Synthesis characterization and photocatalytic act

Authors: Gauri Bandekar N S Rajurkar I S Mulla U P Mulik D P Amalnerkar P V Adhyapak

Publish Date: 2013/01/11

Volume: 4, Issue: 2, Pages: 199-208

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Abstract

In the present study ZnO nanostructures have been successfully synthesized by hydrothermal sonochemical and precipitation methods using polyvinyl pyrrolidone PVP as the capping agent The ZnO nanoparticles were characterized by Xray diffraction XRD field emission scanning electron microscopy FESEM UV–Visible spectroscopy and photoluminescence PL techniques The XRD results revealed the hexagonal wurtzite structure of the ZnO nanostructures for all the samples Furthermore the morphology of the ZnO particles was obtained from FESEM micrographs Particles prepared by hydrothermal method were found to be rice grain shaped and that prepared by precipitation and sonochemical methods were spherical shaped Sunlight driven photocatalytic degradation of methylene blue MB was studied for ZnO nanostructures synthesized by various methods The ZnO nanostructures were further decorated with Ag nanoparticles to enhance its dye degradation efficiency The Ag decorated ZnO nanoparticles exhibited a higher degradation rate as compared to pure ZnO nanoparticles which was independent of pH Since this process of dye degradation relies on the degradation of dye due to oxidation by highly reactive hydroxyl radicals there are many factors which affect the efficiency of this process Hence a study was conducted on the effect of various parameters on ZnO viz amount of catalyst reaction pH and concentration of MB dyeThe dyes are extensively used in textile paper printing food processing cosmetics pharmaceutical and leather industries In recent years environmental pollution due to the release of toxic chemicals from industrial sectors has been a major concern Out of the 450000 tons of organic dyes annually produced worldwide more than 11  find its way in effluents during manufacture and application processes Forgas et al 2004 This toxic effluent causes various environmental and health hazards Parsons 2004 To overcome this issue researchers are working on various dye removal methods such as biodegradation coagulation adsorption membrane process and advanced oxidation processes AOPs Among these processes the AOP is a balance process which includes widely studied sunlight driven photocatalysis process for removal of dyes In this process electrons in conduction band and holes in valence band are generated using light energy These holes and electrons interact with H2O and O2 respectively to yield OH and OOH radicals Behnajady et al 2007 Akyol and Bayramoglu 2008 Anandan et al 2007 These hydroxyl radicals are highly reactive with very high oxidation potential and can oxidize the dyes completely It is a fast clean and destructive method of dye degradation in which no secondary waste is generated Fernandez et al 2010Zinc oxide ZnO is one of the ntype semiconductors extensively used for AOP due to its wide band gap 33 eV and large exciton binding energy 60 meV It is also abundant in nature and is environmental friendly Wahab et al 2011 Moreover ZnO is a better semiconductor as compared to commonly used TiO2 because widespread use of TiO2 has been proved to be uneconomical for large scale water treatment operations Sobana and Swaminathan 2007 Nevertheless ZnO can also effectively degrade dyes in acidic or basic medium and is cheaper than TiO2 Yang et al 2010 Xie and Wu 2010 The utmost advantage of ZnO is that it absorbs more light quanta in the UV spectrum as compared to TiO2 Behnajady et al 2006Uptill now a variety of methods have been developed for the synthesis of ZnO nanostructures which include hydrolysis in polyol medium Poul et al 2001 template method Kou et al 2006 chemical precipitation method Wang et al 2007a b thermal oxidation process Zhang et al 2007 hydrothermal Zhou et al 2007 and microwave synthesis Siddiquey et al 2008 These synthetic methods subsequently affect the morphology size crystalline form and photocatalytic activity of ZnO nanostructures Zhang et al 2007 Shi et al 2004 Mohajerani et al 2008 Therefore ZnO nanostructures with diverse morphologies like nanowire Huang et al 2001 nanobelt Pan et al 2001 ellipsoidal Liu et al 2006 Xie et al 2006 Pu et al 2007 flower like Tanner and Yu 2008 tube like Yu et al 2005 have been successfully synthesized Effects of morphology and particle size on the photocatalytic activity of ZnO have also been studied and reported in the literature Wang et al 2007a b Jing et al 2005 Dodd et al 2006 For example Jing et al studied ZnO nanoparticles with three different sizes 14 19 26 nm prepared by precipitation process High surface oxygen vacancies higher ESR intensity stronger PL signal and higher photocatalytic activity was observed for the smaller sized ZnO nanoparticles Jing et al 2005 Dodd et al 2006 reported the synthesis of ZnO nanoparticles within 28–57 nm range by mechanical milling and heat treatment with maximum photocatalytic activity for 33 nm sized ZnO particle Li et al 2005 also obtained ZnO nanoparticles with average sizes 21–90 nm by varying the calcinations temperature between 200 and 1000 °C and found that prismatic aggregated obtained by calcination at 800 °C demonstrated the highest photocatalytic activity More recently Wang et al 2007a b showed that photocatalytic properties of various particle sizes of ZnO 15 50 200 and 1000 nm synthesized by thermal evaporation and chemical deposition in UVinduced degradation of methyl orange depended on size morphology and preparation method of ZnO Thus according to previous reports it is observed that the efficiency of photocatalytic process can be maximized by optimizing the particle size shape and preparation conditions/methods of ZnO photocatalystsWe herein report synthesis of ZnO nanostructures using three different methods viz hydrothermal precipitation and sonochemical in presence of polyvinyl pyrrolidone PVP as a capping agent The effect of synthetic methodology on the morphology and size of the nanostructures was studied The sunlight driven photocatalytic activity of synthesized ZnO nanostructures was demonstrated using methylene blue MB as a representative dye These ZnO nanostructures were further decorated with silver nanoparticles to enhance its photocatalytic activity The degradation process is optimized with respect to catalyst dose dye concentration and pH The higher photocatalytic activity was achieved for ZnO with rice grain shape morphology obtained from hydrothermal process To the best of our knowledge the photocatalytic activity is significantly higher than the other reportsZinc acetate CH3COO2 Zn·2H2O 6 g and PVP 5 g were dissolved separately in 15 mL methanol These two solutions were mixed and were poured in a 250 mL Teflon vessel A solution of NaOH was made by dissolving NaOH 6 g pellets in 30 mL methanol This NaOH solution was then slowly added to the above solution in Teflon vessel A white precipitate was formed Further 5 mL of distilled water was added to this solution The Teflon vessel was sealed and placed in a stainless steel autoclave This stainless steel autoclave was placed in an oven at 70 °C for 24 h The resultant precipitate was centrifuged and washed several times with distilled water and finally with acetone to remove the excess capping of PVP After sufficient washings the residue was dried at 70 °C for 24 h and ground to form a fine powder Sample code ZN1

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