Photosensitization of ZnO with Ag3VO4 and AgI

Advanced Powder Technology 27 (2016) 1427–1437

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Photosensitization of ZnO with Ag3VO4 and AgI nanoparticles: Novel ternary visible-light-driven photocatalysts with highly enhanced activity Behrouz Golzad-Nonakaran, Aziz Habibi-Yangjeh ⇑ Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

a r t i c l e

i n f o

Article history: Received 6 January 2016 Received in revised form 21 April 2016 Accepted 1 May 2016 Available online 11 May 2016 Keywords: ZnO/Ag3VO4/AgI Tandem n–n-heterojunctions Visible-light-driven Dye pollutants

a b s t r a c t This work reports novel ternary nanocomposites consisting of ZnO, Ag3VO4, and AgI as visible-lightdriven photocatalysts with highly enhanced activity. The resultant samples were characterized using XRD, EDX, SEM, TEM, UV–vis DRS, FT-IR, BET, and PL techniques to reveal their microstructure, purity, morphology, texture, and spectroscopic properties. Photocatalytic activity of the samples was evaluated by degradation of rhodamine B (RhB) and methylene blue (MB) under visible-light irradiation. The effect of silver iodide weight percent, preparation time, and calcination temperature on the degradation reaction was investigated and the optimized values were obtained. Among the prepared samples, the ZnO/ Ag3VO4/AgI (10%) nanocomposite exhibited the superior activity, which is about 5.1 and 3.6-fold higher than those of the ZnO/Ag3VO4 in degradations of RhB and MB, respectively. The enhanced activity was mainly attributed to reduction of electron–hole recombination, due to the formation of tandem n–n heterojunctions in the nanocomposite. Finally, it was revealed that superoxide ions are the predominant reactive species in the degradation reaction. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Semiconductor-based photocatalytic processes, as eco-friendly technology, have been drawn increasing attention owing to their potential applications in different disciplines such as industrial production of fine materials, water and air pollution treatments, production of hydrogen by splitting of water, and reduction of carbon dioxide to different fuels [1–4]. Due to their appealing properties such as low cost, non-toxicity, and good stability, TiO2 and ZnO have been regarded as the most popular photocatalysts. However, the key problems that hinder widespread applications of these photocatalysts are their wide band gaps of about 3.2 eV, resulting in poor activity under solar irradiation and quick recombination of photogenerated electron–hole pairs, leading to inferior photocatalytic activity [5–7]. Hence, designing efficient visiblelight-driven photocatalysts based on TiO2 and ZnO is the major challenge in this research area. One effective strategy to overcome these shortcomings is combining narrow band gap semiconductors with these wide band gap semiconductors [8]. In the few years ago, great efforts have been made to prepare TiO2 and ZnO-based nanocomposites to enhance their photocatalytic activities under visible-light irradiation to meet their practical applications. Among ⇑ Corresponding author. Tel.: +98 045 33514702; fax: +98 045 33514701. E-mail address: [email protected] (A. Habibi-Yangjeh).

the prepared photocatalysts, nanocomposites of TiO2 and ZnO with silver containing narrow band gap semiconductors have been very attractive, due to their great activities [9–24]. Very recently, we prepared novel ZnO/Ag3VO4 nanocomposites with n–n heterojunction and applied them in photocatalytic degradation of rhodamine B (RhB) under visible-light irradiation [24]. The results showed that the nanocomposite with 30 wt% of Ag3VO4 exhibited the best activity. On the other hand, silver iodide is another n-type semiconductor with narrow band gap [10,23]. Hence, it is expected that by combination of AgI with the ZnO/Ag3VO4 nanocomposite, tandem n–n heterojunctions could be formed, leading to more separation of the photogenerated charge carriers, resulting in enhanced photocatalytic activity for the resultant ZnO/Ag3VO4/AgI nanocomposites under visible-light irradiation. Herein, we present the preparation of novel ternary ZnO/Ag3VO4/AgI nanocomposites with different weight percents of AgI as highly enhanced visible-light-driven photocatalysts. The resultant samples were characterized 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 (DRS), Fourier transform-infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET), and photoluminescence (PL) techniques. Photocatalytic activity of the nanocomposites was evaluated by visible-light degradation of RhB and the optimal weight percent of silver iodide was

http://dx.doi.org/10.1016/j.apt.2016.05.001 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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B. Golzad-Nonakaran, A. Habibi-Yangjeh / Advanced Powder Technology 27 (2016) 1427–1437

determined. To reveal ability of the optimal photocatalyst to degrade other dye pollutant, degradation of methylene blue (MB) was also studied. It was demonstrated that ultrasonic irradiation time and calcination temperature have considerable influence on the degradation reaction. To disclose the role of the reactive species in the degradation reaction, a series of scavengers for the reactive species were employed. The photocatalyst exhibited reasonable stability during four recycling experiments. 2. Experimental 2.1. Materials All reagents were of analytical grade and used without further purification. Deionized water was used throughout this study.

age 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 Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) models with nitrogen adsorption– desorption isotherms collected by Belsorp apparatus at 196 °C. Prior to the experiments, the samples were degassed at 120 °C for 15 h. The PL spectra of the samples were provided 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. 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.

2.2. Instruments

2.3. Preparation of the nanocomposites

The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Ka radiation (k = 0.15406 nm), employing scanning rate of 0.04°/s in the 2h range from 10° 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 volt-

Typically for preparation of the ZnO/Ag3VO4 nanocomposite with 30 wt% of silver vanadate, 2.284 g of zinc nitrate tetrahydrate and 0.384 g of silver nitrate were dissolved in100 mL of water under stirring at room temperature. Then, aqueous solution of sodium hydroxide (5 M) was slowly added to the solution until the pH of the solution reached 10. Afterward, an aqueous solution of ammonium metavanadate (0.08 g dissolved in 25 mL of water) was added dropwise to the dark brown suspension under stirring at room temperature. Then, the former orange suspension was sonicated for 120 min in a cylindrical Pyrex reactor provided with

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

Fig. 1. XRD patterns for the ZnO, ZnO/AgI, ZnO/Ag3VO4, and ZnO/Ag3VO4/AgI nanocomposites with different weight percents of silver iodide.

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water circulation arrangement to maintain its temperature at 25 °C. The formed orange suspension was centrifuged to get the precipitate out and washed two times with water and ethanol and dried in an oven at 60 °C for 24 h.

For preparation of the ZnO/Ag3VO4/AgI (10%) nanocomposite, where 10% is weight percent of AgI, 0.45 g of the ZnO/Ag3VO4 nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 10 min. Then, 0.036 g of silver nitrate was added

(a) Zn

Zn

(c)

(b)

(d)

(V)

(I) (f)

(e)

(g)

(Ag)

(Zn)

(O)

Fig. 2. (a) EDX spectra for the ZnO, ZnO/Ag3VO4, and ZnO/Ag3VO4/AgI (10%) samples. (b)–(g) EDX mapping for the ZnO/Ag3VO4/AgI (10%) nanocomposite.

(a)

(b)

Ag3VO4 & AgI

300 nm

ZnO

Fig. 3. (a) SEM and (b) TEM images of the ZnO/Ag3VO4/AgI (10%) nanocomposite.

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to the suspension and stirred for 120 min at room temperature. Afterward, an aqueous solution of sodium iodide (0.032 g in 50 mL of water) was dropwise added to the suspension at ambient temperature and sonicated for 60 min in the Pyrex reactor. The molar ratio of silver cations to iodide ions was 1:1. The formed olive colored suspension was then centrifuged to remove the precipitate and washed two times with water and ethanol and dried in an oven at 60 °C for 24 h. Similarly, weight percent of AgI was changed to prepare the ZnO/Ag3VO4/AgI (5%), ZnO/Ag3VO4/AgI (7.5%), ZnO/Ag3VO4/AgI (15%), and ZnO/Ag3VO4/AgI (20%) nanocomposites. In order to investigate the effect of preparation time, the ZnO/Ag3VO4/AgI (10%) nanocomposite was prepared by ultrasonic irradiations for 30, 60, 120, 180, and 240 min. Finally, to evaluate the effect of calcination temperature on the photocatalytic activity, the ZnO/Ag3VO4/AgI (10%) nanocomposite was calcined for 2 h at 200, 300, 400, and 500 °C. The schematic diagram for preparation of the nanocomposites is illustrated in Scheme 1. 2.4. 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. The degradation reactions of RhB and MB solutions were separately investigated. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL aqueous solution of RhB (2.5 10 5 M) or MB (1.9 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 and 664 nm corresponding to the maximum absorption wavelengths of RhB and MB, respectively. 3. Results and discussion The crystal structures of the resultant samples were determined by XRD patterns and the results are displayed in Fig. 1. For the pure ZnO, the diffraction peaks appeared at 31.81°, 34.57°, 36.43°, 47.30°, 56.80°, 63.00°, 66.50°, 68.00°, 69.20°, 71.69°, and 76.30°, matching well with the planes of the wurtzite hexagonal structure

(a)

(b)

H—O

V—O

H—O

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


[24]. In the case of the ZnO/Ag3VO4 nanocomposite, the XRD pattern clearly related to the planes of ZnO and Ag3VO4 phases. As can be seen, the diffraction peaks of Ag3VO4 appeared at 20.06°, 20.68°, 32.81°, 34.57°, 34.61°, 36.43°, 38.42°, 41.40°, 49.30°, 50.20°, 51.40°, and 56.80°. It is evident that the XRD patterns of the ZnO/Ag3VO4/AgI nanocomposites composed of the peaks corre-

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sponding to ZnO, Ag3VO4, and AgI counterparts of the nanocomposites. In addition, for the AgI counterpart, its diffraction peaks appeared at 21.86°, 24.00°, 39.14°, 45.58°, 52.10°, 54.00°, 57.00°, 59.60°, 61.20°, 63.00°, 73.90°, and 75.18°. Purity of the ZnO, ZnO/Ag3VO4, and ZnO/Ag3VO4/AgI (10%) samples were verified by EDX spectra and the results are shown in

Fig. 5. (a) Photodegradation of RhB over the ZnO, ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI nanocomposites with different weight percents of silver iodide. UV–vis spectra for degradation of RhB under visible-light irradiation over the (b) ZnO, (c) ZnO/AgI, (d) ZnO/Ag3VO4, and (e) ZnO/Ag3VO4/AgI (10%) samples.

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Fig. 6. (a) The degradation rate constant of RhB over the ZnO, ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI nanocomposites with different weight percents of silver iodide. (b) PL spectra for the ZnO, ZnO/AgI, ZnO/Ag3VO4, ZnO/Ag3VO4/AgI (10%), and ZnO/Ag3VO4/AgI (20%) samples.

Fig. 2. As can be seen, the peaks of ZnO sample are clearly ascribed to Zn and O elements. For the ZnO/Ag3VO4 nanocomposite, the peaks are related to Zn, O, Ag, and V elements. In the case of the ZnO/Ag3VO4/AgI (10%) nanocomposite, the spectrum confirms evidently purity of this nanocomposite. In addition, formation of the ternary nanocomposites was also confirmed by elemental mapping for the ZnO/Ag3VO4/AgI (10%) nanocomposite and the results are

displayed in Fig. 2b–g. As can be seen, Zn, O, Ag, V, I elements have homogenously distributed over the sample, demonstrating coexistence of ZnO, Ag3VO4, and AgI counterparts in the prepared nanocomposite. Moreover, weight percents of Zn, O, Ag, V, and I elements in this nanocomposite are about 54.1%, 14.9%, 21.7%, 3.37%, and 5.91%, respectively, which are close to the corresponding theoretical values of 50.5%, 16.8%, 24.0%, 3.20%, 5.50%, respectively. Fig. 3a and b shows SEM and TEM images of the ZnO/Ag3VO4/ AgI (10%) nanocomposite. It is evident that in this nanocomposite particles of Ag3VO4 and AgI have been deposited on the rod-like ZnO. As an important factor, optical properties of the resultant samples were provided using UV–vis DRS spectra and the results are shown in Fig. 4a. It can be seen that the pure ZnO has strong absorption in UV region with a band edge of 410 nm. Hence, it does not have considerable absorption in visible range. However, the ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI nanocomposites have intensive absorptions in visible range. Consequently, the nanocomposites could have reasonable activity under visible-light irradiation. Moreover, visible-light harvesting ability of the ZnO/ Ag3VO4/AgI nanocomposites slightly enhances with increasing weight percent of AgI. FT-IR spectra of the ZnO, ZnO/Ag3VO4, and ZnO/Ag3VO4/AgI (10%) samples were provided in the range of 400–4000 cm 1 and the results are shown in Fig. 4b. For the all samples, the broad absorption bands around 3300 cm 1 are ascribed to the O–H stretching vibration of adsorbed water. In addition, bending vibrations of the adsorbed water are clearly seen at 1370 and 1645 cm 1. For the ZnO sample, the band at around 570 cm 1 is assigned to the stretching vibration of Zn–O bond [24]. In the case of the ZnO/Ag3VO4 and ZnO/Ag3VO4/AgI (10%) nanocomposites, the absorption bands at 732 and 876 cm 1 could be assigned to silver vanadate. For these samples, the position of the characteristics peak for Zn–O bond was changed because of the strong interaction between constituents of the nanocomposites. Photocatalytic activity of the prepared samples was investigated by degradation of RhB, as a model dye pollutant, under visible-light irradiation. Fig. 5a displays changes of RhB absorbance over different samples as a function of irradiation time. For comparative purposes, photolysis (without using any photocatalyst) and dark (without the light irradiation) experiments were carried out. As can be seen, without using any photocatalyst, RhB degradation under the light illumination is very slow. However, by using photocatalysts and under the light irradiation, the degradation reaction was significantly increased. It is evident that degradations of RhB over the ZnO/Ag3VO4/AgI nanocomposites are higher than those of the ZnO, ZnO/Ag3VO4, and ZnO/AgI samples. The UV–vis absorption spectra of RhB solution during the degradation reaction over the ZnO, ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI (10%) samples at different irradiation times are shown in Fig. 5b–e. As can be seen, the characteristic main absorption peak of RhB is located at 553 nm. Intensity of this absorption peak decreases with the increase of the light irradiation time without any changes in position of the peak. Moreover, new absorption peaks are not formed during the degradation reaction. Hence, it can be concluded

Table 1 The values of absolute electronegativity (v), valence band energy (VB), conduction band energy (CB), and energy gap (Eg) for counterparts of the ternary nanocomposite. Semiconductor

v (eV)

VB energy (eV)

CB energy (eV)

Eg (eV)

ZnO AgI Ag3VO4

5.76 5.47 5.64

+2.86 +2.38 +2.24

0.34 0.42 +0.04

3.2 2.8 2.2

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NHE (eV) 2H+

H2O2 + RhB → CO2 + H2O

eˉ .

-1.0

e- e0.0

Ef

Ef Ef

1.0

2.8 eV

2.2 eV 3.2 eV Dye

2.0

+ + + +

Ag3VO4 3.0

_ _ _ _

h+ h+

+ h+ h

Products

e- e-

e- e-

_ _ _ _

O2-

O2

+

+ + +

Ag3VO4 AgI

AgI

ZnO ZnO Before junction

After junction

Fig. 7. A plausible diagram for separation of electron–hole pairs in the ZnO/Ag3VO4/AgI nanocomposites.

that decomposition mechanism involving cleavage of the whole conjugated chromophore structure of RhB and stable intermediates are not formed by photocatalytic degradation of this pollutant over the photocatalysts [25,26]. To quantitatively analyze the degradation reaction of RhB over the samples, the pseudo-first-order kinetic model was applied and observed first-order rate constants (kobs) was calculated [27] and the results are shown in Fig. 6a. As can be seen, the degradation rate constant increases with increasing weight percent of silver iodide up to 10% and then decreases. The degradation rate constants over the ZnO, ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI (10%) samples are 6.90 10 4, 68.8 10 4, 32.8 10 4, and 355 10 4 min 1, respectively. Consequently, activity of the ZnO/ Ag3VO4/AgI (10%) nanocomposite is about 51, 5.1, and 10.8-folds higher than those of the ZnO, ZnO/Ag3VO4, and ZnO/AgI samples, respectively. To compare extent of the photogenerated electron– hole pair separations, PL spectra for the ZnO, ZnO/Ag3VO4, ZnO/AgI, ZnO/Ag3VO4/AgI (10%), and ZnO/Ag3VO4/AgI (20%) samples were provided and the results are shown in Fig. 6b. A PL spectrum with a weaker intensity represents that recombination of the charge carriers is low [28]. As can be seen, compared with the ZnO, ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI (20%) samples, the ZnO/Ag3VO4/AgI (10%) nanocomposite shows a weaker PL intensity, suggesting more effective separation of the photogenerated electron–hole pairs for the ZnO/Ag3VO4/AgI (10%) nanocomposite which prolong the life time of the photogenerated charge carriers, resulting in enhanced photocatalytic activity. Hence, decrease of the photocatalytic activity for the ZnO/Ag3VO4/AgI (20%) nanocomposite relative to the ZnO/Ag3VO4/AgI (10%) nanocomposite is related to poor separations of the charge carriers. Similar to many reports, further increase in weight percent of silver iodide leads to agglomeration of the excess particles, leading to formation of fewer junctions between its counterparts at higher loading of AgI [29,30]. In order to understand the formation of tandem n–n heterojunctions between the three n-type semiconductors, positions of valence band (VB) and conduction band (CB) energies for

ZnO, Ag3VO4, and AgI were calculated by Butler and Ginley model and the results are shown in Table 1. Being as naturally n-type semiconductors, Fermi levels of ZnO, Ag3VO4, and AgI are located close to their CB levels. The Fermi level of ZnO is more negative than that of the Ag3VO4. Hence, after contacting of these semiconductors with each other, electrons flow from ZnO to Ag3VO4 until their Fermi levels coincide [31]. Consequently, the whole energy bands of Ag3VO4 would be raised up whereas those of the ZnO would be declined until equilibrium state of Fermi levels was obtained. As a result, positive charges are accumulated on ZnO and negative charges are accumulated on Ag3VO4. Hence, n–n heterojunction with the same Fermi levels are formed between ZnO and Ag3VO4 (Fig. 7). Moreover, the Fermi level of AgI is more negative than that of the Ag3VO4. Then, after contacting of these semiconductors with each other, electrons flow from AgI to Ag3VO4 until their Fermi levels coincide and internal electrostatic field, directed from AgI to Ag3VO4, is formed. As a result, tandem n–n heterojunctions between AgI and Ag3VO4 in one side and Ag3VO4 with ZnO in other side are formed. Under the visible-light irradiation, electron–hole pairs are generated on Ag3VO4 and AgI counterparts of the ZnO/Ag3VO4/AgI nanocomposites, due to their narrow band gaps. The photogenerated electrons on the CB of Ag3VO4 reinforce to flow to the CB of ZnO and AgI by the help of the produced internal electrostatic field, due to the formation of tandem n–n heterojunctions. However, the VB energies of Ag3VO4 and AgI are less positive than that of the ZnO; hence, the photogenerated holes cannot transfer from Ag3VO4 and AgI to VB of ZnO. As a consequence, holes and electrons are spatially separated from each other, resulting in an increased lifetime for the electron–hole pairs. It is simply evident that separation of the charge carriers in Ag3VO4 cannot take place without formation of tandem n–n heterojunctions, because the CB level of Ag3VO4 is lower than those of the AgI and ZnO. However, as confirmed by the PL spectra, separation of the charge carriers in the nanocomposite takes place, leading to enhanced photocatalytic activity. As a result, tandem n–n heterojunctions have formed between these n-type semiconductors.

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Fig. 8. The degradation rate constant of RhB over the ZnO/Ag3VO4/AgI (10%) nanocomposite in presence of various scavengers.

To elucidate role of the reactive species on the degradation reaction of RhB over the ZnO/Ag3VO4/AgI (10%) nanocomposite, the trapping experiments were performed. For this purpose, ben-

zoquinone, ammonium oxalate, and 2-PrOH were used as scavengers of O2 , h+, and OH, respectively [32]. The degradation rate constants of RhB in presence of the selected scavengers are shown in Fig. 8. It is evident that decrease of the rate constant in the presence of benzoquinone is very higher than those of the ammonium oxalate and 2-PrOH. Consequently, it was revealed that superoxide ions are the predominant reactive species in the degradation reaction rather than hydroxide radicals and holes. The preparation time of photocatalysts could considerably affect their properties such as morphology, size, and extent of aggregation [33]. In order to study the effect of preparation time, the ZnO/Ag3VO4/AgI (10%) nanocomposite was prepared by ultrasonic irradiations for 30, 60, 120, 180, and 240 min and the results are shown in Fig. 9a. As can be seen, photocatalytic activity of the nanocomposite prepared by ultrasonic irradiation for 120 min is higher than those of the other samples. To compare morphology of the prepared nanocomposite at different ultrasonic irradiation times, SEM images of the nanocomposite prepared by ultrasonic irradiations for 120 and 240 min were provided and the results are shown in Fig. 9b and c. It is evident that in the nanocomposite prepared by 120 min of ultrasonic irradiation, there are many junctions between the counterparts. However, in the case of the nanocomposite prepared by ultrasonic irradiation for 240 min, agglomeration of the nanocomposite takes place, leading to destruction of the junctions. Hence, separation of the charge carriers in the interfaces of the nanocomposite cannot easily occur,

500

(a)

kobs (min-1) × 10-4

450

400

350

300

250

200 30

60

120

180

240

Ultrasonic irradiation time (min)

(b)

(c)

Fig. 9. (a) The degradation rate constants of RhB over the ZnO/Ag3VO4/AgI (10%) nanocomposite prepared at different ultrasonic irradiation times. SEM images of the ZnO/ Ag3VO4/AgI (10%) nanocomposite prepared by ultrasonic irradiations for (b) 120 min and (c) 240 min.

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tion temperature [34]. Hence, calcination of the nanocomposite could affect its surface properties. For this reason, BET surface areas and pore properties of the nanocomposite before and after calcination at 500 °C were determined using nitrogen adsorption–desorption measurements and the plots are presented in Fig. 10b and the calculated results are displayed in Table 2. As can be seen, the samples have typical IV isotherms, demonstrating presence of mesopores. The values of specific surface areas for the calcined and uncalcined nanocomposite are 6.44 and 8.61 m2 g 1, respectively. In addition, it can be seen that pore diameter and total pore volume of the nanocomposite are decreased with calcination. As a result, decrease of the photocatalytic activity of the nanocomposite with calcination could be attributed to decrease of its surface area, pore diameter, and pore volume. It is well known that ability of photocatalysts to degrade different pollutants is very important from economical view point of designing photocatalytic processes. To show activity of the ZnO/ Ag3VO4/AgI (10%) nanocomposite for degradation of different pollutants, degradation of MB under visible-light irradiation over the ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI (10%) samples was considered and the results for degradations of RhB and MB are displayed in Fig. 11. The photocatalytic activity of the ternary nanocomposite in degradations of MB is about 3.6 and 37-folds greater than those of the ZnO/Ag3VO4 and ZnO/AgI samples, respectively. As a consequence, ability of the nanocomposite for efficiently degradation of different dye pollutants is confirmed. It is clearly evident that stability of photocatalysts is another vital consideration in photocatalytic processes. To evaluate stability of the ZnO/Ag3VO4/AgI (10%) nanocomposite, five degradation experiments of RhB over the nanocomposite were conducted and the result is shown in Fig. 12. After each run, the photocatalyst was recycled by washing with ethanol and drying at 60 °C for 24 h. It can be observed that after using the photocatalyst in five successive cycles, the ternary photocatalyst did not exhibit significant loss of its activity. Hence, the ternary nanocomposite has reasonable activity and stability in photocatalytic degradation of the pollutant. 4. Conclusions

Fig. 10. (a) The degradation rate constants of RhB over the ZnO/Ag3VO4/AgI (10%) nanocomposite calcined at different temperatures. (b) Nitrogen adsorption–desorption data for the ZnO/Ag3VO4/AgI (10%) nanocomposite without calcination and calcined at 500 °C.

resulting in decrease of the photocatalytic activities at higher ultrasonic irradiation times. To investigate the effect of calcination temperature on the photocatalytic activity of the ZnO/Ag3VO4/AgI (10%) nanocomposite, the nanocomposite was calcined for 2 h at 200, 300, 400, and 500 °C and the degradation rate constants are displayed in Fig. 10a. It is evident that the degradation rate constant decreases with increasing the calcination temperature and the best activity is observed for the nanocomposite without any calcination. It is well known that grain size of particles gradually increases with calcina-

In summary, novel ternary ZnO/Ag3VO4/AgI nanocomposites, as visible-light-driven photocatalysts, were successfully prepared using ultrasonic-irradiation method. Photocatalytic activity of the as-prepared samples was investigated by degradations of RhB and MB under visible-light irradiation. It was revealed that photocatalytic activity increases with weight percent of silver iodide up to 10% and then decreases. Activity of this nanocomposite is about 5.1 and 3.6-folds higher than those of the ZnO/Ag3VO4 nanocomposite in degradations of RhB and MB, respectively. As confirmed by PL spectra, the superior activity was mainly attributed to more suppression of electron–hole pairs from recombinations, due to formation of tandem n–n heterojunctions. The results revealed that the nanocomposite prepared by ultrasonic irradiation for 120 min showed the best activity. Moreover, it was observed that the photocatalytic activity decreases with increasing calcination temperature. Based on the trapping experiments, it was showed that superoxide ions have main role in the degradation reaction.

Table 2 Texture properties of the ZnO/Ag3VO4/AgI (10%) nanocomposite with and without calcination. Condition

Surface area (m2 g

Without calcination With calcination

8.61 6.44

1

)

Mean pore diameter (nm)

Total pore volume (cm3 g

14.04 10.87

0.0302 0.0175

1

)

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Fig. 11. The degradation rate constants of RhB and MB over the ZnO/Ag3VO4, ZnO/AgI, and ZnO/Ag3VO4/AgI (10%) samples under visible-light irradiation.

[7]

[8]

[9]

[10]

[11]

[12]

[13] [14]

[15] Fig. 12. Reusability of the ZnO/Ag3VO4/AgI (10%) nanocomposite. [16]

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