Ag2CrO4 nanoparticles modified on ZnO nanorods as an efficient plasmonic photocatalyst under visible light

Accepted Manuscript Title: Ag/Ag2 CrO4 nanoparticles modified on ZnO nanorods as an efficient plasmonic photocatalyst under visible light ¨ Authors: Nuray Guy, ¨ Mahmut Ozacar PII: DOI: Reference:

S1010-6030(18)31042-6 https://doi.org/10.1016/j.jphotochem.2018.10.035 JPC 11551

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

21-7-2018 17-10-2018 18-10-2018

¨ Please cite this article as: Guy ¨ N, Ozacar M, Ag/Ag2 CrO4 nanoparticles modified on ZnO nanorods as an efficient plasmonic photocatalyst under visible light, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.10.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ag/Ag2CrO4 nanoparticles modified on ZnO nanorods as an efficient plasmonic photocatalyst

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Nuray Güya and Mahmut Özacara,b*

Sakarya University, Science&Arts Faculty, Department of Chemistry, 54187 Sakarya, Turkey.

Sakarya University, Biomedical, Magnetic and Semiconductor Materials Research Center

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b

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under visible light

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(BIMAS-RC), 54187 Sakarya, Turkey.

* Corresponding author. Tel: +90 264 295 60 41; fax: +90 264 295 59 50. E-mail address: [email protected] (M. Özacar)

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Highlights

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Graphical Abstract

The Ag/Ag2CrO4/ZnO plasmonic photocatalyst was first time synthesized.

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The Ag/Ag2CrO4/ZnO exhibited remarkably enhanced IC degradation activity.

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The effectiveness of Ag/Ag2CrO4/ZnO is due to heterogeneous structure and SPR effect.

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Abstract

The highly efficient plasmonic photocatalysts have drawn attention with their effective charge carrier separation capabilities and visible light absorption features, which makes them immensely favorable material in the organic contaminants eliminating for wastewater

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treatment. Thus, the goal of this study is to improve visible light driven plasmonic photocatalysts using Ag/Ag2CrO4 and ZnO. A highly efficient nanoheterojunction structured been prepared

by precipitation-photoreduction method and

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Ag/Ag2CrO4/ZnO has

characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM),

Fourier

transform

infrared

spectroscopy

(ATR-FTIR),

electrochemical

efficiencies

of

ZnO,

Ag2CrO4,

Ag/Ag2CrO4,

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photocatalytic

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measurements and UV-Vis diffuse reflectance/absorbance spectroscopy (DRS). When Ag2CrO4/ZnO

and

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Ag/Ag2CrO4/ZnO are compared, Ag/Ag2CrO4/ZnO exhibits the best efficiency for the

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degradation of indigo carmine (IC) under visible-light. The reasons for the highly increased

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performance of Ag/Ag2CrO4/ZnO may be related to the synergistic effect of Ag/Ag2CrO4 doped on ZnO surface. Thus, the visible light absorption capability of ZnO can be increased

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and the recombination of charge carriers can be effectively hindered. The nano photocatalyst also showed high stability even after five cycles. Therefore, the Ag/Ag2CrO4/ZnO can be

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effectively used as active plasmonic photocatalysts under visible light and it exhibits a great

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potential in removing environmental contamination.

Key words: Ag2CrO4, Ag/Ag2CrO4/ZnO composite photocatalyst, Surface plasmon resonance effect, Plasmonic photocatalyst, Reactive oxygen species.

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1. Introduction

The wastes containing various organic contaminants caused by textile, pharmaceutical, dyeing, paper and pulp industries are damaging natural water resources. These organic pollutants cause serious ecological problems by consuming dissolved oxygen in the water [1–

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3]. For this reason, these industrial wastes need to be removed effectively and economically.

Traditional wastewater treatment methods such as flocculation, adsorption, membrane process

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have some limitations and disadvantages [1]. Recently, photocatalysis as a type of solution for the issues of energy shortage and environmental pollution is one of the most promising

technologies. Particularly, as a “green” technology, semiconductor-based photocatalysis has

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become more important for decomposition of organic contaminants in wastewater [1,4,5].

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Zinc oxide as a semiconductor photocatalyst which has excellent optical and electronic

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features, nontoxicity, photostability, and low cost has been commonly examined. But, its

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photocatalytic activity is considerably restrained since it has a wide band gap (3.2 eV)

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corresponding to only ∼4% of solar energy [6]. Many methods have been tried to shift the photoactivity of ZnO to the visible light region and to increase its quantum yield [6]. One of

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them is to produce extra energy levels between the conduction band (CB) and the valence band (VB) by deposition of metal or non-metal [7,8]. Thus, shift in absorbance is observed

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towards visible region. However, these methods are inadequate in the environmental cleanup

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technologies where solar light is used [6].

The visible-light-driven plasmonic photocatalysts are the most effective ones among other known photocatalysts [9,10]. Silver based compounds such as AgBr [11], Ag2O [12], AgI [13], Ag3VO4 [14], Ag2CO3 [15], Ag2S [16], Ag3PO4 [17] and Ag2CrO4 [18] have been reported to indicate advanced photocatalytic influential under visible light. The generation of

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Ag on the silver salts which facilitates influential charge separation is one of the reasons for the high photocatalytic efficiency of silver salts or silver based composites under visible light [8,19–22]. The synergistic effects of the combination of these silver-based components with metal oxide photocatalysts such as ZnO and TiO2 increase light absorption and photocatalytic performance. Nowadays, especially plasmonic photocatalysts Ag/AgX (X = Cl, Br, I) which

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have been improved, attract great interests due to their photocatalytic efficiency under visible

light [23]. This is largely owing to the surface plasmon resonance (SPR) of metallic Ag

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nanoparticles and the synergistic effect between Ag and AgX [24]. The strong absorption of visible light provided by the SPR effect of Ag nanoparticles and the polarization caused by AgX retard electron-hole recombination and accelerate charge transfer.

In the Ag/AgX

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composite, Ag nanoparticles may also suppress the decomposition of AgX, while they

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significantly increase visible light absorption [23]. In addition, Ag nanoparticles effectively

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inhibit the recombination of pairs of charge carriers due to the Schottky barrier formed at the

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metal-semiconductor interface [21]. As a result, Ag NPs play an important role in providing

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influential migration of electrons from the metal nanoparticle (NP) to the conduction band of the semiconductor and intensifying the light. Thus, the Ag NPs-semiconductor heterostructure

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becomes an significant factor in the formation of visible driven photocatalysts [25]. Ag2CrO4 which has a narrow band gap of ~1.80 eV is a new photocatalyst. It is considered a good

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candidate for decomposition of organic contaminant under visible light because of its crystal structure and electronic properties. ZnO and Ag2CrO4 are n-type semiconductors. Combined

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with ZnO and Ag2CrO4, n-n heterojunction structures will be composed. The n-n heterojunctions could stimulate internal electric fields between components and give rise to influentially delay of recombination of the charge carriers. As a consequence, it is thought that higher photocatalytic efficiency of nano photocatalysts will be observed compared to ZnO [24,26]. So far, the modifying of Ag/Ag2CrO4 on ZnO has not yet been evaluated to

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develop their photocatalytic activity. Here, we refer about the synthesizing of ternary Ag/Ag2CrO4/ZnO plasmonic photocatalysts using a precipitation-photoreduction method. When Ag/Ag2CrO4/ZnO plasmonic photocatalysts are compared with other photocatalyts, the synergistic effect between the components and the strong SPR effect of Ag leads to the advanced photocatalytic efficiency. Besides, the possible separation of charge carriers

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mechanism has also been proposed for visible-light excited indigo carmine photodegradation

based on Ag/Ag2CrO4/ZnO nanostructures. Thus, this study can shed light on both scientists

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and their workings in the field of photocatalysis.

2. Materials and Methods

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2.1. Materials

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Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, Sigma Aldrich), sodium hydroxide (NaOH,

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Merck), sodium borohydride (NaBH4, Merck), silver nitrate (AgNO3, Merck), potassium

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chromate (K2CrO4, Merck), hexamethylenetetramine (HMT, Sigma Aldrich), indigo carmine (IC, commercial grade), and ethanol (Merck) were supplied. All chemicals except IC were of

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reagent grade.

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2.2. Preparation of photocatalysts

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The ZnO nanorods were synthesized via microwave-assisted hydrothermal method. The synthesis procedure of ZnO nanorods involves the following steps: Firstly, 3 mmol of Zn(NO3)2.6H2O was dissolved in 100 mL of distilled water and then 3 mmol HMT was added and stirred for 1 h. The formed solutions were transferred into a Teflon autoclave (100 mL) and heated at 170 °C for 10 min under temperature-controlled mode in a microwave furnace

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(CEM Mars 5) operating at 700 W and then cooled [27–31]. The prepared photocatalyts were centrifugated, and then washed with distilled water and absolute ethanol for three times and finally dried at 60 °C.

For synthesis of the Ag2CrO4/ZnO, 0.35 g of the synthesized ZnO powders was dispersed in

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150 mL of distilled water by ultrasonication for 30 min. Subsequently, 0.154 g of AgNO3 was

introduced into the above the mixture and stirred for 60 min. 100 mL of K2CrO4 (0.15 M) was

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introduced drop-by-drop to the mixture and refluxed at 96 0C. The resulting dark purple suspension was centrifugated and washed three times with distilled water and absolute ethanol

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and then dried at 60 °C.

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For preparation of the Ag/Ag2CrO4/ZnO, some of the Ag+ ions were reduced on

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Ag2CrO4/ZnO via a photo-reduction method. The Ag2CrO4/ZnO composites were

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photoinduced by 128 W UV light (λ ≥ 365 nm) for 2 hours. Ag/Ag2CrO4/ZnO composites

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were fabricated.

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2.3. Characterization of photocatalysts

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The photocatalysts were verified by powder X-ray diffraction (XRD, PANalytical Empyrean diffractometer with Cu Kα (λ=1.54 Å) in the 2θ angles ranging from 10 to 90. The Xray

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photoelectron spectra (XPS, Specs-Flex) were recorded using Al Kα radiation. The morphologies of nanocomposites were detected by a field emission scanning electron microscopy (FESEM, FEI QUANTA FEG 450) and transmission electron microscope (TEM, FEI TALOS F200S TEM 200 kV). The surface compositions of the products were analyzed by energy dispersive spectroscopy (EDS). The metal contents of Ag/Ag2CrO4/ZnO were

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examined by atomic absorption spectrometer (AAS, A Shimadzu AA6701F). The UV–Vis absorption spectra of the IC solution and photocatalysts were examined by using a UV-visible spectrophotometer (UV-Vis, Shimadzu UV-2600). The diffuse reflectances of the samples were determined by using a UV-Vis spectrophotometer fitted with an integrating sphere attachment. The band gap energies of the nanophotocatalysts were detected by the Kubelka–

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Munk function, F(R) and by plotting the [F(R)hv]1/2 versus photon energy (hν). Mott– Schottky measurements were carried out in 0.1 mol L-1 KCl solution using an electrochemical

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workstation (CHI 660C) with a traditional three electrode system. The glassy carbon, Pt-wire and Ag/AgCl electrodes were used as working, counter and reference electrodes, respectively. Similarly, the photocurrent measurements were carried out by a CHI 660D workstation in a

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three-electrode cell with 0.1M Na2SO4 electrolyte solution under visible light irradiation from

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a Xenon lamp with 300 W as light source. All these experiments were performed at room

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temperature. Fluorescence measurements were performed by Hitachi S-7000 fluorescence

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spectrophotometer. The total organic carbon content was measured with a Shimadzu TOC-L

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Total Organic Analyser.

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2.4. Photocatalytic testing

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Photocatalytic performances of as-synthesized products were analyzed by degradation of aqueous solution of IC under visible light of 128 W Xenon lamp. For each degradation

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system, 50 mg of photocatalyst was introduced into aqueous solution of IC (100 mL of 16 mg/L). Before the visible light illamunation, the suspension of stirration was carried out in the dark conditions for 30 min to provide the equilibrium adsorption/desorption of IC on the photocatalyst surface. 5 mL of the aliquots were taken at certain time intervals and centrifuged. The absorbance changes of IC at the maximum absorption wavelength of 610 nm

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were examined by UV-Vis spectrophotometer. The percentage of photodegradation was detected by the Eqs. (1) and (2) [6]:

degradation (%) =

C0

A0 −A A0

x 100

(1)

x 100

(2)

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=

C0 −C

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where C0 and C indicate concentrations before irradiation and after irradiation time t, respectively. A0 and A indicate absorbances of the IC at 610 nm before irradiation and after

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irradiation time t, respectively.

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One of the significant phenomenons for the photocatalysis proccess is generation of the strong

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reactive oxygen species (ROS) [18]. So, ROS trapping experiments were achieved with

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specific scavengers by applying the same photocatalytic degradation process in this work.

3. Results and Discussion

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3.1. Characterization of photocatalysts

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The crystal structure, phase features, and purity of the as-synthesized photocatalyts were analyzed by XRD. Fig. 1 demonstrates XRD patterns of as-obtained products. In the case of the Ag2CrO4, the diffraction peaks are attributed to (120), (200), (220), (002), (012), (122),

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(240), (222), (051), (400), (242), (213), (430) and (402) planes, which belong to orthorhombic phase of Ag2CrO4 (ICSD database No. 98-001-6298). The diffraction peaks in the pattern of ZnO can be related to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) crystal planes of wurtzite hexagonal phase (ICSD database No. 98-005-7478) [32-34]. For Ag2CrO4/ZnO nanocomposite, the diffraction peaks are well matched to (100), (002), 9

(101), (102), (110), (113), (200), (112) and (202) planes of wurtzite hexagonal crystalline ZnO and orthorhombic phase of Ag2CrO4 [35]. The diffraction peaks marked with “#” matched with the the (111), and (220) crystalline planes of metallic Ag. These two peaks can be attributed to metallic Ag (ICSD database No. 98-018-0878). These results reveal that the surface of Ag/Ag2CrO4/ZnO is not clean. Moreover, metallic Ag nanoparticles and

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Ag/Ag2CrO4 have been deposited on the surface of ZnO nanorods. When the diffraction peaks of the products are compared, impurities and other phases such as AgO, Ag2O and Ag2CrO4

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were not seen in the pattern.

To determine the surface chemical compositions and valance states of Ag/Ag2CrO4/ZnO, the

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XPS measurements performed and the results were illustrated in Fig. 2. In Fig. 2a, the survey

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XPS spectrum for Ag/Ag2CrO4/ZnO indicates that the composite mainly consists of Ag, Cr,

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Zn and O atoms and no any other elements can be detected. For Zn 2p spectrum, the two

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peaks located at 1022.2 eV and 1045.4 eV are attributed Zn 2p3/2 and Zn 2p1/2, respectively,

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proving Zn presents in the form Zn2+ chemical state on the nanocomposite surface (Fig. 2b) [36]. For Ag 3d spectrum, the two peaks at 368.0 and 373.9 eV are assigned to Ag 3d5/2 and

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Ag 3d3/2, respectively, indicating the presence of Ag+ species on the Ag/Ag2CrO4/ZnO (Fig. 2c). As shown in Fig. 2d, the Cr 2p XPS spectrum of the Ag/Ag2CrO4/ZnO nanocomposite

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draw the attention to two peaks at 578.5 eV and 588.5 eV, which correspond to characteristic peaks of Cr 2p3/2 and Cr 2p1/2, respectively, demonstrating the presence of Cr6+ within

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nanocomposite [37,38]. From Fig. 2e, the O 1s spectra can be divided into two conspicuous peaks around 530 and 531.5 eV. The peak located at 530 eV corresponds to lattice oxygen of nanocomposite, which may be related to both Cr–O and ZnO chemical bonding. The later peak is ascribed to the surface absorbed oxygen [36]. Consequently, the peaks for Zn2+, O2-, Ag+ and Cr6+ have slight shifts compared with the values in pure Ag2CrO4 and ZnO. All these

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findings distinctly confirm stronger interaction between these atoms in the Ag/Ag2CrO4/ZnO nanocomposite and heterojunction structure is formed. [37,38]. Furthermore, the determined XPS measured surface atomic percentages for Zn, O, Ag and Cr in the Ag/Ag2CrO4/ZnO nanocomposite were given in Table 1 and they were compared with EDS and AAS results.

detailed

morphologies

of

the

ZnO,

Ag2CrO4,

Ag/Ag2CrO4,

Ag2CrO4/ZnO,

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The

Ag/Ag2CrO4/ZnO were analyzed by FESEM and their images are displayed in Fig. 3. As

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shown in Fig. 3a, the morphologies of ZnO structures are irregular nanorods. Fig. 3b depicts

that Ag2CrO4 nanoparticles are roughly spherical structures with smooth surface. Fig. 3c displays the distribution of metallic Ag on Ag/Ag2CrO4 surface. For the Ag2CrO4/ZnO

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nanocomposite, Ag2CrO4 nanoparticles are observed around the ZnO nanorods (Fig. 3d). As a

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result, Fig. 3e shows Ag2CrO4 and metallic Ag nanoparticles have distributed around the ZnO

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nanoplates and nanorods. Similarly, TEM images at different magnifications (Fig. 4a-d)

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confirm that Ag/Ag2CrO4/ZnO heterostructures consist of metallic Ag nanoparticles, spherical

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Ag2CrO4 nanoparticles and ZnO nanorods. HRTEM image of Ag/Ag2CrO4/ZnO heterojunction is shown in Fig. S1 (Supplementary data). As can be seen in Fig. S1, the lattice

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fringes of ZnO are estimated to be 0.27 nm which corresponds to (100) lattice plane and lattice distance of 0.235 nm corresponds to the d-spacing of the (111) crystal plane of Ag

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[39,40]. This indicates the strong interfacial interaction between Ag/Ag2CrO4 and ZnO, which was useful to separate the photogenerated charge carriers within Ag/Ag2CrO4/ZnO

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structure in comparison with individual ZnO or Ag/Ag2CrO4 structures [37].

Table 1 illustrates EDS analysis results for the as-prepared photocatalysts to verify purity and element compositions of them. The results for the Ag2CrO4 illustrate Ag, Cr and O elements while it does Zn and O elements for ZnO. Ag/Ag2CrO4/ZnO was formed of Ag, Cr, O and Zn

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elements which demonstrated existence of Ag2CrO4 and ZnO particles. The amount of atomic Ag in the Ag/Ag2CrO4 and Ag/Ag2CrO4/ZnO composites is higher than that of Ag2CrO4 and Ag2CrO4/ZnO, respectively, as evidence of doping with silver. Furthermore, in the EDS results of the other products no other impurities were appeared [26]. Meanwhile, the metal content values for Ag/Ag2CrO4/ZnO were detected by AAS. The amounts of Ag, Cr and Zn

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content in Ag/Ag2CrO4/ZnO nanocomposite were determined to be 16.15%, 3.82% and 49.81%, respectively from AAS analysis results. According these results, the rates of changes

determined by AAS are compatible for Ag/Ag2CrO4/ZnO.

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of atomic percentages determined by EDS and XPS and weight percentages of the elements

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To obtain more structural information, FTIR spectra of ZnO, Ag2CrO4, Ag/Ag2CrO4,

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Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO nanophotocatalysts were supplied and the spectra is

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shown in Fig. 5. For the ZnO including products, the peak at 550 cm-1 is related to the

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stretching vibration of the Zn–O bond. For the products with Ag2CrO4, the peaks were

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observed at 860 and 910 cm-1, equaled to stretching vibrations of the Cr–O bond. For the Ag/Ag2CrO4/ZnO, the peaks belonging to Zn–O and Cr–O bonds are evidently seen in the

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same wave number for similar samples [5,41,42].

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Optical features of the nanophotocatalysts were supplied by UV–Vis DRS and the obtained data in the range of 200–800 nm are displayed in Fig. 6. ZnO shows a strong absorption only

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in the UV spectrum owing to its large band gap. Contrary to the ZnO photocatalyst, Ag/Ag2CrO4, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO nanocomposites show absorption in the visible region owing to Ag2CrO4 with narrow band gap. The electron-hole pairs are formed in photocatalysts when a photocatalyst absorbs a higher energy light than its band gap. Therefore, Ag/Ag2CrO4, Ag2CrO4/ZnO, Ag/Ag2CrO4/ZnO nanocomposites produce the large

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amount of charge carriers under visible light. Compared with Ag2CrO4, nanocomposite structures containing ZnO have higher band gap energies. The band gap energy of ZnO active in the UV region is 3.25 eV, for Ag2CrO4, Ag/Ag2CrO4, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO photocatalysts are 1.78, 1.76, 2.50 and 2.48, respectively.

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The investigation of the electronic properties of the photocatalysts was additionally evaluated

by Mott–Schottky analysis from capacitance measurements, which are generally used to

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analyze the carrier type and the state of the flat band potential experimentally [43]. Flat band potential of products were calculated using Mott-Schottky equation [44,45].

2

2

0 εε0 Nd

kT

) [(E-EFB )- e ] πr2 0

(3)

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N

C

=( e

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1

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wherein e0 is the electronic charge, ε the dielectric constant of ZnO, ε0 the permittivity of vacuum, Nd the donor density, V the applied potential, EFB the flat band potential, and kT/e0 is

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a temperature-dependent term [44]. The positive slope and negative slope of the MottSchottky (M-S) plots indicate n-type and p-type semiconductor, respectively. The flat band

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potential was detected from the x-intercept of the 1/C2 versus E plots. The M-S plots are

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shown in Fig. 7 for the photocatalysts. The slopes of the curves are positive, demonstrating the n-type semiconductors. To predict flat band potential (EFB), the linear portion of the data was extrapolated to 1/C2 = 0 [34]. The EFB for ZnO, Ag2CrO4, Ag/Ag2CrO4, Ag2CrO4/ZnO

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and Ag/Ag2CrO4/ZnO products determined from M-S plots were -0.64 V, 0.8 V, 0.52 V, 0.50 and 0.3 V (vs. Ag/AgCl), respectively, which equal to -0.35 V, 1.09 V, 0.81 V 0.79 V and 0.59 V (vs. NHE), correspondingly. Changing of negative values of the VFB in nanocomposites displays the better capability of the photocatalysts to provide the electron-

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hole separation in photocatalysis [45]. The valence energy levels were determined by the following equation:

EVB =ECB + Eg

(4)

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where EVB, ECB and Eg are the VB and CB potentials and band gap energies, respectively. The values of EVB for ZnO, Ag2CrO4, Ag/Ag2CrO4, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO are 2.6,

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2.87, 2.57, 3.29 and 2.97, respectively (vs. NHE).

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3.2. Photocatalytic activity

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Silver based photocatalysts have widespread practices in wastewater treatment and

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decomposition of dye contaminants. The photocatalytic efficiencies of the as-synthesized photocatalysts were determined by photodegradation of indigo carmine under visible-light.

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For comparison, the degradation efficiencies of ZnO, Ag2CrO4, Ag/Ag2CrO4, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO were also studied. The concentrations of IC after photodegradation

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were merely identified from the maximum absorbance by UV−Vis spectra. In the presence of

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different photocatalysts, the change in IC concentration versus to the irradiation time is displayed in Figure 8a. The blank experiments proved that insignificant IC was decomposed in the absence of nanophotocatalyst. The degradation of dye is mostly occurred by

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photocatalytic process in the presence of nanophotocatalysts under visible-light [24]. As can be seen from Fig. 8a, the degradation efficiency is only 35.8% within 30 min when ZnO is used as a photocatalyst under visible-light. About 73.47, 75.15, 96.22, and 99.73% of IC was decomposed

using

Ag2CrO4,

Ag/Ag2CrO4,

Ag2CrO4/ZnO

and

Ag/Ag2CrO4/ZnO,

respectively, as photocatalysts under the same irradiation time. Ag2CrO4 is an active

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photocatalyst in visible light. ZnO doping and the SPR effect of Ag nanoparticles increase the photocatalytic activity of Ag2CrO4. Among these composites, Ag/Ag2CrO4/ZnO demonstrates the greatest photocatalytic activity. The result is in agreement with the photodegradation of IC, which proves Ag/Ag2CrO4/ZnO composite is efficient visible-light nanophotocatalyst for the decomposition of dyes and contaminants. The kinetic analysis of IC degradation was

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confirmed by the first order reaction kinetics, called as Langmuir-Hinshelwood (L-H) model

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[24].

ln(C0/C) = kt

(5)

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where k is the apparent first-order rate constant, C0 and C are the concentrations of IC before

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irradiation and after irradiation time t. Fig. 8b displays the curves of ln (C0/C) against t. A

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clear linear correlation between ln (C0/C) and t means that the reaction is a typical first-order.

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The pseudo-first order rate constants of the photocatalyts are shown in Table 2. The

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Ag/Ag2CrO4/ZnO composites demonstrated the biggest rate constant (k=158.81 min−1) in all of photocatalysts for the photodegradation of IC. The great visible light photocatalytic

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efficiency of the Ag/Ag2CrO4/ZnO can be related to (i) the SPR of Ag nanoparticles, (ii) retardation of the recombination of the charge carriers by electron transport at the interface

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between Ag2CrO4 and ZnO, (iii) enlargement of ZnO into visible light region with absorption

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range Ag2CrO4 and Ag/Ag2CrO4 [24,46–48].

In order to investigate behavior of the photoexcited e--h+ pairs, PL spectra for the products were provided under excitation at 300 nm. The reason for choosing 300 nm excitation wavelength is reported to be appropriate for Ag2CrO4 composites in the literature [5,23,42,49]. The results are illustrated in Fig. 9(a). Intensity of a PL spectrum displays rate of

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recombination of the e--h+ pairs. As can be shown in Fig. 9a, the peak intensity of Ag/Ag2CrO4/ZnO is weaker than the others. A weaker intensity of peak is associated with retarding recombination of charge carriers and elongation of lifetime of theirs, and the advanced photocatalytic performance of Ag/Ag2CrO4/ZnO [18,42,49]. The PL measurements support the photocatalytic degradation data. Furthermore, as can be seen in Fig. 9b, the

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photocurrent transient response curves of photocatalysts demonstrated the achievement of the

charge separation procedure. In Fig. 9b, it is seen that the photocurrent intensities rapidly

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increase when visible light is turned on, and then decrease rapidly when the lamp is turned off [38]. Ag/Ag2CrO4/ZnO has exhibited the highest advanced photocurrent response among other photocatalysts. A higher photocurrent response corresponds a higher charge separation

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performance, so leading to a greater photocatalytic efficiency [38,50]. Both PL and

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photocurrent measurements confirm the high charge transfer and recombination inhibition of

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Ag/Ag2CrO4/ZnO heterostructure compared to other photocatalysts.

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The enhancement in the amount of reactive oxygen species in the amount of reactive oxygen species, results in influential separation of electron-hole pairs. To detect the ROS, the trapping

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experiments were performed in the photocatalysis process of Ag/Ag2CrO4/ZnO. The obtained results are exhibited in Fig. 10. Benzoquinone (BZQ) as the superoxide radical (•O2-)

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scavenger, tert-butanol (t-BuOH) as hydroxyl radical (•OH) scavenger and disodium ethylenediaminetetraacetate (EDTA-Na2) as hole (h+) scavenger were used [51]. By the

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addition of these scavengers into the photocatalytic system, the photodegradation performances of IC were obviously decreased. According to these results, •O2-, •OH and h+ radicals have major effect on photodegradation. The generation rates of the radicals can be summarized as follows: •O2- > h+ >•OH [18,52].

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3.3. Proposed photocatalytic mechanism

When the results have been taken into account, the possible mechanism of the photodegradation of IC for Ag/Ag2CrO4/ZnO nanophotocatalyst has been proposed. Scheme 1 depicts a profile of the photodegradation process. In this process, the photogenerated electrons

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from the plasmon-induced Ag0 nanoparticles could flow to the conduction band (CB) of ZnO rather than Ag2CrO4. Since ZnO and Ag2CrO4 are n-type semiconductors, the fermi levels are

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near to the CB levels. After their contact with each other, a n-n heterojunction with the same level of fermi occurs. The fermi energy level of Ag2CrO4 is more positive than ZnO, so electrons must migrate from ZnO to Ag2CrO4, leaving positively charged holes and negative

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charged electrons in the heterojunction surfaces on ZnO and Ag2CrO4, respectively. But,

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under visible light, Ag and Ag2CrO4 would be induced to fabricate photogenerated e--h+ pairs

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because of the dipolar characteristic of the SPR of Ag nanoparticles and the visible light

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absorption ability supplied by Ag2CrO4, respectively [5,26,49]. Therefore, the photoexcited

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electrons of Ag2CrO4 could migrate from Ag2CrO4 to Ag then to the CB of ZnO, delaying the charge carriers recombination. The electrons in the CB of ZnO reduce adsorbed molecules of -

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O2 to form superoxide radical anions •O2 , since the CB potential of ZnO (-0.35 V) is slightly -

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more negative than potential of O2/•O2 (-0.33 eV). The photogenerated holes in VB of Ag2CrO4 also can oxidize H2O to hydroxyl radicals •OH since the VB potential (2.87 V) is more positive than •OH/H2O potential (2.27 V) [26,39,49,51–57]. These radical species such -

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as •O2 and •OH have important roles in the process of IC degradation. These strong radicals thrust into dye molecules and entirely decompose the dye to H2O, CO2 and mineral acids [32,36,58]. The possible mechanisms as follows:

Ag2CrO4 + hv → Ag2CrO4(e−) + Ag2CrO4(h+)

(6) 17

Ag0 + hv → Ag∗

(7)

Ag∗ + ZnO → ZnO(e−) + •Ag+

(8)

-

ZnO(e−) + O2 → •O2 + ZnO

(9)

•Ag+ + H2O → HO• + H+ + Ag

(10)

Ag2CrO4(e−) + Ag → Ag2CrO4 + Ag(e−)

(11)

-

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e− + O2 → O2 •

(12)

Ag2CrO4(h+) + H2O → HO• + H+ + Ag2CrO4 -

O2 • + HO• + indigo carmine → CO2 + H2O + mineral acids

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(13)

(14)

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So, the heterostructure and electron motion of the Ag/Ag2CrO4/ZnO nanophotocatalyst can

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influentially hinder recombination of the photogenerated charge carriers and advance the

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A

degradation efficiency.

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Total organic carbon (TOC) analysis was carried out for detect the degree of mineralization of the IC reached throughout the photodegradation. Since the disapperance of dye colour is not

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sufficient evidence for detect the full mineralization of the dye, TOC analysis is important. This analysis was performed on the dye solutions with Ag/Ag2CrO4/ZnO for specific time

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intervals. As can be seen in Fig. 11, the color removal of dye was quicker than the degree of mineralization. The highest TOC removal was almost 96% for 120 min. This may be due to the division of the azo bond. The high TOC value may be related to the hardship in converting

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the N atom of the dye into oxidized nitrogen derivatives. As the result, the dye molecules were converted to other intermediate products which still presence in the solution regardless of the dye decolourization. For complete mineralization of the dye, a 120 minute degradation process is needed [59–61].

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To analyze the photostability and reusability of the Ag/Ag2CrO4/ZnO, five repetitive photodegradation experiments. After every cycle, the Ag/Ag2CrO4/ZnO was separeted by centrifugation and suspended into the IC solution. As can be displayed Fig. 12a, there no important changes in the photocatalytic efficiency of Ag/Ag2CrO4/ZnO, exhibiting it has great stability and reusability. Furthermore, the crystal structure and purity of the Ag/Ag2CrO4/ZnO

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was analyzed by XRD (Fig. 12b) after recycling measurements. As can be seen Fig. 12b, when the diffraction peaks are compared, a few low intensity peaks are disapeared and the

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other peaks are not changed. As a result, crystal structure of Ag/Ag2CrO4/ZnO remains stable after five repetitive experiments.

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4. Conclusions

followed

by

photoreduction.

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precipitation,

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Photocatalytically active two component system has been effectively produced via a The

Ag/Ag2CrO4/ZnO

nanocomposite

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photocatalysts display enhanced photocatalytic efficiency compared with pristine Ag2CrO4, Ag/Ag2CrO4, Ag2CrO4/ZnO under visible-light. Despite after five cycles, the photocatalytic

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performance does not display any major decreases. When Mott Schottky curves are taken into consideration, the CB and VB energy levels of ZnO are more negative than those of Ag2CrO4,

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so electrons are expected to flow ZnO to Ag2CrO4. However, Ag and Ag2CrO4 produce charge carriers because of SPR of Ag and the light absorption capability of Ag2CrO4 under

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visible light. The increased photocatalytic efficiency of Ag/Ag2CrO4/ZnO may attribute the synergistic effect between Ag2CrO4/ZnO based heterostructured semiconductor photocatalysts and SPR of Ag nanoparticles. Thus, we believe that Ag/Ag2CrO4/ZnO will be an ideal photocatalyst for the decomposition of dyes and other organic contaminants under visible light. It can be suggested that this type of Ag/Ag2CrO4/ZnO plasmonic photocatalyst may

19

provide alternative insights for producing Ag NPs-based heterostructured semiconductor photocatalysts in wastewater pollution eliminating. To develop the new photocatalyst and to apply in the degradation processes, we still study on the synthesizing different photocatalysts and will use these photocatalysts for the different applications such as colorless phenol or

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other pollutants as probe, and we will report the next investigation in the future.

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Acknowledgments

This investigation has been supported by the Scientific Research Projects Commission of Sakarya University (Project number: 2016-02-04-042, 2016-02-04-010, 2017-50-02-010 and

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2018-3-12-49). M.Ö. thanks Turkish Academy of Sciences (TUBA) for partial support.

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A. Shet, V.S. K, ScienceDirect Solar light mediated photocatalytic degradation of

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[1]

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Figure Captions

Fig. 1. XRD patterns of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO. Fig. 2. XPS spectra of Ag/Ag2CrO4/ZnO nanocomposite: (a) XPS survey spectra, (b) Zn 2p spectra, (c) Ag 3d spectra, (d) Cr 2p spectra and (d) O 1s spectra.

Ag/Ag2CrO4/ZnO photocatalyts.

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Fig. 4. TEM images of Ag/Ag2CrO4/ZnO at different magnifications.

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Fig.3. FESEM images of (a) ZnO, (b) Ag2CrO4, (c) Ag/Ag2CrO4, (d) Ag2CrO4/ZnO and (e)

Fig. 5. FTIR spectra of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO. Fig. 6. Plots of [F(R∞)hν]2 versus hν for the determination of band gap energies of Ag2CrO4,

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Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO.

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Fig. 7. Mott–Schottky curves of the Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and

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Ag/Ag2CrO4/ZnO.

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Fig. 8. (a) Photocatalytic degradation of samples and (b) pseudo-first-order reaction rate

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constants in visible light.

Fig. 9. (a) PL emission spectra of the as-prepared photocatalysts with an excitation

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wavelength of 300 nm; (b) transient photocurrent density of photocatalysts under visible light (λ > 420 nm) irradiation.

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Fig. 10. Photodegradation efficiency of IC over different scavengers under visible light. Fig. 11. Percent TOC removal by Ag/Ag2CrO4/ZnO photocatalyst.

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Fig. 12. (a) Photodegradation efficiency of IC over Ag/Ag2CrO4/ZnO in different recycle runs, (b) XRD spectra of Ag/Ag2CrO4/ZnO after recycling measurements. Scheme 1. Schematic illustration showing photocatalytic mechanism of Ag/Ag2CrO4/ZnO under the visible light irradiation.

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Table 1. The EDS analysis of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO. Table 2. Kinetics data of IC photodegradation in the presence of Ag2CrO4, Ag/Ag2CrO4, ZnO,

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Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO.

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Fig. 1. XRD patterns of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO.

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Fig. 2. XPS spectra of Ag/Ag2CrO4/ZnO nanocomposite: (a) XPS survey spectra, (b) Zn 2p

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spectra, (c) Ag 3d spectra, (d) Cr 2p spectra and (d) O 1s spectra.

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Fig. 3. FESEM images of (a) ZnO, (b) Ag2CrO4, (c) Ag/Ag2CrO4, (d) Ag2CrO4/ZnO and (e)

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Ag/Ag2CrO4/ZnO photocatalysts.

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Fig. 4. TEM images of Ag/Ag2CrO4/ZnO at different magnifications.

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Fig. 5. FTIR spectra of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO.

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Fig. 6. Plots of [F(R∞)hν]2 versus hν for the determination of band gap energies of (a) ZnO

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(b) Ag2CrO4/ZnO, (c) Ag/Ag2CrO4/ZnO, (d) Ag2CrO4 and (e) Ag/Ag2CrO4.

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Fig. 7. Mott–Schottky curves of the Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and

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Ag/Ag2CrO4/ZnO.

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Fig. 8. (a) Photocatalytic degradation of samples and (b) pseudo-first-order reaction rate

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constants in visible light.

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Fig. 9. (a) PL emission spectra of the as-prepared photocatalysts with an excitation wavelength of 300 nm, (b) transient photocurrent density of photocatalysts under visible light

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(λ > 420 nm) irradiation.

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IP T SC R

A

CC E

PT

ED

M

A

N

U

Fig. 10. Photodegradation efficiency of IC over different scavengers under visible light.

39

IP T SC R

A

CC E

PT

ED

M

A

N

U

Fig. 11. Percent TOC removal by Ag/Ag2CrO4/ZnO photocatalyst.

40

IP T SC R

Fig. 12. (a) Photodegradation efficiency of IC over Ag/Ag2CrO4/ZnO in different recycle

A

CC E

PT

ED

M

A

N

U

runs, (b) XRD spectrum of Ag/Ag2CrO4/ZnO after recycling measurements.

41

IP T SC R

Scheme 1. Schematic illustration showing photocatalytic mechanism of Ag/Ag2CrO4/ZnO

A

CC E

PT

ED

M

A

N

U

under the visible light irradiation.

42

Table 1. The EDS analysis of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO.

Ag2CrO4

Ag/Ag2CrO4

ZnO

Ag2CrO4/ZnO

(EDS)

Ag/Ag2CrO4/ZnO EDS

XPS

AAS

(at.%)

(at.%)

(w.%)

IP T

Elements/(at.%)

43.01

42.8

39.36

33.91

37.72

39.92

29.87

Ag

34.17

35.18

-

8.63

9.40

7.63

16.50

Cr

22.82

22.02

-

6.64

6.54

6.05

3.82

Zn

-

-

60.64

50.82

46.34

46.4

49.81

Total

100.00

100.00

100.00

100.00

100.00

100

100.00

A

CC E

PT

ED

M

A

N

U

SC R

O

43

Table 2. Kinetics data of IC photodegradation in the presence of Ag2CrO4, Ag/Ag2CrO4, ZnO, Ag2CrO4/ZnO and Ag/Ag2CrO4/ZnO.

Samples

Rate constants, k (min−1 )

Photodegradation ratios

Regression coefficients,

after 30 min irradiation

R2

73.47

Ag/Ag2CrO4

41.40x10-3

75.15

ZnO

5.65x10-3

35.80

Ag2CrO4/ZnO

108.15x10-3

96.22

Ag/Ag2CrO4/ZnO

158.81x10-3

99.73

0.9834 0.9441

N A M ED PT CC E A

44

0.9413

SC R

9.66x10-3

U

Ag2CrO4

IP T

(%)

0.9698 0.8365