Research Article Efficient Charge Transfer Mechanism in ...downloads.hindawi.com/journals/jnm/2014/608572.pdf · (c) F : XRD patterns of (a) ZnO NRs, (b) pure PFO, and (c) PFO/ZnO

Research ArticleEfficient Charge Transfer Mechanism in PolyfluoreneZnONanocomposite Thin Films

Bandar Ali Al-Asbahi12 Mohammad Hafizuddin Haji Jumali1 and Rashad Al-Gaashani3

1 School of Applied Physics Faculty of Science and Technology Universiti Kebangsaan Malaysia (UKM) 43600 BangiSelangor Malaysia

2 Department of Physics Faculty of Science Sanarsquoa University Sanarsquoa Yemen3Department of Physics Thamar University Thamar Yemen

Correspondence should be addressed to Bandar Ali Al-Asbahi alasbahibandargmailcomand Mohammad Hafizuddin Haji Jumali hafizhjukmmy

Received 23 January 2014 Accepted 16 April 2014 Published 7 May 2014

Academic Editor Tianxi Liu

Copyright copy 2014 Bandar Ali Al-Asbahi et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

The optical properties and charge transfer mechanism of poly (991015840-di-n-octylfluorenyl-27-diyl) (PFO)ZnO thin films have beeninvestigatedTheZnOnanorods (NRs)were prepared via amicrowave techniqueThe solution blendingmethodwas used to preparethe PFOZnO nanocomposites X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) were used todetermine the structural properties while UV-Vis and photoluminescence (PL) were employed to investigate the optical propertiesof the films XRD patterns confirmed that there was no variation in the structure of both PFO and ZnO NRs due to the blendingprocess FE-SEM micrographs displayed that ZnO NRs were well coated by PFO in all nanocomposite films The absorptionspectra of the nanocomposite thin films exhibited a red-shift indicating the increment in conjugation length of the PFOZnOnanocomposite Significant quenching in the emission intensity of PFOwas observed in fluorescence spectra of the nanocompositefilmsThis quenching was attributed to efficient charge transfer in the PFOZnO nanocomposites which was further supported bythe shorter PL lifetime of PFOZnO than that of the PFO thin filmThe continuous decline in PL intensity of these nanocompositesis attributed to homogenous dynamic quenching between PFO and ZnO NRs

1 Introduction

The growing interest in polymer based photoelectric devicesin particular conjugated polymer can be attributed to severalfactors such as environmentally friendly adjustable bandgaps high optical absorption coefficient and low-cost fab-rication All of these factors can be achieved because of theflexibility in organic synthesis hence the chemical tailoringtowards the desired properties is possible For examplepoly(3-hexylthiophene) and poly(p-phenylene vinylene) andtheir derivatives have been reported to absorb light at variouswavelengths [1 2]

There are great concerns about the photo stability andthermal stability of the polymers although other pressingproblems are associated with polymer based photoelectric

devices which are poor electron-hole separation and trans-port properties [3 4] While the former concern may beovercome by introducing a more stable polymer the laterproblems aremore challenging Several approaches have beensuggested to inhibit these problems [5ndash7] but a well acceptedstrategy is via hybridization with inorganic semiconductingnanostructures such as metal oxides [8ndash11] Successful initialattempts motivate researchers to look for the right combina-tion between a stable polymer and metal oxide and thus theiroptimum ratio

In terms of stable polymers there are a limited numberof suitable candidates and one of them is poly[991015840-di-n-octylfluorenyl-27-diyl] (PFO) Earlier work reported thatPFO displayed better photo stability and thermal stabilitythan those of the poly(phenylene vinylene)s ldquoPPVsrdquo [12]

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2014 Article ID 608572 8 pageshttpdxdoiorg1011552014608572

2 Journal of Nanomaterials

which are commonly studied PFO is a semicrystalline con-jugated polymer [13ndash15] with lowest unoccupied molecularorbital (LUMO) and highest occupied molecular orbital(HUMO) of minus29 and minus59 eV respectively [16]

As one of the most widely investigated materials ZnO iscommonly used as electron acceptor in polymerinorganicnanocomposites [17ndash20] This metal oxide exhibits greatertendency to stabilize in wurtzite hexagonal form with con-duction band valence band and energy gap of minus42 minus76and 34 eV respectively [18] Most of the earlier works onpolymerZnOnanocomposites demonstrated the photolumi-nescence (PL) quenching which was conveniently claimeddue to efficient charge transfer effect [17ndash19]

One of the prerequisites for good photoelectric mate-rials is efficient charge transfer which in turn depends onthe comparable band structures Benefitting from quantumconfinement effect with close consideration on suitabilitybetween the band structures the combination between amore stable PFO and ZnO nanorods (NRs) is believed toresult in better materials characteristics for photoelectricapplication A common indicator for charge transfer isPL emission quenching Unfortunately observation in PLquenching alone is not sufficient to suggest efficient chargetransfer Additional evidence via measuring the fluorescencelifetime is required However theoretical approach via cal-culating fluorescence lifetime [21] is conveniently used as analternative method especially for researchers having limitedequipment at their disposal The current work is not onlycalculated fluorescence lifetime but also other importantparameters namely the quenching rate constant [22] andphoto-induced electron transfer rate [23] which are usedtogether as supplementary evidence to indicate tendencies ofdetermined systems

Evidence for charge transfer in PFOZnO nanocompos-ites primarily from PL quenching was demonstrated in thiswork In addition values for the aforementioned parametersin case of dry thin films at room temperature were calcu-lated to provide additional evidence for the effective chargetransfers The quenching type and quenching efficiencywere also determined in order to have more comprehensiveinformation on the suitability of this nanocomposite as anactive layer in the photoelectric devices

2 Experimental Details

ZnO NRs were synthesized using the microwave techniqueA microwave oven (Sanyo Electric model no EM-G430245GHz 1000W) was used in this work Deionized water(resistivity 182MΩsdotcm) was used to prepare zinc acetatedihydrate 015M (C

4H6O4Znsdot2H

2O) (Aldrich Fluka) aque-

ous solution (A) and 333M of NaOH aqueous solution (B)at room temperature The zinc acetate aqueous solution (A)was stirred for 5min and then the aqueous solution (B) wasadded dropwise under steady stirring until a white solutionwas formed Then the final white solution was exposedto microwave radiation in the oven for 5min before beingleft to cool naturally to 30∘C Subsequently the solutionwas centrifuged at 4000 rpm for 5min to collect the white

precipitate produced after radiation The precipitate waswashed with alcohol and deionized water alternately for fourtimes and then was dried under atmosphere at 60∘C for 24 hFinally the obtained powder was heated at 500∘C for 2 h in aconventional oven

The PFO (product from Sigma Aldrich USA) withaverage molecular weight of 58200 was used in this study Toprepare the nanocomposites firstly the PFO was dissolvedin tetrahydrofuran (THF) at a concentration of 30mgmL(PFOTHF) The PFOZnO nanocomposites were preparedin solution form by blending the fixed concentration of ZnO(30mgmL) with various ratios of PFO (0 10 30 50 and 70wt) followed by sonication for 5min Then the solutionwas deposited onto a glass substrate (12 cm times 2 cm) to formthree layers by a spin coating technique (rotational speedof 2000 rpm for 30 s) Finally it was left to dry at roomtemperature

The structural characterization of the films was carriedout by X-ray diffraction (Bruker-AXS D8 Advance X-raydiffractometer using Cu-K120572 radiation source with 120582 =15418 A)The nanostructure of the films was observed usinga field emission scanning electron microscope (FE-SEM)model Supra 55VP with operating voltage of 300 kV Theabsorption and emission spectra of the films were recordedusing UV-Vis spectrophotometer (Perkin Elmer LAMBDA900) and Luminescence spectrometer (Perkin Elmer LS55)respectively

3 Results and Discussion

The X-ray diffractograms for the ZnO NRs PFO andPFOZnO nanocomposite thin films are shown in Figure 1The diffractogram of ZnO NRs shows that all peaks can beindexed to the wurtzite hexagonal structure of ZnO crystaland no other crystalline peaks of any impuritieswere detected(Figure 1(a)) By comparison the XRD diffractograms forPFO consist of a narrow peak at 7∘ and broad peaks inbetween 10∘ and 30∘ (2120579) which can be deconvoluted intothree broad peaks at 155∘ 200∘ and 231∘ This observationconfirmed that the PFO is in semicrystalline phase as sug-gested by earlier reports [13ndash15] The X-ray diffractogramsfor nanocomposite thin films consist of broad PFO peakswith several narrow peaks representing ZnO NRs Theintensities of ZnO NRs peaks in these nanocomposite filmsare enhanced as more ZnO NRs are added into the polymer(Figure 1(c)) More importantly the structure of both PFOand ZnO NRs remained unchanged even after the blendingprocess

Figure 2 displays the morphology of ZnO NRs andPFOZnO nanocomposites The ZnO existed as wurtzitehexagonal nanorods (pencil-like) having an average diameterof about 95 nm and length of 800 nm (Figure 2(a)) Thenanorods were closely packed which were grown from acommon seed The smooth hexagonal structures of the ZnONRs confirm that the nanorods were well crystalline aswurtzite unit cell

The general view of nanocomposite film (Figure 2(b))reveals the uniformity of the deposited film A closer view of

Journal of Nanomaterials 3

10 20 30 40 50 60

(110

)

(102

)

(002

)(100

)

Inte

nsity

(au

)(101)

2120579 (∘)

(a)

10 20 30 40 50

Inte

nsity

(au

)

2120579 (∘)

(b)

10 20 30 40 50 60

(110

)

(102

)

(101

)(0

02)

Inte

nsity

(au

)

(100

)

70wt

50wt

10wt

30wt

2120579 (∘)

(c)

Figure 1 XRD patterns of (a) ZnO NRs (b) pure PFO and (c) PFOZnO nanocomposite films

the films showed that the majority of ZnO NRs were brokeninto smaller rods and covered by PFO in all nanocompositesthin films (Figures 2(c)ndash2(f)) At 10 wt ZnO NRs allnanorods were covered and uniformly distributed As theZnONRs content increased the nanorods were thinly coatedby the PFO At 70wt there were many uncoated nanorodbundles which are believed to be due to the lack of PFO

The absorption spectra of all films were presented inFigure 3TheUV-Vis spectrum of PFO film displays a 120582max at404 nm which corresponds to a Π-Πlowast transition [24] anda shoulder around 435 nm as the fingerprint for 120573-phaseof PFO [25] By comparison the absorption edge of ZnONRs was located in the UV region (lt360 nm) which issimilar to an earlier report [21] Upon addition of ZnO NRsthe absorbance of the nanocomposite films was dramaticallyreduced No additional absorption peak was observed in thespectra confirming the absence of interaction between PFOand ZnO NRs at the ground states [19]

Relative to pure PFO all absorption peaks of PFOZnOnanocomposite thin films were red-shifted (around 11 nm)

signifying the extension in the conjugation length of thenanocomposites [26ndash28]

Figure 4 shows the fluorescence spectra of ZnONRs purePFO and PFOZnO nanocomposite thin films with variousZnO contentsTheUVemission in the fluorescence spectrumof ZnO NRs (Figure 4(a)) comprises two overlapping bandsThe peak of the first band centered at 378 nm which is relatedto the near-band edge emission of the wide band gap ZnONRs This band was due to the recombination of excitoniccentres in the nanorod [29] Additionally a broad green bandat 460ndash550 nm was observed which is mainly caused by theintrinsic defects or oxygen vacancies in the ZnO NRs andresults in the recombination of a photo-generated hole withthe single ionized charge state of this defect [21 30]

Figure 4(b) reveals the emission peaks of PFO at 427 442465 and 500 nm which were assigned to 0-0 0-1 0ndash2 and 0ndash3 vibronic transition respectively As in absorption spectraaddition of ZnO NRs resulted in red-shift of the emissionpeaks position More importantly a dramatic quenching ofPFO emission intensity was observed and above 30wt of


100 nm

(a)

2120583m

(b)

100 nm

(c)

100 nm

(d)

100 nm

(e)

100 nm

(f)

Figure 2 FE-SEM images of ZnO NRs and PFOZnO nanocomposites (a) ZnO NRs (b) distribution of PFOZnO nanocomposite on thinfilm at 90 10 wt (c) 90 10 wt (d) 70 30wt (e) 50 50wt and (f) 30 70wt nanocomposite thin films

ZnO NRs content the 427 nm peak of pure PFO completelydisappeared

Since there was no overlap between the emission spec-trum of PFO (Figure 4(b)) and the absorption spectrumof ZnO NRs (Figure 3(a)) dramatic PL quenching in PFOin the hybrid system was due to charge transfer from thepolymer to the metal oxide NRs [31] Due to the presence of

ZnO NRs recombination of exciton in PFO was inhibitedInstead the electron-hole pairs in the polymer dissociated atthe PFOZnO interface with the electrons moving across theinterface into ZnO NRs while the holes remain in the PFO

This argument is supported by the fact that the con-duction and valence bands of ZnO are relatively lower thanthose of PFO Therefore from an energy level point of


320 330 340 350 360000

002

004

006

008

Abso

rban

ce (a

u)

120582 (nm)

(a)

350 400 450 500 550 600000

005

010

015

020

025

030

035

040

PFO

Abso

rban

ce (a

u)

120582 (nm)

70wt50wt

10wt30wt

(b)

Figure 3 Absorption spectra of PFOZnO nanocomposite films

350 400 450 500 55000

02

04

06

08

10

12

Nor

mal

ized

int

(au

)

378

120582 (nm)

(a)

400 450 500 5500

50

100

150

200

250

300

Fluo

resc

ence

int

(au

)

Pure PFO

120582 (nm)

70wt50wt

10wt30wt

(b)

Figure 4 Fluorescence spectra of (a) pure ZnO NRs excitation at 320 nm and (b) pure PFO and PFOZnO nanocomposite films excitationat 355 nm

PFOZnO NRs

HOMO VB

CB

PFOZnO NRs

Interfacial charge transfer and separation

minus59 eV

minus42 eV

minus76 eV

h

Figure 5 The interfacial charge generation transfer and separation between PFO and ZnO NRs (Open circle hole closed circle electron)


0 5 10 15 20 25

1

2

3

4

5

6

Concentration of ZnO NRs (mM)

I 0I

KSV = 19327 Mminus1

R2 = 0988

Figure 6 Stern-Volmer plot for fluorescence quenching of 3mgmL(PFO) by ZnO NRs

view the effective transfer of electron across the interface ispossible (Figure 5) The dissociation of electron-hole pairsat interface and subsequent efficient electrons transfer toZnO NRs suggested that the PFOZnO nanocomposite is apromising p-n-type nanocomposite material

Effective charge transfer between PFO and ZnO NRs canbe further confirmed from important parameters such asquenching rate constant (119870SV) lifetime (120591) of excited PFO inthe presence of ZnO NRs bimolecular quenching rate (119896

119902)

and photo-induced electron transfer rate (119870ET) 119870SV is thevalue of the slope in Stern-Volmer plot The 120591 119896

119902 and 119870ET

can be calculated once the119870SV value is obtainedUtilizing Stern-Volmer relationship [22] a linear Stern-

Volmer plot with 1198772 = 0988 can be drawn and presented inFigure 6The linear relationship is a very important indicatorof homogenous dynamic quenching of this hybrid materialThe slop of the plot was sim19327Mminus1 which was comparablewithMEH-PPVTiO

2nanocomposite material [31]This119870SV

value signified that 50 of the PFO emission quenched atZnO NRs concentration of 52mM

In homogeneous dynamic quenching the Stern-Volmerequation can be rewritten as [22]

120591

0

120591

= 1 + 119870SV [119876] (1)

where 120591 and 1205910are the excited state lifetime of PFO in the

presence and absence of ZnO NRs respectively and [119876] isthe concentration of ZnO NRs

Table 1 shows the effect of quencher (ZnONRs) onthe photoluminescence lifetime of PFO All nanocompositematerials exhibited shorter lifetime than the pristine PFOthin film 120591

0= 0346 ns [32] confirming the efficient

charge transfer from PFO to ZnO NRs at the interfaces[21] Furthermore compatible band structure between bothmaterials induced the electron transfer from LUMO of PFOto CB of ZnO (Figure 5)

The bimolecular quenching rate (119896119902) and the photo-

induced electron transfer rate (119870ET) for PFOZnO nanocom-posite films can be calculated using (2) [33] and (3) [34] asfollows

119870SV = 1205910119896119902 (2)

119870ET =1

120591

minus

1

120591

0

(3)

Using (2) the 119896119902has been evaluated as 559 times 1011Mminus1sdotSminus1

This value is significantly greater than the minimum valuefor efficient quenching which is 1 times 1010Mminus1sdotSminus1 [22] Thehigh value of 119896

119902obtained in this work proves the good com-

bination between the ZnO NRs and PFO Equally importanthigh 119896

119902value signified the admirable quality of the interface

between these two materials The values of 119896ET calculatedusing (3) were constantly increased from 153 times 109 to 1273 times109 Sminus1 as more ZnO NRs are added into PFO (Table 1) Thisfinding provides additional evidence illustrating significantimprovement in quenching efficiency and charge transferin PFOZnO nanocomposite upon increment of ZnO NRsThe calculated 119896ET values in this work are comparable withother organic-metal oxide nanocomposite systems such asphycocyaninTiO

2[23] In other words the photo-induced

electron transfer rate from PFO monomers to ZnO NRs canbe controlled with the ZnO NRs content High ZnO NRscontent has a high surface area which gives an increaseto a lot of defects and raises the electron transfer rateConsequently more efficient PFOZnO nanocomposite canbe obtained as an active layer in photoelectric devices

4 Conclusion

This work reports on the study of charge transfer mechanismin PFOZnO nanocomposite thin films ZnO NRs weresuccessfully synthesized using the microwave techniquewhereas PFOZnO nanocomposites were successfully pre-pared via solution blending method prior to producing thefilms using spin coating technique The X-ray diffractogramsconfirmed the purity of ZnO NRs and the existence ofamorphous phase of PFO and crystalline phase of ZnO NRsAs the ZnO NRs content increased the emission intensity ofPFO reduced suggesting enhanced quenching fluorescence ofPFOThe quenching rate (119870SV) and the bimolecular quench-ing rate (119870

119902) constants were estimated to be about 19327Mminus1

and 559 times 1011Mminus1sdotSminus1 respectively In addition as the ZnONRs content increased from 10 to 70wt the photo-inducedelectron transport rate increased from 153 times 109 to 1273 times109 Sminus1 Moreover it was found that the photoluminescencelifetime for the PFOZnO thin films is much shorter thanthat of the pure PFO thin film These findings confirmed theefficient electron transfer across PFOZnO interface whichare very crucial for greater improvement in the performanceof the PFOZnO based photoelectric devices


Table 1 Photo-induced electron transport rate and lifetime values of PFOZnO nanocomposite thin films

The weight ratio of ZnO NRs(wt)

Concentration of ZnO NRs [119876](mM)

Lifetime of PFO120591 (ns)

Photo-induced electrontransport rate 119870ET (S

minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273

Conflict of Interests

The authors declare that they have no conflict of interestsregarding the publication of this paper

Acknowledgment

The authors would like to thank the Universiti KebangsaanMalaysia (UKM) for providing excellent research facilitiesunder the Research University Grants DPP-2013-048 andDLP-2013-012

References

[1] F Padinger R S Rittberger and N S Sariciftci ldquoEffectsof postproduction treatment on plastic solar cellsrdquo AdvancedFunctional Materials vol 13 no 1 pp 85ndash88 2003

[2] Q Zhou Q Hou L Zheng X Deng G Yu and Y CaoldquoFluorene-based low band-gap copolymers for high perfor-mance photovoltaic devicesrdquoApplied Physics Letters vol 84 no10 pp 1653ndash1655 2004

[3] W U Huynh J J Dittmer and A P Alivisatos ldquoHybridnanorod-polymer solar cellsrdquo Science vol 295 no 5564 pp2425ndash2427 2002

[4] D Haarer ldquoPhotoconductive polymers structure mechanismsand propertiesrdquoDie Angewandte Makromolekulare Chemie vol183 no 1 pp 197ndash220 1990

[5] S R Forrest ldquoThe road to high efficiency organic light emittingdevicesrdquo Organic Electronics Physics Materials Applicationsvol 4 no 2-3 pp 45ndash48 2003

[6] F Li Y Du Y Chen L Chen J Zhao and P Wang ldquoDirectapplication of P3HT-DOPOZnO nanocomposites in hybridbulk heterojunction solar cells via grafting P3HT onto ZnOnanoparticlesrdquo Solar Energy Materials and Solar Cells vol 97pp 64ndash70 2012

[7] T P Nguyen C W Lee S Hassen and H C Le ldquoHybridnanocomposites for optical applicationsrdquo Solid State Sciencesvol 11 no 10 pp 1810ndash1814 2009

[8] J K Lee W L Ma C J Brabec et al ldquoProcessing additivesfor improved efficiency from bulk heterojunction solar cellsrdquoJournal of the American Chemical Society vol 130 no 11 pp3619ndash3623 2008

[9] A J Moule and K Meerholz ldquoControlling morphology inpolymer-fullerene mixturesrdquoAdvancedMaterials vol 20 no 2pp 240ndash245 2008

[10] N Z Yahya and M Rusop ldquoInvestigation on the optical andsurface morphology of conjugated polymer MEH-PPV ZnOnanocomposite thin filmsrdquo Journal of Nanomaterials vol 2012Article ID 793679 4 pages 2012

[11] M Alam N M Alandis A A Ansari and M R Shaik ldquoOpti-cal and electrical studies of polyanilineZnO nanocompositerdquoJournal of Nanomaterials vol 2013 Article ID 157810 5 pages2013

[12] A M Assaka P C Rodrigues A R M de Oliveira et alldquoNovel fluorine containing polyfluorenes with efficient blueelectroluminescencerdquo Polymer vol 45 no 21 pp 7071ndash70812004

[13] G C Farial T S Plivelic R F Cossiello et al ldquoAmultitechniquestudy of structure and dynamics of polyfluorene cast films andthe influence on their photoluminescencerdquo Journal of PhysicalChemistry B vol 113 no 33 pp 11403ndash11413 2009

[14] M H H Jumali B A Al-Asbahi C C Yap M M Salleh andM S Alsalhi ldquoOptoelectronic property enhancement of conju-gated polymer in poly(991015840-di-n-octylfluorenyl-27-diyl)titaniananocompositesrdquoThin Solid Films vol 524 pp 257ndash262 2012

[15] MHH Jumali B AAl-Asbahi C C YapMM Salleh andMS Alsalhi ldquoOptical properties of poly (991015840-di-n-octylfluorenyl-27-diyl)amorphous SiO

2nanocomposite thin filmsrdquo Sains

Malaysiana vol 42 no 8 pp 1151ndash1157 2013[16] M Bajpai R Srivastava M N Kamalasanan R S Tiwari and

S Chand ldquoCharge transport and microstructure in PFOMEH-PPV polymer blend thin filmsrdquo Synthetic Metals vol 160 no15-16 pp 1740ndash1744 2010

[17] I Park Y Lim S Noh et al ldquoEnhanced photovoltaic perfor-mance of ZnO nanoparticlepoly(phenylene vinylene) hybridphotovoltaic cells by semiconducting surfactantrdquo Organic Elec-tronics Physics Materials Applications vol 12 no 3 pp 424ndash428 2011

[18] LQian Y Zheng K R Choudhury et al ldquoElectroluminescencefrom light-emitting polymerZnO nanoparticle heterojunc-tions at sub-bandgap voltagesrdquo Nano Today vol 5 no 5 pp384ndash389 2010

[19] R Jetson K Yin K Donovan and Z Zhu ldquoEffects of surfacemodification on the fluorescence properties of conjugated poly-merZnO nanocompositesrdquo Materials Chemistry and Physicsvol 124 no 1 pp 417ndash421 2010

[20] K Yuan F Li L Chen and Y Chen ldquoApproach to a blockpolymer precursor from poly(3-hexylthiophene) nitroxide-mediated in situ polymerization for stabilization of poly(3-hexylthiophene)ZnO hybrid solar cellsrdquo Thin Solid Films vol520 no 19 pp 6299ndash6306 2012

[21] Y Y Lin C W Chen W C Yen W F Su C H Ku and JJ Wu ldquoImproved performance of polymerTiO

2nanorod bulk

heterojunction photovoltaic devices by interface modificationrdquoApplied Physics Letters vol 92 no 5 Article ID 053312 4 pages2008

[22] J R Lakowicz Principles of Fluorescence Spectroscopy SpringerNew York NY USA 3rd edition 2008


[23] A Kathiravan and R Renganathan ldquoPhotosensitization of col-loidal TiO

2nanoparticles with phycocyanin pigmentrdquo Journal

of Colloid and Interface Science vol 335 no 2 pp 196ndash202 2009[24] Z Han J Zhang X Yang H Zhu and W Cao ldquoSynthesis

and application in solar cell of poly(3-octylthiophene)titaniananotubes compositerdquo Organic Electronics Physics MaterialsApplications vol 11 no 8 pp 1449ndash1460 2010

[25] R Palacios P Formentin E Martinez-Ferrero J Pallaresand L F Marsal ldquo120573-phase morphology in ordered poly(99-dioctylfluorene) nanopillars by template wetting methodrdquoNanoscale Research Letters vol 6 no 1 pp 1ndash5 2011

[26] G Chen and P Wang ldquoAlteration of the optical properties ofpoly 991015840-dioctylfluorene by TiO

2nanocrystallinerdquo Journal of

Non-Crystalline Solids vol 352 no 23ndash25 pp 2536ndash2538 2006[27] T W Hagler K Pakbaz K F Voss and A J Heeger ldquoEnhanced

order and electronic delocalization in conjugated polymersoriented by gel processing in polyethylenerdquo Physical Review Bvol 44 no 16 pp 8652ndash8666 1991

[28] K Pichler D A Halliday D D C Bradley P L BurnR H Friend and A B Holmes ldquoOptical spectroscopy ofhighly ordered poly(p-phenylene vinylene)rdquo Journal of PhysicsCondensed Matter vol 5 no 38 pp 7155ndash7172 1993

[29] M H Huang Y Wu H Feick N Tran E Weber and P YangldquoCatalytic growth of zinc oxide nanowires by vapor transportrdquoAdvanced Materials vol 13 no 2 pp 113ndash116 2001

[30] D K Bhat ldquoFacile synthesis of ZnO nanorods by microwaveirradiation of zinc-hydrazine hydrate complexrdquo NanoscaleResearch Letters vol 3 no 1 pp 31ndash35 2008

[31] T Vats S N Sharma M Kumar et al ldquoComparison of photo-stability optical and structural properties of TiO

2conjugated

polymer hybrid composites prepared via different methodsrdquoThin Solid Films vol 519 no 3 pp 1100ndash1105 2010

[32] B A Al-Asbahi M H H Jumali C C Yap M H Flaifel andM M Salleh ldquoPhotophysical properties and energy transfermechanism of PFOFluorol 7GA hybrid thin filmsrdquo Journal ofLuminescence vol 142 pp 57ndash65 2013

[33] F Ricchelli ldquoPhotophysical properties of porphyrins in biologi-cal membranesrdquo Journal of Photochemistry and Photobiology BBiology vol 29 no 2-3 pp 109ndash118 1995

[34] J Zimmermann J von Gersdorff H Kurreck and B RoderldquoDetermination of the electron transfer parameters of a cova-lently linked porphyrin-quinonewithmesogenic substituentsmdashoptical spectroscopic studies in solutionrdquo Journal of Photochem-istry andPhotobiology B Biology vol 40 no 3 pp 209ndash217 1997

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Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Nano

materials


Journal ofNanomaterials


which are commonly studied PFO is a semicrystalline con-jugated polymer [13ndash15] with lowest unoccupied molecularorbital (LUMO) and highest occupied molecular orbital(HUMO) of minus29 and minus59 eV respectively [16]

As one of the most widely investigated materials ZnO iscommonly used as electron acceptor in polymerinorganicnanocomposites [17ndash20] This metal oxide exhibits greatertendency to stabilize in wurtzite hexagonal form with con-duction band valence band and energy gap of minus42 minus76and 34 eV respectively [18] Most of the earlier works onpolymerZnOnanocomposites demonstrated the photolumi-nescence (PL) quenching which was conveniently claimeddue to efficient charge transfer effect [17ndash19]

One of the prerequisites for good photoelectric mate-rials is efficient charge transfer which in turn depends onthe comparable band structures Benefitting from quantumconfinement effect with close consideration on suitabilitybetween the band structures the combination between amore stable PFO and ZnO nanorods (NRs) is believed toresult in better materials characteristics for photoelectricapplication A common indicator for charge transfer isPL emission quenching Unfortunately observation in PLquenching alone is not sufficient to suggest efficient chargetransfer Additional evidence via measuring the fluorescencelifetime is required However theoretical approach via cal-culating fluorescence lifetime [21] is conveniently used as analternative method especially for researchers having limitedequipment at their disposal The current work is not onlycalculated fluorescence lifetime but also other importantparameters namely the quenching rate constant [22] andphoto-induced electron transfer rate [23] which are usedtogether as supplementary evidence to indicate tendencies ofdetermined systems

Evidence for charge transfer in PFOZnO nanocompos-ites primarily from PL quenching was demonstrated in thiswork In addition values for the aforementioned parametersin case of dry thin films at room temperature were calcu-lated to provide additional evidence for the effective chargetransfers The quenching type and quenching efficiencywere also determined in order to have more comprehensiveinformation on the suitability of this nanocomposite as anactive layer in the photoelectric devices

2 Experimental Details

ZnO NRs were synthesized using the microwave techniqueA microwave oven (Sanyo Electric model no EM-G430245GHz 1000W) was used in this work Deionized water(resistivity 182MΩsdotcm) was used to prepare zinc acetatedihydrate 015M (C

4H6O4Znsdot2H

2O) (Aldrich Fluka) aque-

ous solution (A) and 333M of NaOH aqueous solution (B)at room temperature The zinc acetate aqueous solution (A)was stirred for 5min and then the aqueous solution (B) wasadded dropwise under steady stirring until a white solutionwas formed Then the final white solution was exposedto microwave radiation in the oven for 5min before beingleft to cool naturally to 30∘C Subsequently the solutionwas centrifuged at 4000 rpm for 5min to collect the white

precipitate produced after radiation The precipitate waswashed with alcohol and deionized water alternately for fourtimes and then was dried under atmosphere at 60∘C for 24 hFinally the obtained powder was heated at 500∘C for 2 h in aconventional oven

The PFO (product from Sigma Aldrich USA) withaverage molecular weight of 58200 was used in this study Toprepare the nanocomposites firstly the PFO was dissolvedin tetrahydrofuran (THF) at a concentration of 30mgmL(PFOTHF) The PFOZnO nanocomposites were preparedin solution form by blending the fixed concentration of ZnO(30mgmL) with various ratios of PFO (0 10 30 50 and 70wt) followed by sonication for 5min Then the solutionwas deposited onto a glass substrate (12 cm times 2 cm) to formthree layers by a spin coating technique (rotational speedof 2000 rpm for 30 s) Finally it was left to dry at roomtemperature

The structural characterization of the films was carriedout by X-ray diffraction (Bruker-AXS D8 Advance X-raydiffractometer using Cu-K120572 radiation source with 120582 =15418 A)The nanostructure of the films was observed usinga field emission scanning electron microscope (FE-SEM)model Supra 55VP with operating voltage of 300 kV Theabsorption and emission spectra of the films were recordedusing UV-Vis spectrophotometer (Perkin Elmer LAMBDA900) and Luminescence spectrometer (Perkin Elmer LS55)respectively

3 Results and Discussion

The X-ray diffractograms for the ZnO NRs PFO andPFOZnO nanocomposite thin films are shown in Figure 1The diffractogram of ZnO NRs shows that all peaks can beindexed to the wurtzite hexagonal structure of ZnO crystaland no other crystalline peaks of any impuritieswere detected(Figure 1(a)) By comparison the XRD diffractograms forPFO consist of a narrow peak at 7∘ and broad peaks inbetween 10∘ and 30∘ (2120579) which can be deconvoluted intothree broad peaks at 155∘ 200∘ and 231∘ This observationconfirmed that the PFO is in semicrystalline phase as sug-gested by earlier reports [13ndash15] The X-ray diffractogramsfor nanocomposite thin films consist of broad PFO peakswith several narrow peaks representing ZnO NRs Theintensities of ZnO NRs peaks in these nanocomposite filmsare enhanced as more ZnO NRs are added into the polymer(Figure 1(c)) More importantly the structure of both PFOand ZnO NRs remained unchanged even after the blendingprocess

Figure 2 displays the morphology of ZnO NRs andPFOZnO nanocomposites The ZnO existed as wurtzitehexagonal nanorods (pencil-like) having an average diameterof about 95 nm and length of 800 nm (Figure 2(a)) Thenanorods were closely packed which were grown from acommon seed The smooth hexagonal structures of the ZnONRs confirm that the nanorods were well crystalline aswurtzite unit cell

The general view of nanocomposite film (Figure 2(b))reveals the uniformity of the deposited film A closer view of


10 20 30 40 50 60

(110

)

(102

)

(002

)(100

)

Inte

nsity

(au

)(101)

2120579 (∘)

(a)

10 20 30 40 50

Inte

nsity

(au

)

2120579 (∘)

(b)

10 20 30 40 50 60

(110

)

(102

)

(101

)(0

02)

Inte

nsity

(au

)

(100

)

70wt

50wt

10wt

30wt

2120579 (∘)

(c)









100 nm

(a)

2120583m

(b)

100 nm

(c)

100 nm

(d)

100 nm

(e)

100 nm

(f)







320 330 340 350 360000

002

004

006

008

Abso

rban

ce (a

u)

120582 (nm)

(a)

350 400 450 500 550 600000

005

010

015

020

025

030

035

040

PFO

Abso

rban

ce (a

u)

120582 (nm)

70wt50wt

10wt30wt

(b)


350 400 450 500 55000

02

04

06

08

10

12

Nor

mal

ized

int

(au

)

378

120582 (nm)

(a)

400 450 500 5500

50

100

150

200

250

300

Fluo

resc

ence

int

(au

)

Pure PFO

120582 (nm)

70wt50wt

10wt30wt

(b)


PFOZnO NRs

HOMO VB

CB

PFOZnO NRs


minus59 eV

minus42 eV

minus76 eV

h



0 5 10 15 20 25

1

2

3

4

5

6


I 0I

KSV = 19327 Mminus1

R2 = 0988




119902)


119902 and 119870ET






120591

0

120591

= 1 + 119870SV [119876] (1)








119870SV = 1205910119896119902 (2)

119870ET =1

120591

minus

1

120591

0

(3)









4 Conclusion










minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273



Acknowledgment


References
























2nanorod bulk

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




10 20 30 40 50 60

(110

)

(102

)

(002

)(100

)

Inte

nsity

(au

)(101)

2120579 (∘)

(a)

10 20 30 40 50

Inte

nsity

(au

)

2120579 (∘)

(b)

10 20 30 40 50 60

(110

)

(102

)

(101

)(0

02)

Inte

nsity

(au

)

(100

)

70wt

50wt

10wt

30wt

2120579 (∘)

(c)









100 nm

(a)

2120583m

(b)

100 nm

(c)

100 nm

(d)

100 nm

(e)

100 nm

(f)







320 330 340 350 360000

002

004

006

008

Abso

rban

ce (a

u)

120582 (nm)

(a)

350 400 450 500 550 600000

005

010

015

020

025

030

035

040

PFO

Abso

rban

ce (a

u)

120582 (nm)

70wt50wt

10wt30wt

(b)


350 400 450 500 55000

02

04

06

08

10

12

Nor

mal

ized

int

(au

)

378

120582 (nm)

(a)

400 450 500 5500

50

100

150

200

250

300

Fluo

resc

ence

int

(au

)

Pure PFO

120582 (nm)

70wt50wt

10wt30wt

(b)


PFOZnO NRs

HOMO VB

CB

PFOZnO NRs


minus59 eV

minus42 eV

minus76 eV

h



0 5 10 15 20 25

1

2

3

4

5

6


I 0I

KSV = 19327 Mminus1

R2 = 0988




119902)


119902 and 119870ET






120591

0

120591

= 1 + 119870SV [119876] (1)








119870SV = 1205910119896119902 (2)

119870ET =1

120591

minus

1

120591

0

(3)









4 Conclusion










minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273



Acknowledgment


References
























2nanorod bulk

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




100 nm

(a)

2120583m

(b)

100 nm

(c)

100 nm

(d)

100 nm

(e)

100 nm

(f)







320 330 340 350 360000

002

004

006

008

Abso

rban

ce (a

u)

120582 (nm)

(a)

350 400 450 500 550 600000

005

010

015

020

025

030

035

040

PFO

Abso

rban

ce (a

u)

120582 (nm)

70wt50wt

10wt30wt

(b)


350 400 450 500 55000

02

04

06

08

10

12

Nor

mal

ized

int

(au

)

378

120582 (nm)

(a)

400 450 500 5500

50

100

150

200

250

300

Fluo

resc

ence

int

(au

)

Pure PFO

120582 (nm)

70wt50wt

10wt30wt

(b)


PFOZnO NRs

HOMO VB

CB

PFOZnO NRs


minus59 eV

minus42 eV

minus76 eV

h



0 5 10 15 20 25

1

2

3

4

5

6


I 0I

KSV = 19327 Mminus1

R2 = 0988




119902)


119902 and 119870ET






120591

0

120591

= 1 + 119870SV [119876] (1)








119870SV = 1205910119896119902 (2)

119870ET =1

120591

minus

1

120591

0

(3)









4 Conclusion










minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273



Acknowledgment


References
























2nanorod bulk

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




320 330 340 350 360000

002

004

006

008

Abso

rban

ce (a

u)

120582 (nm)

(a)

350 400 450 500 550 600000

005

010

015

020

025

030

035

040

PFO

Abso

rban

ce (a

u)

120582 (nm)

70wt50wt

10wt30wt

(b)


350 400 450 500 55000

02

04

06

08

10

12

Nor

mal

ized

int

(au

)

378

120582 (nm)

(a)

400 450 500 5500

50

100

150

200

250

300

Fluo

resc

ence

int

(au

)

Pure PFO

120582 (nm)

70wt50wt

10wt30wt

(b)


PFOZnO NRs

HOMO VB

CB

PFOZnO NRs


minus59 eV

minus42 eV

minus76 eV

h



0 5 10 15 20 25

1

2

3

4

5

6


I 0I

KSV = 19327 Mminus1

R2 = 0988




119902)


119902 and 119870ET






120591

0

120591

= 1 + 119870SV [119876] (1)








119870SV = 1205910119896119902 (2)

119870ET =1

120591

minus

1

120591

0

(3)









4 Conclusion










minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273



Acknowledgment


References
























2nanorod bulk

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0 5 10 15 20 25

1

2

3

4

5

6


I 0I

KSV = 19327 Mminus1

R2 = 0988




119902)


119902 and 119870ET






120591

0

120591

= 1 + 119870SV [119876] (1)








119870SV = 1205910119896119902 (2)

119870ET =1

120591

minus

1

120591

0

(3)









4 Conclusion










minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273



Acknowledgment


References
























2nanorod bulk

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









minus1) times109

0 0 0346 mdash10 273 0226 15330 87 0129 48650 154 0087 86070 230 0064 1273



Acknowledgment


References
























2nanorod bulk

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials

















2conjugated












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

Research Article Efficient Charge Transfer Mechanism in ...downloads.hindawi.com/journals/jnm/2014/608572.pdf · (c) F : XRD patterns of (a) ZnO NRs, (b) pure PFO, and (c) PFO/ZnO