Introduction

Phtocatalytic treatment of water pollution using semiconductor-based materials has been considered as one of the most promising technologies to remove of toxic compounds from the environment. Over the past decades, the development of new visible-light photoactive materials has been pursued to improve the practical capacity of semiconductors under natural solar energy. To achieve this purpose, variant strategies such as doping the elemental impurities, coupling different semiconductors, and the synthesis of new visible-light photoactive catalysts have been served (Zhang et al. 2010; Lee et al. 2010; Yin et al. 2010; Kudo et al. 1998; Wang et al. 2008, 2010a, b; An et al. 2010).

Recently, due to their unique crystal structures and interesting photocatalytic properties, the ability of silver-based compounds such as Ag3VO4 (Konta et al. 2003; Hu and Hu 2007), Ag2CO3 (Dai et al. 2012), and Ag3PO4 (Yi et al. 2010; Bi et al. 2012) have been widely investigated for the degradation of organic pollutions under visible illumination. In spite of having suitable photoactivity, the high rate of electron–hole recombination makes the practical application of such materials limited. Moreover, corrosion of the semiconductor surface can take place during the photoreaction because of the conversion of Ag+ ions to metallic Ag particles. To address this issue, recently, much effort has been focused on the development of new series of heterostructure materials, named plasmonic photocatalysts, which can lessen the mentioned limitations (Xiang et al. 2010; Chen et al. 2008). According to the reports the presence of metallic Ag and Au nanoparticles on the surface of Ag-based photocatalysts can improve their stability for repeated using (Kakuta et al. 1999; Wang et al. 2008, 2009). Inspired by these findings, many attempts have been performed to fabricate the highly efficient plasmonic silver-based photocatalysts (Kuai et al. 2010; An et al. 2010; Xu et al. 2009).

In this paper we report our new attempts to prepare the visible light Ag–AgBr/α-Ag3VO4 photocatalysts by a facile precipitation method at the presence of a water-soluble ionic liquid which acted as a complexing agent and bromide source. Studying their photocatalytic activity for the degradation of AB92 and E. coli gram-negative bacteria revealed that the plasmonic nanostructures had higher activity than the pure α-Ag3VO4 sample. In addition, cycling experiments were performed to investigate the stability of as-prepared nanostructures after repeated using.

Experimental

Materials, instruments, and methods

AgNO3 and NH4VO3 were used as silver and vanadium source, respectively. Tetra methyl ammonium hydroxide (TMAOH) was used as a template agent. During the synthesis procedure, 1-butyl-3-methylimidazolium bromide ([BMIM]Br) ionic liquid acted as the bromide source. All of the chemicals were purchased from Merck Co. and used without further purification. Double distilled water (DDW) was used during the preparation steps.

1.02 g of AgNO3 was dissolved in 20 mL of DDW and the solution added drop by drop to a beaker contains 50 mL of water and 0.5 mL of TMAOH. The obtained yellow suspension was stirred vigorously while the color of precipitation changed to brown and then black. After 2 h of stirring at room temperature, a clear solution containing 0.233 g of NH4VO3 and [BMIM]Br (0.2 mL) in 30 mL of DDW was added slowly to the as-prepared suspension. This resulted in the formation of a yellow precipitation. The reaction medium was stirred for 0.5 h. The final product (Ag–AgBr/α-Ag3VO4) was filtered, washed repeatedly with DDW and methanol, and dried at 60 °C overnight. Preparation of pure α-Ag3VO4 was carried out in the same manner without adding any [BMIM]Br.

In all the experiments 0.02 g of photocatalysts was used and the used solvent was DDW. The visible illumination source was a 250-W OSRAM lamp. Total concentrations of the dye solution were easily determined using an UV spectrophotometer set at the λ max of AB92 dye (574 nm).

Results and discussion

Characterization of the nanostructures

The formation process of the nanostructures can be described as follows:

$${({\rm{C}}{{\rm{H}}_3})_4}{{\rm N}^ + }{{\rm OH}^ - }\left( {{{\rm{H}}_2}{\rm{O}}} \right) \to {\left( {{\rm{C}}{{\rm{H}}_3}} \right)_4}{{\rm N}^ + }\left( {{\rm{aq}}} \right) + {{\rm OH}^ - }\left( {{\rm{aq}}} \right)$$
(1)
$${\rm Ag}^{ + } {\rm (aq)} + 2\text{OH}^{ - }{\rm (aq)} \to {\rm Ag(OH)}_2^{ - }{{\rm (aq)}}$$
(2)
$$2{\rm Ag(OH)}_2^{ - }{{\rm (aq)}} \to {\rm Ag}_{2} {\rm O}_{{(\text{s})}} + 2{\rm OH}^{ - }{{\left( {\text{aq}} \right)}} + {\rm H}_{2} {\rm O}$$
(3)
$${\rm VO}_{4}^{3 - } + {\rm H}_{2} {\rm O} \to {\rm HVO}_{4}^{2 - } + {\rm OH}^{ - }$$
(4)
$${\rm HVO}_{4}^{2 - } + 3{\rm Ag(OH)}_{2}^{ - } \to \alpha - {\rm Ag}_{3} {\rm VO}_{4} + 5{\rm OH}^{ - } + {\rm H}_{2} \text{O}$$
(5)
$${\rm Ag}_{2} {\rm O}_{{(\text{s})}} + 3{\rm (CH_{3})}_{4} {\rm N}^{ + } {\rm OH}^{ - } + {\rm Ag}^{ + } + {\rm H}_{2} {\rm O} \to 2{\rm Ag(OH)}_{2}^{ - } + {\rm AgOH} + 3{\rm (CH_{3} )}_{4} {\rm N}^{ + }$$
(6)
$${\rm AgOH} + {\rm OH}^{ - } \to {\rm Ag(OH)}_{2}^{ - }$$
(7)

Based on Eqs. 1–7, TMAOH facilitates the formation of Ag(OH) −2 intermediate which is a necessary step to complete the formation of the final products. In the next step, unstable Ag(OH) −2 species can convert to silver oxide and/or react with HVO −24 to form α-Ag3VO4. Besides, the solid Ag2O particles react with TMAOH to prompt the formation of α-Ag3VO4.

Because of the presence of [BMIM]Br ionic liquid in synthesis medium and the high reactivity of silver ions toward combination with the bromide anions, the following reactions could be occurred:

$$2 {\text{Ag}}^{ + } + {\text{NO}}_{3}^{ - } + {\text{OH}}^{ - } + [{\text{BMIM]}}^{ + } {\text{Br}}^{ - } \to {\text{Ag}} - {\text{OH}} - {\text{AgBr}} \ldots [ {\text{BMIM]}}^{ + } + {\text{NO}}_{3}^{ - }$$
(8)
$${\text{Ag}} - {\text{OH}} - {\text{Ag}} - {\text{Br}} + {\text{OH}}^{ - } \to {\text{Ag(OH)}}_{2}^{ - } + {\text{AgBr}}$$
(9)

According to Eqs. 8 and 9, the formation of [BMIM] contained Ag–OH–Ag–Br species and its later reaction with the hydroxide ions promotes the formation of silver bromide phase of the photocatalysts.

The XRD patterns were used to investigate the phase structure of the samples and the results are shown in Fig. 1. All the sharp peaks at 2θ = 32.5°, 33.06°, 33.7°, 26.6°, 27.2°, and 47° in the pattern of silver methavanadate are indexed to the crystalline phase of α-Ag3VO4 ((JCPDS 43-054)). In addition, the peaks at 32.5°, 44.9°, and 55.1° are assigned to the silver bromide crystal phase structure which has a significant intensity compared to the characteristic peaks of α-Ag3VO4. Two observable peaks at 64.9° and 74° in the XRD pattern of Ag–AgBr/α-Ag3VO4 are the characteristic peaks of Ag° crystalline phase. However, we did not observe any characteristic peaks assigned to the silver nanocrystals in the pattern of α-Ag3VO4.

Fig. 1
figure 1

The XRD patterns of prepared nanostructures

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The FTIR spectrum of the final product (Ag–AgBr/α-Ag3VO4) recorded at 400–3500 cm−1 is shown in Fig. 2. The strong absorption bands at 400–650 cm−1 and the sharp one at 862 cm−1 are assigned to the symmetric and asymmetric vibrations of V–O–V bonds in the structures. Furthermore, the band at 920 cm−1 is the characteristic band of the bending vibrations of the terminal VO −34 groups (Chao-Ming et al. 2009). Peaks at wavenumbers of 1465 and 1655 cm−1 are assigned to the C=C and C=N stretching vibrations in the imidazolium ring of [BMIM]Br.

Fig. 2
figure 2

The FTIR spectrum of prepared Ag–AgBr/α-Ag3VO4 sample

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In order to determine the optical response of the prepared samples, we studied the UV–vis absorption spectra. From Fig. 3, the absorption onset is around 582 and 498 nm for α-Ag3VO4 and AgBr, respectively. This absorption extends into the higher wavelength (nearly 690 nm) for the Ag–AgBr/α-Ag3VO4 heteojunction photocatalyst. This observation is in agreement with the obtained results for the better photocatalytic activity of Ag–AgBr/α-Ag3VO4 under visible irradiation.

Fig. 3
figure 3

The UV–vis spectra of the prepared photocatalysts

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BET analysis was carried out to explain the porosity of the prepared samples and the results exhibited that the specific surface area is 31.00 and 19.84 m2 g−1 for Ag–AgBr/α-Ag3VO4 and α-Ag3VO4 pure sample, respectively. Moreover, the results for the BJH analysis showed that the maximum distribution of the pore size in the final product is around 1.85 nm. Typical nitrogen adsorption–desorption isotherm and BJH curve of the final product is shown in Fig. 4a, b, respectively.

Fig. 4
figure 4

A typical adsorption–desorption isotherm (a) and a BJH plot for Ag–AgBr/α-Ag3VO4 (b)

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The surface morphology and microstructure of the samples have been examined by SEM. From Fig. 5, the uniform spherical nanostructures exhibit the regular morphology and the average size of the particles on the surface is around 57 nm. C.

Fig. 5
figure 5

Different magnification of SEM image of Ag–AgBr/α-Ag3VO4 sample

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Photocatalytic removal of the bacteria and organic pollutants

The obtained results for the photodegradation of AB92 solution over the α-Ag3VO4-based nanostructures under visible illumination are shown in Fig. 6a, b. Comparing the rate and efficiency for bleaching of the dye solutions, we found that Ag–AgBr/α-Ag3VO4 plasmonic photocatalyst has better activity than the pure α-Ag3VO4 sample. As shown in Fig. 6a, the former case was able to completely destroy the chromophoric structure of AB92 in 15 min. As shown in Fig. 6b, which represents the UV spectrum of the dye solution illuminated in the presence of Ag–AgBr/α-Ag3VO4, the absorption in λ max decreased dramatically by the treatment time.

Fig. 6
figure 6

a The results for the photodegradation of AB92 under illumination (a, b) and dark (c, d). b UV–vis spectrum of AB92 solution during the photocatalytic experiment (dye concentration: 20 ppm, catalyst dosage: 0.02 g, temperature: 27 °C)

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According to the reports, a simplified Langmuir–Hinshelwood model could well describe the photoreaction kinetics of dye decomposition (Konta et al. 2003; Hu and Hu 2007; Dai et al. 2012; Yi et al. 2010; Bi et al. 2012). Based on this model, depicting Ln(C/C0) versus time gives a linear curve (Fig. 7) having the slope of 0.145 s−1, where k is the pseudo-first-order rate constant. C0 and C stand for the initial concentration and the concentrations after a certain reaction time, respectively.

Fig. 7
figure 7

Pseudo-first order kinetic of AB92 photodegradation over Ag–AgBr/α-Ag3VO4

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To study the stability of the prepared nanostructures as a very important issue in the photocatalysis, we carried out the cycling tests by evaluating the decreased concentration of AB92 after repeated reactions while all the parameters were kept constant during the experiments. From Fig. 8, the rate and efficiency were smoothly decreased after four times of repeated using when we used Ag–AgBr/α-Ag3VO4 as photocatalyst and the activity just changed afterward. As a whole, the results demonstrate that the plasmonic Ag–AgBr/α-Ag3VO4 sample is a very stable visible light photocatalyst and it is promising for the industrial applications.

Fig. 8
figure 8

The results of reuse-ability tests over Ag–AgBr/α-Ag3VO4 sample

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From the equation E g  = 1239.8/λ (Anandan et al. 2009), the band gap energies were calculated to be 2.13 and 2.48 eV for α-Ag3VO4 and AgBr, respectively. To estimate the band structure of α-Ag3VO4 and AgBr we used the empirical formulas EVB = χ − Ee+ 0.5E g and ECB = EVB − E g , where EVB and ECB are the edge potential of valance and conduction bands (Katsumata et al. 2014; Bi et al. 2011). χ is the semiconductor electronegativity which is defined as the geometric mean of the electronegativity of the constituent atoms. Electronegativity of each atom is also defined as the arithmetic mean of its electron affinity and first ionization energy. The amount of χ was 6.19 and 5.8 eV for Ag3VO4 and AgBr, respectively. E e (the energy of free electrons on the hydrogen scales) is about 4.5 eV versus NHE. As a result, EVB and the corresponding ECB were calculated to be 2.75 and 0.62 eV for Ag3VO4, and 2.54 and 0.06 eV for AgBr, respectively. From the UV–vis spectrum of Ag–AgBr/α-Ag3VO4, the absorption edge shifted to the higher wavelengths or lower energy which is in agreement with its better photoactivity. From Scheme 1, under irradiation, the electron–hole pairs are photogenerated on the surface of the photocatalyst. Because of the band gap energies (2.13 and 2.48 eV) both α-Ag3VO4 and AgBr can be photoexcited. The electrons can migrate to the conduction band of α-Ag3VO4 due to the more negative potential of AgBr conduction band (Qing et al. 2013). This process reduces the electron–hole recombination and enhances the photoactivity.

Scheme 1
scheme 1

Photodegradation of the wastewater over the prepared nanostructures

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In order to prove the high photocatalytic capacity of Ag–AgBr/α-Ag3VO4, a real wastewater sample was obtained from the textile industry, diluted with DDW, and treated under the same conditions for the degradation experiment. As shown in Fig. 9, the absorption in all the wavelengths decreased dramatically which confirmed the incredible photocatalytic properties of Ag–AgBr/α-Ag3VO4 nanostructure.

Fig. 9
figure 9

The UV–vis spectrum for the photodegradation of a real wastewater under the same conditions over Ag–AgBr/α-Ag3VO4

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The obtained results for the antibacterial experiments over the prepared samples indicated that the nanostructures had high bactericidal effect on E. coli in water under visible irradiation. The results of the light control experiment indicate that no photolysis of the bacterial cells occurred under visible light. According to Fig. 10, the number of colonies formed in the cultivation medium decreased impressively up to 30 and 60 min when we used Ag–AgBr/α-Ag3VO4 and α-Ag3VO4 as photocatalyst, respectively. Super-active species included \({\rm{O}}{{\rm{H}}^ \cdot }\), \({{\text{HO}}^{\cdot}}_{2}\), and \({\text{HO}}_{2}^{\cdot}\) radicals produced on the surface of nanostructures during the irradiation which can attack the microorganisms and destroy the cell walls. This well shows that the prepared nanostructures can be applied for the removal of pathogenic microorganisms from the wastewater under sun light. Furthermore, under dark conditions, the number of surviving bacteria was almost similar to that of observed for the control sample. This demonstrates that Ag–AgBr/Ag3VO4 has no toxic effect on E. coli cells. However, α-Ag3VO4 showed a significant antibacterial activity in the absence of visible light which may arise from the release of free silver ions bonded to the structure into the medium. To support this matter, we analyzed a filtered suspension treated by α-Ag3VO4 sample in the absence of irradiation source by Atomic Absorption Spectroscopy (Younglin, AAS8200 Model) to determine the silver content. However, the result of this analysis implied that there was not any free Ag in the solution. To explain the antibacterial mechanism of α-Ag3VO4 under dark, TEM analysis was performed on the damaged cell walls during the treatment time. Based on the TEM micrographs (Fig. 11), aggregation of the nanostructures around the outer membrane of the cell wall is the most probable mechanism for the bacteria inactivation. We suggest that bonding the VO −34 groups of the nanostructures to the lipopolysaccharides constituents of the bacterial cell wall blocked some vital channels where the microorganism nourishes via them. Lipopolysaccharides are located in the outer membrane of the cell wall and predominantly contain a specific lipid, named lipid A which contains many types of polar groups such as phosphate, hydroxyl, amino, and carboxylic. All of these functional segments are capable of making strong bonds with the vanadate groups of the nanostructures. This process is schematically shown in Scheme 2.

Fig. 10
figure 10

The results of the antibacterial tests over the prepared photocatalysts

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Fig. 11
figure 11

TEM images of E.coli cell wall after the treatment time with α-Ag3VO4

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Scheme 2
scheme 2

The antibacterial mechanism over α-Ag3VO4 under dark conditions

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Conclusions

Plasmonic Ag–AgBr/α-Ag3VO4 nanostructures were prepared by an ionic liquid-assisted precipitation method. XRD, DRS, SEM, BET, and FTIR analysis methods well confirmed the formation of crystalline phase of the structures. From the UV–vis spectra, the absorption onset of pure α-Ag3VO4 was significantly changed in the presence of AgBr and its band gap shifted to the higher wavelength. The results for the photocatalytic degradation of AB92 and E.coli bacterium revealed that the prepared nanostructures had incredible activity for the removal of wastewater under visible irradiation. In addition, according to the results of cycling experiments, the photocatalysts were stable under illumination which makes them suitable for the practical applications. To guarantee the photoactivity of the prepared catalysts toward decomposition of the real wastewaters under sun light, photodegradation of a wastewater sample produced by the textile industries was investigated and we found that the absorption in all wavelengths decreased significantly during the photocatalytic treatment. To explain the antibacterial mechanism of pure α-Ag3VO4 sample, TEM analysis was performed and the results showed that the most important reason for the antibacterial activity at dark is the aggregation of solid particles around the outer membrane of the cell wall which disturbs the microorganism nourishment.