Ag

Journal of Colloid and Interface Science 491 (2017) 216–229

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Ultrasonic-assisted preparation of plasmonic ZnO/Ag/Ag2WO4 nanocomposites with high visible-light photocatalytic performance for degradation of organic pollutants Mahsa Pirhashemi, Aziz Habibi-Yangjeh ⇑ Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

g r a p h i c a l a b s t r a c t t = -60 min t = 0 min t = 30 min t = 60 min t = 90 min t = 120 min t = 150 min t = 180 min t = 210 min t = 240 min t = 270 min

1.0

Absorbance

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0.6

0.4

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0.0 200

250

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a r t i c l e

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Article history: Received 17 October 2016 Revised 15 December 2016 Accepted 17 December 2016 Available online 21 December 2016 Keywords: ZnO/Ag/Ag2WO4 Ternary nanocomposites Plasmonic photocatalyst Visible-light-driven photocatalyst

a b s t r a c t In this work, plasmonic ternary ZnO/Ag/Ag2WO4 nanocomposites as efficient visible-light-driven photocatalysts prepared by a facile ultrasonic-irradiation method. The as-prepared samples were characterized by XRD, SEM, TEM, EDX, XPS, UV–vis DRS, FT-IR, and PL techniques. The photocatalytic performance of the prepared ZnO/Ag/Ag2WO4 nanocomposites were evaluated by photodegradations of rhodamine B, methylene blue, methyl orange, and fuchsine under visible-light irradiation. The optimal nanocomposite with 15 wt% of Ag/Ag2WO4 to ZnO showed the highest photocatalytic activity for RhB degradation, which is about 95 and 19 times higher than those of the Ag/Ag2WO4 and ZnO samples, respectively. The highly enhanced activity of the ZnO/Ag/Ag2WO4 (15%) nanocomposite was attributed to the surface plasmon resonance effect of metallic silver and the formation of heterojunctions between the counterparts, which effectively suppresses recombination of the photogenerated charge carriers. Lastly, the plasmonenhanced photocatalytic mechanism associated with the ZnO/Ag/Ag2WO4 nanocomposites was discussed. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (A. Habibi-Yangjeh). http://dx.doi.org/10.1016/j.jcis.2016.12.044 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

Parallel with population growth and civilization, productions of different synthetic compounds have increased. On the other hand, discharging of these stable compounds in the environment has

M. Pirhashemi, A. Habibi-Yangjeh / Journal of Colloid and Interface Science 491 (2017) 216–229

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ZnO has wide band gap of 3.20 eV and low quantum efficiency, limiting its practical widespread applications under the solar-energy illumination [11]. Hence, continuous attempts are carried out to enhance photocatalytic activity of ZnO under visible-light irradiation [9,11]. One effective strategy to overcome these drawbacks is combining of ZnO with metallic silver to prepare plasmonic photocatalysts [12–14]. Due to the surface plasmon resonance effect of metallic silver, highly enhanced photocatalytic activity is usually observed under visible-light illumination [15]. In order to produce metallic silver, Ag-containing semiconductors can be used as precursor. In this regard, different semiconductors such as AgBr, AgCl, Ag2O, Ag3PO4, and Ag2SO4 have been used for preparation of different plasmonic photocatalysts such as ZnO/Ag/AgBr, ZnO/Ag/AgCl, ZnO/Ag/Ag2O, ZnO/Ag/Ag3PO4, and ZnO/Ag/Ag2SO4 [16–20]. Silver tungstate (Ag2WO4) has a band gap of about 3.1 eV, which has been demonstrated to be an outstanding photocatalyst for degradation of pollutants [21,22]. In recent years, Ag2WO4 has been applied for preparation of some plasmonic photocatalysts [23–25]. Plasmonic Ag/Ag2WO4 was recently applied for photoreduction of carbon dioxide under visible light [23]. In addition, Dai et al., prepared plasmonic g-C3N4/Ag2WO4/Ag photocatalysts and applied them for photocatalytic degradation of methylene blue

produced huge amounts of wastewaters. These wastes generally contain different compounds such as pesticides, herbicides, fertilizers, detergents, different drugs, and synthetic dyes, which large amounts of them are annually produced worldwide [1]. These wastewaters are detrimental to humans, aquatic life, the environment, and microorganisms. Therefore, removal of these pollutants from wastewaters is of particular importance to protect the environment and human beings. Among various physical, chemical, and biological treatment methods, heterogeneous photocatalytic processes have been regarded as the most realistic solutions to environmental pollution, due to their abilities to unselective degradation of different pollutants, low-energy need, and environmentally-friendly procedure [2–5]. Beyond photocatalytic degradation of pollutants, heterogeneous photocatalysts have promising applications in different disciplines such as splitting of water to produce hydrogen gas, photoreduction of carbon dioxide, disinfection of microorganisms, and synthesis of organic compounds [6–8,4,9]. Hence, nowadays, investigations about photocatalytic processes is an active research area worldwide [9]. In the field of photocatalytic processes, ZnO has emerged as leading candidate because of its unique characteristics, such as low cost, nontoxicity, and good thermal and chemical stability [10]. However,

Scheme 1. Schematic illustration for the preparation procedure of the ZnO/Ag/Ag2WO4 nanocomposites.

ZnO

Ag2WO4

Ag

(13 (26 5) 2)

Fig. 1. XRD patterns for the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 nanocomposites with different weight percents of silver tungstate.

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(MB) under visible-light irradiation [24]. Very recently, Liv et al., reported surface plasmonic enhanced Ag2WO4/Ag/Bi2MoO6 photocatalyst with remarkable activity in degradation of MB under visible-light irradiation [25]. However, literature review showed that there is no reports about preparation and investigation photocatalytic activity of the plasmonic ZnO/Ag/Ag2WO4 nanocomposites. Hence, in this work, we successfully prepared visible-lightdriven ZnO/Ag/Ag2WO4 photocatalysts with different compositions via a facile ultrasonic-irradiation method. The phase structure, morphology, optical properties, and stability of the prepared samples were investigated by X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared spectroscopy (FT-IR), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), and photoluminescence spectroscopy (PL) techniques. The ZnO/ Ag/Ag2WO4 nanocomposites showed enhanced photocatalytic activity for degradation of several organic pollutants including rhodamine B (RhB), MB, methyl orange (MO), and fuchsine under visible-light irradiation. Moreover, possible enhancement mechanism of photocatalytic activity proposed and discussed in detail.

2. Experimental 2.1. Materials All of the reagents used in this experiment were analytical grade and they used without further purifications. Zinc nitrate (Zn(NO3)24H2O), silver nitrate, sodium hydroxide, and benzoquinone were purchased from Loba Chemie. Sodium tungstate (Na2WO42H2O), 2-propanol, ammonium oxalate, benzoquinone, RhB, MB, MO, fuchsine, and absolute ethanol were obtained from Merck. Deionized water was used throughout the work.

2.2. Instruments 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 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

(a)

Zn

ZnO/Ag/Ag₂WO₄(15%) Ag/Ag₂WO₄

Intensity ( a.u.)

ZnO

O Au W

Ag

Zn

Ag

W

Zn

W Ag O

Ag

Zn

Ag

WW

O Zn 0

2

4

6

Zn

8

10

Energy (keV)

(b)

(d)

(c)

7 μm

7 μm

Zn

7 μm

(f)

(e)

W

Ag

7 μm

O

7 μm

Fig. 2. EDX spectra for the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples. (b–f) EDX mapping of the ZnO/Ag/Ag2WO4 (15%) nanocomposite.


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 Philips CM30 instrument with an acceleration voltage of 150 kV. Also, the X-ray photoelectron spectroscopy (XPS) data were recorded on a X-ray 8025 BestTec spectrometer and all binding energies were corrected by using the contaminant carbon (C 1s = 284.6 eV) as an internal standard. The UV–vis DRS 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 275 nm. The conditions were fixed in order to compare the PL intensities. The UV–vis spectra for the degradation reaction were studied using a Cecile 9000 spectrophotometer. The pH of solutions was measured using a Metrohm digital pH meter of model 744. The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz). 2.3. Sample preparation 2.3.1. Preparation of the ZnO sample In a typical procedure for preparation of the ZnO sample, 5.220 g of zinc nitrate tetrahydrate was dissolved in 150 mL of water under stirring at room temperature. Then, aqueous solution of NaOH (5 M) was slowly added dropwise into the solution under stirring at room temperature until pH of the solution reached to 10. After that, the suspension was sonicated in air for 120 min. The resultant white suspension was centrifuged to get the precipitate out and washed two times with water and ethanol to remove the unreacted reagents and dried in an oven at 60 °C for 24 h. 2.3.2. Preparation of the ZnO/Ag/Ag2WO4 nanocomposite For preparation of the ZnO/Ag/Ag2WO4 (15%) nanocomposite, where 15% is weight percent of Ag/Ag2WO4, 0.425 g of the prepared ZnO was dispersed into 150 mL of water by ultrasonic irradiation for 10 min. Then, 0.055 g of silver nitrate was added to the suspension and stirred for 60 min. Afterwards, an aqueous solution of sodium tungstate (0.053 g in 50 mL of water) was dropwise

(a)

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added to the suspension and sonicated in air for 60 min. The formed light grey 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. The schematic diagram for preparation of the ZnO/Ag/Ag2WO4 nanocomposites is illustrated in Scheme 1. 2.4. Photocatalysis experiments Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 mL capacity. The reactor was provided with water circulation arrangement to maintain the temperature at 25 °C. The solution was magnetically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. A LED lamp of 50 W was used as visible-light source. The emission spectrum of the source has high intensity in visible range and its intensity rapidly decreases in wavelengths near to UV and IR ranges [26]. The source was fitted on the top of the reactor. The distance between the liquid surface and the source was about 20 cm. The photocatalyst was dispersed for 6 min in an ultrasonic bath before using. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL aqueous solution of RhB (1.0 105 M), MB (1.3 105 M), MO (1.05 105 M), or fuchsine (0.77 105 M) was continuously stirred in the dark for 60 min, to attain adsorption-desorption equilibrium. Samples were taken from the reactor at regular intervals and the photocatalyst removed before analysis by the spectrophotometer at 553, 664, 464, and 540 nm corresponding to the maximum absorption wavelength of RhB, MB, MO, and fuchsine, respectively. 2.5. Reactive species trapping experiments For detecting the reactive species produced during the photocatalytic degradation reaction, some sacrificial agents, such as 2propanol, ammonium oxalate, and benzoquinone were used as the scavengers of hydroxyl radical (OH), hole (h+), and superoxide anion radical (O 2 ), respectively. The method was similar to the former photocatalytic activity process with the addition of 1 mmol/L of the quencher in the presence of 250 mL of RhB (1.0 105 M).

(c)

(b)

(d) ZnO Ag2WO4 Ag

150 nm

Fig. 3. SEM images of the (a) ZnO, (b) Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples. (d) TEM image of the ZnO/Ag/Ag2WO4 (15%) nanocomposite.

220


(a)

(b)

Zn 2p3/2

(c)

Ag 3d5/2 Ag 3d3/2 366.63 eV

Zn 2p1/2

372.54 eV

368.04 eV

W 4f7/2 W 4f 5/2

373.13 eV

(e)

(d)

O 1s 531.78 eV 530.02 eV

Fig. 4. XPS spectra for the ZnO/Ag/Ag2WO4 (15%) nanocomposite: (a) survey scan, and high-resolution spectra for: (b) Zn 2p, (c) Ag 3d, (d) W 4f, and (e) O 1s.

3. Results and discussion 3.1. Characterization of the prepared samples The XRD patterns providing information about the crystalline structure of the as-prepared samples are shown in Fig. 1. It could be observed that the XRD pattern of ZnO sample is in good agreement with the wurtzite hexagonal crystalline phase (JCPDS No. 36-3411) [27]. All diffraction peaks of the Ag/Ag2WO4 sample correspond to (0 2 1), (0 2 2), (2 2 0), (1 3 0), (2 2 1), (0 1 3), (0 4 0), (1 2 3), (0 4 2), (0 0 4), (0 5 1), (1 3 3), (0 2 4), (2 3 3), (2 4 2), (0 6 0), (2 2 4), (3 4 1), (1 2 5), (4 4 0), (1 3 5), and (2 6 2) planes of Ag2WO4 (JCPDS No. 33-1195) [28]. It can be observed that in the pattern

of Ag/Ag2WO4 sample, small peaks at 38.15° and 78.35° can be observed, which assigned to (1 1 1) and (3 1 1) reflections of cubic Ag (JCPDS No. 65-2871) [24]. For the ternary samples, the peaks of ZnO, Ag2WO4, and metallic silver are observed, manifesting preparation of the ZnO/Ag/Ag2WO4 nanocomposites. Moreover, all the characteristic peaks belonging to ZnO do not show any shifts, which indicate that the introduction of Ag2WO4 did not change the crystal phase of ZnO. The composition and distribution of elements in the samples were examined by EDX instrument. According to the EDX spectra, Fig. 2a presents that the ZnO sample is composed of Zn and O elements. It can be seen that in the case of the Ag/Ag2WO4 sample, only Ag, W, and O elements exist. The EDX spectrum for the

Transmittance (a.u.)


Zn—O

H—O O—H

400

ZnO Ag/Ag WO ZnO/Ag/Ag WO (15%)

W— O

900

1400

1900

2400

2900

3400

3900

Wavenumber (cm-1) Fig. 5. FT-IR spectra for the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples.

1.6 1.4

Absorbance

1.2

ZnO/Ag/Ag ZnO/Ag/Ag ZnO/Ag/Ag ZnO/Ag/Ag Ag/Ag WO ZnO

WO WO WO WO

600

700

(30%) (20%) (15%) (10%)

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

800

Wavelength (nm) Fig. 6. UV–vis DRS for the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 nanocomposites with different weight percents of silver tungstate.

ZnO/Ag/Ag2WO4 (15%) nanocomposite consists of Zn, Ag, W, and O as the major elements. The element mapping images (Fig. 2b–f) are illustrated to identify the element distributions in the ZnO/Ag/Ag2WO4 (15%) nanocomposite, in which four elements of Zn (Fig. 2c), Ag (Fig. 2d), W (Fig. 2e), and O (Fig. 2f) are observed to be distributed homogeneous and uniformly within the selected area. All these results further indicated the formation of heterojunctions between ZnO, Ag2WO4, and Ag counterparts of the ternary nanocomposite. Morphology of the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples were examined by SEM images. Fig. 3a shows that the ZnO sample exhibited spindle-like morphology with diameter of about 200 nm and length of nearly 700 nm. From Fig. 3b, it is evident that the Ag/Ag2WO4 sample is mainly made up rods with different diameters mainly in micron size. As can be seen,

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due to strong interaction between counterparts of the ZnO/Ag/ Ag2WO4 (15%) nanocomposite, surface morphology of the ternary nanocomposite is considerably different from the ZnO and Ag/ Ag2WO4 samples (Fig. 3c). Furthermore, it is evident that size of particles in the ternary nanocomposite strongly decreased after formation of the nanocomposite. For the ZnO/Ag/Ag2WO4 (15%) nanocomposite, aggregated spindle-like particles of ZnO near to particles of Ag2WO4 are clearly seen in its TEM image (Fig. 3d). The surface chemical composition and chemical states of the elements in the ZnO/Ag/Ag2WO4 (15%) nanocomposite were analyzed by XPS technique and the results are displayed in Fig. 4. The photoelectron peaks of Zn, Ag, W, and O elements are clearly observed, which is consistent with the EDX result (Fig. 4a). The XPS peak of C 1s at 284.6 eV is assigned to residual carbon from the XPS instrument. Two peaks appeared at binding energies of 1021.7 and 1045.1 eV are assigned to Zn 2p3/2 and Zn 2p1/2, respectively, indicating the presence of Zn2+ [17,29] (Fig. 4b). As shown in Fig. 4c, two characteristic peaks, located at 367.03 and 372.9 eV, could be assigned to Ag 3d5/2 and Ag 3d3/2, respectively. After decomposing the above two peaks, the strong peaks at 366.63 and 372.54 eV are ascribed to Ag+, while another two weak peaks at 368.04 and 373.13 eV are owing to the presence of metallic silver [24,30]. This observation is consistent with the XRD patterns, confirming the presence of AgO in the nanocomposites. In the high resolution spectrum of W 4f in Fig. 4d, there are two peaks at 35.21 and 37.35 eV, which correspond to W 4f7/2 and W 4f5/2, respectively in oxidation state of +6. As indicated in Fig. 4e, for oxygen element, it is clear that the peak deconvoluted into two bands at 530.02 and 531.78 eV, related to the lattice oxygen and the oxygen of adsorbed water molecules, respectively [30]. Fig. 5 shows FT-IR spectra of the ZnO, Ag/Ag2WO4, and ZnO/Ag/ Ag2WO4 (15%) samples. For these samples, the broad absorption band around 3420 cm1 and narrow peak at 1664 cm1 are related to the OAH stretching and bending vibrations of adsorbed water molecules on surface of the samples [31]. For the ZnO sample, the absorption peak at 570 cm1 is due to the vibration of ZnAO bond [32]. The peak at 868 cm1 for the Ag/Ag2WO4 sample is assigned to the stretching vibration of WAO bond in WO2 4 species [33]. Finally, in the case of the ZnO/Ag/Ag2WO4 nanocomposite, the similar absorption bands for ZnAO and WAO bonds could also be observed. The UV–vis DRS spectra in the range of 300–800 nm for the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 samples are depicted in Fig. 6. As observed in these spectra, the DRS spectrum of ZnO sample only exhibits the fundamental absorption band in the UV region. The absorption edge of ZnO is approximately 357 nm (Eg 3.2 eV), without any absorption in visible wavelengths. The spectrum related to the Ag/Ag2WO4 sample has one absorption in UV region with a maxima at 342 nm and one broad absorption in visible range. The absorption in UV region is related to Ag2WO4 counterpart and the absorption in visible range is ascribed to surface plasmon resonance effect of metallic silver [15]. As can be seen, the absorption ability of the ternary nanocomposites in visible range slightly enhances with increasing weight percent of Ag/Ag2WO4, which could be mainly attributed to the presence of the Ag° particles. In fact, because of the presence of Ag°, the white color of ZnO sample gradually changes with increasing weight percent of Ag2WO4 in the ternary nanocomposite and reaches to dark gray color in the ZnO/Ag/Ag2WO4 (30%) nanocomposite. Hence, parallel with the XRD patterns and XPS results, the UV–vis DRS spectra confirmed preparation of the plasmonic ternary nanocomposites. As a result, the ternary nanocomposites could show noticeable photocatalytic activity under visible-light irradiation.

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under intense stirring after 1 h. Fig. 7a shows the photocatalytic performance of the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 nanocomposites under visible-light irradiation. The photolysis of RhB in the absence of photocatalysts is negligible within the test period, representing its considerable stability under the visiblelight illumination. In addition, in the presence of the light irradiation, it can be seen that over the ZnO and Ag/Ag2WO4 samples, only 24.4% and 17.1% of RhB were decomposed after irradiation for

3.2. Evaluation of photocatalytic activity Photocatalytic activities of the prepared samples were evaluated by comparing the degradation rates of RhB in aqueous solution under visible-light irradiation. Before the photocatalytic reaction test, dark treatment was performed to reach the adsorption-desorption equilibrium state. The adsorption–desorption balance was achieved between the photocatalysts and RhB

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Ag/Ag WO

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Irradiation time (min) t = -60 min t = 0 min t = 30 min t = 60 min t = 90 min t = 120 min t = 150 min t = 180 min t = 210 min t = 240 min t = 270 min

Absorbance

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t = -60 min t = 0 min t = 30 min t =60 min t = 90 min t = 120 min t = 150 min t = 180 min t = 210 min t = 240 min t = 270 min

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t = -60 min t = 0 min t = 30 min t = 60 min t = 90 min t = 120 min t = 150 min t = 180 min t = 210 min t = 240 min t = 270 min

(d)

1.0

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0.4

0.2

0.0 450

500

550

600

650

Wavelength (nm) Fig. 7. (a) Photodegradation of RhB over the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 nanocomposites with different weight percents of silver tungstate. UV–vis spectra for degradation of RhB under visible-light irradiation over the (b) ZnO, (c) Ag/Ag2WO4, and (d) ZnO/Ag/Ag2WO4 (15%) nanocomposite.

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140

3.0

(a)

Photolysis Dark

120

ZnO

2.5

Ag/Ag WO ZnO/Ag/Ag WO (10%)

100

ZnO/Ag/Ag WO (15%)

kobs (min-1) × 10-4

ln (C0/C)

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ZnO/Ag/Ag WO (20%) ZnO/Ag/Ag WO (30%)

1.5

1.0

80

60

40 0.5 20 0.0 0

50

100

150

200

250

300 0

Irradiation time (min)

Without

Ag/Ag WO ZnO/Ag/Ag WO (15%)

AO

BQ

Type of scavengers

(b)

ZnO

2-PrOH

Fig. 9. The degradation rate constants of RhB over the ZnO/Ag/Ag2WO4 (15%) nanocomposite in presence of various scavengers.

Intensity (a.u.)

Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples. From Fig. 7b–d, it is clear that the absorption intensity of RhB only decreases a little with extension of the exposure time when photocatalytic reaction is performed over the ZnO and Ag/Ag2WO4 samples under the light irradiation. However, for the degradation reaction over the ZnO/ Ag/Ag2WO4 (15%) nanocomposite, the characteristic absorption peak of RhB decreases rapidly with the increase of irradiation time and the peak almost was disappeared after the light irradiation for 270 min. To have a better understanding of the reaction kinetics, the experimental data were fitted by a pseudo-first-order model as expressed by Eq. (1). 320

350

380

410

440

470

Wavelength (nm) Fig. 8. (a) Plots of ln([C]0/[C]t) versus time for the degradation reaction over the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 nanocomposites with different weight percents of silver tungstate. (b) PL spectra for the ZnO, Ag/Ag2WO4, and ZnO/Ag/ Ag2WO4 (15%) samples.

300 min, respectively. However, all the ZnO/Ag/Ag2WO4 nanocomposites present enhanced photocatalytic degradation performance. The ZnO/Ag/Ag2WO4 (15%) nanocomposite performed the highest photocatalytic activity and completely decomposed RhB within 270 min. With further increasing weight percent of Ag/Ag2WO4 up to 30%, the photocatalytic activity did not enhance but decreased instead. As a result, it can be concluded that with the excessive Ag/Ag2WO4 counterpart of the nanocomposite, separation of the photogenerated electron-hole pairs does not take place efficiently, due to destruction of the formed heterojunction between ZnO and Ag/Ag2WO4 counterparts of the ternary nanocomposite. Hence, the over-accumulated electrons on surface of the Ag/Ag2WO4 are attracted by the photogenerated holes, so the photocatalytic activity decreases. The results mentioned above implied that the amount of the Ag/Ag2WO4 counterpart in the ternary nanocomposites plays a key role in the photocatalytic performance of the ZnO/Ag/Ag2WO4 nanocomposites. In addition, the degradation process was monitored by providing UV–vis absorption spectra of RhB during the degradation reaction over the ZnO,

lnðC 0 =C t Þ ¼ kobs t

ð1Þ

where Ct is the real-time concentration of RhB, C0 is initial concentration of RhB, kobs is observed pseudo-first-order rate constant (min1), and t represents the irradiation time. The degradation rate constants of RhB were calculated to be 6.16 104, 1.27 104, and 121 104 min1 over the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples, respectively (Fig. 8a). Hence, the photocatalytic activity of the ZnO/Ag/Ag2WO4 (15%) nanocomposite is about 19 and 95 times higher than those of the ZnO and Ag/Ag2WO4 samples, respectively. This result clearly indicates that the combination of ZnO and Ag/Ag2WO4 significantly enhances the photocatalytic activity under the visible-light irradiation. Photoluminescence (PL) spectra have been performed to reveal the charge carrier trapping, migration, and recombination processes of photocatalysts, since PL emission arises from the recombination of charge carriers [32]. In a PL spectrum, the weaker emission intensity represents the lower recombination efficiency of the photogenerated charge carriers and the higher photocatalytic activity. As shown in Fig. 8b, it is clear to see that both of the ZnO and Ag/Ag2WO4 samples have wide and strong peaks, which possess relatively high recombination rates for the photoinduced charge carriers. It was clearly observed that the ZnO/Ag/Ag2WO4 (15%) nanocomposite has the lowest peak intensity than other samples, implying that the formation of heterojunction between ZnO and Ag/Ag2WO4 counterparts contributed in decreasing the recombination of electron-hole pairs effectively and

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enhancing the charge separation efficiency, leading to considerably enhanced photocatalytic activity of the ternary nanocomposite relative to its counterparts. Therefore, based on the results obtained from UV–vis DRS and PL spectra, it was concluded that the enhanced photocatalytic activity of the ternary nanocomposites was attributed to more visible-light absorption and effective separation of photogenerated charge carriers. During the photocatalytic degradation reactions of organic pollutants, hydroxyl radicals (OH), holes (h+), and superoxide anion radicals (O 2 ) are generally produced as the main active species. So the reactive species trapping experiments in the photocatalytic degradation reaction of RhB over the ZnO/Ag/Ag2WO4 (15%) nanocomposite were conducted to find major reactive species. Hence, different scavengers were used to consume the corresponding active species. For this purpose, 2-propanol, ammonium oxalate, and benzoquinone were used for trapping of OH, h+, and O2 species, respectively. So according to the changes in the photocatalytic reaction rate constant, the effects of different active species in the reaction system were distinguished and the results are shown in Fig. 9. As can be seen, when these scavengers were added into the reaction system, the photocatalytic degradation reaction experienced a fast deactivation, suggesting that OH, h+, and O 2 have remarkable influence on the degradation reaction. However, the deactivation sequence is as benzoquinone > ammonium oxalate > 2-propanol, implying that the role of O 2 in the degradation reaction is higher than those of the other scavengers. In order to fully understand the enhanced photocatalytic activity of the ZnO/Ag/Ag2WO4 nanocomposites, the conduction (CB)

and valence band (VB) edge potentials, represented as ECB and EVB, were calculated from the following equations [34]:

EVB ¼ X Ee þ 0:5Eg

ð2Þ

ECB ¼ EVB Eg

ð3Þ

in which X is the absolute electronegativity of the semiconductor, determined by the geometric mean of the absolute electronegativity of constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV); and Eg is the band gap of the semiconductor. The calculated values of ECB and EVB for ZnO are 0.34 and +2.86 eV, respectively, while those for Ag2WO4 are 0.07 and 3.03 eV, respectively. On the basis of the alignment of their energy levels, an illustration of possible electron transfer behavior is shown in Fig. 10. According to the wide band gap of ZnO and Ag2WO4 semiconductors, they cannot be excited by visible-light irradiation. However, Ag0 particles would generate electron-hole pairs under visible-light irradiation due to the surface plasmon resonance effect [35–37]. Then, the excited electrons transfer to the CB of Ag2WO4 and ZnO counterparts. The injected electrons react with adsorbed molecules of oxygen over the CB of ZnO to produce O2 species, because CB of ZnO is more negative than the potential of O2/O2 (E0(O2/O2) = 0.33 eV/ NHE) [38]. Whereas, the CB edge potential of Ag2WO4 (0.07 eV vs. NHE) is more positive than the standard redox potential of O2/O2, suggesting that the electrons at CB of Ag2WO4 cannot reduce molecules of oxygen to O 2 [39]. However, the photogenerated

Electric field Electric cloud

---

+ ++

+ ++ … + H2O + CO2 ← RhB + OH

---

•

Ag sphere •

OH + RhB → CO2 + H2 O + … H+

e-

• + * Dye - - /Dye

e e e

LUMO

e-

O2-

-

e

H2O2

O2

-

-

-

e e e

O2 + H+ SPR

e- e- e-

CB=-0.34 eV

-

e

e-

+ * Dye - /Dye - -

e e e

LUMO

CB=-0.07 eV

Dye 3.2 eV

Products 3.1 eV

HOMO

Dye+/Dye

HOMO VB=+2.86 eV

Dye+/Dye ZnO

VB=+3.03 eV

Fig. 10. A plausible diagram for separation of electron-hole pairs in the Ag ZnO/Ag/Ag 2WO42WO4 nanocomposites.

225


140

(a)

(b)

Prepared at 120 min

120

Prepared at 60 min

Intensity (a.u.)

kobs (min-1) × 10-4

100 80 60 40 20 0 30

45

60

90

10

120

20

30

40

50

60

70

80

2Theta (deg.)

Ultrasonic irradiation time (min)

(c)

(d)

Intensity (a.u.)

(e)

Prepared at 2h-ultra ZnO/Ag WO (15%)

320

350

380

410

440

470

Wavelength (nm) Fig. 11. (a) The degradation rate constants of RhB over the ZnO/Ag/Ag2WO4 (15%) nanocomposite prepared at different ultrasonic irradiation times. (b) XRD patterns, (c and d) SEM images, and (e) PL spectra for the ZnO/Ag/Ag2WO4 (15%) nanocomposite prepared by ultrasonic irradiations for 60 min and 120 min.

electrons on the CB of ZnO and Ag2WO4 react with adsorbed molecules of oxygen to produce H2O2 molecules; because the CB levels of these semiconductors are more negative than E0(O2/H2O2) (+0.682 eV vs. NHE) [40]. After that, the produced H2O2 molecules react with electrons to produce active OH radicals. In addition, the photogenerated holes degrade pollutants directly. Indeed, anchoring Ag/Ag2WO4 over surface of the ZnO particles could increase separation efficiency of the photogenerated electron-hole pairs, which is benefit for enhancing photocatalytic activity. The degradation reaction of dyes over the ZnO/Ag/Ag2WO4 nanocomposites takes place by multiple charge transfer pathways under visible-light irradiation, as shown in Fig. 10. The other mechanism is based on the excitation of dye under visible light [41,42]. The dye absorbs some of photons form visible light and subse-

quently electrons are excited from HOMO to LUMO orbitals of RhB. After that, electrons are transfer from the dye to CB of ZnO and Ag2WO4. The transferred electrons generate different reactive species in the next steps. The ZnO/Ag/Ag2WO4 (15%) nanocomposite was prepared by ultrasonic irradiations for various times in order to investigate the effect of the preparation time on the photocatalytic activity of the nanocomposite. As shown in Fig. 11a. photocatalytic activity of the nanocomposite prepared by ultrasonic irradiation for 60 min remarkably is higher than those of the other preparation times. To show the effect of the ultrasonic irradiation time on the photocatalytic activity, the XRD patterns and SEM images of the nanocomposite prepared by ultrasonic irradiations for 60 and 120 min were provided and they are shown in Fig. 11b–d. As can be seen,

226


140

(a)

(b)

Calcined at 400 °C

120

Without calcination

Intensity (a.u.)

kobs (min-1) × 10-4

100 80 60 40 20 0 Room temp.

100

200

300

400

10

20

30

40

50

60

70

80

2Theta (deg.)

Calcination temperature (°C)

(d)

(c)

(e)

Calcined at 400 °C

Intensity (a.u.)

Without calcination

320

350

380

410

440

470

Wavelength (nm) Fig. 12. (a) The degradation rate constants of RhB over the ZnO/Ag/Ag2WO4 (15%) nanocomposite calcined at different temperatures. (b) XRD patterns, (c and d) SEM images, and (e) PL spectra for the ZnO/Ag/Ag2WO4 (15%) nanocomposite without calcination and calcined at 400 °C.

intensity of the XRD peaks was increased with increasing the preparation time, implying that size of the crystallites increases. In addition, it is clearly evident that when the ultrasonic time was extended to 120 min, particle agglomeration was significantly increased, resulting in formation of larger particles. Fig. 11e shows PL spectra of the nanocomposite prepared by ultrasonication for 60 and 120 min. Interestingly, the PL emission intensity for the ZnO/ Ag/Ag2WO4 (15%) nanocomposite prepared at 60 min ultrasonic irradiation time is lower than that of the nanocomposite prepared at 120 min irradiation. Hence, it is concluded that the electron-hole pairs in the nanocomposite prepared by 60 min of ultrasonic irradiation has longer life time than the nanocomposite fabricated at 120 min. As a result, it was concluded that in the nanocomposite prepared at higher ultrasonic irradiation time, the formed hetero-

junctions between the counterparts are destructed in some extent. Therefore, separation of the charge carriers in the interfaces of the nanocomposite cannot easily take place. Hence, photocatalytic activity of the nanocomposite decreases at higher ultrasonic irradiation times. Photocatalytic activity of the ZnO/Ag/Ag2WO4 (15%) nanocomposite after calcinations at various temperatures was evaluated through photocatalytic degradation of RhB under visible-light irradiation. Dependence of the degradation rate constant of RhB over the ZnO/Ag/Ag2WO4 (15%) nanocomposite prepared by ultrasonic irradiation for 60 min and calcined for 2 h at different temperatures is presented in Fig. 12a. It can be seen that the calcination temperature has a great effect on the photocatalytic activity. It is well known that size of particles increases with increasing


140

105

MB

RhB

90

100

kobs (min-1) × 10-4

kobs (min-1) × 10-4

120

80 60 40

60 45 30 15

0

0

60

35

MO 50

kobs (min-1) × 10-4

kobs (min-1) × 10-4

75

20

30

227

25 20 15 10

Fuchsine

40 30 20 10

5

0

0

Fig. 13. The degradation rate constants of RhB, MB, MO, and fuchsine over the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples under visible-light irradiation.

1.0 Run 1

Absorbance

0.8

Run 2

0.6

0.4

Run 3

0.2 Run 4

0.0 0

200

400

600

800

1000 1200 1400 1600

Irradiation time (min) Fig. 14. Reusability of the ZnO/Ag/Ag2WO4 (15%) nanocomposite.

calcination temperature of photocatalysts [43,44]. In Fig. 12b–d, the XRD patterns and SEM images for the calcined and uncalcined nanocomposite are compared. It is evident that after calcination of

the nanocomposite, the formed crystallites are larger in size, which can be attributed to the thermally promoted crystallite growth [43]. As a result, due to growth of particle sizes, photocatalytic activity decreases with increasing calcination temperature. Furthermore, Fig. 12e illustrates PL spectra of the ZnO/Ag/Ag2WO4 (15%) nanocomposite before and after calcination at 400 °C. As seen from the spectra, the PL intensity increased as the nanocomposite calcined at 400 °C. Hence, it was concluded that by increasing size of the particles, separation of the photogenerated charge carriers was decreased, leading to the decrease of the photocatalytic activity. Wastewaters usually contain different pollutants. Hence, ability of photocatalysts to degrade unselectively different pollutants is very important parameter in photocatalytic processes. In order to evaluate ability of the ZnO/Ag/Ag2WO4 (15%) nanocomposite for degradation of other dye pollutants, photocatalytic degradations of MB and fuchsine (as cationic dyes) and MO (as anionic dye) over the nanocomposite under the visible-light irradiation are shown in Fig. 13. It is clearly evident that activity of this nanocomposite is much higher than those of the ZnO and Ag/Ag2WO4 samples. For example, the degradation rate constants of MB over the ZnO, Ag/ Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples are 6.31 104, 8.30 104, and 93.9 104 min1, respectively. Moreover, the rate constants for MO degradation over the ZnO, Ag/Ag2WO4, and ZnO/Ag/Ag2WO4 (15%) samples are 3.17 104, 3.44 104, and 32.7 104 min1, respectively. Therefore, it is evident that

228


ZnO/Ag/Ag WO

1 ZnO/Ag VO

ZnO/AgI

0.8

Absorbance

ZnO/Ag VO /Ag CrO

ZnO/AgBr/Fe O /Ag VO

0.6

Fe O /ZnO/AgBr/Ag PO

0.4

Fe O @AgBr-ZnO

ZnO/Ag VO /Fe O

0.2 Fe O /ZnO/Ag VO /AgI

g-C N /Fe O /Ag VO /Ag CrO

0 -60

0

60

120

180

240

300

360

Irradiation time (min) Fig. 15. Photodegradation of RhB over the ZnO/Ag/Ag2WO4 (15%) nanocomposite and some recently introduced photocatalysts under visible-light irradiation.

Table 1 Weight percent of silver and degradation rate constant of RhB over different photocatalysts. Photocatalyst

Silver weight%

kobs (min1) 104

ZnO/Ag3VO4 Fe3O4/AgBr/ZnO ZnO/Ag3VO4/Fe3O4 ZnO/AgI g-C3N4/Fe3O4/Ag3VO4/Ag2CrO4 ZnO/Ag/Ag2WO4 ZnO/Ag3VO4/Ag2CrO4 Fe3O4/ZnO/AgBr/Ag3PO4 ZnO/AgBr/Fe3O4/Ag3VO4 Fe3O4/ZnO/Ag3VO4/AgI

44.3 20.5 38.9 18.4 40.5 6.98 39.6 26.0 17.8 27.3

54.4 77.6 95.1 100 108 121 129 155 156 192

activity of the ZnO/Ag/Ag2WO4 (15%) nanocomposite in degradation of MB is nearly 14.8 and 11.3-folds greater than those of the ZnO and Ag/Ag2WO4 samples, whereas in the case of MO degradations, the activity is 10.3 and 9.5-folds higher than those of the ZnO and Ag/Ag2WO4 samples, respectively. Finally, the rate constants for fuchsine degradations over the ZnO, Ag/Ag2WO4, and ZnO/Ag/ Ag2WO4 (15%) samples are 7.92 104, 6.64 104, and 55.2 104 min1, respectively. Hence, activity of the ZnO/Ag/Ag2WO4 (15%) nanocomposite in photodegradation of fuchsine is 7.0 and 8.3-folds higher than those of the ZnO, Ag/Ag2WO4 samples, respectively. As a result, it was concluded that the plasmonic nanocomposite has highly enhanced activity in degradation of different pollutants. It is well known that except high photocatalytic activity, the stability of photocatalysts is practically important. Durability of the ZnO/Ag/Ag2WO4 (15%) nanocomposite was investigated by cycled degradation of RhB with identical conditions for four times under visible-light irradiation. The used nanocomposite at each experiment was centrifuged, washed with ethanol and dried. As shown in Fig. 14, the photocatalyst maintained stable and active after the four-cycle test. The results suggest that the ZnO/Ag/Ag2WO4 (15%) nanocomposite may have practical application potentials as an effective and stable photocatalyst for degradations of different dyes under visible-light irradiation.

In order to show advantage of the ZnO/Ag/Ag2WO4 (15%) nanocomposite relative to other very recently introduced photocatalysts, their photocatalytic activities against degradation of RhB under visible-light irradiation were performed and the results are displayed in Fig. 15. As can be seen, photocatalytic activity of the ZnO/Ag/Ag2WO4 (15%) nanocomposite is higher than those of five photocatalysts, but its activity is lower than four ones. As well known, silver is an expensive element. Hence, weight percent of silver element was calculated in these photocatalysts and the results were shown in Table 1. It is evident that weight percent of silver in the ZnO/Ag/Ag2WO4 (15%) nanocomposite is very lower than those of the other photocatalysts. Hence, the ZnO/Ag/Ag2WO4 (15%) photocatalyst could be used in large scale applications.

4. Conclusions In summary, ultrasonic-assisted method was applied for preparation of novel ternary ZnO/Ag/Ag2WO4 nanocomposites, as plasmonic visible-light-driven photocatalysts. These nanocomposites were verified by different techniques including XRD, SEM, TEM, EDX, XPS, DRS, FT-IR, and PL. The experimental results demonstrated that the optimal ZnO/Ag/Ag2WO4 (15%) nanocomposite showed the highest photocatalytic performance, which manifested a noteworthy 19 and 95-folds enhancements in RhB degradation relative to the ZnO and Ag/Ag2WO4 samples, respectively. In addition, according to the trapping experiments, it was found that the deactivation sequence is as benzoquinone > ammonium oxalate > 2-propanol, implying that the role of O 2 in the degradation reaction is higher than those of h+ and OH species. In order to show ability of the ternary nanocomposite to degrade other dye pollutants, photocatalytic degradations of three more dyes were evaluated. Interestingly, it was found that activity of the ZnO/Ag/ Ag2WO4 (15%) nanocomposite in degradation of MB is nearly 14.8 and 11.3-folds greater than those of the ZnO and Ag/Ag2WO4 samples, respectively, whereas in the case of MO the activity is 10.3 and 9.50-folds higher than those of the ZnO and Ag/Ag2WO4 samples, respectively. Hence, the results suggested that the ternary plasmonic photocatalyst may have practical applications as an


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