Facile Synthesis and Recyclability of Thin Nylon Film-Supported n

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Facile Synthesis and Recyclability of Thin Nylon FilmSupported n-Type ZnO/p-Type AgO Nano Composite for Visible Light Photocatalytic Degradation of Organic Dye 2

Hairus Abdullah, Dong-Hau Kuo, Yen-Rong Kuo, Fu-An Yu, and Kuo-Bin Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12153 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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The Journal of Physical Chemistry

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Facile Synthesis and Recyclability of Thin Nylon Film-Supported n-Type

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ZnO/p-Type Ag2O Nano Composite for Visible Light Photocatalytic

3

Degradation of Organic Dye

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Hairus Abdullah1, Dong-Hau Kuo*,1, Yen-Rong Kuo1, Fu-An Yu1, Kou-Bin Cheng2

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1

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No.43, Sec. 4, Keelung Road, Taipei 10607, Taiwan

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Department of Fiber and Composite Materials, Feng Chia University, No. 100 Wenhwa Road, Taichung 40724, Taiwan

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*Fax: +011-886-2-27303291. E-mail: [email protected] . 1

ACS Paragon Plus Environment


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ABSTRACT: ZnO/Ag2O nano composite powder had been successfully synthesized by co-

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precipitation method and been embedded into thin nylon film as well as tested for the

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degradation of organic dye solution under visible light illumination. After being embedded into

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nylon film, the as-prepared nylon-ZnO/Ag2O composite hybrid film was very stable and reusable

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for six consecutive runs without any refreshing treatments. Although the photocatalytic activities

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of the hybrid films were lower than those of nano composite powder, the reusability of hybrid

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film was more stable than that of nano composite powder. The facile synthesis and

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photocatalytic performances of ZnO/Ag2O nano composite catalyst powders and nylon-

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ZnO/Ag2O hybrid composite films were investigated in this work.

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1. INTRODUCTION

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The toxic organic dyestuffs from textile, paper, pulp, tannery, and pharmaceutical industries that

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dumped into natural water resources without any treatment are the urgent concerns for the human

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living environment nowadays. Discharging of dye-contained effluents without going through a

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treatment system before dumping into aquatic environment is an unacceptable manner, since

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their breakdown products can be more toxic, carcinogenic and mutagenic to aquatic life, mainly

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due to carcinogens, such as benzidine, naphthalene, and other aromatic compounds.1-3 The

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treatments of dye-containing wastewater are very crucial, since without sufficient treatments,

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dyes can remain in the environment for a long period of time, i.e., 92 years are needed to

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hydrolyze Reactive Blue 19 at pH 7 and 25°C.4 To remediate the non-biodegradable pollutants,

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adsorption and coagulation methods had been used, however those methods caused another

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secondary solid pollutant in wastewater and further treatment was required to alleviate it.5 Many

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research articles suggested that photosensitized degradation by using semiconductor materials

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could be useful for discoloration of the organic dye pollutants.6-10

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Among the semiconductor photocatalysts, TiO2 and ZnO are known as good photocatalysts

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for alleviating many environmental contaminants due to their high stability, low toxicity, low

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cost, and photosensitivity. In some works, photocatalytic activities of ZnO were found to be as

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effective as TiO2 in visible light photocatalytic degradation of organic compounds in aqueous

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solution with a low cost.11,12 The advantages of using ZnO as photocatalyst were its larger

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fraction of solar light spectrum absorbance,6 easier crystallization at lower processing

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temperature, and anisotropic growth.13-14 Therefore, it is preferable to use ZnO for antibacterial

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applications,15 self-cleaning materials,16 and water splitting.17-18

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To modify wide bandgap n-type ZnO19 become a visible light active material, it should be

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coupled with a p-type and a lower band gap material that acts as a sensitizer to excite the photo-

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induced electron and hole pairs. The concept of forming the p-n junction has been applied in

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other research works and the efficient separation of the photo-induced electron and hole pairs has

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been demonstrated.20-27 One of the famous p-type materials is Ag2O that has been widely used in

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industrial applications such as electrode materials, cleaning agent, catalyst for alkane activation,

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colorant, olefin epoxidation, and preservation.28,29 The band gap values of Ag2O, varying from

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1.2 - 1.6 eV, depend on the preparation procedure as reported by other research works.30-32 One

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of the Ag2O advantages as a photocatalyst is its mechanism in photocatalytic activity. There

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were not only photogenerated O2- and OH• radical as common agents that involved in

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photocatalytic degradation of organic pollutants but also •O3- (ozone radical) and other

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superoxide anion radical involved in the photocatalytic mechanisms. It is believed that the

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recombination rate of the photo-generated electron and hole pairs is retarded since they are used

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to produce a strong oxidizer of ozone radical therefore they can work together in degrading

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organic pollutant.30 In addition, the coupling with other p-type nanoparticles makes ZnO easier

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to be recycled after using for photodegradation of organic pollutant, since to the best of our

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knowledge, pure ZnO nanoparticle is hard to be collected after use. It may due to the high

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wettability of ZnO surfaces in aqueous solution. However, after depositing Ag2O on ZnO

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surfaces, the nano composite particles were easier to be collected so that the recyclability of

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composite powders was considerably improved.

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Although photocatalyst in the form of powder can have a higher efficiency in degrading dye

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pollutants, it also suffers from some drawbacks in: (i) low utility of sunlight due to the limited

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penetration of sunlight into deep water,33,34 (ii) time consuming and high cost for the post4


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treatment separation processes35,36 and (iii) the spreading of catalyst powder may harm human

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being.37 To overcome these problems, immobilization of photocatalyst powder on various

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substrates has been done in many research works.38-42 The efficiency and surface area of

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photocatalyst powder are sacrificed by the photocatalyst immobilization in a specific matrix. The

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photocatalytic performance of those immobilized photocatalysts could be lower compared to the

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performance of photocatalysts in powder form.

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Polymer supported photocatalyst is one of the good alternatives for solving the post-

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treatment problems and has been widely used in many research works. D.K. Hwang et al. studied

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the immobilized Au-TiO2 on polycarbonate that was casted on glass substrate for acetaldehyde

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photo decomposition.40 J. Zheng et al. immobilized TiO2 on cellulose matrix through the

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electrostatic and hydrogen bonding to prevent the leaching of catalyst powders, leading to a

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portable photocatalyst for easy recyclability.41 E. Pajootan et al. used the carbon nanotube-coated

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electrodes and the immobilized TiO2 for dye degradation in a continuous photocatalytic-electro-

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Fenton process.42 There are many advantages of using polymer as immobilized substrate such as

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low cost and ease of availability, simple preparation process, good affinity with catalysts and

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pollutants, a high specific surface area, no photocatalyst leaching, long-term stability, and easy

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utilization.43 The polymer that has been used as substrate also provides a buoyancy force to the

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photocatalyst for floating near water surface and enhanced the efficiency of photocatalytic

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degradation of dye solution.

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In this work, the n-type ZnO/p-type Ag2O photocatalyst was synthesized in the forms of

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powders and thin hybrid films and used to degrade Methylene Blue (MB) dye, which was used as

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a model of organic pollutant under visible light illumination. Similar work with the system of

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ZnO/Ag2O had also been done,44 however their photocatalytic ability of composite catalyst was 5



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degraded after exposing to the UV light during photo degradation of organic dye. In the present

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work, the same catalyst system was used and it was quite stable and had a better efficiency in

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visible light photocatalysis. The effect of the different amounts of Ag2O on the ZnO/Ag2O

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composite powder and the nylon-ZnO/Ag2O hybrid composite film had not been studied yet. The

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objectives of this work are to optimize photocatalytic activity of ZnO/Ag2O by varying the

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deposition amount of Ag2O on ZnO surfaces and to prevent the leaching of catalyst powder

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during photodegradation thus improving the recyclability of photocatalyst by embedding

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ZnO/Ag2O nano composites into nylon film. The amounts of 10%, 20%, and 30% Ag2O were

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deposited on ZnO to obtain the composite powders with different amounts of Ag2O to degrade

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MB dye solution. Those composite powders with the best photocatalytic activity were chosen to

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be imbedded into nylon film to improve the recyclability and to avoid the leaching of catalyst

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powder. The photocatalytic activities of ZnO/Ag2O composite catalysts in the forms of powder

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and thin hybrid film were demonstrated in this work.

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2. EXPERIMENTAL PROCEDURE Materials. In this work, chemical compounds are commercially available without any purification treatment.

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Preparation of ZnO/Ag2O Powder. Powder samples were prepared by co-precipitation

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method. Depositing Ag2O on ZnO nanoparticles was executed as follows: 0.035 g NaOH in 150

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ml DI water was first prepared. 1 g of the commercially as-received ZnO was then dispersed in

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AgNO3 (0.1466 g in 150 ml DI water) aqueous solution in a different vessel followed by treating

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in sonication bath for 1 h. The reaction was initiated by slowly adding the diluted NaOH solution

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into the mixed solution of ZnO and AgNO3 under vigorous stirring. The reaction solution was 6


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stirred for 1h at room temperature until the color of solution changed to brown color. The

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obtained precipitate was then washed three times by using alcohol and collected by

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centrifugation. At last, the precipitate product of ZnO/Ag2O composite powder was dried in a

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rotary evaporator. The as-prepared ZnO/Ag2O powder contains 10% wt of Ag2O. To obtain

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ZnO/Ag2O composite powder with 20% and 30% Ag2O, the appropriate amounts of AgNO3 and

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NaOH were introduced into the reaction solution with the same procedure.

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Fabrication of Nylon/ZnO/Ag2O Hybrid Film. To prepare nylon-ZnO/Ag2O hybrid film,

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650 mg nylon pellet (Elvamide® Nylon 8061) was first dissolved in 60 ml ethanol by heating at

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80 ⁰C for 2h. Three different compositions of nylon-ZnO/Ag2O hybrid films were prepared by

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adding 33 mg, 50 mg and 66 mg ZnO/Ag2O into nylon solution to form a slurry solution. The

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total weight for each composition (nylon + nano composite powder) was set to 200 mg. To form

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the hybrid films, the slurry solution of nylon and nano composite powder was casted into Petri

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dish and dried at room temperature for overnight. Finally, the hybrid films were carefully peeled

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off by immersing the film in water and dried in the air at room temperature. All of the steps for

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the fabrication of nylon-ZnO/Ag2O hybrid films are shown in Figure 1.

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Figure 1. Fabrication of ZnO/Ag2O nano composite catalysts and their hybrid films

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Characterization. The surface and cross sectional morphologies of hybrid film as well as

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surface morphologies of ZnO/Ag2O catalyst powder were examined by field-emission scanning

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electron microscopy (FE-SEM, JSM 6500F, JEOL, Tokyo, Japan). The element mapping in

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composite catalyst powder was evaluated by STEM(Tecnai F20 G2, Philips, Netherlands).

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Powder X-ray diffraction (XRD) pattern of ZnO/Ag2O catalyst powder was recorded by Bruker

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D2-phaser diffractometer using Cu Kα radiation with a wavelength of 1.5418 Å. The UV-Vis

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diffuse reflectance spectra (DRS) and peak absorbance of MB dye degradation at the wavelength

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of 655 nm were recorded using a Jasco V-670 UV-Visible-Near IR spectrophotometer. The

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weight losses of pure nylon film and hybrid composite films were analyzed using a

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thermogravimetric analyzer (TA Instrument Q500, New Castle, DE, USA) at a heating rate of 20

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⁰

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Jasco FP-8500 UV-Vis spectrometer.

C min−1 up to 600◦C in air. The photoluminescence (PL) emission spectra were examined by a

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Photodegradation Experiments. The MB dye photodegradation experiments were

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conducted with composite catalyst in the form of powder and hybrid composite film. The

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photocatalytic activities of ZnO/Ag2O powders and its hybrid composite films were tested for

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their ability to degrade 100 ml of 10 ppm and 5 ppm MB dye solution, respectively under visible

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light illumination. Photodegradation experiments were done in a reactor equipped by an air

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channel and air was continuously pumped via the air channel to the reactor during the

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photodegradation testing to provide enough oxygen for MB dye degradation. The halogen lamp

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of 150 W that was used as source of visible light was jacketed by the water-cooled quartz glass

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vessel to avoid the overheating from halogen light. To test the MB dye photodegradation by

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using nano composite powders, the composite catalyst powder was dispersed into dye solution

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and kept in a dark condition under vigorous stirring for 30 min before illuminated by halogen 9



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lamp to ensure the adsorption and desorption equilibrium between composite catalyst and dye

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solution. As pure nylon film had high adsorption of dye, the hybrid films were immersed in 50

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ppm MB dye solution for 12 h to ensure the nylon was saturated with dye. This procedure was

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done to confirm the MB dye solution was degraded by photocatalytic activity instead of

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adsorption mechanisms. After drying at room temperature, the dye-saturated hybrid films were

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then immersed into 5 ppm dye solution under vigorously stirring for 30 min before illuminated

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by halogen light. During the photodegradation testing of catalyst powder and hybrid film, the

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halogen lamp was immersed into dye solution. The surface of hybrid film was arranged to face

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the light source in order to have effective light illumination. The time when the catalyst was

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added into dye solution was set to t = -30 min, meanwhile the time when the light was turned on

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was set to t = 0 min. To monitor the degradation of MB dye, 4.5 ml aliquots taken from dye

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solution at different time intervals during the photodegradation experiments were monitored by

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UV-Vis absorbance intensity at 655 nm.

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3. RESULTS AND DISCUSSION

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X-Ray Diffraction Pattern of As-prepared ZnO/Ag2O Photocatalyst. Figure 2 shows the

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XRD patterns of ZnO, Ag2O, and ZnO/Ag2O composite powders before and after dye

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degradation. The peaks of ZnO are representing the hexagonal wurtzite-type ZnO with lattice

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constants of a = 3.249 Å and c = 5.206 Å and well agreeing to the peaks listed in the reference

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profile of PDF#65-3411. The Ag2O peaks are corresponding to the reference of PDF#41-1104

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with the lattice constant of 4.726Å. All the peaks for ZnO/Ag2O nano composite in XRD pattern

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are contributed only from ZnO and Ag2O nanoparticles. After four consecutive tests for

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photodegradation, two XRD peaks of Ag2O were disappeared and two peaks of Ag as indicated 10


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in Figure 2 had shown up. These peak variations indicate that after illuminated by visible light,

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the Ag+ atoms in the lattice reacted with photogenerated electron to form the reduced Ag

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nanoparticles.45

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Figure 2. XRD pattern of ZnO, Ag2O, ZnO/Ag2O and the used ZnO/Ag2O nano composite powders.

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Figure 3. TEM images of (a) low and (b) high magnifications of ZnO/Ag2O nano composite catalysts.

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Morphology and Microstructure Analyses of ZnO/Ag2O Composite Powder. Figure 3

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shows the TEM images of low and high magnifications of as-prepared ZnO/Ag2O nano

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composite. The nano composite powder contained the irregular shape particles with the size

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range between 20 nm – 70 nm. Due to the much smaller Ag2O nanoparticles compared to the

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ZnO nano particles, it made the interfaces between them become very stable and not easy to be

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damaged under ultrasonication treatment. The bigger particles in Figure 3 are ZnO with Ag2O

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nanoparticles decorated on their surfaces. It showed that the size of Ag2O nanoparticle was less

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than 15 nm and there was no Ag2O aggregation outside the ZnO particles. Some of the Ag2O

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nanoparticles were very small (less than 5 nm) and coated on ZnO surfaces.

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Figure 4. (a) HAADF image of ZnO/Ag2O nano composite catalysts with their element mapping of (b) Zn, (c) Ag, and (d) O as well as (e) interfaces of ZnO and Ag2O (smaller particle) nanoparticles with (f) their selected area electron diffraction pattern (SAED). 13



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Panel a, b, c and d of Figure 4 show the HAADF image and its element mapping images of

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zinc, silver, and oxygen, respectively. HAADF image showed that ZnO nanoparticles were

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coated with tiny Ag2O nano particles on their surfaces. The STEM element mapping confirmed

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the positions of Zn, Ag, and O elements on nano composites. The nano composites contained

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ZnO as a dominant component and their surfaces were covered by Ag2O nanoparticles. Figure 4e

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and 4f show the HRTEM image for the interface between ZnO and Ag2O nanoparticles and the

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selected area diffraction pattern (SAED), respectively. Figure 4f simultaneously shows the ZnO

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and Ag2O diffraction spots. The typical diffraction spots of the ZnO hexagonal structure were

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shown up together with other weaker spots that related to Ag2O diffraction spots.

241 0.5

(a) 242

8.00E-038

(b)

0.4

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ZnO; Eg = 3.20 eV Ag2O; Eg = 1.30 eV

6.00E-038

0.3 (α hv)2

Absorbance

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ZnO Ag2O

0.2

244 4.00E-038

ZnO/Ag2O

245 2.00E-038

0.1

246 0.0

300

400

500

600

Wavelength (nm)

700

800

0.00E+000

1

247

2

3

4

hv (eV)

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Figure 5. (a) Diffuse reflectance spectra (DRS) of ZnO, Ag2O and ZnO/Ag2O nano composite powders as well as (b) the ZnO and Ag2O bandgaps determination by absorbance spectrum fitting.

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Diffuse reflectance spectra and bandgap measurement of ZnO/Ag2O nano composite

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powder. Figure 5a simultaneously shows the diffuse reflectance spectra (DRS) of ZnO, Ag2O,

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and ZnO/Ag2O nano composite powders. ZnO nanoparticle had a strong absorbance of light

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from the UV light region until 400 nm. Ag2O nanoparticle had a strong and continuous 14


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absorbance of light in the UV and visible light regions. The coupling effects of ZnO and Ag2O

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make the nano composite powder increase the light absorbance in visible light region and have a

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little red shift in the absorption edge. Tauc plot was determined by the formula of αhν = A(hν –

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Eg)m for Eg> hν and αhν = 0 for Eg< hν. In where α is the material optical absorbance, hν is the

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photon energy, Eg is bandgap energy value, and m = 0.5 and 2 for materials with direct and

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indirect allowed transition bandgap, respectively.19 ZnO and Ag2O had direct allowed transition

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bandgap with the m value of 0.5. The tauc plot was delineated in Figure 5b with the optical

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bandgaps of ZnO and Ag2O which were respectively calculated to be 3.2 eV and 1.3 eV. The

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measured bandgap energy values of ZnO and Ag2O were closed to the reported works.46,47

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Photocatalytic Activities of ZnO/Ag2O Composite Powder and its Reusability.

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Photocatalytic activities of 10%, 20%, and 30% Ag2O-loaded ZnO/Ag2O nano composite

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catalysts were confirmed by degradation of MB dye under visible light illumination. There was

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no degradation of MB dye under illumination of visible light without any catalyst added in the

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dye solution. The catalyst powder of 20 mg was used to test its photocatalytic capability in

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degrading 100 ml of 10 ppm MB dye solution. After adding the catalyst into dye solution at t = -

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30 min, the catalyst-dispersed solution was stirred at a dark condition and it was found the

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catalyst at t = 0 min lowered the concentration of dye solution by 20% as shown in Figure 6a.

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Figure 6a also shows photodegradation of 100 ml of 10 ppm MB dye solution by using pure

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Ag2O, pure ZnO, and 10%, 20%, and 30% Ag2O-loaded ZnO/Ag2O composite catalysts. The

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results showed the synergetic effect of ZnO/Ag2O nano composite to enhance photocatalytic

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activities. Almost 100% MB dye solution could be degraded in 20 min in the presence of

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ZnO/Ag2O photocatalyst under visible light illumination. The inset in Figure 6 shows the

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decreasing peak intensity of MB dye solution at 655 nm in UV-Vis-NIR spectrophotometry after 15



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adding 20% Ag2O-loaded ZnO/Ag2O composite catalyst to MB dye solution under visible light

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illumination. The photodegradation kinetic reaction satisfied the pseudo-first order kinetics that

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could be described as ln(Co/C) = kt, where k was kinetic constant and t was the irradiation time.

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Since the value of ln(Co/C) is the function of irradiation time (t), therefore k will provide a

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specific constant for determining performances of different reactions. As calculated from the

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photodegradation in Figure 6a, the kinetic constants (k) of pure ZnO, Ag2O, and 10%, 20% and

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30% Ag2O-loaded ZnO/Ag2O composite catalysts were 0.05438 min-1, 0.1200 min-1, 0.1699 min-

286

1

, 0.1816 min-1, and 0.1845 min-1, respectively. The kinetic constants indicated the 20% - 30%

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Ag2O loading in composite catalysts had better performance in MB dye photodegradation. These

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results revealed that the photocatalytic performances of composite catalyst with the composition

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of 20% Ag2O improved 3.3 times that of commercial ZnO and 1.5 times that of Ag2O.

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Figure 6. Photocatalytic activities of (a) 10%, 20% and 30% Ag2O-loaded ZnO/Ag2O composite catalyst powder and (b) 33 mg, 50 mg, and 66 mg ZnO/20%Ag2O-loaded nylon film in degrading MB dye under visible light illumination.

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In order to test the reusability of ZnO/Ag2O composite catalyst, the 20% Ag2O-loaded

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ZnO/Ag2O powder was prepared for four consecutive runs by degrading the same concentration 17



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of 10 ppm MB dye solution under visible light illumination. Figure 7a shows the photocatalytic

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performance of 20% Ag2O-loaded ZnO/Ag2O composite catalyst in reusability testing. The

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results indicated after first run, the photocatalytic activities of composite catalyst was quite stable

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in degrading MB dye solution, although during the photodegradation, some of Ag2O

300

nanoparticles attached to ZnO was photo decomposed to Ag nanoparticles, as confirmed from its

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XRD pattern. The more positive potential of Ag+/Ag (0.7991 V vs SHE) compared to O2/HO2 (-

302

0.046 V vs SHE) caused photogenerated electrons were preferably drifted to Ag+ to form Ag

303

nanoparticles.46 The first run of reusability testing was faster than others due to the more ozone

304

anion radical formation from Ag2O lattice decomposition, that will be further elucidated in other

305

section. The great photocatalytic efficiencies of ZnO/Ag2O nano composite were due to the

306

preparation process of Ag2O that provided a good contact on ZnO surfaces to enhance electron

307

transfer between two oxides and the synergetic effect of n-type ZnO and p-type Ag2O.

18


Page 19 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


308 309

Figure 7. Reusability of (a) ZnO/20%Ag2O nano composite catalyst powder and (b) 50 mg

310

ZnO/20%Ag2O-loaded nylon film for MB dye photodegradation under the visible light

311

illumination.

312

Morphology and Microstructure Analyses of Nylon-ZnO/Ag2O Hybrid Films. Figure 8

313

shows FE-SEM cross sectional and surface images of nylon-ZnO/Ag2O hybrid films. Figure 8a

314

shows the thin pure nylon film of ~2 μm without loading any composite catalyst in it. After 19



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Page 20 of 35

315

being embedded the composite catalyst into nylon film, the thicknesses of hybrid films gradually

316

increased from ~2 μm to (b) 6 μm, (c) 11 μm, and (d) 12 μm, as the amounts of composite

317

catalysts increased from 0 mg to 33 mg, 50 mg, and 66 mg in nylon films, respectively. By

318

comparing the thicknesses and the catalyst amounts in hybrid films, the hybrid film with 66 mg

319

catalyst loading was found to be the densest one. The inset of surface image in each figure also

320

shows the increased amount of catalyst, which is indicated by more granular particles found from

321

the contrast difference on the surfaces. The dense cross section in each figure showed the

322

composite catalyst particles were well embedded into nylon film, therefore it could prevent

323

particles from leaching during photodegradation and post treatment. Although the cross sections

324

of hybrid films were quite dense, their FE-SEM images showed that they also provided some

325

voids that let the dye solution pass through the hybrid films and have more interactions with the

326

embedded composite catalyst during photodegradation.

327 328 329 330

Figure 8. FE-SEM cross-sectional images of (a) pure nylon film and the hybrid films with (b) 33 mg, (c) 50 mg, and (d) 66 mg ZnO/20%Ag2O-loaded nylon film. The inset in each image is its own surface morphology.

331 20


Page 21 of 35

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332

Photocatalytic Activities of Nylon-ZnO/Ag2O Hybrid Films. To confirm the

333

photocatalytic activities of nylon-ZnO/Ag2O hybrid films, the 33 mg, 50 mg, and 66 mg

334

ZnO/20%Ag2O-loaded nylon films were prepared and tested for their photocatalytic ability to

335

degrade 100 ml of 5 ppm MB dye solution under visible light illumination. Before taking for

336

photodegradation testing, each hybrid film was immersed into 50 ppm MB dye solution for 12 h

337

to ensure the dye adsorption saturation of hybrid film. It was done to confirm that the

338

degradation of dye solution was caused by the photo reaction of hybrid film instead of its dye

339

adsorption. After immersing in 50 ppm dye solution, the hybrid films were dried at room

340

temperature and directly tested for photodegradation experiments, where the hybrid films were

341

then immersed in 100 ml of 5 ppm MB dye solution and the solution was vigorously stirred for

342

30 min in dark condition to ensure again the equilibrium of adsorption and desorption between

343

hybrid films and dye solution. The equilibrium between hybrid films and dye solution was

344

confirmed by the increasing dye concentration in solution measured at t = 0 min, as shown in

345

Figure 6b due to the dye desorption from hybrid films. Meanwhile, air was pumped continuously

346

via a channel immersed into the dye solution to provide sufficient amount of dissolved oxygen in

347

solution for a maximum photo degradation reaction. In order to obtain a maximum photocatalytic

348

performance, hybrid films were arranged to face against visible light source during the

349

photodegradation testing. Figure 6b shows the photocatalytic performance of 33 mg, 50 mg, and

350

66 mg composite catalyst-loaded nylon films, accompanied by that of pure nylon film in

351

degrading 5 ppm MB dye solution for a comparative purpose. Pure nylon did not have

352

photocatalytic capability, therefore it released the adsorbed dye of 80% in 90 min during

353

photodegradation experiment. The pseudo-first order kinetic constants of 33 mg, 50 mg, and 66

354

mg composite catalyst-loaded nylon films were 0.01843 min-1, 0.0371 min-1, and 0.02072 min-1, 21



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Page 22 of 35

355

respectively, as calculated from Figure 6b. By comparing the k values, the photocatalytic

356

performance of 50 mg composite catalyst-loaded nylon film was 1.6 and 1.4 times higher than

357

those of 33 mg and 66 mg composite catalyst-loaded nylon films, respectively. The great

358

photodegradation performance of 50 mg composite catalyst-loaded nylon film was attributed to

359

the higher amount of embedded composite catalyst in nylon film, compared to that in the 33 mg

360

catalyst-loaded film, and more voids in hybrid film, compared to that in 66 mg catalyst-filled

361

film, because the 50 mg composite catalyst-filled nylon film had a lower density, as discussed in

362

previous section. Therefore, dye solution could pass through the hybrid film and have more

363

interactions with a higher amount of composite catalyst in nylon film. The denser 66 mg

364

composite catalyst-filled nylon film also limited the light to pass through due to higher amount

365

of embedded composite catalyst, therefore the photo reactions were restricted.

366

Reusability of Nylon-ZnO/Ag2O Hybrid Films. The 50 mg composite catalyst-loaded

367

nylon film was used to demonstrate the photocatalytic reusability of hybrid film in degrading 100

368

ml of 5 ppm MB dye solution under visible light. The reusability experiment was conducted for 6

369

consecutive runs by degrading the same concentration of 5 ppm MB dye solution with the same

370

hybrid film catalyst. After finishing each run of reusability experiment, the hybrid film was taken

371

out from solution and dried at room temperature without any further washing treatment for the

372

next run. Figure 7b shows the photocatalytic activities of hybrid film in degrading MB dye

373

solution for six runs. The good performance of hybrid film was related to the flowability of dye

374

solution through the voids in hybrid film, therefore dye molecules could constantly interact with

375

nano composite catalyst inside the film during photodegradation. Furthermore, thin hybrid film

376

also let the light pass through and therefore more sites in hybrid film become active under photo

377

reactions. 22


Page 23 of 35

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378

To clearly show the differences between embedded (hybrid composite film) and non-embedded

379

(nano composite powder) photocatalyst systems, specific rate constants that obtained from

380

reusability experiments were provided in Table 1.

381 382 383

Table 1. Photo degradation reaction rate constants of embedded system and non-embedded photocatalyst system obtained from reusability experiments. Specific rate constants First run Second run Third run Fourth run Fifth run Sixth run

Embedded system 16.5% Wt 25% Wt 33% Wt 0% Wt photocatalyst photocatalyst photocatalyst photocatalyst 0.01843 min-1 -

0.02963 min-1 0.04093 min-1 0.02531 min-1 0.03793 min-1 0.03312 min-1 0.03135 min-1

0.02072 min-1 -

0 min-1 -

Nonembedded system 0.1816 min-1 0.1439 min-1 0.1196 min-1 0.0975 min-1 -

384 385

As shown in Table 1, the degradation rate constants of embedded systems are much lower

386

compared to those of non-embedded systems due to the non-active surface area of photocatalyst

387

that embedded in nylon matrix. As an example, based on the fourth run degradation rate

388

constants in Table 1, only about 20% photocatalytic activity was still remained in the embedded

389

system, as compared to the non-embedded system. However, as observed in the Table 1, the

390

photocatalytic activities of non-embedded system decreased faster than those of embedded one.

391

Although the efficiency of embedded system was not as high as that of non-embedded one, the

392

recyclability of embedded system was much better. The long-term durability and feasible

393

recyclability are the advantages of our hybrid composite films.

394

Mechanisms of ZnO/Ag2O Photocatalytic Activity. The coupling of high bandgap n-type

395

ZnO and low bandgap p-type Ag2O makes the photocatalytic reaction more effective than those

396

of single phases of pure ZnO and Ag2O nano particles in degrading MB dye. After photo

397

excitation, the photo-induced carriers would diffuse on the catalyst surfaces and interact with dye 23



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Page 24 of 35

398

solution, then the photogenerated electron and hole would respectively provide the reduction and

399

oxidation reactions. The degradation of organic dye is dominantly due to the role of

400

photoinduced hole in providing oxidation and the dye is expected to be mineralized to CO2 and

401

H2O as final products. To have high efficient photocatalytic activities, the recombination rate

402

between photo-induced electron and hole after photo excitation is very critical. The lower the

403

recombination rate, the more efficient the photocatalytic activities are. To understand the

404

coupling effects of p-type Ag2O and n-type ZnO, further information about energy band

405

structures of those materials are needed. The n-type ZnO has a wide bandgap with the work

406

function of 5.2 eV and electron affinity of 4.3 eV,47 meanwhile the p-type Ag2O has a narrow

407

bandgap with the work function of > 5 eV

408

information and the as-calculated bandgap values of Ag2O and ZnO that obtained from our DRS

409

measurement can be used to depict the bandgap structure in Figure 9a. After the formation of

410

heterojunction between ZnO and Ag2O nanoparticles, the electron from ZnO will diffuse to

411

Ag2O and holes will diffuse to ZnO until their Fermi energy levels are aligned. The bandgap of

412

Ag2O is much smaller than that of ZnO, therefore it causes more electron and hole pairs in Ag2O

413

conduction and valence bands respectively under visible light irradiation, as confirmed by DRS

414

measurement that Ag2O had high absorbance of light in visible regions (Figure 5). After photo

415

excitation, the electron is easily drifted from p-type Ag2O to n-type ZnO and the photogenerated

416

hole from n-type ZnO to p-type Ag2O due to the built-in electric field in the depletion zone

417

between the p-type and n-type semiconductors. The mechanism of the built-in electric field

418

obviously retards the photo carrier recombination rate after photo excitation as supported by

419

photoluminescence (PL) data shown in Figure 9b and prolongs their life time in providing photo

420

reduction and oxidation on semiconductor surfaces. The PL spectra were obtained after light

48

and ionization potential of 5.3 eV.46 All these

24


Page 25 of 35

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421

excitation at the wavelength of 325 nm. ZnO powder emitted photon at 375 nm with the intensity

422

much higher than that of ZnO after coupling with Ag2O. These data indicate the photogenerated

423

electron and hole pairs in ZnO can easily recombine thus emitting photon with high intensity.

424

After coating with Ag2O, the recombination rate abruptly decreases, as confirmed by the missing

425

peak at 375 nm in PL spectra. The photogenerated electron and hole pairs can either recombine

426

and dissipate energy as heat or be used to induce water absorption and oxidation on catalyst

427

surfaces to form hydroxyl radicals and to form oxygen radicals, respectively. The hydroxyl and

428

oxygen radicals then mineralize dye to carbon dioxide and water as final products.49

429

Furthermore, one of the advantages in employing Ag2O as photocatalyst is the formation of

430

ozone radical (•O3-), a strong oxidizer agent.30 During the photodegradation, Ag2O lattice

431

decomposes to Ag nanoparticles and O or O2- as evidenced by the Ag XRD peak of after being

432

used catalyst, followed by the reaction with the photogenerated hole to form ozone radical.50

433

This ozone anion radical was extremely reactive and could oxidize organic pollutant quickly.51

434

The strong oxidation of ozone anion radical together with the photogenerated hole can degrade

435

organic pollutant. However, the supply of ozone radicals from Ag2O lattice decomposition

436

gradually decreases, because Ag2O transforms to Ag nanoparticle during the photocatalytic

437

degradation.

438 439 440 441 442 443

25



444 445

ZnO 446

(a)

Ag2O

Evac

WF = 5 eV

EA = 4.3 eV

447

IP = 5.3 eV

O 2 O 2-

WF = 5.2 eV

O 2 O 2-

448

CB

EF (Ag2O) EF (equilibrium)

CB 449

EF(ZnO)

VB OH-

450

•OH-

451

VB

OH-

452

electron hole

•OH-

453 454

(b)

800

455 600

456 457

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

ZnO ZnO@Ag 2O

400 200

458 459

0 400

460

450

500 550 600 650 Wavelength (nm)

700

750

800

461 462 463 464

Figure 9. (a) Energy band diagram of p-type Ag2O/n-type ZnO heterojunction with proposed photocatalytic mechanism of as-prepared nano composite catalyst under visible light irradiation. (b) Photoluminescence emission spectra of ZnO and ZnO/Ag2O nano composite catalyst.

465 466

4. CONCLUSIONS

26


Page 27 of 35

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467

ZnO/Ag2O nano composite powder had been successfully synthesized via co-precipitation

468

method for Ag2O and been embedded into nylon film for considering its recyclability. The as-

469

prepared nano composite powder and its hybrid film had been characterized. The photocatalytic

470

activities of n-type ZnO/p-type Ag2O toward 100 ml of 10 ppm MB dye degradation under

471

visible light significantly increased than those of single phases of pure ZnO and Ag2O

472

nanoparticles. The as-prepared nano composite powder was reusable up to 4 consecutive runs

473

with almost 100% degradation in 30 min. ZnO nanoparticles with higher work function can

474

facilitate the photo-induced electron drifted from Ag2O to ZnO to accomplish the charge

475

separation. The fast photo degradation of MB dye involved not only the conventional

476

photogenerated hole and hydroxyl radical but also the super oxidizing agent of ozone anion

477

radical. The as-prepared 50 mg ZnO/Ag2O-loaded nylon film showed good photocatalytic

478

activities in degrading 100 ml of 5 ppm MB dye under visible light in ~60 min. The hybrid film

479

was also stable and reusable without appreciable loss in photocatalytic activities up to 6

480

consecutive runs. The good photocatalytic capability of thin hybrid films is related to the

481

formation of many voids in the films to provide the dye-accessible tunnels for the contact

482

reactions between the dye and the embedded ZnO/Ag2O nano powder. Owing to the good

483

photocatalytic hybrid film, this nylon-supported film is prospective and easily recyclable for

484

removing hazardous organic compounds in wastewater.

485 486

ACKNOWLEDGEMENTS

487

This work was supported by the Ministry of Science and Technology of Taiwan under Grant no.

488

MOST 104-2218- E-035-004.

489 490

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Facile Synthesis and Recyclability of Thin Nylon Film-Supported n

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