, L. Wang and J. J. Zou, Appl. Catal., B, 2016, 198, 154–161.
31. R. D. Holtz, B. A. Lima, A. G. Souza Filho, M. Brocchi and O. L. Alves, Nanomedicine: Nanotechnology, Biology and Medicine, 2012, 8, 935–940.
32. K. Naddafi, H. Jabbari and M. Chehrehei, Iran. J. Environ. Health Sci. Eng., 2010, 7, 223–228.
33. B. S. Atiyeh, M. Costagliola, S. N. Hayek and S. A. Dibo, Burns, 2007, 33, 139–148.
34. O. Choi, K. K. Deng, N. J. Kim, L. Ross, R. Y. Surampalli and Z. Hu, Water Res., 2008, 42, 3066–3074.
35. P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei and M. H. Whangbo, Angew. Chem., Int. Ed., 2008, 47, 7931–7933.
36. P. Wang, B. B. Huang, X. Y. Zhang, X. Y. Qin, H. Jin, Y. Dai, Z. Y. Wang, J. Y. Wei, J. Zhan, S. Y. Wang, J. P. Wang and M. H. Whangbo, Chem.–Eur. J., 2009, 15, 1821–1824.
37. C. Hu, T. W. Peng, X. X. Hu, Y. L. Nie, X. F. Zhou, J. H. Qu and H. He, J. Am. Chem. Soc., 2010, 132, 857–862.
38. X. F. Wang, S. F. Li, H. G. Yu, J. G. Yu and S. W. Liu, Chem.–Eur. J., 2011, 17, 7777–7780.
39. C. W. Xu, Y. Y. Liu, B. B. Huang, H. Li, X. Y. Qin, X. Y. Zhang and Y. Dai, Appl. Surf. Sci., 2011, 257, 8732–8736.
40. P. S. Saud, Z. K. Ghouri, B. Pant, T. An, J. H. Lee, M. Park and H. Y. Kim, Carbon Lett., 2016, 18, 30–36.
41. R. Konta, H. Kato, H. Kobayashi and A. Kudo, Phys. Chem. Chem. Phys., 2003, 5, 3061–3065.
42. X. X. Hu, C. Hu and J. H. Qu, Mater. Res. Bull., 2008, 43, 2986–2997.
43. H. Xu, H. M. Li, G. S. Sun, J. X. Xia, C. D. Wu, Z. X. Ye and Q. Zhang, Chem. Eng. J., 2010, 160, 33–41.
44. C. M. Huang, K. W. Cheng, G. T. Pan, W. S. Chang and T. C. K. Yang, Chem. Eng. Sci., 2010, 65, 148–152.
45. L. Zhang, Y. M. He, P. Ye, Y. Wu, W. H. Qin and T. H. Wu, Mater. Sci. Eng., B, 2013, 178, 45–52.
46. L. Zhang, Y. M. He, P. Ye, Y. Wu and T. H. Wu, J. Alloys Compd., 2013, 549, 105–113.
47. S. M. Wang, D. L. Li, C. Sun, S. G. Yang, Y. Guan and H. He, Appl. Catal., B, 2014, 144, 885–892.
48. X. J. Zou, Y. Y. Dong, X. D. Zhang and Y. B. Cui, Appl. Surf. Sci., 2016, 366, 173–180.
49. J. X. Wang, X. Yang, J. Chen, J. J. Xian, S. G. Meng, Y. Zheng, Y. Shao and D. Z. Li, J. Am. Ceram. Soc., 2014, 97, 267–274.
50. C. M. Huang, G. T. Pan, Y. C. M. Li, M. H. Li and T. C. K. Yang, Appl. Catal., A, 2009, 358, 164–172.
51. K. S. Raju, M. K. P. Seydei and S. A. Suthanthiraraj, Mater. Lett., 1994, 19, 65–68.
52. C. Hu, T. W. Peng, X. X. Hu, Y. L. Nie, X. F. Zhou, J. H. Qu and H. He, J. Am. Chem. Soc., 2010, 132, 857–862.
53. H. S. Shin, H. C. Choi, Y. Jung, S. B. Kim, H. J. Song and H. J. Shin, Chem. Phys. Lett., 2004, 383, 418–422.
54. H. Xu, H. M. Li, L. Xu, C. D. Wu, G. S. Sun, Y. G. Xu and J. Y. Chu, Ind. Eng. Chem. Res., 2009, 48, 10771–10778.
55. X. H. Yang, H. T. Fu, X. Z. An, X. C. Jiang and A. B. Yu, RSC Adv., 2016, 6, 34103–34109.
56. H. B. Rao, Z. W. Lu, X. Liu, H. W. Ge, Z. Y. Zhang, P. Zou, H. Hua and Y. Y. Wang, RSC Adv., 2016, 6, 46299–46307.
57. S. N. Gu, W. J. Li, F. Z. Wang, S. Y. Wang, H. L. Zhou and H. D. Li, Appl. Catal., B, 2015, 170, 186–194.
58. S. M. Wang, Y. Guan, L. P. Wang, W. Zhao, S. G. Yang, H. He and C. Sun, J. Comput. Theor. Nanosci., 2015, 12, 5016–5021.

---