Journal of Colloid and Interface Science333(2009)128–134Contents lists available at ScienceDirectJournal of Colloid and Interface Science/locate/jcisSynthesis of poly(3-hexylthiophene)grafted TiO2nanotube composite Ming-De Lu a,Sze-Ming Yang b,∗a Photovoltaics Technology Center,Industrial Technology Research Institute,Chutung31040,Taiwanb Department of Chemical and Materials Engineering,National Central University,Chung-Li32054,Taiwana r t i c l e i n f o ab s t r ac tArticle history:Received22May2008Accepted30January2009 Available online6February2009Keywords:TitaniaPoly(3-hexylthiophene) NanotubeSolar cell material A composite of poly(3-hexylthiophene)(P3HT)grafted on TiO2nanotubes was synthesized.It was characterized using XRD,TEM,TGA,FTIR and XPS.Cyclic voltammetry(CV)was used to elucidate the electrochemical behavior and evaluate the HOMO and LUMO energy levels.Photoluminescence(PL) measurements show that the emission intensity of P3HT mixed with TiO2nanotubes was one third of that of random P3HT,while that of P3HT grafted onto TiO2nanotubes was10%of random P3HT. The results show that the P3HT grafted onto TiO2nanotubes is more efficient in photoinduced charge transfer than a physical mixture of P3HT and TiO2nanotubes,indicating this composite has potential for the fabricating hybrid organic–inorganic solid state solar cells.Crown Copyright©2009Published by Elsevier Inc.All rights reserved.1.IntroductionAmong various organic solar cells,the organic–inorganic hy-brid solar cell is one of the most promising,since it has not only a large interface area where excitons,bound electron–hole pairs, may effectively dissociate,but also two separate channels for ef-ficient electron and hole transport.In an organic–inorganic hybrid solar cell,an inorganic semiconductor such as titanium oxide acts as an electron acceptor,and a conducting polymer such as poly-thiophene can be adopted to absorb light and transport holes. Excitons formed after light is absorbed in organicfilms typically travel less than20nm before recombination[1,2],so electron acceptors must be intermixed on the nanometer scale with or-ganic semiconductors,to obtain a high charge separation yield. The smallness of nanoparticles results in a large interface between the conducting polymer and the inorganic semiconductor,and in-creases the tunability and efficiency of hetero-junctions.Studies of nanocrystalline titanium oxide and conducting polymer photo-voltaic cells have recently been published[3–8].Most studies on conjugated polymer–TiO2PV cells have involved sintering together TiO2nanocrystals andfilling the pores with a conjugated polymer. Fan et al.reported the fabrication of MEH-PPV/TiO2photovolatic device,which had a power conversion efficiency of1.6%[9].Qiao et al.described the fabrication of a water-soluble polythiphene/TiO2 photovoltaic device,which had a power conversion efficiency of 0.13%[10].Huisman et al.reported the fabrication of a poly(3-octylthiphene)/TiO2photovoltaic device,which had a power con-version efficiency of0.06%[11].Watanabe et al.reported the fab-*Corresponding author.Fax:+88634252296.E-mail address:smyang@.tw(S.M.Yang).rication of a TiO2/poly(3-hexylthiophene)heterojunction solar cell, with a power efficiency of0.061%in air[12].Additionally,many other studies have been carried out on the TiO2and regioregular P3HT blend system[13–18].Huynh et al.reported that in a solar cell with P3HT/CdSe nanorods[19],electron transport was faster in the nanorods than in the nanoparticles,so the power conver-sion efficiency in the nanorods exceeded that in the nanoparticles.Since TiO2is insoluble in organic solvents,and the compos-ite formed by mixing conducting polymer solution with TiO2 nanoparticle is heterogeneous,phase segregation occurs easily.Re-cent studies on obtaining titania nanotubes from titania pow-der in a simple chemical process have attracted much interest [20].Electron transfer in titania nanotubes is more efficient than in nanoparticles.This investigation synthesizes conducting poly-mer on a highly crystalline titania nanotube.The composite thus formed is homogeneous and without phase segregation.The highly crystalline titania nanotubes facilitate electron transport.2.Experimental2.1.MaterialsAnatase powder obtained from the TAYCA Corporation of Japan(AMT-100).Regioregular poly(3-hexylthiophene)was pur-chased from Aldrich.The other chemicals were purchased in their reagent grade and used without further purification including3-aminopropyltriethoxysilane(Acros,99%),3-thiophene acetic acid (Acros,98%),1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hy-drochloride(Acros,98%),FeCl3(Showa,97%)and3-hexylthiophene (Aldrich,99%).0021-9797/$–see front matter Crown Copyright©2009Published by Elsevier Inc.All rights reserved. doi:10.1016/j.jcis.2009.01.073M.-D.Lu,S.-M.Yang/Journal of Colloid and Interface Science333(2009)128–1341292.2.Methods2.2.1.Synthesis of TiO2tubesAnatase powder(3.0g)was added to100mL of a10M NaOH aqueous solution in a250mL Teflonflask,was placed in an oil bath that was maintained at110–130◦C for24h.The mixture was treated with distilled water and then centrifuged to separate the product from the solution.This procedure was repeated until the pH of the supernatant reached10.Next,1000mL of0.1M HCl aqueous solution was added and ultrasonically dispersed.The ma-terials were repeatedly washed with distilled water until the pH of the supernatant reached4.2.2.2.Synthesis of P3HT-grafted TiO2tubesSilanization To a250mLflask were added 1.5g of TiO2nan-otubes,toluene(100mL)and5g3-aminopropyltriethoxysilane (APS);the mixture was refluxed for24h in an atmosphere of nitrogen to react with the hydroxyl groups on the TiO2surface. The mixture was washed in toluene and methanol,and then cen-trifuged to separate the product from the solution.The sample was dried in a vacuum oven at60◦C for24h.Condensation The silanized TiO2nanotubes(1g)were sus-pended in150mL of the solvent acetone.Then,0.35g of3-thiophene acetic acid(T3A)and0.7g1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride(EDCI)were added,and the mixture was stirred at room-temperature for24h.The mix-ture was washed with acetone by centrifugation at2000rpm for 20min.The sample was dried in a vacuum oven at60◦C for24h. The sample was called thiophene-TiO2nanotubes.Grafting(1)Random-P3HT grafted TiO2nanotube.0.1g thiophene–TiO2nanotubes(from section“Condensation”)were stirred at room temperature in150mL chloroform.Then,0.4g of FeCl3and0.1 g3-hexylthiophene were added,and the mixture was stirred for 24h.The reaction product was precipitated with methanol and the residue FeCl3was removed by dissolution in methanol.Fi-nally,the product was added to methanol and stirred at65◦C for24h.All samples were dried in a vacuum oven at60◦C for 24h.(2)Regioregular-P3HT grafted TiO2nanotube.The same pro-cedure was adopted for Random-P3HT grafted TiO2nanotubes,but 3-hexylthiophene monomer was replaced with regioregular P3HT.2.3.CharacterizationThe morphology of the nanotubes was observed using a JEOL JEM2000FX II transmission electron microscope(TEM).Samples for TEM were prepared by the ultrasonic dispersion of a small amount of sample in water;a drop of the solution was then dis-persed to a copper microgrid with a carbonfilm.X-ray diffraction(XRD)experiments were performed using a Bruker D8A X-ray diffractometer with a Cu Kαradiation source. The IR spectra were obtained using a Jasco FT/IR410spectrometer in the range4000–400cm−1from samples that were dispersed in KBr pellets.TGA analyses were carried out in N2and air,the molecular weight of P3HT grafted TiO2nanotubes was determined by thermal gravimetric analysis(TGA)on thermal analyzer(Perkin–Elmer TGA7).Samples were heated to800◦C from50◦C at the speed of10◦C/min.Cyclic voltammograms of the sample were obtained with a working electrode of a spin-coatedfilm on Pt,and using an Ag/AgCl reference electrode.0.1M[CH3(CH2)3]4N·ClO4in acetoni-trile was the electrolyte and the scanning rate was50mV/s in the range−2.0to+1.2V.Photoluminescence spectra were obtained using a Cary Eclipse spectrometer by photoexcitation at430nm.Absorption spectra were obtained using a Cary300Bio spectrom-eter in the range400–800nm.X-ray photoelectron spectroscopic(XPS)analysis of tubes was performed using a Thermo VG Scientific Sigma Probe with a base pressure of10−9mbar(UHV)and monochromatized Al(Kα)radi-ation(hν=1486eV).The binding energy was calibrated using an Au4f7/2standard;the same fwhm and Lorentzian/Gaussian mix val-ues were used in the curvefitting of all peaks.3.Results and discussion3.1.MorphologyFig.1(a)shows TEM images of the TiO2nanotubes.Titania particles have been transformed into nanotubes with an inner diameter of5nm,an outer diameter of12nm,and a length of200–300nm.Figs.1(b)and1(c)present TiO2nanotubes af-ter silanization and thiophene–TiO2nanotubes.The tubular shape remains.Fig.1(d)presents random-P3HT mixed with TiO2nan-otubes;the composite formed exhibits phase segregation and the polymer distribution is non-uniform.Fig.1(e)shows random-P3HT grafted TiO2nanotubes;the covalent bonding of the P3HT grown in situ provides for higher coverage of P3HT on the nanotubes,and no phase segregation occurs.3.2.FTIR measurementsThe FTIR spectrum of titania nanotube in Fig.2(d)has a broad band between3500and3000cm−1,which indicates the presence of–OH groups on the surface of the titania.Two peaks at approx-imately3411and1627cm−1,which are characteristic of stretch-ing and bending vibrations of water molecules,reveal molecularly adsorbed water on the dried titania nanotubes.The spectrum of TiO2nanotube after silanization in Fig.2(a)includes a peak at 953cm−1,which is ascribed to Ti–O–Si[21–23],and signals that correspond to organic fragments of APS.The band at500cm−1 was associated with the Ti–O–Ti stretching mode of TiO2[24]. The C–O and N–H stretching absorption of the thiophene–TiO2 nanotube at1660and1559cm−1support the successful incor-poration of3-thiophene acetic acid units into the TiO2nanotube after silanization(Fig.2(b)).The bands at1450and1540cm−1 were attributed to the symmetric C–C stretching mode;the band at3097cm−1was associated with the C–H stretching mode of the thiophene rings.Fig.2(e)presents the FT-IR spectrum of random-P3HT,which includes the following main characteristic absorption bands;1450cm−1band of symmetric C=C stretching,1488cm−1 band of C–H stretching in the thiophene ring,1156and1215cm−1 bands of C–H bending;the peak at788cm−1is assigned to C–H out-of-plane stretching vibration,while the peaks at997and 695cm−1are indications of C–S stretching in the thiophene ring. Fig.2(c)presents IR spectra of random-P3HT grafted TiO2nan-otubes with peaks of C=C stretching,C–H stretching and bending, C–S stretching and Ti–O–Ti stretching.The IR spectra show that P3HT is grafted onto TiO2nanotubes.3.3.X-ray photoelectron spectroscopyQuantitative XPS analysis of titania nanotubes and random-P3HT grafted titania nanotubes yielded the binding energies that are presented in Table1.Fig.3presents the high resolution spectra of the titania nanotubes and titania nanotubes after silanization. The TiO2nanotubes after silanization contain N1s and Si2p peaks, verifying the presence of the APS surface modifier.The Ti2p1/2and Ti2p3/2spin-orbital splitting photo-electrons in both samples are located at binding energies of463.8±0.1eV and458.1±0.1eV,respectively,as presented in Fig.4(A).Fig.4(B)130M.-D.Lu,S.-M.Yang /Journal of Colloid and Interface Science 333(2009)128–134Fig.1.TEM image of the (a)TiO 2nanotubes,(b)TiO 2nanotube silanization,(c)thiophene–TiO 2nanotube,(d)random-P3HT mixed with TiO 2nanotube,inset in panel (d)shows a TEM image of low P3HT area,(e)random-P3HT grafted TiO 2nanotube.Fig.2.FT-IR spectrum of (a)TiO 2nanotube silanization,(b)thiophene–TiO 2nan-otube,(c)random-P3HT grafted TiO 2nanotube,(d)TiO 2nanotube,(e)random-P3HT.presents the O 1s peak.Table 1lists the binding energies of the peaks.The spectrum of untreated TiO 2nanotubes (Fig.4(B))in-clude a peak at 529.5eV with a shoulder located toward the higher binding energy side (531.4eV).The shoulder at higher binding energy was assigned to OH species on the surface [32].The O 1s peak at 532.0eV in the spectrum of TiO 2nanotube after silaniza-tion is consistent with the reported O 1s binding energy of the Si–O–Ti species [23,32],indicating the bonding of APS to surface of the TiO 2nanotubes.The O 1s signal from random-P3HT grafted TiO 2nanotubes (Fig.4(B)(d))included four peaks at 529.5,530.5,532.0and 533.2eV.Additionally,the high-resolution N 1s spec-trum (Fig.4(C)(b))includes peaks at binding energies 399.8and 401.6eV.The two peaks were attributed to terminal-protonated amines NH +3(401.6eV)and to terminal NH 2groups (399.8eV)[25,29].The grafting of thiophene onto TiO 2nanotubes reduced the amount of NH +3and NH 2(Fig.4(C)(c)).Additionally,thiophene-TiO 2nanotubes have a new peak at a binding energy of 399.9eV;this new peak was attributed to –NHCO–groups [30].This result is consistent with a reaction between the terminal amine group of silane and the COOH group of T3A.The element composition and the chemical state of composite surface were determined by XPS.Intensity of N 1s is decreasing from (b)to (d),because the ti-tania was covered with P3HT (N element was covered with P3HT).The high-resolution S 2p spectrum of thiophene–TiO 2nanotubesTable 1Binding energy of titania nanotubes and its nanocomposites.SampleTi (eV)S (eV)O 1s (eV)2p3/22p1/22p3/22p1/2Ti–O–Ti Ti–OH Ti–O–Si TiO 2nanotubes458.1463.8––529.5531.4–Silanized TiO 2nanotubes 458.2463.9––529.5530.8532.0Thiophene–TiO 2nanotubes458.2463.9164.5165.8529.5530.8532.0Random-P3HT grafted TiO 2nanotubes458.2463.7164.1165.3529.5530.5532.0M.-D.Lu,S.-M.Yang/Journal of Colloid and Interface Science333(2009)128–134131Fig.3.XPS spectrum of N1s,Si2p from(a)the silanized TiO2nanotube and(b)TiO2 nanotube.(Fig.4(D))includes two peaks at164.5and165.8eV for S2p3/2and S2p1/2,indicating the bonding of T3A to NH2of the silanized TiO2 nanotube.The binding energies of S2p in random-P3HT-grafted TiO2nanotubes are164.1eV and165.3eV for S2p3/2and S2p1/2 (Fig.4(D)).The signal intensities of N1s,Ti2p,O1s and S2p in the random-P3HT-grafted TiO2nanotubes were reduced when titania was covered with P3HT.3.4.Thermal analysisFig.5(c)presents the thermograms of random-P3HT-grafted on titania nanotubes.The polymer undergoes two major,but not com-pletely distinct,steps of weight loss in air in the temperature range 50–800◦C.Thefirst step is primarily loss of the alkyl side-chain at 300–475◦C.The second weight loss step,in the temperature range 475–650◦C,corresponds to degradation of the polymer backbone. Thermogram was also adopted to determine the amount of P3HT in the nanocomposites.The amount of P3HT in the nanocompos-ites was determined from the mass loss between200◦C,at which the adsorbed H2O and solvents were evaporated,and800◦C,at which the polymer was completely decomposed.Table2presents the weight gain of each reaction step and the calculated molar ra-tio:W H2OW TiO2W H2O=4wt%, W aW TiO2+W H2O+W a=6wt%, W bW TiO2+W H2O+W a+W b=5wt%,W cW TiO2W H2OW a W b W c=40wt%,where W TiO2:weight of TiO2nanotube,W H2O:weight of adsorbedH2O and solvents,W a:weight increase of TiO2nanotube after silanization,W b:weight increase after grafting thiophene on TiO2 nanotube,W c:weight increase after polymerization of3HT on thiophene grafted TiO2nanotube.Define W TiO2=1,so W H2O=0.0417,W a=0.0664,W b=0.0583,W c=0.7776.From Scheme1,we assumed that3HT grafted on each thiophene on the TiO2nanotube.The thiophene–TiO2nanotube had a thiophene to silane molar ratio of0.41,in-dicating not every NH2group from silane was bonded to T3A. The result is consistent with the S/N ratio of0.45,obtained from XPS.P3HT-grafted TiO2nanotubes have a molar ratio of3HT to thiophene9.96.If each thiophene of thiophene–TiO2nanotubes is assumed to be grafted to3-hexylthiophene,then random-P3HT-grafted TiO2nanotubes have an average of103-hexylthiophene units per chain.3.5.X-ray diffractionAnatase crystal outperforms rutile and brookite in organic so-lar cells[26].The XRD patterns and the above FT-IR spectra show the formation of pure TiO2nanotubes with an anatase crystalline phase following calcination at300◦C.Fig.6presents TiO2nan-otubes and P3HT-grafted TiO2nanotubes;all samples had a peak at 2θ=25.2◦,which peak corresponded well to the value of25.3◦in the literature for the(101)crystalline face of anatase.Other peaks at38.1◦,48.1◦and54.2◦correspond to the anatase(112),(200) and(211)crystal planes,respectively.3.6.Optical and electrochemical propertiesThe energy levels of P3HT and TiO2were analyzed using cyclic voltammetry and from spectroscopic absorption data.Fig.7 presents the absorption spectra,the onset points of1.91–1.95eV and3.26eV were used the approximate band gaps of the P3HT and TiO2,respectively.Random-P3HT physically mixed TiO2nanotubes have a longer absorption wavelength than random-P3HT.Random-P3HT-grafted TiO2nanotubes have the longest absorption wave-length.The result indicates that the random-P3HT-grafted TiO2 nanotubes have the longest conjugation length.Fig.8presents cyclic voltammograms of the composites.The compositefilm on a Pt disk electrode(working electrode)was scanned(scan rate:50mV/s)anodically and cathodically in a so-lution of TBAClO4in acetonitrile(0.1M)with Ag/AgCl as the ref-erence electrode.In Fig.8(a),regioregular P3HT yields two clearly separated oxidation peaks with maxima at625and950mV(vs. Ag/AgCl).Thefirst oxidation step of regioregular P3HT at625mV is abrupt and covers a narrower range of potentials than random-P3HT.This behavior can be considered to be a manifestation of the higher structural homogeneity of the regioregular P3HT com-pared to random-P3HT,and therefore a more isoenergetic polymer chain.The second step(950mV)is associated with the doping re-action,so the HOMO level of P3HT is determined by this step.The HOMO and LUMO energy levels of P3HT and TiO2were analyzed using cyclic voltammetry and from spectroscopic absorption data. The HOMO and LUMO levels can be determined from the onset oxidation potential and onset reduction potential,with reference to the energy level of ferrocene(4.8eV below the vacuum level, which is defined as zero)[27,28].E HOMO/LUMO=−E ox/redonset−E onset(Fc/Fc+)−4.8eV,(1) where E onset(Fc/Fc+)is the potential of the external standard, which is the ferrocene/ferricenium ion(Fc/Fc+)couple.The value of E onset(Fc/Fc+)determined under the same conditions as for P3HT and P3HT-grafted TiO2nanotubes,is about400mV vs. Ag/AgCl.As presented in Fig.8(a),regioregular-P3HT-grafted TiO2 nanotubes have oxidation peaks at630and1000mV,and on-set points of500and760mV,which correspond to oxidation in ordered lamellae and in the doping reaction[31],respectively. Regioregular-P3HT-grafted TiO2nanotubes have a higher oxida-tion potential peak is compared to regioregular-P3HTfilm.The difference between the cyclic voltammograms indicates that the TiO2causes a partial loss of crystallinity reducing the conjugation length of regioregular P3HT by disrupting the ordered structure. In Fig.8(c),random-P3HT has onset points of920mV associated with the doping reaction.The random-P3HT-grafted TiO2nanotube yields an oxidation peak of1100mV,and onset points of780mV correspond to the doping reaction.Random-P3HT physically mixed TiO2nanotubes have an onset point of880mV.Random-P3HT has132M.-D.Lu,S.-M.Yang /Journal of Colloid and Interface Science 333(2009)128–134Fig.4.XPS spectrum of (A)Ti 2p signals,(B)O 1s signals,(C)N 1s signals,(D)S 2p signals from (a)TiO 2nanotube,(b)silanized TiO 2nanotube,(c)thiophene–TiO 2nanotube and (d)random-P3HT grafted TiO 2nanotube.a higher oxidation potential than random-P3HT-grafted or physi-cally mixed TiO 2nanotubes.The oxidation potential indicates that random-P3HT-grafted on TiO 2nanotubes has the longest conjuga-tion.The result agrees with those obtained from UV–vis spectra.The presence of TiO 2nanotubes in random-P3HT increases the conjugation length of random-P3HT.The LUMO level of the TiO 2was derived from the reduction peaks in the cyclic voltammogram of random-P3HT physically mixed TiO 2nanotubes.Random-P3HT physically mixed TiO 2nanotubes yield reduction peaks,and onset points of −200mV,corresponding to a LUMO energy level of TiO 2(Fig.8(d)).The absorption data and obtained band energy levels,shown in Table 3,indicate that the P3HT and TiO 2nanotube com-posite has potential as an electron donor–acceptor material.3.7.Photoluminescence spectraFig.9presents the PL spectra of neat random-P3HT,random-P3HT mixed TiO 2nanotubes and random-P3HT-grafted TiO 2nano-tubes under incident light with a wavelength of 430nm.Adding TiO 2markedly reduced the PL intensity of random-P3HT.Although the absorption intensity of P3HT is similar to P3HT-grafted or mixed TiO 2(Fig.7),the P3HT mixed TiO 2nanotubes have on third of the emission intensity of random P3HTs,while P3HT-grafted TiO 2nanotubes have 10%of the emission intensity.The results show charge transfer from P3HT to TiO 2,and P3HT-grafted TiO 2nanotubes is more efficient than in the physical mixture.The pho-toluminescence intensity of P3HT-grafted TiO 2nanotubes is less than one third of that of the physical mixture.The in situ growth of P3HT provides improved quenching than the physical mixture because the thiophene monomers can diffuse more easily between the nanosized tubes compared to the long polymers,therefore pro-viding a more intimate mixture of the polymer and TiO 2.Addition-ally,the covalent attachment of the P3HT to the TiO 2nanotubes will help to improve charge transfer,increasing the amount of quenching thatoccurs.Fig.5.Thermogravimetric analysis of (a)TiO 2nanotube silanization,(b)thiophene–TiO 2nanotube,(c)random-P3HT grafted TiO 2nanotube,(d)P3HT.Table 2Weight increase wt%,molar ratio from TGA.TiO 2nanotube after silanizationThiophene–TiO 2nanotube P3HT grafted TiO 2nanotube Net TGA weight increase %6wt%5wt%40wt%Molar ratio–R 1=0.41R 2=9.96The net weight increase wt%was calculated from 200◦C to 800◦C.R 1is molar ratio of thiophene to silane,R 2is molar ratio of 3HT to thiophene.M.-D.Lu,S.-M.Yang /Journal of Colloid and Interface Science 333(2009)128–134133Scheme 1.Synthesis of random poly(3-hexylthiophene)grafted TiO 2nanotubecomposite.Fig. 6.XRD patterns of (a)TiO 2nanotube,(b)TiO 2nanotube silanization,(c)thiophene–TiO 2nanotube,(d)random-P3HT grafted TiO 2nanotube.Fig.7.UV–vis spectrums of thin films,(a)random-P3HT grafted TiO 2nanotube,(b)random-P3HT mixed TiO 2nanotube,(c)random-P3HT and (d)TiO 2nanotube.Fig.8.Cyclic voltammogram of (1)regioregular P3HT,(2)regioregular P3HT-grafted TiO 2nanotube,(3)random-P3HT,(4)random-P3HT grafted TiO 2nanotube,(5)random-P3HT mixed with TiO 2nanotube,(6)TiO 2nanotube,scanning rate of 50mV/s.4.SummarySurface characterization of the titania nanotubes and titania nanotubes P3HT composites by XPS and FTIR indicated NH 2and thiphene groups on silanized titania nanotube and thiophene–titania nanotube surfaces.A homogeneous organic–inorganic con-ducting polymer composite of P3HT grafted on TiO 2nanotubes was synthesized.The TGA results show 40wt%P3HT in the random-P3HT-grafted TiO 2nanotubes.P3HT,grafted on TiO 2nanotubes,was formed and PL studies indicated an interaction between P3HT and titania nanotubes in the composite.The in situ growth of P3HT provides improved quenching than the physical mixture because the thiophene monomers can diffuse more easily between the nanosized tubes compared to the long polymers,therefore provid-ing a more intimate mixture of the polymer and TiO 2.Additionally,the covalent attachment of the P3HT to the TiO 2nanotubes will help to improve charge transfer,increasing the amount of quench-ing that occurs.This suggests that nanocomposites are a promisingTable 3Cyclovoltammetry data,HOMO and LUMO levels,E g of donor and acceptor materials.MaterialHOMO of P3HT [eV]/[V]LUMO of P3HT d [eV]HOMO of TiO 2d [eV]LUMO of TiO 2[eV]/[V]E opt .g[eV]Regioregular-P3HT−5.18/0.78c −3.30–– 1.88a Regioregular-P3HT grafted TiO 2nanotube −5.16/0.76c −3.25–– 1.89a Random-P3HT−5.32/0.92c −3.37–– 1.95a Random-P3HT grafted TiO 2nanotube −5.18/0.78c −3.27––1.91aRandom-P3HT mixed TiO 2nanotube−5.28/0.88c −3.35−7.46−4.20/−0.20c1.93a ,3.26ba P3HT.b TiO 2.c V vs.Ag/AgCl.dLUMO level =HOMO level +E optg .134M.-D.Lu,S.-M.Yang/Journal of Colloid and Interface Science333(2009)128–134Fig.9.Photoluminescence spectrum of(a)random-P3HT,(b)random-P3HT mixed TiO2nanotube and(c)random-P3HT grafted TiO2nanotube.material in fabricating hybrid organic–inorganic solid state solar cells.AcknowledgmentsWe thank the assistance of S.J.Ueng and R.M.Huang from the instrumental center,National Central University for TEM and XPS. 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