Novel magnetically separable ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with tandem n–n heterojunctions as highly efficient visible-light-driven photocatalysts

Abstract

In the present work, novel magnetically separable ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percentages of Ag₃VO₄ were successfully prepared. The preparation method is a facile large-scale method with a low temperature of 96 °C. Characterization techniques of XRD, EDX, SEM, TEM, UV-vis DRS, FT-IR, PL, and VSM were applied to study the microstructure, purity, morphology, spectroscopic, and magnetic properties of the resultant samples. Photocatalytic activity was evaluated by degradation of rhodamine B under visible-light irradiation. The results indicated that the nanocomposite with 10% Ag₃VO₄ has the best activity. The photocatalytic activity of this nanocomposite is about 2.8 and 3.5-fold higher than those of the ZnO/AgBr/Fe₃O₄ and ZnO/Ag₃VO₄/Fe₃O₄ nanocomposites, respectively. The highly enhanced activity was attributed to the formation of tandem n–n heterojunctions between ZnO, AgBr, and Ag₃VO₄ which suppresses recombination of the photogenerated electron–hole pairs. Furthermore, the enhanced activity of the nanocomposite for degradation of two more dye pollutants and a colorless pollutant was confirmed. Finally, the magnetic photocatalyst was recycled using an external magnetic field with reasonable activity after four runs.

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Introduction

It is believed that heterogeneous photocatalytic processes, as a green and cost-effective technology, have potential to address different challenges in the field of environmental pollution, energy shortage, and global warming facing human being in the present century.^1–3 In order to utilize solar energy as the source for photocatalytic processes, it is essential that visible-light-driven photocatalysts with reasonable activity are provided. However, traditional photocatalysts such as TiO₂ and ZnO have wide band gaps. Consequently, they can mainly absorb the UV fraction of solar radiation, accounting for only about 4% of solar energy, which leads to their poor photocatalytic activities under visible-light irradiation.^4–6 Moreover, owing to their fast recombination of electron–hole pairs, they have low photocatalytic activity. Hence, preparation of highly enhanced visible-light-driven photocatalysts based on TiO₂ and ZnO has been the subject of many articles and at the present it is an interesting and attractive topic in the field of photocatalysis research.^7–20 In order to increase visible-light photocatalytic activity, two strategies can be used. The first one is enabling photocatalysts to absorb more energy from the solar irradiation and subsequently produce more photogenerated electron–hole pairs. The second one is preventing the photogenerated charge carriers from undesirable recombination, resulting in enhancement of electron–hole separation efficiency.^21–24 However, finding one-component photocatalyst with considerable activity under visible-light irradiation is almost impossible. Hence, we have to design multi-component photocatalysts with wide visible-light absorption range, low recombination of charge carriers, non-selective degradation ability for different pollutants, and reasonable stability. Recently, it was found that by combining two n-type semiconductors, n–n heterojunctions could be formed at the interface of them.^25–28 Due to formation of internal electric field at the interface of these semiconductors, separation of the photogenerated electron–hole pairs could be greatly enhanced. Being as n-type semiconductors, by combining ZnO, AgBr, and Ag₃VO₄ with each other, n–n heterojunctions could be formed, resulting in enhanced photocatalytic activity.

On the other hand, after using photocatalysts in the reaction system, they cannot be entirely recycled, leading to generation of secondary pollution. Consequently, a major problem in large-scale utilization of photocatalysts is removal of them from the treated solutions. Magnetic photocatalysts, as new generation of photocatalysts, could tackle the above mentioned problem.²⁹ However, to the best of our knowledge, there is no report about the preparation and photocatalytic activity of the magnetic ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with tandem n–n heterojunctions.

Based on the above considerations, we report novel magnetically separable ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄ prepared using a facile refluxing method at a low temperature of 96 °C. The microstructure, purity, morphology, spectroscopic, and magnetic properties of the prepared samples were studied using X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform-infrared spectroscopy (FT-IR), photoluminescence spectroscopy (PL), and vibrating sample magnetometry (VSM) techniques. Photocatalytic activity of the nanocomposites was investigated by degradations of rhodamine B (RhB), methylene blue (MB), methyl orange (MO), and phenol under visible-light irradiation. To further elucidate the photocatalytic degradation mechanism, a series of scavengers for the reactive species were employed. It was demonstrated that weight percent of Ag₃VO₄, refluxing time, and calcination temperature have considerable influence on the degradation reaction. Finally, the nanocomposite was magnetically separated from the treated solution after four successive cycles.

Experimental

Materials

Zinc nitrate (Zn(NO₃)₂·4H₂O), ferric chloride (FeCl₃·6H₂O), ferrous chloride (FeCl₂·4H₂O), silver nitrate, sodium bromide, ammonium-metavanadate, ammonia, and sodium hydroxide were purchased from Loba chemie. RhB, MO, MB, 2-propanol, ammonium oxalate, benzoquinone, and absolute ethanol were obtained from Merck and employed without further purification. Deionized water was used throughout this study.

Instruments

The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm), employing scanning rate of 0.04° per s in the 2θ range from 20° to 80°. Surface morphology and distribution of particles were studied by LEO 1430VP SEM, using an accelerating voltage of 15 kV. The purity and elemental analysis of the products were obtained by EDX on the same SEM instrument. For SEM and EDX experiments, samples mounted on an aluminum support using a double adhesive tape coated with a thin layer of gold. The TEM investigations were performed by a Zeiss-EM10C instrument with an acceleration voltage of 80 kV. The DRS spectra were recorded by a Scinco 4100 apparatus. The FT-IR spectra were obtained by a Perkin Elmer Spectrum RX I apparatus. The PL spectra of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer with an excitation wavelength of 300 nm. The conditions were fixed in order to compare the PL intensities. UV-vis spectra for the degradation reaction were studied using a Cecile 9000 spectrophotometer. Magnetic properties of the samples were obtained using an alternating gradient force magnetometer (model AGFM, Iran). The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz). The pH of solutions was measured using a Metrohm digital pH meter of model 691.

Preparation of the nanocomposites

Nanoparticles of Fe₃O₄ were prepared by chemical co-precipitation method that the detailed procedure was described elsewhere.³⁰ For preparation of the ZnO/AgBr/Fe₃O₄ nanocomposite, 0.128 g of Fe₃O₄ nanoparticles was dispersed into 150 mL of water by ultrasonic irradiation for 30 min in a cylindrical Pyrex reactor provided with water circulation arrangement to maintain its temperature at 25 °C. Then, 1.305 g of zinc nitrate and 0.263 g of silver nitrate were added to the formed dark brown suspension of Fe₃O₄ with mechanical stirring continued for 30 min. Afterwards, aqueous solution of sodium hydroxide (5 M) was slowly added to the solution under stirring until pH of the solution reached 10. Then, aqueous solution of sodium bromide (0.319 g dissolved in 20 mL of water) was dropwise added to the dark brown suspension under stirring. Afterwards, the formed orange suspension was stirred for 60 min and refluxed at about 96 °C for 60 min. The light brown product was centrifuged to get the precipitate and washed two times with water and ethanol and the product was magnetically separated. The precipitate was dried in an oven at 70 °C for 24 h.

In a typical procedure for preparation of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite, where 10% is the weight percent of Ag₃VO₄, 0.450 g of ZnO/AgBr/Fe₃O₄ nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 10 min. Then, silver nitrate (0.058 g) was dissolved in the suspension and mechanically stirred for 60 min. After that, an aqueous solution of NaOH (5 M) was dropwise added into the solution under stirring at room temperature until the pH of the solution reached to 10. Then, an aqueous solution of ammonium metavanadate (0.013 g in 50 mL of water) was slowly added to the formed suspension. Finally the suspension was refluxed at 96 °C for 60 min. The suspension was centrifuged to get the precipitate out and washed twice with water and ethanol and dried in an oven at 60 °C for 24 h. The schematic diagram for preparation of the nanocomposites can be illustrated in Scheme 1.

Scheme 1 The schematic diagram for preparation of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites.

Photocatalysis experiments

Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 mL capacity. Temperature of the reactor was maintained at 25 °C using a water circulation arrangement. The solution was mechanically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. An LED source with 50 W was used as a visible-light source. The source was fitted on the top of the reactor. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL aqueous solution of RhB (1 × 10⁻⁵ M), MB (1.3 × 10⁻⁵ M), MO (1.05 × 10⁻⁵ M), or phenol (5.0 × 10⁻⁵ M), was continuously stirred in the dark for 60 min, to attain adsorption equilibrium. Samples were taken from the reactor at regular intervals and the photocatalyst removed before analysis by the spectrophotometer at 553, 664, 477, and 270 nm corresponding to the maximum absorption wavelengths of RhB, MB, MO, and phenol, respectively.

Results and discussion

Fig. 1 exhibits XRD patterns of the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄. For pure Fe₃O₄, the diffraction peaks well match with the (220), (311), (400), (331), (422), (333), and (531) planes of the face-centered cubic structure (JCPDS file number of 75-1610).³¹ In the case of the ZnO/Fe₃O₄ nanocomposite, the diffraction peaks are ascribed to face-centered cubic Fe₃O₄ and wurtzite hexagonal crystalline ZnO (JCPDS file number of 65-3411).³² For the ZnO/AgBr/Fe₃O₄ and ZnO/Ag₃VO₄/Fe₃O₄ nanocomposites, the diffraction patterns verify co-existence of all counterparts of the nanocomposites. Furthermore, in the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites, diffraction peaks of Ag₃VO₄ are clearly observed that well matched with its monoclinic phase (JCPDS no. 43-0542).³³

Fig. 1 XRD patterns for the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄.

Purity of the samples was verified by EDX spectra and the results for the ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples are shown in Fig. 2a. It is evident that the samples have reasonable purity and all of the peaks are simply ascribed to elements of the resultant samples. Other peaks in these spectra are related to the elements applied for sputter coating of the samples on the EDX stage. Weight percents of Zn, O, Ag, Br, Fe, and V elements in the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite are 37.5, 23.8, 17.0, 3.8, 11.4, and 6.4, respectively. Furthermore, EDX mapping was employed to display the element distributions in the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite and the results are shown in Fig. 2b–h. It is evident that there is homogeneous distribution for the elements. Hence, it was concluded that the counterparts of the nanocomposite have uniformly combined to each other.

Fig. 2 (a) EDX spectra for the ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples. (b)–(h) EDX mapping of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite.

Morphology and junction between constituents of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite was investigated by SEM and TEM techniques and the results are shown in Fig. 3. For this nanocomposite, particles of Fe₃O₄, AgBr, and Ag₃VO₄ are clearly seen on ZnO sheets (Fig. 3a). Moreover, the TEM images of the nanocomposite confirm distribution of Fe₃O₄, AgBr, and Ag₃VO₄ particles on the ZnO plates (Fig. 3b and c).

Fig. 3 (a) SEM image of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite. (b) and (c) TEM images of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite with two magnifications.

It is believed that photocatalytic activity is closely related to extent of generation, separation, and transfer of the photogenerated charge carriers.³⁴ UV-vis spectra of photocatalysts give useful information about extent of electron–hole pairs generated under the light irradiation. Hence, UV-vis DRS spectra of the resultant samples were provided and the results are shown in Fig. 4a. As can be seen, the ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites have intense absorption in the visible-light range. Consequently, under visible-light irradiation, more electron–hole pairs are generated on the nanocomposites, resulting in enhanced photocatalytic activity.

Fig. 4 (a) UV-vis DRS spectra for the ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄. (b) FT-IR spectra for the Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples.

To characterize structural properties of the resultant samples, FT-IR spectra of the Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples were provided and the spectra are shown in Fig. 4b. For the all samples, the broad absorption bands around 3200–3400 cm⁻¹ are related to the O–H stretching vibration of adsorbed water. For the Fe₃O₄ nanoparticles, two characteristic peaks at 435 and 610 cm⁻¹ are related to the vibrations of Fe–O bond.²⁹ For the samples with ZnO, the peak at 560 cm⁻¹ is ascribed to the stretching vibration of the Zn–O bond.²⁸ In the case of the samples containing Ag₃VO₄, the peaks for Ag–V and V–O bonds were appeared about at 770, 880, and 920 cm⁻¹.²⁴ For the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite, the peaks for Fe–O, Zn–O, Ag–V, and V–O bonds are clearly seen in the wavenumbers similar to its counterparts.

It is well known that magnetic properties of photocatalysts affect on magnetic separability of them. Hence, magnetization of the pure Fe₃O₄ nanoparticles and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite were investigated and the results are shown in Fig. 5. As can be seen, saturated magnetization for the Fe₃O₄ nanoparticles and the nanocomposite at 8500 Oe are 55.5 and 6.26 emu g⁻¹, respectively. Decrease of the magnetization of the nanocomposite is simply related to the presence of the nonmagnetic ZnO, AgBr, and Ag₃VO₄ particles along with the magnetic Fe₃O₄ nanoparticles. As can be seen in inset of Fig. 5, saturated magnetization of the nanocomposite is high enough to separate it from the solution after decontamination processes.

Fig. 5 Magnetization curves for the Fe₃O₄ nanoparticles and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite.

To investigate visible-light photocatalytic activity of the prepared samples, RhB, a widely used dye, was selected as a target pollutant. The degradation reaction was performed over the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄ and the results are displayed in Fig. 6a. Furthermore, photolysis of RhB under the visible-light irradiation (in absence of any photocatalyst) was carried out and the results are shown in this figure. It is evident that RhB has reasonable stability under the light irradiation for 360 min. It is observed that photocatalytic activity of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄ is higher than those of the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, and ZnO/Ag₃VO₄/Fe₃O₄ samples. Among the prepared samples, the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite has the superior activity in the degradation reaction. After the light irradiation for 105 min, about 21, 16, 69, 65, and 98% of RhB were degraded over the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples, respectively. Fig. 6b–e shows plots of absorbance versus wavelength for degradation of RhB over the ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples at various irradiation times. It is evident that intensity of the absorption peaks continuously decreases and there are not any blue-shifts for RhB absorption peaks centered at 554 nm. Therefore, it was concluded that degradations of RhB over the nanocomposite takes place by aromatic ring-opening mechanism.^35,36

Fig. 6 (a) Photodegradation of RhB over the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percents of Ag₃VO₄. UV-Vis spectra for degradation of RhB under visible-light irradiation over the (b) ZnO/Fe₃O₄, (c) ZnO/AgBr/Fe₃O₄, (d) ZnO/Ag₃VO₄/Fe₃O₄, and (e) ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite.

In order to describe quantitatively photocatalytic activity of the resultant samples, observed first-order rate constants (k_obs) of the degradation reaction over the samples was calculated using the slope of equation ln[RhB] = ln[RhB]_o − k_obst,¹¹ in which [RhB] and [RhB]_o are concentrations of RhB in time of t and zero, respectively and the results are shown in Fig. 7a. As can be seen, the degradation rate constant increases with weight percent of Ag₃VO₄ up to 10 and then decreases. The degradation rate constants of RhB over the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples 4.92 × 10⁻⁴, 4.74 × 10⁻⁴, 102 × 10⁻⁴, 82.6 × 10⁻⁴, and 290 × 10⁻⁴ min⁻¹, respectively. Hence, activity of the nanocomposite is about 59, 61, 2.8, and 3.5-fold larger than those of the Fe₃O₄, ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, and ZnO/Ag₃VO₄/Fe₃O₄ samples, respectively. The emission intensity in a PL spectrum is closely related to recombination of photogenerated electron–hole pairs. Consequently, PL spectra have useful information about separation of photogenerated charge carriers. To disclose the enhanced activity of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite relative to the ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (5%), and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (20%) samples, their PL spectra were provided and the results are shown in Fig. 7b. As can be seen, compared with the other samples, intensity of the PL spectrum for the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite shows significant decrease, suggesting efficiently separation of the charge carriers in this nanocomposite relative to the other samples. This result clearly indicates that hybridization of the ZnO/Fe₃O₄ with AgBr and Ag₃VO₄ particles promotes the separation of electron–hole pairs in the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites. However, similar to many multi-component photocatalysts,^37–39 further increase in weight percent of Ag₃VO₄ leads to a decrease in the degradation rate constant. This may be due to agglomeration of the excess Ag₃VO₄ particles on the nanocomposites, which reduces the heterojunctions between Ag₃VO₄ and ZnO/AgBr/Fe₃O₄ counterparts, thereby separation of the charge carriers decreases. Hence, the amount of Ag₃VO₄ in the nanocomposites needs an optimum value.

Fig. 7 (a) The degradation rate constants of RhB over different samples. (b) PL spectra for the ZnO/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (5%), ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%), and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (20%) nanocomposites.

To describe the enhanced activity of the nanocomposite, a hypothesized mechanism for separation of the charge carriers is presented (Fig. 8). The conduction band (CB) and valence band (CB) energies for ZnO, AgBr, and Ag₃VO₄ at the point of zero charge were calculated using Butler and Ginley model⁴⁰ and the results are shown in Table 1. Being as n-type semiconductors, the Fermi levels for ZnO, AgBr, and Ag₃VO₄ are close to their CB levels. The Fermi level of ZnO is more negative than those of AgBr and Ag₃VO₄. Hence, after contacting of ZnO with AgBr, electrons simply flow from ZnO to AgBr until their Fermi levels coincide.^41,42 As a result, negative and positive charges are accumulated on AgBr and ZnO counterparts of the nanocomposite, respectively. Hence, an n–n heterojunction with the same Fermi level is formed (Fig. 8). Separation of the charge carriers by n–n heterojunction produces an internal electrostatic field directed from the ZnO to AgBr. Moreover, similar n–n heterojunction is formed after contacting ZnO with Ag₃VO₄. In this region, electrons transfer from ZnO to Ag₃VO₄. After reaching to equilibrium state, an electric field is formed at the interface area, directed from ZnO to Ag₃VO₄. As a result, two n–n heterojunctions are formed in the contacting region between ZnO with AgBr in one side and ZnO with Ag₃VO₄ in the other side. Hence, tandem n–n heterojunctions between these n-type semiconductors are formed.²⁸ Due to their narrow band gaps, under the visible-light irradiation, electron–hole pairs are generated on AgBr and Ag₃VO₄ counterparts of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites. The photogenerated electrons on the CB levels of AgBr and Ag₃VO₄ simply transfer to the CB of ZnO with the help of the internal electrostatic field produced by tandem n–n heterojunctions. On the other hand, the VB energies of AgBr and Ag₃VO₄ are less positive than that of the ZnO. Consequently, the holes cannot transfer from the VB levels of AgBr and Ag₃VO₄ to that of the ZnO. Finally, after transfer of the electrons, holes and electrons are accumulated on the narrow band gap semiconductors and ZnO, respectively. As a result, due to formation of tandem n–n heterojunctions between ZnO with AgBr and ZnO with Ag₃VO₄, the charge carriers are spatially separated from each other, resulting in an increased lifetime for the electron–hole pairs. Afterwards, the separated charge carriers in the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites effectively migrate to surface of the photocatalyst and participate in the degradation reactions, leading to enhanced activity relative to their counterparts. It is noteworthy that separation of the photogenerated charge carriers in AgBr and Ag₃VO₄ could not take place without formation of tandem n–n heterojunctions; because, the CB levels of AgBr and Ag₃VO₄ are less negative than the CB of ZnO. Hence, the photogenerated electrons cannot transfer from AgBr and Ag₃VO₄ to ZnO, resulting in more recombination of the photogenerated electron–hole pairs and high intensity of the PL spectrum. However, separation of the charge carriers in the nanocomposite was demonstrated by the PL spectra. Hence, formation of the tandem n–n heterojunctions between the three n-type semiconductors is confirmed.

Fig. 8 Hypothesized mechanism for separation of the charge carriers in the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites via tandem n–n heterojunctions.

Table 1 The values of absolute electronegativity (χ), valence band energy (VB), conduction band energy (CB), and energy gap (E_g) for counterparts of the nanocomposite

Semiconductor	χ (eV)	VB energy (eV)	CB energy (eV)	Eg (eV)
ZnO	5.76	+2.86	−0.34	3.20
AgBr	5.80	+2.60	0.00	2.60
Ag3VO4	5.64	+2.24	+0.04	2.20

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In order to explore the role of reactive species responsible for degradation of RhB over the photocatalyst, the degradation reaction was conducted in the presence of a series of scavengers and the results are shown in Fig. 9. In the present work, benzoquinone, ammonium oxalate, and 2-PrOH were used as scavengers of ˙O₂⁻, h⁺, and ˙OH, respectively.⁴³ It is evident that decrease of the rate constant in the presence of scavengers is as benzoquinone > ammonium oxalate > 2-PrOH. Hence, it is concluded that the effect of superoxide ions and holes on the degradation reaction is higher than that of hydroxyl radicals.

Fig. 9 The degradation rate constant of RhB over the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite in presence of various scavengers.

It is well known that preparation time of photocatalysts can considerably affect morphology, crystallinity, aggregation, and size of particles. Hence, the preparation time could remarkably affect photocatalytic activity. To find optimum preparation time, the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite prepared by refluxing for 15, 30, 60, 120, and 240 minutes and the results are shown in Fig. 10a. It can be seen that the degradation rate constant increases with increasing the preparation time up to 60 min and then decreases. Decrease of the photocatalytic activity for the samples prepared with higher refluxing time could be related to more aggregation of the particles, resulting in decreased surface area of the photocatalyst.

Fig. 10 (a) The degradation rate constants of RhB over the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite prepared at different refluxing times. (b) The degradation rate constants of RhB over the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite calcined at different temperatures.

To investigate the effect of calcination temperature on the photocatalytic activity, the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite was calcined at 200, 300, 400, 500, and 600 °C for 2 h and the degradation reactions were carried out over them and the rate constant are shown in Fig. 10b. It can be seen that the degradation rate constant enhances with increasing calcination temperature up to 500 °C and then promptly decreases. To explore structure of the photocatalysts calcined at 500 and 600 °C, XRD patterns of the nanocomposite before and after calcinations are shown in Fig. 11a. As can be seen, after calcination of the nanocomposite at 500 °C, metallic silver is produced by decomposition of silver containing semiconductors in the nanocomposite.⁴⁴ The produced silver particles can increase transfer of electrons from the CB of Ag₃VO₄ and AgBr to that of the ZnO,^45,46 resulting in more separation of the charge carriers. However, activity of the nanocomposite calcined at 600 °C is very lower than those of the other samples. Morphologies of the nanocomposite calcined at 500 and 600 °C are compared in Fig. 11b and c. It is evident that there is a great changing in morphology of the nanocomposite after calcination at 600 °C. As can be seen, after calcination at 600 °C, plates of ZnO have tightly aggregated to each other and particles of other counterparts of the nanocomposite have considerably separated from the ZnO plates, resulting in decrease of the contact surface between counterparts of the nanocomposite. Hence, the photogenerated electron–hole pairs could not be separated efficiently from each other, leading to decrease the activity of the calcined photocatalyst at 600 °C.

Fig. 11 (a) XRD patterns for the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite before and after calcinations at 500 and 600 °C. (b) SEM images of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite calcined at 500 and 600 °C.

Generally, wastewaters contain mixtures of different pollutants. Hence, it is essential that photocatalysts can degrade these pollutants non-selectively. To show ability of the nanocomposite calcined at 500 °C for degradation of different pollutants, degradations of RhB, MB, MO, and phenol over the nanocomposite was studied under the visible-light irradiation. Fig. 12 shows plots of the degradation rate constants for RhB, MB, MO, and phenol over the calcined ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) samples. It is evident that photocatalytic activity of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite is much higher than those of the ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄ samples. Hence, it is confirmed that the magnetic ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites has considerable activity in degradations of three different dye pollutants and one colorless pollutant.

Fig. 12 The degradation rate constants of RhB, MB, MO, and phenol over the ZnO/Fe₃O₄, ZnO/AgBr/Fe₃O₄, ZnO/Ag₃VO₄/Fe₃O₄, and ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposites calcined at 500 °C.

For large-scale utilization of photocatalysts, besides photocatalytic activity, stability is another important factor from economical viewpoint. To investigate stability of the photocatalyst, the degradation experiments were carried out and the results are shown in Fig. 13. In each cycle, after using the nanocomposite, it was magnetically separated, washed with ethanol and dried at 60 °C for 24 h. It can be seen that the photocatalyst has reasonable stability during the degradation reaction and magnetic recycling from the treated solution. To investigate stability of the photocatalyst after fourth recycling, the XRD patterns of the nanocomposite before and after using for four cycles are shown in Fig. 13b. It is evident that very small amount of metallic silver is produced by photocorrosion of AgBr and Ag₃VO₄. Hence, it can be concluded that the magnetic nanocomposite has reasonable stability during the degradation processes.

Fig. 13 (a) Reusability of the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite for four successive runs. (b) The XRD patterns of the nanocomposite before and after fourth recovery.

Conclusions

Briefly, novel magnetically separable ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites, as highly efficient visible-light-driven photocatalysts, were successfully prepared by a facile large-scale procedure. Photocatalytic activity of the resultant samples was investigated by degradations of three dye pollutants and phenol under visible-light irradiation. Among the prepared samples, the ZnO/AgBr/Fe₃O₄/Ag₃VO₄ (10%) nanocomposite displayed the best activity. Activity of this nanocomposite was about 3.5 and 2.8-fold higher than those of the ZnO/Ag₃VO₄/Fe₃O₄ and ZnO/AgBr/Fe₃O₄ nanocomposites in degradation of RhB, respectively. The photocatalytic activity increases with increasing refluxing time up to 60 min and then decreases. Moreover, the results showed that the nanocomposite calcined at 500 °C has the best activity. Activity of the calcined nanocomposite in degradations of MB and MO is about 4 and 10-fold higher than that of the ZnO/AgBr/Fe₃O₄ nanocomposite, respectively. Hence, ability of the nanocomposite for efficiently degradation of different dye pollutants and phenol is confirmed. To describe the enhanced activity, a hypothesized mechanism for separation of the charge carriers was presented based on formation of tandem n–n heterojunctions between ZnO with AgBr and ZnO with Ag₃VO₄. Finally, it was confirmed that the magnetic ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites has considerable activity after four successive runs.

Acknowledgements

The authors wish to acknowledge University of Mohaghegh Ardabili, for financial support of this work.

Notes and references

1. K. Li, X. An, K. H. Park, M. Khraisheh and J. Tang, Catal. Today, 2014, 224, 3.
2. S. Dong, J. Feng, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun and J. Sun, RSC Adv., 2015, 5, 14610.
3. H. Ahmad, S. K. Kamarudin, L. J. Minggu and M. Kassim, Renewable Sustainable Energy Rev., 2015, 43, 599.
4. M. Pelaez, N. T. Nolan, S. C. Pillai, M. K. Seery, P. Falaras, A. G. Kontos, P. S. M. Dunlop, J. W. J. Hamilton, J. A. Byrne, K. O'Shea, M. H. Entezari and D. D. Dionysiou, Appl. Catal., B, 2012, 125, 331.
5. S.-M. Lam, J.-C. Sin, A. Z. Abdullah and A. R. Mohamed, Desalin. Water Treat., 2012, 41, 131.
6. M. Tahir and N. S. Amin, Energy Convers. Manage., 2013, 76, 194.
7. Y. Zang and R. Farnood, Appl. Catal., B, 2008, 79, 334.
8. K. Vignesh, A. Suganthi, M. Rajarajan and S. A. Sara, Powder Technol., 2012, 224, 331.
9. B. Krishnakumar, B. Subash and M. Swaminathan, Sep. Purif. Technol., 2012, 85, 35.
10. F. Xu, Y. Yuan, H. Han, D. Wu, Z. Gao and K. Jiang, CrystEngComm, 2012, 14, 3615.
11. M. Pirhashemi and A. Habibi-Yangjeh, Appl. Surf. Sci., 2013, 283, 1080.
12. C. An, W. Jiang, J. Wang, S. Wang, Z. Ma and Y. Li, Dalton Trans., 2013, 42, 8796.
13. K. Dai, D. Li, L. Lu, Q. Liu, C. Liang, J. Lv and G. Zhu, Appl. Surf. Sci., 2014, 314, 864.
14. C. Dong, K.-L. Wu, M.-R. Li, L. Liu and X.-W. Wei, Catal. Commun., 2014, 46, 32.
15. F. Chang, J. Zhang, Y. Xie, J. Chen, C. Li, J. Wang, J. Luo, B. Deng and X. Hu, Appl. Surf. Sci., 2014, 311, 574.
16. J. Yi, L. Huang, H. Wang, H. Yu and F. Peng, J. Hazard. Mater., 2015, 284, 207.
17. J. Ma, K. Wang, L. Li, T. Zhang, Y. Kong and S. Komarneni, Ceram. Int., 2015, 41, 2050.
18. S. Shaker-Agjekandy and A. Habibi-Yangjeh, Mater. Sci. Semicond. Process., 2015, 34, 74.
19. S. Yi, X. Yue, D. Xu, Z. Liu, F. Zhao, D. Wang and Y. Lin, New J. Chem., 2015, 39, 2917.
20. C. Liu, C. Cao, X. Luo and S. Luo, J. Hazard. Mater., 2015, 285, 319.
21. Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar and J. He, Nanoscale, 2013, 5, 8326.
22. C.-C. Shen, Q. Zhu, Z.-W. Zhao, T. Wen, X. Wang and A.-W. Xu, J. Mater. Chem. A, 2015, 3, 14661.
23. W.-S. Wang, H. Du, R.-X. Wang, T. Wen and A.-W. Xu, Nanoscale, 2013, 5, 3315.
24. Q. Zhu, W.-S. Wang, L. Lin, G.-Q. Gao, H.-L. Guo, H. Du and A.-W. Xu, J. Phys. Chem. C, 2013, 117, 5894.
25. Y. Yan, H. Guan, S. Liu and R. Jiang, Ceram. Int., 2014, 40, 9095.
26. M. Xu, T. Ye, F. Dai, J. Yang, J. Shen, Q. He, W. Chen, N. Liang, J. Zai and X. Qian, ChemSusChem, 2015, 8, 1218.
27. F. Kiantazh and A. Habibi-Yangjeh, Mater. Sci. Semicond. Process., 2015, 39, 671.
28. M. Pirhashemi and A. Habibi-Yangjeh, Ceram. Int., 2015, 41, 14383.
29. S.-Q. Liu, Environ. Chem. Lett., 2012, 10, 209.
30. R. Massart, IEEE Trans. Magn., 1981, 17, 1247.
31. J. Safari and Z. Zarnegar, J. Mol. Struct., 2014, 1072, 53.
32. B. G. Mishra and G. R. Rao, J. Mol. Catal. A: Chem., 2006, 243, 204.
33. C.-M. Huang, G.-T. Pan, Y.-C. M. Li, M.-H. Li and T. C.-K. Yang, Appl. Catal., A, 2009, 358, 164.
34. H. H. Mohamed and D. W. Bahnemann, Appl. Catal., B, 2012, 128, 91.
35. X. Li and J. Ye, J. Phys. Chem. C, 2007, 111, 13109.
36. A. Martinez-dela Cruz and U. M. Garcia Perez, Mater. Res. Bull., 2010, 45, 135.
37. C. Xing, Z. Wu, D. Jiang and M. Chen, J. Colloid Interface Sci., 2014, 433, 9.
38. S.-W. Cao, X.-F. Liu, Y.-P. Yuan, Z.-Y. Zhang, Y.-S. Liao, J. Fang, S. C. J. Loo, T. C. Sum and C. Xue, Appl. Catal., B, 2014, 147, 940.
39. L. Zhang, X. Wang, Q. Nong, H. Lin, B. Teng, Y. Zhang, L. Zhao, T. Wu and Y. He, Appl. Surf. Sci., 2015, 329, 143.
40. S. R. Morrison, Electrochemistry at semiconductor and oxidized metal electrode, Plenum, New York, 1980.
41. C. Shifu, Z. Wei, L. Wei, Z. Huaye, Y. Xiaoling and C. Yinghao, J. Hazard. Mater., 2009, 172, 1415.
42. W. Wang, J. Wang, Z. Wang, X. Wei, L. Liu, Q. Ren, W. Gao, Y. Liang and H. Shi, Dalton Trans., 2014, 43, 6735.
43. Z. He, Y. Shi, C. Gao, L. Wen, J. Chen and S. Song, J. Phys. Chem. C, 2015, 118, 389.
44. M. Shekofteh-Gohari and A. Habibi-Yangjeh, Solid State Sci., 2015, 48, 177.
45. C. Liu, C. Cao, X. Luo and S. Luo, J. Hazard. Mater., 2015, 285, 319.
46. W. Cao, L. Chen and Z. Qi, J. Mol. Catal. A: Chem., 2015, 401, 81.

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