Surface & Coatings Technology 231 (2013) 301–306

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Surface & Coatings Technology

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Preparation and characterization of high-refractive-index polymer/inorganic hybridfilms containing TiO2 nanoparticles prepared by 4-aminobenzoic acid

Bo-Tau Liu ⁎, Pei-Shan LiDepartment of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan

⁎ Corresponding author at: Department of Chemical andYunlin University of Science and Technology, 123 Univ. RdTaiwan, ROC. Tel.: +886 5 534 2601; fax: +886 5 531 207

E-mail address: liubo@yuntech.edu.tw (B.-T. Liu).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2012.03.011

a b s t r a c t

a r t i c l e i n f o

Available online 8 March 2012

Keywords:High refractive indexTiO2 nanoparticlesChelating agentEpoxy

In this study we used 4-aminobenzoic acid (4ABA) as the chelating ligand to synthesize anatase TiO2 nano-particles (NPs) in an aqueous solution. The chelating behaviors of 4ABA with respect to TiO2 NPs were eval-uated by infrared spectroscopy, zeta potential analysis, x-ray photoelectron survey spectra, and differentialscanning calorimetry. The pre-made TiO2 NPs were incorporated into epoxy resin with various NP contentsto fabricate high-refractive-index hybrid films at a low film-forming temperature. We found that the hybridfilms featuring TiO2 NPs prepared by 4ABA had higher refractive indices than those by acetic acid—a tradi-tional chelating ligand. The difference can reach to 0.044 at 80% NP content. We suspect that the increasein refractive indices may arise from the fact that 4ABA features a higher refractive index and less demanddue to the higher coordinating stability.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polymer–inorganic nanocomposites have attracted attention as newmaterials because of their novel mechanical, electrical, and optical prop-erties. Especially in the application of optics [1–4], polymer–inorganicnanocomposites often provide superior performance due to the high re-fractive indices of inorganic materials. However, special agents (calledcoupling agents, chelating agents or modifiers) are usually used on thesurface of inorganic nanoparticles (NPs) to connect the two differentphases because of the compatibility issue between inorganic NPs andor-ganicmatrix [5]. Two methods are mainly used to fabricate polymer–inorganic hybrid materials, including in situ synthesis and ex situsynthesis [6]. In situ synthesis represents one-step fabrication ofthe nanocomposites with in-situ generating inorganic NPs; Ex situmethod is to incorporate pre-made inorganic NPs into monomers,and subsequently polymerize the monomers to form nanocompo-sites. Either in situ or ex situ methods, chelating agents must beused to cover the surface of NPs in order to avoid the aggregationof NPs and control the size of NPs.

In recent years, there are a great number of reports in describing thesyntheses, characteristics, and applications of polymer–inorganic hy-brid materials exhibiting high refractive indices [7–13]. The chelatingagents commonly used include acetylacetone [14,15], acetic acid [16],silane coupling agents [17–19], propionic acid [20], acrylic acid[21,22], and so on. Acid group is regarded as the best chelating ligand

Materials Engineering, National,., Sec. 3, Douliou, Yunlin 64002,1.

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for metal alkoxides such as titanium alkoxide. Nevertheless, in the pre-vious study [23], we found that the large amounts of chelating agentswith low refractive index could reduce the maximum attainable refrac-tive indices of the hybrid nanocomposites. The problem leads to twoquestions: are there other chelating agents with higher refractive indi-ces? Are there othermore efficient ligands to reduce the amount of che-lating agents in use? Arrrachart et al. showed that titanium atoms maybe chelated to amino groups or carboxylic acids [24]. Nakayama andHayashi [20] used various long-chain alkyl amines to improve the dis-persion of TiO2 NPsmodified by propionic acid in the different solvents.The results lead us to infer that the chelating agents with both aminogroups and carboxylic acid may be more efficient to chelate metal ions.

In this study, we attempted to use 4-aminobenzoic acid (4ABA) asa chelating agent to synthesize TiO2 NPs. Besides higher refractiveindex than other chelating agents commonly used, every 4ABA mole-cule possesses two chelating ligands: an amino group and a carboxyl-ic acid. The competition for the two functional groups chelating TiO2

NPs was observed by infrared spectroscopy, zeta potential analysis,x-ray photoelectron survey spectra, and differential scanning calo-rimetry. We incorporated the pre-made TiO2 NPs into epoxy resin atvarious particle contents to fabricate polymer–inorganic hybridfilms and thenmonitored the effects of the chelating agents on the re-fractive index of the hybrid films.

2. Experimental

2.1. Materials

Titanium(IV) isopropoxide (TIPO, 97%, Aldrich)was used as the pre-cursor for the TiO2 NPs. Diglycidyl ether of Bisphenol A (DGEBA; EEW:

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170; Alfa Aesar), methyl hexahydro phthalic anhydride (MHHPA,Chingtide Co.), and 1-methylimidazole were used to prepare theepoxy resin. 4ABA (99%, Alfa Aesar), acetic acid (99.9%, J. T. Baker), nitricacid (61%, Katayama Chemical), and N,N-dimethylacetamide (DMAc,99%, Mallinckrodt) were used as received. Deionized water (DI water,>18 MΩ·cm) was used throughout the experiments.

2.2. Synthesis of TiO2 NPs

TIPO (9.75 g) and4ABA (0.274 g)weremixed for 1 h and then addedinto DIwater (166 g). The aqueous solution became turbid immediately.After stirring at room temperature for 12 h, nitric acid (1.66 g) wasadded to the aqueous solution, which was then heated at 82 °C for1.5 h, resulting in a stable light-yellow titania sol (nTiO2). To evaluatethe effects of 4ABA on the preparation of titania, the acetic acid-modified TiO2 NPs (oTiO2) were synthesized as reported previously[23], serving as a control group.

2.3. Preparation of high-refractive-index hybrid films

The as-prepared titania sols (nTiO2 or oTiO2) were purified by theultra-filtration method using a stirred cell (XFUF 07601, Millipore)with the ultrafiltration membrane (Omega™ 10 k). The washed solswere isolated by centrifugation at 57,430g for 70 min. The wet precip-itate was redispersed in DMAc through ultrasonication for 1 h.Various amounts of epoxy resin were added into the titania–sol solu-tion as shown in Table 1. The epoxy resin consisted primarily ofDGEBA and MHHPA (molar ratio: 1:1) with a small amount of 1-methylimidazole as a promoter. Silicon wafer and glass substrateswere cleaned using an O2 plasma (PDC-23G, Harrick Plasma; 18 W,20 min). The cleaned substrates were dipped in the various mixturesfor 10 min and then raised from the dipping bath. The coated sub-strates were placed in an oven at 60 °C for 10 min and then at140 °C for 2 h. A temperature of 140 °C was selected for this process,slightly below the initial decomposition temperature determinedthrough thermogravimetric analysis (TGA). To avoid uniformity is-sues, the thickness of the hybrid films coated on substrate for themeasurement of refractive indices was controlled to much largerthan the size of TiO2 NPs, as shown in Table 1.

2.4. Measurements

The morphology of the TiO2 NPs was examined using a high-resolution transmission electron microscope (JEM-2010, JEOL). Thecrystalline phase of the as-prepared TiO2 NPs was characterizedthrough X-ray diffraction (XRD) using an X-ray diffractometer (Mini-flex II, Rigaku) and Cu Kα radiation. Adsorption of 4ABA on the TiO2

NPs was studied by Fourier transform infrared (FTIR) spectroscopy(Spectrum One, PerkinElmer). The particle size distribution of theas-prepared TiO2 NPs was determined using a dynamic light scatter-ing (DLS) analyzer (90 Plus, Brookhaven). Before mixing with KBrfor FTIR spectroscopic analysis, the TiO2 NPs were dried at a vacuumoven of 60 °C for 2 h to evaporate the solvents and avoid the

Table 1Compositions and properties of epoxy/TiO2 hybrid films.

Code TiO2 NPs(wt%)

Epoxy resin(wt%)

Thicknessc

(nm)Refractiveindexd

N40a (O40b) 40 60 79 (234) 1.696 (1.662)N50 (O50) 50 50 123 (111) 1.711 (1.680)N60 (O60) 60 40 130 (145) 1.736 (1.701)N80 (O80) 80 20 67 (142) 1.781 (1.737)

a Samples prepared from nTiO2.b Samples prepared from oTiO2.c Thickness of the hybrid films determined through an ellipsometer.d Refractive indices of the hybrid films at 633 nm.

occurrence of side reactions. The amount of chelating agents con-tained in the as-prepared TiO2 NPs was determined using a thermo-gravimetric analyzer (TGA 2050, TA Instruments) operated at aheating rate of 10 °C/min under an air flow. The chelating behaviorof 4ABA on the surface of TiO2 NPs was analyzed by binding energy(PHI 5000 VersaProbe, ULVAC-PHI) and zeta potential (90 Plus,Brookhaven Instruments). The reactivity of the as-prepared TiO2

NPs was examined by differential scanning calorimetry (DSC Q50,TA Instruments) at a heating rate of 10 °C/min under an N2 atmo-sphere. The refractive indices of the prepared films were determinedover the wavelength range from 300 to 800 nm using an ellipsometer(GES-5E, SOPRA).

3. Results and discussion

Fig. 1a shows the TEM image and the selected-are electron diffrac-tion (SAED) pattern of the as-prepared nTiO2 NPs. Irregular shape andsmall size of nTiO2 NPs can be observed in the TEM image. No largeparticles or aggregates were found under TEM observation. TheSAED pattern reveals the nTiO2 NPs to be well crystallized. The XRDpattern of nTiO2 NPs shown in Fig. 1b indicates dominant peakswith values of 2θ located at 25.2, 37.8, 47.6, 54.4, and 62.7°, corre-sponding to the (101), (004), (200), (105), and (204) planes, respec-tively, of the anatase phase. Thus, the as-prepared nTiO2 NPs were ofpure anatase phase. The average crystallite size was 5.3 nm calculatedfrom the peak of (101) reflection using Sherrer's equation. Fig. 1c re-veals that the particle size of the as nTiO2 NPs were about 10.5 nm inthe distribution range of 9 to 13 nm, consistent with the observationof the TEM image. Because the particle size measured from DLS islarger than that from XRD, we speculate that nTiO2 NPs were com-posed of some small crystals or grains. Although shape and crystalli-zation of the nTiO2 NPs were similar to those of the TiO2 NPsprepared from acetic acid (oTiO2) [23], the particle size of nTiO2 NPsis smaller than that of oTiO2 NPs, which may imply that 4ABA has bet-ter coordinating stability for titania than acetic acid. Anyway, bothnTiO2 NP and nTiO2 NP solutions were well dispersed and have welltransmittance; such systems would be appropriate for opticalapplication.

Fig. 2 presents FTIR spectra of 4ABAandnTiO2NPs; both exhibit a ab-sorption at 2800–3000 cm−1 for C\H units and a peak near 1633 cm−1

for H\O\H bending vibrations, indicating water adsorption [25]. Thebroad and strong adsorption band at about 450–1000 cm−1 in the spec-trum of the nTiO2 NPs corresponds to Ti\O\Ti stretching vibrations.Adsorptions at 1511 and 1406 cm−1 (asymmetric and symmetricstretching vibrations of COO− units, respectively) appear in the spec-trum of the nTiO2 NPs. The shifting and splitting of the band at1669 cm−1 for the C_O groups in free 4ABA indicate the formation ofchemical bonds between the carboxylic groups and titanium atoms[25,26]. The spectrum of 4ABA features a pair of sharp adsorptionbands at 3459 and 3358 cm−1 for N\H stretching vibrations and an ad-sorption peak at near 1600 cm−1 for N\H bending vibrations. If theabove-mentioned sharp bands for N\H stretching vibrations corre-sponding to free 4ABA become broad and the band for N\H bending vi-brations is shifted to lower frequencies, the amino groups may beionized and coordinated to titanium atoms [24,27]. However, thebroad adsorption at about 3100-2700 cm−1 for ammonium ion is hardlyobserved in nTiO2 spectrum due to the adsorption range overlappingwith the O\H stretching vibrations. The adsorption peak at near1600 cm−1 shown in the spectrum of 4ABA disappears in the nTiO2

spectrum. According to these observations, it is difficult to concludethe interaction behavior between amino groups and titanium atomsunder FTIR analysis. Interestingly, a very strong unidentified adsorptionband at 1384 cm−1 is found in the spectrum of the nTiO2 NPs, whichmay arise from certain interaction between 4ABA and titania.

To more clearly comprehend the interaction between 4ABA and ti-tania in nTiO2 NPs, we recorded XPS spectra of 4ABA and the nTiO2

Fig. 1. (a) TEM image with electron diffraction, (b) XRD pattern, and (c) particle sizedistribution of the as-prepared nTiO2 NPs.

Fig. 2. FTIR spectra of (1) 4ABA and (2) the as-prepared nTiO2 NPs.

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NPs. Fig. 3a reveals three primary peaks in the survey spectrum of4ABA: two intense signals for carbon atoms (C1s) and oxygen (O1s)near 285 eV and 532 eV, respectively, and a weaker signal for

nitrogen (N1s) near 400 eV. Besides titanium atoms, the surface ofthe nTiO2 NPs also features these three chemical elements. This im-plies that 4ABA may exist on the surface of nTiO2 NPs. Fig. 3b showsthat the spectrum peak of Ti 2p3/2 is near symmetrical and the dis-tance of peaks between Ti 2p1/2 and Ti 2p3/2 is 5.8 eV. The results in-dicate the titanium mainly existed as Ti4+ [28]. Besides the peak at530.2 eV ascribed to oxygen atoms in the O\Ti bonds, the O 1 s XPSspectrum of the nTiO2 NPs shows one single peak at 531.6 eV insteadof two peaks at 532.1 and 533.6 eV in the XPS spectrum of 4ABA forthe oxygen atoms in the O_C and O\C bonds, respectively(Fig. 3c). It implies that the carboxyl group was bound symmetricallythrough its two oxygen atoms onto the TiO2 surface [29], consistentwith the results from FTIR analysis. Fig. 3d shows that the binding en-ergy of N 1 s in the XPS spectrum of nTiO2 is shifted toward higher en-ergy and split into two peaks at 400.4 and 407.1 eV in contrast to onepeak at 399.5 eV in the XPS spectrum of free 4ABA. We speculatedthat the shift and split of binding energy were attributed to thechange of the local chemical environment of N atoms due to thelone pair of electrons of N atoms coordinating to titanium atoms.

To obtain more detailed features on chemical coupling of 4ABA totitania, we measured zeta potentials for the nTiO2 and oTiO2 NPs.Fig. 4 displays the variations in zeta potentials as a function of pHvalues. Compared with the oTiO2 NPs, the zeta-potentials of thenTiO2 NPs shift slightly to the positive-potential direction under thesame pH value over the entire pH range, and consequently the iso-electric point shifts to a higher pH value. If only one kind of functionalgroup (amino group or carboxyl group) of 4ABA chelates the surfaceof TiO2 NPs, the remaining functional group will be exposed out ofthe surface of nTiO2 NPs and lead to a large shift of zeta potentialover 60 mV [30]. Therefore, both amino group and carboxylic acidare possible to chelate titanium atoms, confirming our inference inXPS analysis. However, it is difficult to imagine that the aminogroup and the carboxyl group chelate simultaneously the surface ofnTiO2 NPs because 4ABA is para-substituted with respect to aminogroup and carboxyl group and possesses a rigid benzyl ring.

The dynamic DSC curves with regard to oTiO2 NPs/DGEBA, nTiO2

NPs/DGEBA, and 4ABA/DGEBA are shown in Fig. 5. 4ABA/DGEBA re-veals one exothermic temperature peak with the maximum peaktemperature of 154 °C, which responds to the reaction of epoxygroups with amino groups [31]. However, no exothermic tempera-ture peak is found in the curves of oTiO2 NPs/DGEBA and nTiO2 NPs/DGEBA. Due to the high reactivity of amino groups to epoxy groups,the DSC analysis suggests that no free amino group is exposed outof the surface of the nTiO2 NPs.

Fig. 3. (a) XPS survey spectra, (b) spectrum of Ti 2p, (c) spectra of O 1 s, and (d) spectra of N 1 s.

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According to the above-mentioned analyses, that is, FTIR, XPS, zetapotential, and DSC, we may conclude that both amino groups and car-boxyl groups were coordinated to titanium atoms in the nTiO2 NP,similar to the results of direct mixed materials of titanium alkoxide

Fig. 4. Variations in zeta potentials of TiO2 NPs as a function of pH values. ■: nTiO2; :oTiO2.

with 4ABA [24]. Due to the linear rigid structure of 4ABA, 4ABA ismounted on TiO2 NPs, unlike general surfactants, which one tail an-chors the surface of particles and the other tail is towards solution.

Fig. 5. Dynamic DSC sans of the mixtures: (1) epoxy/oTiO2 NPs, (2) epoxy/nTiO2 NPs,and (3) epoxy/4ABA.

Fig. 6. TGA curves of the (1) oTiO2 NPs and (2) nTiO2 NPs.

Fig. 8. Photograph images (a) and the corresponding reflection spectra (b) of blankglass (1) and the N60 sample (2).

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Fig. 6 displays TGA curves of the nTiO2 and oTiO2 NPs. The char yieldsof the nTiO2 and oTiO2 NPs at 600 °C were 87.2 and 84.5%, respective-ly, indicating the solid content of titania in the TiO2 NPs. The resultsimply that, compared with acetic acid, 4ABA as a chelating agent forthe preparation of TiO2 NPs required less amount. The reduction of re-quired amount may result from enhancement of stability of TiO2 NPsdue to tail-anchoring stabilization replaced by whole body-mountingor -pasting stabilization. In addition, such large weight loss in TGAanalysis indicates that the chelating agents (either 4ABA or aceticacid) exist not only on the surface but also in the interior of TiO2 NPs.

According to our previous study [23], the refractive index of thechelating agent and its residual in the as-prepared TiO2 NPs play animportant role in the refractive-index characterization of hybridnanocomposites. Fig. 7 and Table 1 reveal that the refractive indicesof the hybrid materials containing nTiO2 NPs were higher thanthose containing oTiO2 NPs. The difference can reach to 0.044 at 80%NP content. The results may arise from the facts that 4ABA featuresa higher refractive index and less demand than acetic acid. Therefore,4ABA may be a better chelating agent that other traditional agentssuch as the compounds only featuring carboxylic acids. Fig. 8 showsthe photo image and the corresponding reflection spectrum of theepoxy/nTiO2 hybrid film. In contrast to blank glass, the epoxy/nTiO2

hybrid film exhibits high optical transparency and high light reflec-tion due to 4ABA with higher refractive index.

4. Conclusions

10.5-nm anatase TiO2 NPs were synthesized through using 4ABAas a new chelating agent. According to the FTIR, zeta potential, XPS,

Fig. 7. Refractive indices of the hybrid films incorporating various TiO2 NP contents at633 nm.

DSC, and TGA analyses, we infer that 4ABA was mounted on the as-prepared TiO2 NPs and existed in the interior of the NPs. The hybridfilms containing TiO2 NPs prepared by 4ABA reveal higher refractiveindices than those by acetic acid and the difference can reach to0.044 at 80% NP content. We suspect that the enhanced refractiveindex in this system resulted from the facts that 4ABA features ahigher refractive index and less demand due to the higher coordinat-ing stability.

Acknowledgment

This study was supported financially by the National ScienceCouncil of the Republic of China.

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