Composites: Part B 45 (2013) 1515–1520

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Composites: Part B

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Polyaniline mediated enhancement in band gap emission of Zinc Oxide

Mansi Dhingra a, Sadhna Shrivastava b,⇑, P. Senthil Kumar a, S. Annapoorni a

a Department of Physics & Astrophysics, University of Delhi, New Delhi 110 007, Indiab Department of Physics, Dyal Singh College, Lodi Road, University of Delhi, New Delhi 110 003, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 May 2012Received in revised form 29 July 2012Accepted 6 September 2012Available online 24 September 2012

Keywords:A. Polymer–matrix compositesB. InterfaceD. Electron microscopyE. Powder processing

1359-8368/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compositesb.2012.09.020

⇑ Corresponding author. Tel.: +91 011 27666834; faE-mail address: sadhna.0793@yahoo.co.in (S. Shriv

Zinc Oxide (ZnO) nanoparticles are prepared using sol–gel technique; while Polyaniline (PANI) is pre-pared via chemical oxidative polymerization of aniline. For the formation of composites fixed weightof ZnO nanoparticles were physically blended, followed by grinding with increasing weight content ofPANI. The mixture so obtained was compressed in the form of pellets for further studies. SEM micro-graphs showed changes in the morphology of ZnO nanoparticles from rods to flakes indicating the forma-tion of composites. Raman studies reveal the presence of hydrogen linkages between ZnO and PANI.Photoluminescence (PL) shows a suppression of defect emission of ZnO nanoparticles and an enhance-ment in band gap emission. The presence of hydrogen linkages between ZnO and PANI leads to suppres-sion of defects in ZnO nanoparticles and hence an enhancement in UV emission is observed. A largenumber of dangling bonds present on surface of nanorods are passivated by the hydrogen frompolyaniline.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Organic–inorganic hybrid material remained an interesting areaof research in the past due to its versatility in the range of applica-tions and device performance. The properties of these hybrid mate-rials are different from the organic part and correspondinginorganic nanoparticles depending on the type of bonding betweenthem, but however capable of exhibiting the best properties ofboth the components [1–3]. The properties of the hybrid vary withthe method of the preparation of the composites. The formation ofhybrid structures and the combination of inorganic particles withpolymers is usually accomplished either by simple mixing or bysurface modification. The mixing of polymers and nanoparticleshelps in tailoring flexible composites that exhibit advantageouselectrical, optical and mechanical properties. It is also seen thatsurface modification of nanoparticles is done by grafting polymersonto it that results in altogether changed properties of the result-ing composites [4,5]. Many applications of the organic–inorganiccomposites have been explored in the recent years. Most commonarea for these applications includes catalytic, luminescent or elec-tronic properties and this depends on the choice of the organic andinorganic component [6–9].

Among the inorganic oxide materials, ZnO has particular impor-tance because of its high exciton binding energy (60 meV) andwide band gap of 3.37 eV [10]. It is widely used as a componentin the pharmaceutical and cosmetic industries because of its good

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x: +91 011 27667061.astava).

absorbing properties in the UV region of visible spectrum. Thus thestudy of the optical properties is of utmost importance for any ofthe above mentioned applications. Photoluminescence (PL) and Ra-man measurements are helpful for understanding the defects andvacancies in the pristine as well as in composites [11,12]. The opti-cal band gap of the material as well as the possible radiative andnon-radiative centers could be understood from these techniques.Kamat et al. showed that nanocrystals of ZnO exhibit differentoptical behaviors when combined with organic molecules [13].Environment friendly and low cost ZnO is being grafted with vari-ous conjugate polymers to fabricate electronic devices [14]. On theother hand, conjugated polymer like PANI stands out because it isone of the typical conducting polymers that can be easily synthe-sized and which exhibits a broad range of conductivity from insu-lating to an almost metallic behavior [15,16]. Depending on theextent of the redox reaction, polyaniline can exist in a range of oxi-dation states viz fully reduced leucoemeraldine, half oxidizedemeraldine, and fully oxidized pernigraniline. The number ofhydrogen atoms bound to the nitrogen attached to the phenyl ringsdifferentiates into the above forms of polyaniline. Hence, polyani-line based conducting polymers have been considered as extre-mely interesting members of polymer family because it can beeasily flipped between oxidized or reduced states [17]. It is beingused widely as a component in the formation of nanocompositeswith classes of oxides and ferrites and is known for complementingthe properties of the composites [18–21]. The hybrid nanocompos-ite mixtures can combine the advantages of both types of materi-als. In situ polymerization has been widely explored by manyresearch groups for the formation of polymer nanocomposites

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[22]. Present work is aimed on mixing doped-PANI and ZnO nano-particles to investigate the interactions occurring between inor-ganic nanoparticles and organic molecules. In this paper, thepreparation technique involves synthesizing the inorganic nano-particles separately and then mixing them with the conductingpolymer. This will help us to understand the hybrid structures inthe form of composites and interfaces. Composites of ZnO and PANIso formed resulted in an altogether different optical and structuralproperty. Raman spectroscopy and photoluminescence study wasemployed as a tool to investigate the hybrid effect between ZnOand PANI. Finally, we report and discuss the possible reasons forthe modified properties.

2. Experimental

2.1. Synthesis of ZnO nanoparticles

ZnO nanoparticles were prepared using the sol–gel process [23].Zinc acetate dihydrate as salt and methanol as a solvent were usedto make 0.6 M solution. The mixture was homogenized using stir-ring at 60 �C for 2 h. A clear transparent solution so obtained wasthen poured into a round bottom flask and was heated at 70 �Cfor 3 h to get solid powder [24]. This solid powder containinghydroxides and unreacted acetates is the precursor complex. Theprecursor complex was further heat treated at 500 �C (as deter-mined by thermogravimetric analysis) for 1 h to get ZnO nanopow-der. The nanopowder so obtained was washed with distilled waterto get ZnO nanoparticles.

2.2. Synthesis of polyaniline

Polyaniline was synthesized by oxidative polymerization of ani-line [25]. 0.1 M aniline monomer was dispersed in 1 M aqueousHCl in a double walled flask. The temperature was maintained at0 ± 1 �C. Aqueous solution of ammonium persulfate was addeddropwise in the double walled flask while stirring. After 5 h, stir-

Fig. 1. XRD pattern of (a) ZnO, (b) ZP1:0.5, (c) ZP1:1, and (d)

ring was stopped and the solution was filtered. The residue waswashed 2–3 times with distilled water and then dried at 60 �C. Thiswas followed by refluxing in methanol for 5 h to remove the olig-omers. The residue was dried at room temperature and pulvarisedin pestle mortar to get doped polyaniline powder, which was usedfor the preparation of the composites.

2.3. Preparation of composites

Composites were prepared by physically mixing ZnO and PANI[26]. Fixed quantity of ZnO (100 mg) nanoparticles were mixedwith different weight of doped polyaniline powder and then com-pressed in the form of pellets by applying a pressure of 100 MP.Four different ratios of ZnO (Z) and Polyaniline (P) were studied la-beled as ZP1:0.5, ZP1:1, ZP1:1.5, and ZP1:2 were prepared with dif-ferent weight ratio as indicated above.

2.4. Characterizations

ZnO, PANI and the composites were characterized by XRD usingMiniflex 2 powder XRD with Cu Ka as source and TEM (FEI TecnaiG2 20, 300KV) techniques. The Raman spectra were obtained usingRenishaw Invia Raman spectrometer, equipped with an argon ionlaser (excitation wavelength 514.5 nm) and the photolumines-cence spectra were taken using Fluorolog (HORIBA JOBIN YVON)spectrofluorometer. Morphological studies were carried out usingSEM (Zeiss EVO MA 15).

3. Results and discussions

3.1. X-ray diffraction studies

Fig. 1 shows the X-ray diffractograms for all the four compositesand ZnO. PANI diffractogram is shown in the inset. Curve (a) showsthe diffractogram for pure ZnO. Here, all the peaks of ZnO matcheswell with the standard JCPDS data. Curve (b–e) are the diffracto-

ZP1:2 (an inset showing the XRD pattern of polyaniline).

Fig. 2. (a) SEM image, (b) TEM image, and (c) SAED pattern of as obtained ZnO nanoparticles.

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grams for the composites with increasing weight content of PANI.It is seen in these curves that all the peaks of ZnO are obtained.However, with increasing weight content of PANI, a slight broaden-ing in all the peaks corresponding to ZnO is observed. The broaden-ing is however more pronounced as the concentration of thepolyaniline increases in the composites. The probable reason forthis is the influence induced on the nanoparticles due to the sur-face effects arising from the surrounding polyaniline [27]. It is fur-ther observed that because of the high intensity peaks ofcrystalline ZnO nanoparticles the contributions from amorphousPANI macromolecules (shown in the inset) get suppressed and isnot visible in the diffractogram of the composites.

3.2. Morphological studies

Fig. 2a shows the SEM of pure ZnO nanoparticles heat treated at500 �C wherein clear rodlike structures are observed. TEM mea-surements also confirm the formation of rod like structures havingan approximate length of 200 nm and diameter of 50 nm as shownin Fig. 2b. The selected area electron diffraction (SAED) is shown inFig. 2c. The presence of spots are indicative of highly crystallinenature of the sample and an oriented growth.

Fig. 3. SEM images (a) ZnO (an inset showing the magn

Fig. 3 shows the SEM images for the pure ZnO and the threecomposites in the pellet form. SEM image of pure ZnO pellet(Fig. 3a) shows compressed nanorods which is also clear fromthe magnified image given as an inset whereas the composites(Fig. 3b–d) clearly reveal the change in morphology of ZnO with in-crease in weight concentration of PANI. In the case of compositewhich has low concentration of PANI viz ZP1:0.5, the ZnO nanorodsseparate as shown in Fig. 3b. ZnO nanorods, believed to possess anegative surface charge. However, in presence of PANI which hasa positive surface charge; the electronegative nanorods of ZnO tendto repel each other [28]. On increasing the polyaniline concentra-tion the presence of both rods and flakes were observed (Fig. 3c).Increase in the amount of PANI in the composite induces gradualformation of flakes which can be attributed to aggregation ofZnO nanorods into densely packed arrangements and decrease inthe surface energy of nano-ZnO. On further increase in the amountof PANI, the rods disappear and only flakes were seen as shown inFig. 3d. This can be explained as follows. As the content of polymerincreases in the vicinity of ZnO nanorods, the surface interactionsoccurring between ZnO nanorods and polymer affects the spatialdistribution of nanorods. This leads to the aggregation of ZnOnanorods by variation of surface energy to energetically stabilize

ified image), (b) ZP1:0.5, (c) ZP1:1, and (d) ZP1:2.

Fig. 4. Photoluminescence spectra of (a) ZnO, (b) ZP1:0.5, (c) ZP1:1, (d) ZP1:1.5, and(e) ZP1:2 excited at 350 nm.

Fig. 5. Luminescence enhancement of ZnO nanoparticles with increasing weightratio of PANI.

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the polar surface of ZnO nanorods [29]. There are two polar sur-faces at the ends of the nanorod, the Zn-terminated ZnO (0001)and the O-terminated ZnO ð000 �1Þ surfaces. The influence of organ-

Fig. 6. A schematic showing possible linkages between ZnO and PANI.

ic molecules coordinated at the surface stabilizes the polar surfacesof ZnO nanorods and passivate the end surface dangling bonds[30]. Therefore, minimization of interfacial energy between thenanorods and the surrounding polymer derives a close packing ofnanorods into flake like structures.

3.3. Photoluminescence (PL) studies

ZnO is known for emitting in the UV and Visible ranges whereUV peak is considered to be originating from recombination of freeexciton through an exciton–exciton collision process. The greenemission generally referred as defect recombination is attributedto the radiative recombination of a photo generated hole with anelectron that belongs to the singly ionized oxygen vacancies (V0)on the surface of nanoparticles [31,32].

V0 ! Vþ0 þ e�;Vþ0 þ e� þ h! Vþ0 þ green

Fig. 4 shows the photoluminescence spectra of the bare ZnOpellet and composites excited at the wavelength of 350 nm. Forthe bare ZnO nanoparticles, a weak ultraviolet peak at �387 nmand a strong emission at 500 nm is observed as seen in the Curve(a) of Fig. 4. An intense green emission for the Zinc Oxide nanopar-ticles has been observed by many research groups. However,among the nanoparticles, nanorods of ZnO are the most commoncandidates where green emission is prominent compared to bandgap emission. This is due to the fact that nanorods possess a highersurface to volume ratio, the relative concentration of the oxygenvacancies on the surface increases giving higher defect recombina-tions [33]. Polyaniline however do not exhibit an emission atkexc = 350 nm.

When these nanorods are introduced in the polyaniline matrix,it is observed that the intensity of green emission falls while anenhancement in band gap emission is observed. This quenchingin the defect emission becomes significant as the PANI content incomposite increases as shown in curve (b–e) of Fig. 4. The ratioof the intensities of band gap to green emission for the pure ZnOand its composites are given in Fig. 5.

It is assumed that UV emission efficiency improved because ofthe passivation of green emission centers [34,5]. This can be dueto the an interaction between the surface defects and the hydrogenfrom polyaniline where a bonding is established between the twospecies (BANHABANHAV0AZnO, where B denotes the benzenoidrings). Fig. 6 gives the schematic of the possible linkages betweenZnO and PANI. Sekigushi et al. has shown the effect of hydrogena-tion on luminescence of ZnO where hydrogen plasma passivatesthe green emission centers and enhances the band edge photolu-minescence [35]. It can be suggested that polyanilne acts as aquencher of green luminescence in ZnO where the hydrogen frompolyaniline combines with the oxygen vacancies on the surface ofthe nanorods. Pan et al. suggested that the hydrogen ion forms ametastable complex with the oxygen vacancies of ZnO at roomtemperature [36]. Dev et al. subsequently observed an enhance-ment in band edge emission and passivation of defect levels onincorporation of hydrogen in ZnO nanowires [37]. The quenchingin defect emission becomes more significant as the concentrationof polyaniline in the composites increases, which is likely due tothe more number of H+ ions that are available for combining withthe surface dangling bonds of ZnO nanorods. The neutralization ofthe oxygen vacancy due to the charge transfer from hydrogen tothe native surface defects could be the possible reason for suchan effect [38]. A theoretical study of such an effect is given byDag et al. where a pseudohydrogen atom passivates the end sur-face dangling bonds of ZnO, leaving no surface states in the bandgap [39]. It is observed here that there is �30 times enhancement

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in the ratio of band gap to green emission for the ZP1:2 to bareZnO.

Fig. 8. Raman spectra of (a) doped PANI, (b) ZP1:0.5, (c) ZP1:1, (d) ZP1:1.5, and (e)ZP1:2 at laser power of 0.25 mW.

3.4. Raman spectroscopic studies

The Raman spectrum of ZnO was recorded at a laser power of25 mW with excitation wavelength of 514.5 nm for pure ZnO andis shown in Fig. 7. The peak at 437 cm�1 corresponding to E2 (high)mode of ZnO dominates in the Raman spectrum; this peak isknown to originate from the oxygen atoms in ZnO [40]. A peak at383 cm�1 corresponding to the A1 (TO) mode of ZnO and 2E2 modeappears at 335 cm�1. The quasi mode corresponding to E1 (LO) orA1 (LO) modes at 584 cm�1 matches well with earlier reportedwork on ZnO thin films from our research group [41].

In the case of ZnO/PANI composites the spectrum was recordedat lower laser power of 0.25 mW to avoid burning of the polymerand again with the excitation wavelength of 514.5 nm. Pure PANIand the spectrum for various compositions are shown in Fig. 8. Ra-man spectra of doped polyaniline in the curve (a) of Fig. 8, has CACstretching vibrations of benzene rings at 1623 cm�1, CAN+ at1339 cm�1, CAN vibrations at 1252 cm�1, CAH bending of benzenerings at 1192 cm�1 and NAH bending at 1530 cm�1 reveals thatpolyaniline is in the form of emeraldine salt [42–44]. However,in ZP1:0.5 (Fig. 8b) a peak at 1600 cm�1 is observed which corre-sponds to the CAC stretching of semi-quinones signifying littleinteraction between ZnO and PANI. On increase in content of PANI,for ZP1:1, shown in Fig. 8c, peak at 1489 cm�1 corresponding toC@N stretching enhances whereas peak at 1600 cm�1 in the com-posite diminishes which signifies that semiquinones disappearsand quinones peak starts (1489 cm�1) arising confirming an in-crease in the linkages of ZnO with PANI. Also, stretching vibrationsbecause of the CAH of benzenoid rings at 1192 cm�1 shift to1189 cm�1 and a peak at 1166 cm�1 due to the CAH bending ofquinoids emerges. This peak grows as the content of PANI in thecomposites increases and peak at 1189 cm�1 vanishes off. Finally,the Raman spectra of composite ZP1:2, shown Fig. 8e, reveals ahigh intensity peak at 1489 cm�1, 1166 cm�1 and 1219 cm�1

which corresponds to the C@N, CAH and CAN stretching of qui-noids respectively [45,46]. These results suggests that C@N, CAN,CAC bonds corresponding to quinoid segments become strongerand bonds corresponding to benzenoid segments becomes weakerso a transition from benzenoid to quionoid can be inferred. A sim-ilar observation was reported by Konyushenko et al. where CNTcoating on PANI showed indications of possible interaction be-

Fig. 7. Raman spectrum of pure ZnO done at laser power of 25 mW.

tween single-wall CNT and PANI in Raman spectroscopy [47]. Also,the diminishing intensities of NAH bonds imply a decrease in theH+ ions in the polyaniline chains and consequently influencingthe surface dangling bonds of ZnO nanorods. This is probably be-cause of the hydrogen linkages between the surface oxygen ofZnO nanorods and the NAH group in the polyaniline macromole-cule. Chang et al. has shown detailed study using FTIR techniquewhere the NAH stretching peak shifts towards the lower wave-number revealing a local hydrogen bonding of PANI with the sur-face oxygen of ZnO [6]. Therefore, an interaction between PANIand the inorganic ZnO nanorods can be ascribed to the hydrogenactions as evident from PL results as well. Also, the lower bandsin the Raman spectra of composites are found to be enhanced withincrease in polymer concentration which can be due to the stericrepulsion of rings in the polymer [48]. This also shows a stronginteraction between ZnO and PANI because of the decreased degreeof orbital overlap between p-electrons of the phenyl rings with thelone pair of the nitrogen atom in the PANI molecule creating dis-tortion in the chains. In the case of composites, ZnO peaks arenot evident due to the lower laser power used.

4. Conclusions

The composites of ZnO and PANI were prepared by physicalmixing. A remarkable change in the optical and structural proper-ties of the composites is due to the interfacial interactions betweenZnO and PANI. The nature of such interactions is due to hydrogenlinkages and the surface effects, as observed by Raman spectros-copy and Photoluminescence studies. A large number of danglingbonds present on surface of nanorods are passivated by the hydro-gen from polyaniline. Defect recombination which is considered asa major loss for band gap emission in ZnO nanoparticles can betuned when the nanoparticles are incorporated in Polymer matrix.So, the integration of ZnO nanoparticles into PANI matrix offers arange of nanocomposites having promising applications in lightingand display devices. The two electronically active components leadto the formation of new class of composites with improvedproperties.

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Acknowledgements

The authors would like to acknowledge Department of Geology;University of Delhi for extending the SEM facility. Authors alsowould like to thank University Science and Instrumentation Centre(USIC), University of Delhi for Raman studies. One of the authors S.Shrivastava would like to acknowledge University Grant Commis-sion (UGC), India for funding through the Project F. No. 32-23/2006 (SR).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.compositesb.2012.09.020.

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