Ag Graphted ZnO Nanorods for _ 2

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Materials FocusVol. 3, pp. 385391, 2014(www.aspbs.com/mat)

Ag Grafted ZnO Nanoplates forPhotocatalytic ApplicationsVidur Raj, Pranauv Balaji, Monika Joshi, and Manish Kumar

Amity Institute of Nanotechnology, Amity University Uttar Pradesh,Sector125 Express Highway, Noida 201303, U.P., India

ABSTRACT

Herein, ZnO nanoplates were synthesized by chemical method and Ag nanoparticles were grafted on ZnOnanoplates surface by sonochemical method. The formation of the nanocomposites was investigated usingX-ray diffractometer (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM). Optical properties ofthe nanocomposite were investigated using Photoluminescence spectroscopy (PL) and UV-Visible spectroscopy(UV-Vis). FESEM investigation confirms that the nanoplates were uniformly distributed and Ag nanoparticleswere grafted on the surface on ZnO nanoplates. The photocatalytic activity of Ag/ZnO nanocomposites wasevaluated in terms of their efficiency of the photo excitation of methyl orange dye (MeO) under UV illuminator.A photo excitation of Ag/ZnO showed enhanced photocatalytic activity compared to pure ZnO Nanoplates. Thisenhanced UV activity of Ag/ZnO nanocomposite makes it a potential candidate for optoelectronic devices forSolar Energy conservation, water splitting and Light Emitting devices etc.

KEYWORDS: Optical Material, Chemical Synthesis, X-Ray Diffraction, Photocatalysis.

1. INTRODUCTIONPhotocatalysis using nanoparticles have potential applica-tions in optical devices like Organic LEDs, Solar energyconservation (Solar cells), Water treatment using UV raysand environmental pollution treatment etc.1 Comparedwith other semiconductor nanostructures, ZnO have beenstudied extensively over the last decades due to theirunique physical and chemical properties. ZnO is one of themost widely used photocatalytic metal-oxide nanostruc-tured semiconductors with a wide band gap of 3.2 eV anda large exciton binding energy of 60 meV.2 It is an intrinsi-cally n-type semiconductor of wurtzite structure and has agood chemical, thermal and catalytic stability at room tem-perature. Due to these properties, ZnO is widely applied invarious applications in optical devices,3 transistors,4 piezo-electric devices,56 gas sensor,78 mechanical devices,9 UVdetectors1011 and photoluminescence devices.12

However, photocatalytic efficiency of pure semiconduct-ing nanomaterial is very low due to the fast recombinationof photo-generated electron hole pair and the inability toutilize visible light due to the large wide band gap. There-fore, considerable efforts have been made for the develop-ment of a photocatalytic system that can be more activein visible light (>400 nm).13 To date, many approaches

Author to whom correspondence should be addressed.Email: [email protected]: 29 May 2014Accepted: 21 August 2014

including (i) functionalization of ZnO with photosensi-tizes that absorbs UV light, (ii) doping with metals and(iii) doping with non metals have been applied to enhancethe photocatalytic activity of semiconductors like ZnO.14

Doping and making composite with metals are widelystudied ways of enhancing the photocatalytic activity ofmetal oxide semiconductor. The interaction of metal withsemiconductor improves the electrical and optical proper-ties due to the shifting of the Fermi level and the for-mation of a Schottky barrier for trapped electrons hasearlier been reported.15 These trapped electrons are trans-ferred to the dye, which are further degraded throughthe hydroxyl and super hydroxyl radicals generated byUV illuminator.16 Therefore, the photocatalytic activity ofmetal-semiconductor shows significant enhancement com-pare to pure ZnO.Noble metal such as (Ag and Au) shows unique electron

storage and light absorption properties.17 Compared withall other metal-semiconductor nanostructures, Ag/ZnOnanocomposites have drawn particular attention, not onlybecause ZnO nanostructures can be easily prepared bya series of simple methods,1821 but also that silver dis-plays the most efficient plasmon resonance in the visibleregion among all metals by virtue of its favorable fre-quency dependence of the real and imaginary parts ofthe dielectric function.22 ZnO nanorods with noble metalsshows improved electrical, optical, magnetic and chemi-cal properties.23 Ag nanoparticles have lots of peculiar andexciting physical chemical and catalytic properties that are

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Ag Grafted ZnO Nanoplates for Photocatalytic Applications Raj et al.ARTICLE

different from massive silver. Therefore, incorporation ofsilver (Ag) can enhance the photocatalytic efficiency ofsemiconducting nanoparticle (ZnO) by acting as an elec-tron sink for the exited electrons and thus increases thegeneration of electron hole pairs in ZnO. Moreover, theenhancement was not due to the electrons generated byvisible light excitation of surface plasmon resonance butdue to the plasmon resonance enhanced local electric fieldof the incident light.24 These enhanced properties havebeen utilized in photovoltaic cells, photo-catalysts, solarcells and optoelectronic devices.In the present article, Ag/ZnO nanocomposites have

been synthesized by varying the concentration of Ag. Thephotocatalytic activity of Ag/ZnO nanocomposite underUV-Visible irradiation was studied by photo degrada-tion of organic dye (MeO). The nanocomposite showsenhanced photocatalytic activity compared to the pureZnO. The results indicated that optical absorption ofAg/ZnO nanocomposite showed enhanced photocatalyticactivity at 10 mM concentration of silver.

2. MATERIALS AND METHODS2.1. Synthesis of ZnO Nanoplates50 ml of 1 M NaOH was prepared and 1 g of poly ethy-lene glycol (PEG) was added and stirred for 1 hr. 0.5 MZn(COOCH32 (H2O)2 (MERCK) was added drop-wise inabove solution. The mixture was then stirred at 60 C for2 hr. A white precipitate was collected and washed severaltimes with D.W. and Ethanol (C2H5OH).

2.2. Grafting of Ag Nanoparticles on ZnO Nanoplates0.2 g of synthesized ZnO Nanoplates were dissolved in50 ml of D.W. and ultrasonicated for 2 hr. 10 mMof Ag(NO3 (AR Grade) and 1 mM Tri sodium citrate(Na3C6H5O7 (MERCK) was added to the above solution.The solution was stirred for 2 hr at room temperature. Theresultant solution was centrifuged and obtained a blackishbrown colored powder. The same experiment was repeatedby varying the concentration of Ag (NO3 to 5 mM and2.5 mM concentrations.

3. CHARACTERIZATION TECHNIQUESThe changes of crystalline phase structure of Ag/ZnOnanocomposite were investigated using X-ray diffractome-ter (Bruker, D-2 Phaser) at room temperature. Figure 1shows the comparative XRD image of uncoated ZnONanoplates and ZnO Nanoplates coated with Ag nanopar-ticles. Three prominent peaks at 2 = 3162, 34.44 and36.10 corresponds to the diffraction of the (100), (002)and (101) planes of ZnO respectively. All diffraction peakscan be indexed as wurtzite structure ZnO. The intensityof (101) plane is higher than other two planes suggestingthat most of the ZnO Nanoplates are with (101) planes.XRD image of ZnO decorated with Ag nanoparticles are

Fig. 1. XRD image of ZnO Nanoplates and Ag/ZnO nanocomposite.

obtained two prominent additional peaks at 2 = 3838and 44.25 corresponds to (111) and (200) planes ofAg nanoparticles with face centered cubic structure (fcc)respectively. It is indicated that the crystallization of ZnONanoplates is not affected by grafting of Ag nanoparti-cles, indicating a good crystalline property of the Ag/ZnOnanocomposites.The synthesized material was investigated by FESEM

(Zeiss Ultra Plus-40-98) at room temperature for itsmorphology and composition. Figures 2(A)(D) showsFESEM micrograph of the synthesized ZnO nanoplates.The Nanoplates formed were evenly distributed through-out the sample. The nanoplates are arranged in the formof beautiful uniform flowers with bush like structure. Thepetals of nanoflowers are pyramidal in shape with a diam-eter of 30 nm70 nm and a length of few micrometers.The images reveal that Ag nanoparticles are sparsely dis-tributed on the ZnO Nanoplates. Figure 2(E) shows theEDX mapping of Ag/ZnO nanocomposite. The EDX map-ping shows the presence of materials like Zn, O, Ag andsome of the other stray molecules like C etc. confirmingthe presence of Ag grafted ZnO. The FESEM micrographand EDX mapping measurement reveals the presence ofAg nanoparticles grafted on to the surface of the ZnONanoplates.

4. RESULTS AND DISCUSSIONThe photocatalytic degradation of organic dyes or toxicpollutant is a commonly used approach to characterizethe efficiency of the photo catalyst. The photo-catalyticactivity of Ag/ZnO nanocomposite was evaluated by thephoto-degradation of methyl orange (MeO) placed underUV illuminator for a fixed interval of time. Figure 3shows a series of UV-Vis absorption spectra of (3A) ZnONanoplates (3B) Ag/ZnO nanocomposite obtained fromthe suspension of 10 M solution of organic dye (MethylOrange (C14H14N3NaO3S)MeO) with 1 mg of each ofthe nanomaterials. On increasing the exposure time of UVirradiation the absorption peak intensity decreases indicat-ing the photo-decomposition of the organic dye (MeO)

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Raj et al. Ag Grafted ZnO Nanoplates for Photocatalytic Applications

ARTICLEFig. 2. FESEM micrograph (A)(D) ZnO Nanoplates with sparsely grafted Ag nanoparticle. (E) EDX mapping evaluation of the synthesized nanopar-

ticle with presence Zn, O, Ag.

used. The two plots clearly indicate that after an exposurefor 12 hr with a time interval of 3 hr the photo-catalyticactivity of pure ZnO Nanoplates has been enhanced bygrafting of Ag on the Nanoplates.The semiconducting metal oxides like ZnO on expo-

sure to UV irradiation excite the material resulting in theformation of electron hole pairs in the system. These elec-tron hole pairs generated will interact with the moleculesattached on their surface forming reactive radicals such asO2 , OH

etc. from the oxygen in the metal oxide net-work and hydroxyl groups on their surface respectively.25

These radicals are responsible for the catalysis of the reac-tion for decomposing the organic pollutant materials. Themain limiting factor in this reaction is the electron holepair recombination in the system. This problem is over-come when grafted or composited with noble metals (Ag,Au) on semiconductor metal oxide materials. The noblemetal (Ag, Au) in the system acts as an electron sinkswhich capture the electron from the generated electronhole pair in semiconductor and prohibits the recombina-tion of them. Thus, enhancing the photo-catalytic activity

of the synthesized nanocomposite compared to the pureZnO nanomaterials.To study the photodegradation behaviour of Ag/ZnO

nanocomosite, an aqueeous solution of MeO was irra-diated in the presence and abscence of nanocomposite.Figure 4(a) shows the degradation of MeO (in terms ofthe UV-visible absorption intensity) as a function of timefor bare ZnO nanorods and Ag/ZnO nanocomosite. TheBlue plot indicates no change in the absorption inten-sity under dark conditions. The Ag/ZnO nanocompositeshows higher photocatalytic activity compared to bare ZnOnanorods. The rate of decolarization of MeO as a functionof absorption intensity can be calculated using the follow-ing equation:

% = C0Ct/C0100Where, C0 and C are the initial and reaction concentrationof MeO after a time t, respectively.Figure 4(b) shows a plot of C/C0 curve as a function of

time of MeO degradation in the presence and abscene ofcatalyst. The photodegradation of the dye does not occur

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Fig. 3. UV-Vis spectra of photodegraded MeO by photocatalyst for (a)bare ZnO and (b) Ag/ZnO nanocomposite, as a function of UV illumi-nation time.

in the abscence of catalyst. The degradation curve of MeOby ZnO and Ag/ZnO shows similar profile, suggesting asimilar way of degradation by both the catalyst. However,the degradation rate of Ag/ZnO nanocomposite is higherthen bare ZnO nanorods. The kinetics of the photocat-alytic efficency of the nanocomposite were evaluated bycalculating k using the first order kinetic model derivedfrom the Langmuir-Hinshelwood Kinetic equation. In thismodel,the the first order rate constant (k) can be calculatedusing the following equation:

lnC/C0= ktThe first order rate constant (k) for zinc oxide and zincoxide grafted with silver were calculated from the slopeof the graphs to be 0.03814 hr1 and 0.16299 hr1

respectively. The results indicated that Ag/ZnO nanocom-posite shows higher degradation rate compare to bareZnO nanoplates. Thus, the photocatalytic activity of ZnOnanoplates enhances after the grafting pf Ag nanoparticles.

5. PHOTO CATALYTIC MECHANISMFigure 5(a) shows the schematic diagram of the mecha-nism of enhanced photocatalytic dye degradation of MeOby Ag/ZnO nanocomposites and Figure 5(b) shows thechemical structure of MeO. The transfer of electron in

Fig. 4. (a) Shows the UV absorbance versus UV illumination time.(b) Shows the photocatalytic degradation of MeO versus time. Insetshows C/C0 versus time at 10 mM of Ag/ZnO nanocomposite, bare ZnO.

metal and semiconductor takes place when they are in thecontact and the direction of the electron transfer dependson the energy states and band structure of the metal andthe semiconductor. In case of Ag/ZnO nanocomosite theFermi level of ZnO lies lower than the Fermi level of Ag.Therefore, the conduction band of the ZnO undergoes ashift, and hence the interference of Ag is responsible forband bending. The process of band bending in Ag/ZnOnanocomposite forms the Schottky barrier for trapped elec-trons. These trapped electrons are transferred to MeO forphoto-degradation.In case of ZnO nanoplates on UV illuminator, ZnO leads

to generate electron and hole pairs (Eq. (1)). The gen-erated electrons can be trapped by electron acceptors toproduce superoxide radical ions (O2 (Eq. (2)) and theholes can be trapped by the hydroxyl group to producesuper hydroxyl radicals (OH) (Eq. (3)).

ZnO+h h++ ec (1)O2+ ec O2 (2)h+OH

OH (3)h++H2O OH+H+ (4)

Further, methyl orange (MeO) on absorption of UV-lightgoes to an excited state (MeO), these excited state can

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Fig. 5. (a) Shows the mechanism of the photocatalysis. (b) Shows thestructure of methyl orange.

either donate or accept electron and generate radical cation(MeO+) or radical anion(MeO).26

The photo generated hole hvb+ can also reacts withwater molecule in solution leads to generate superoxideradical anion (Eq. (4)). These super hydroxyl radicals andholes are responsible for decomposition of MeO and pro-duce colorless MeO.In case of ZnO, most of the photo generated charges

can recombine, thus the photodecomposition efficiency ofMeO decreases. On the other hand, in case of Ag/ZnOnanocomposites Ag acts as an electron acceptor causes afast removal of photo generated electron to the conductionband of ZnO and this process decreases the recombinationrate of photo generated electronhole pair. Therefore, thephotocatalytic activity of Ag/ZnO composite shows sig-nificant enhancement over pure ZnO. In this process ofphoto-degradation a small quantity of Ag/ZnO nanocom-posite can initiate the degradation, which can further useusing recycle process.The PL spectrum of uncoated ZnO Nanoplates is shown

in Figure 6(a) which was investigated using Shimadzu RF5301PC at room temperature. A sharp peak at 362 nmis observed that corresponds to radiative recombination,occurs due to the recombination between the electrons inthe conduction band and the holes in the valence band.27

This peak is also known as a UV range emission peak.28

The two small peaks at 387 nm and 469 nm are alsoobserved that corresponds to UV range emission and bluerange emission.2930 The blue emission is considered to bedue to the deep level defect produced during the growth.Oxygen vacancies and zinc interstitial related defects areconsidered as the most probable region for generation ofblue emission.31 Figure 6(b) shows the PL spectra of theAg/ZnO nanocomposite. The UV emission band of the

Fig. 6. (a) PL spectra of ZnO Nanoplates; (b) PL spectra of Ag/ZnOnanocomposite.

nanocomposite decreases on increasing the percentage ofAg nanoparticles. This indicates that due to the surfaceplasmon resonance (SPR) affect of Ag the electron holepair recombination decreases.32 At 10 mM concentrationof Ag two sharp peaks at around 361 nm and 470 nmwere observed. The change of the peaks towards right handside (red shift) is due to change in interaction betweenAg and ZnO Nanoplates. The red shift suggested that Agnanoparticles are very small (few nm) in size and easilyincorporate on the ZnO surface. On modification of ZnONanoplates by Ag nanoparticles PL spectra in Figure 6(b)shows decrement in the visible region peak whereas anincrement in the UV region peak. Also decreasing theconcentration of Ag has decreased the intensity of visibleregion of PL spectra. At 25 mM conc. of Ag the visibleband due to structural defects almost diminished and onlyUV peak was obtained. The PL result also suggested thatat 10 mM concentration of Ag the nanocomposite haveonly point defects, shows very good crystal quality, whichis a good agreement with XRD results.The optical absorption spectra of pure ZnO nanoplates

and AgZnO nanocomposites were shown in Figures 7(a)and (b) respectively. Figure 7(a) shows the absorptionspectra of ZnO nanoplates. A broad peak at around 360 nmcorresponds to the characteristic peak of ZnO Nanoplates.The optical absorbance peak of noble metals, Ag or Au

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Fig. 7. (a) Optical absorption spectra of ZnO Nanoplates; (b) opticalabsorption spectra of Ag/ZnO nanocomposite.

is always sharp and intense compare to the nanocompos-ite. For pure Ag the absorbance peak at 420 nm whichcorresponds to the surface plasma resonance effect. Theposition and broadening of the peak depends on the sizeof the metal and the semiconductor.The aim of the grafting of Ag nanoparticles on ZnO

nanoplates is to enhance the photocatalytic efficiency inthe visible region. After the grafting of silver nanopar-ticles, new broad peaks at around 380 nm, 395 nmand 440 nm appears in all samples containing Ag withmolar ratio of 2.5 mM, 5 mM and 10 mM respectively(Fig. 7(b)), which corresponds to the surface plasmon res-onance of Ag nanoparticles.33 In comparison to the nar-row absorption peak of pure ZnO Nanoplates, the observedbroad bands are evidence of strong interactions betweenZnO and silver nanoparticles. After the grafting of Agnanoparticles the peaks were shifted towards the visiblerange compared to the pure ZnO nanoplates. These resultssuggested that the grafting of silver nanoparticles helps toreduce the band gap. The results indicated that for 10 mMconcentration of Ag a broad peak appears at 430 nm. Theabsorption at 430 nm originates with a good incorporationof small size Ag nanoparticles (few nm) in ZnO matrix.111

From the absorption curve it was concluded that at 10 mMconcentration surface plasmon resonance (SPR) is maxi-mum, which is a good agreement with PL results.

6. CONCLUSIONIn summary, a simple method was developed to synthe-size Ag/ZnO nanocomposite and their photocatalytic activ-ities were evaluated by the degradation of Methyl Orange(MeO) under UV light irradiation. The grafting of Agnanoparticles was found to be enhanced the photocatalyticactivity compared to bare ZnO Nanoplates. The enhancedphotocatalytic activity of Ag/ZnO nanocomposite is dueto the behavior of Ag nanoparticles as a sink for elec-trons from the electron hole pair generated by irradiationin metal-oxide semiconductor material. The XRD resultsreveals that the grafting of Ag nanoparticles does not affectthe crystalline of the ZnO Nanoplates but the morphologyof the nanocomposites varied with the increased amountof Ag. Optical properties of Ag/ZnO nanocomposites wereimproved compared to the pure ZnO Nanoplates. Theoptical properties of Ag nanoparticles on ZnO surfaceswere investigated by varying the concentration of Ag. Theamount of Ag content was found to control the absorptionband due to the surface plasmon resonance effect. Fromthe Photo luminescence and absorption spectra it is con-cluded that at 10 mM concentration of Ag the compositefilm shows best optical properties.

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Ag Graphted ZnO Nanorods for _ 2

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