Modification of TiO2 Nanoparticles with Oleyl Phosphate via Phase

Jan 4, 2017 - Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku,...
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Modification of TiO2 Nanoparticles with Oleyl Phosphate via Phase Transfer in the Toluene−Water System and Application of Modified Nanoparticles to Cyclo-Olefin-Polymer-Based Organic−Inorganic Hybrid Films Exhibiting High Refractive Indices Shiori Takahashi,† Shuhei Hotta,† Akira Watanabe,‡ Naokazu Idota,§ Kimihiro Matsukawa,∥ and Yoshiyuki Sugahara*,†,§ †

Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan ∥ Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *

ABSTRACT: Oleyl-phosphate-modified TiO2 nanoparticles (OP_TiO2) were prepared via phase transfer from an aqueous phase containing dispersed TiO2 nanoparticles to a toluene phase containing oleyl phosphate (OP, a mixture of monoester and diester), and employed for the preparation of OP_TiO2/ cyclo-olefin polymer (COP) hybrid films with high-refractive indices. The modification of TiO2 by OP was essentially completed by reaction at room temperature for 8 h, and essentially all the TiO2 nanoparticles in the aqueous phase were transferred to the toluene phase. The infrared and solidstate 13C cross-polarization and magic-angle spinning (CP/ MAS) NMR spectrum of OP_TiO2 showed the presence of oleyl groups originating from oleyl phosphate. The solid-state 31P MAS NMR spectrum of OP_TiO2 exhibited new signals at −1.4, 2.1, and 4.8 ppm, indicating the formation of Ti−O−P bonds. CHN and inductively coupled plasma analyses revealed that the major species bound to the TiO2 surface was tridentate CH3(CH2)7CHCH(CH2)8P(OTi)3. These results clearly indicate that the surfaces of the TiO2 nanoparticles were modified by OP moieties via phase transfer. OP_TiO2/COP hybrid films exhibited excellent optical transparency up to 19.1 vol % TiO2 loading, and the light transmittance of the hybrid films with 19.1 vol % TiO2 loading was 99.8% at 633 nm. The refractive index of these hybrid films rose to 1.83. KEYWORDS: TiO2 nanoparticle, oleyl phosphate, surface modification, phase transfer, cyclo-olefin polymer, organic−inorganic hybrid, refractive index

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INTRODUCTION Polymers with lightness, transparency, and flexibility are suitable candidates for optical materials, including antireflective coatings1,2 and optical waveguides.3,4 Since the refractive indices of common polymers are relatively low, generally in the range of 1.3−1.7, however, their application for optical materials is limited.5 To overcome their limitations in terms of refractive indices, inorganic nanofillers such as TiO2 (n = 2.7 (rutile)) with high refractive indices have been introduced into the materials.1−5 One of the important issues in the hybrid preparation is the reduction of transparency caused by the addition of TiO2 nanoparticles; upon the introduction of TiO2 nanoparticles into the polymers, aggregation and phase separation of the nanoparticles occurs because the surfaces of the TiO2 nanoparticles are hydrophilic,6 leading to light © XXXX American Chemical Society

scattering. It is therefore appropriate for the preparation of TiO2 nanoparticles/polymer hybrids to achieve dispersion of the TiO2 nanoparticles in an organic solvent, which also acts as an excellent solvent for polymers,7 via surface modification, as well as to use TiO2 nanoparticles with diameters less than 40 nm to suppress the scattering.8 Compounds such as carboxylic acids,9 silane coupling reagents,10 and phosphorus coupling reagents9,11−22 are commonly employed to modify the surfaces of TiO2 nanoparticles. Phosphorus coupling reagents are attractive for binding organic groups because the Ti−O−P bonds formed Received: October 17, 2016 Accepted: December 20, 2016

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DOI: 10.1021/acsami.6b13208 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Overview of Surface Modification of TiO2 Nanoparticles with OP via Phase Transfer in the Toluene−Water System for Preparation of COP-Based Organic−Inorganic Hybrid Films with High Refractive Indices

We report here the surface modification of TiO2 nanoparticles with OP as a coupling reagent to achieve hydrophobic surfaces using the phase-transfer technique, and the preparation of an oleyl-phosphate-modified TiO2 nanoparticle (OP_TiO2)/ nonpolar solvent dispersion. OP_TiO2/COP hybrid films were then prepared and their refractive indices were measured as a function of the TiO2 content.

are highly stable with respect to hydrolysis, and homocondensation between the phosphorus coupling reagents is unlikely to occur under moderate conditions.9 Among phosphorus coupling reagents, esters of phosphoric acid are particularly attractive since functional groups exhibiting affinity to both polymers and solvents can be easily introduced. Since oleyl phosphate (OP), generally available as a mixture of monoester [(C18H35O)PO(OH)2] and diester [(C18H35O)2PO(OH)], exhibits sufficient hydrophobicity, OP was employed to achieve excellent dispersion of TiO2 nanoparticles in nonpolar solvents.20,22 The characteristics required for optical polymers are transparency, low birefringence, thermal stability, and low hygroscopicity. Poly(methyl methacrylate) (PMMA) has been commonly used as an optical polymer because of its excellent transparency and small birefringence. Since low thermal stability and large hygroscopicity have been drawbacks for PMMA, the cyclo-olefin polymer (COP) is currently attracting attention as an alternative optical polymer.23 COP exhibits higher Tg than that of PMMA, and the hygroscopicity of COP is much lower than that of PMMA. Of particular note, its low hygroscopicity should contribute to suppressing deformation and refractive index change upon exposure to atmospheric moisture. Thus, it is important to improve its properties through hybrid formation by introducing inorganic nanofillers, but a very limited number of studies has been reported; silica/ cyclo-olefin copolymer hybrids exhibited improved oxygen barrier properties,24 and the mechanical properties of clay/ COP hybrids were examined.25 So far, no attempt has been reported for the preparation of COP-based hybrids for improved optical properties, such as a high refractive index, despite their possible use as optical polymers. Among various techniques for surface modification of inorganic nanoparticles, phase transfer, which involves two immiscible phases, water and an organic solvent, can be employed.26,27 Phase transfer can be utilized for the transfer of nanoparticles in both directions, from an aqueous phase to an organic phase and vice versa. By using the phase-transfer technique, nanoparticles dispersed in an aqueous phase can be transferred into an organic phase with organically modified surfaces. Phase transfer has been extensively employed for metal nanoparticles in an aqueous phase,26,27 but its application in surface modification of oxide nanoparticles has been limited; so far, the phase-transfer technique can be employed for the surface modification of Fe3O428−34 and TiO2.35−37 Since COP dissolves well in nonpolar solvents, the phase-transfer technique is highly suitable for the preparation of oxide nanoparticles with high dispersibility in COP.

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EXPERIMENTAL PROCEDURE

Instrumentation. Inductively coupled plasma (ICP) emission spectrometry was performed with a Varian VISTA-MPX CCD simultaneous ICP-OES instrument after dissolving the samples (about 10 mg) in a mixture of 2.5 mL of HNO3 (69−70 mass %), 2.5 mL of H2SO4 (>96 mass %), and 5 mL of HF (46−48 mass %) at 200 °C for 2 h. Infrared (IR) spectra were recorded on a JASCO FT/ IR-460 Plus spectrometer using the KBr disk technique. A carbon-13 NMR spectrum was obtained with a JEOL JNM-EX500 spectrometer (solvent, CDCl3; frequency, 125.65 MHz). A solid-state 13C CP/MAS (cross-polarization and magic-angle spinning) NMR spectrum was recorded on a JEOL ECX-400 spectrometer (spinning rate, 8.0 kHz; frequency, 100.54 MHz; pulse delay, 5.0 s; contact time, 1.5 ms). A phosphorus-31 NMR spectrum was obtained with a JEOL JNMEX500 spectrometer (solvent, CDCl3; frequency, 100 kHz). A solidstate 31P MAS NMR spectrum was recorded on a JEOL CMX-400 spectrometer (spinning rate, 8.0 kHz; frequency, 160.26 MHz; pulse delay, 5.0 s). CHN analysis was performed with a Perkin Elmer PE2400 instrument. A transmission electron microscopic (TEM) image was obtained with a JEOL JEM-1011 microscope operating at 100 kV. Dynamic light scattering (DLS) measurement was performed with a Nikkiso Nanotrac Wave-UT151 instrument. Ultraviolet−visible (UV−vis) spectra were recorded on a JASCO V630 spectrophotometer. The interference fringe patterns that overlapped with the UV−vis spectra of thin films on glass substrates were removed by fitting using the Cauchy exponential model in a commercial optical thin film analysis software package (Film Wizard, SCI). Film thickness measurement was performed with a Kosaka Surf-corder ET3000 instrument. Refractive indices were determined by light interference method using an Otsuka Electronics FE-3000 refractive film thickness monitor, and the least-square method was adopted with an algorithm of curve fittings employed. Materials. An aqueous dispersion of TiO2 nanoparticles (TiO2/ H2O dispersion (15 mass %)) and OP (a mixture of monoester (mono-OP) and diester (di-OP)) were kindly supplied by Sakai Chemical Industry Co., Ltd. (Osaka, Japan) and COP (ZEONEX480R) was kindly supplied by Zeon Corporation (Tokyo, Japan). The median diameter estimated by DLS measurement was 4.9 nm (Figure S1 of the Supporting Information). Ethanol and toluene employed as a poor solvent and dispersant, respectively, were used without further purification (purity, 99.5%). Surface Modification of TiO2 Nanoparticles. For preparation of OP-modified TiO2 nanoparticles, 5 mL of TiO2/H2O dispersion and 15 mL of dilute nitric acid (pH 2.1) were added to a 100 mL beaker, B

DOI: 10.1021/acsami.6b13208 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces and 20 mL of toluene containing 0.49 mL of OP ([OP]/[TiO2] = 0.16), which is equal to the amount of OP for the maximum graft density on TiO2 nanoparticles on the basis of the size of PO4 groups (see the “Characterization of OP-Modified TiO2 Nanoparticles” section for details), was added to make a two-phase liquid in the beaker. The two-phase liquid was stirred at 220 rpm at room temperature. Attention was paid to avoid formation of bubbles at the interface and to maintain the phase separation of the aqueous phase and the toluene phase. The reaction periods were 1, 4, 8, and 24 h. Since the toluene phase contained unreacted OP, the nanoparticles were washed as follows: after the toluene layer was separated, ethanol was added to cause precipitation. The solid was separated by ultracentrifugation (14 000 rpm, 10 min). The solid was dispersed in toluene, and ethanol was added to form a precipitate, which was separated again by ultracentrifugation. The product was labeled as x_OP_TiO2 (x, reaction period). After the products were dispersed in toluene, all the volatiles were removed using an evaporator and were then redispersed in 20 mL of toluene to form an OP_TiO2/toluene dispersion (Scheme 1). Preparation of TiO2/COP Hybrid Films. Two hundred milligrams of COP was added to an OP_TiO2/toluene dispersion. The total volume of solvent was kept at 5.3 mL, and the amounts of TiO2 added were adjusted to 2.4−11.9 vol % of COP. After the solution was stirred for 2 h at room temperature, a homogeneous dispersion containing COP was obtained. Each hybrid film was prepared by spincoating on a glass substrate at 2000 rpm for 20 s and subsequent heating at 100 °C for 1 h.

absorption bands were assignable to the oleyl group; C−H stretching vibration of −HCCH− at 3003 cm−1, C−H stretching vibration of CH3 and CH2 at 2850−2952 cm−1 and bending vibration of CH2 at 1461 cm−1.11 The 13C NMR spectrum of OP and the solid-state 13C CP/ MAS NMR spectrum of 8h_OP_TiO2 are shown in Figure 2.

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RESULTS AND DISCUSSION Characterization of OP-Modified TiO2 Nanoparticles. The change in yields of TiO2 nanoparticles in the toluene layer with the reaction period is shown in Figure 1. Typical

Figure 2. Solid-state 13C CP/MAS NMR spectra of 8h_OP_TiO2 and 13 C NMR spectra of OP. Measurement conditions (solid-state 13C CP/MAS NMR): spinning rate, 8.0 kHz; frequency, 100.54 MHz; pulse delay, 5.0 s; contact time, 1.5 ms. (13C NMR): solvent, CDCl3; frequency, 125.65 MHz.

In the spectrum of 8h_OP_TiO2, signals were observed at 130, 68, 33, 31, 28, 24, and 15 ppm, and all the signals attributable to the oleyl group derived from OP were observed, as shown in the inset.38,39 The 31P NMR spectrum of OP and the solid-state 31P MAS NMR spectrum of 8h_OP_TiO2 are shown in Figure 3. In the spectrum of 8h_OP_TiO2, an intense signal was observed at −1.4 ppm with weak shoulders at around 2.1 and 4.8 ppm. The observed spectrum resembled that of surface modification of TiO2 nanoparticles by OP in the homogeneous system,22 indicating the formation of Ti−O−P bonds. CHN and ICP analyses showed the C/P ratio to be 18.3, indicating that monoOP was mainly bound to the TiO2 nanoparticle surface. Considering that the ν(PO) absorption band disappeared and the ν(P−O) absorption band was broadened in the IR spectrum of 8h_OP_TiO2 (Figure S2),11 the major environment was thought likely to be tridentate. The shoulder of the signal may be assignable to a bidentate environment of monoOP moiety, a monodentate environment of mono-OP moiety, and/or a di-OP environment involving the PO groups.22 These results clearly demonstrate that the surfaces of the TiO2 nanoparticles were modified by oleyl phosphonate moieties using a liquid−liquid two-phase system. The fractional coverage of 8h_OP_TiO2 by OP moieties is discussed using the sizes of the terminal binding −PO32− and oleyl groups. When assuming that bare TiO2 nanoparticles consist of uniform spheres with the diameter of 4.9 nm determined by the DLS results, the number of nanoparticles and its total surface area in the products can be calculated using

Figure 1. Variation in yield of x_OP_TiO2 as a function of reaction times. The yields of x_OP_TiO2 were determined by ICP results. Inset: photographs of the reaction behavior of TiO2 nanoparticles with OP at toluene (upper) and water (lower) interfaces at a certain period.

photographs are also shown. The yield was calculated by calculating the amount of Ti in samples from ICP results. The yield gradually increased as follows: 1h_OP_TiO2, 70%; 4h_OP_TiO2, 89%; 8h_OP_TiO2, 98%; and 24h_OP_TiO2, 99%. Correspondingly, the colorless, transparent toluene layer (initial stage) was changed to a clear, pale liquid. When the reaction period was equal to or more than 4 h, on the other hand, the aqueous layer was changed to a clear, colorless solution. On the basis of the yields, it is concluded that essentially all the TiO2 nanoparticles in the aqueous phase were transferred into the toluene phase by reacting for at least 8 h. We therefore describe the analytical results for 8h_OP_TiO2 hereafter. The IR spectra of 8h_OP_TiO2, TiO2, and OP are shown in Figure S2. In the spectrum of 8h_OP_TiO2, the following C

DOI: 10.1021/acsami.6b13208 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. TEM image of 8h_OP_TiO2/toluene dispersion. Sample: evaporated 8h_OP_TiO2 dispersion (0.2 mL) distilled with 5 mL of toluene on a TEM grid. Measurement condition: acceleration voltage, 100 kV.

Figure 3. Solid-state 31P MAS NMR spectrum of 8h_OP_TiO2 and 31 P NMR spectrum of OP. Measurement conditions (solid-state 31P MAS NMR): spinning rate, 8.0 kHz; frequency, 160.26 MHz; pulse delay, 5.0 s. (31P NMR): solvent, CDCl3; frequency, 100 kHz.

density of rutile-type TiO2 (4.27 g/cm3) and the Ti contents determined by the ICP results. A Ti/P ratio based on ICP results also provides the number of OP moieties on the nanoparticles. Considering the molecular structure of OP moieties, the oleyl groups can be regarded as a cylinder with a diameter of 0.36 nm2,40 which is larger than the occupied area for the −PO32− groups based on the PO4 groups (0.24 nm2).41 Thus, it is likely that the amount of OP is governed by the size of the bulky oleyl groups, and the fractional coverage can be estimated as 68% for 8h_OP_TiO2 from the area occupied by the oleyl groups on the TiO2 nanoparticles. The surface projected fraction covered by the oleyl groups could be increased further by tilting the oleyl groups. A TEM image of 8h_OP_TiO2 is shown in Figure 4. The OP_TiO2 particle size was 5−20 nm. The particle size distribution obtained by DLS using 8h_OP_TiO2/toluene dispersion is shown in Figure 5. The particle size distribution was narrow, and the median diameter was estimated to be 12.8 nm. Since the median diameter of raw TiO2 nanoparticle was ∼5 nm, the TEM image and DLS measurements of the 8h_OP_TiO2/toluene dispersion indicate that 8h_OP_TiO2 contained aggregates consisting of 2 or 3 nanoparticles. Even though some aggregates formed, OP_TiO2 was dispersed in toluene at estimated particle sizes of much below 40 nm. Also, gaps of a few nanometers were present between the TiO2 nanoparticles in the TEM image. Since the maximal length of the oleyl phosphate should be around 2 nm based on other oleyl-bearing molecules,40,42 the gaps are likely ascribable to a single-layer OP moiety bound to the TiO2 particle surfaces. The DLS measurement results for 8h_OP_TiO2 were compared with our previous study on surface modification with OP in a homogeneous system.22 It was found that the median diameter was smaller by 3.3 nm in the present study, indicating that the aggregation of TiO2 nanoparticles was suppressed by use of the phase-transfer technique. Characterization of OP_TiO2/COP Hybrid Films. The UV−vis spectra of 8h_OP_TiO2/COP hybrid films are shown

Figure 5. Particle size distribution for 8h_OP_TiO2/toluene dispersion obtained by DLS measurement.

in Figure 6. The transmittance with 19.1 vol % TiO2 content was 99.8% at 633 nm, showing that the films were highly transparent. The thicknesses of the 8h_OP_TiO2/COP hybrid films are shown in Figure S3. The film thicknesses were 230−330 nm between 2.4 and 19.1 vol % TiO2 content. The film thickness increased as the TiO2 content was increased, possibly because of an apparent viscosity increase due to hydrophobic interaction between OP and COP. The refractive indices of 8h_OP_TiO2/COP hybrid films are shown in Figure 7. At 19.1 vol % TiO2 content, a rise in the refractive index to 1.83, an increment of 0.31 from the refractive index of COP, 1.52, was observed. In our previous study, PMMA/TiO2 hybrid films have been prepared using OPmodified TiO2 nanoparticles to improve their refractive D

DOI: 10.1021/acsami.6b13208 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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phase transfer. OP_TiO2/COP hybrid films exhibited excellent optical transparency up to 19.1 vol % TiO2 content when 99.8% light transmittance at 633 nm was achieved. The refractive index was increased to 1.57−1.83 linearly between 2.4 and 19.1 vol % TiO2 content, and an increment of 0.31 was achieved from the refractive index of 1.52 of COP.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13208. Particle size distribution for bare TiO2 nanoparticle, IR spectra of 8h_OP_TiO2 and their starting materials, and variation in thickness of 8h_OP_TiO2/COP hybrid films (PDF)

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Figure 6. UV−vis spectra of 8h_OP_TiO2/COP hybrid films with different TiO2 content. The interference fringe patterns that overlapped with the UV−vis spectra of thin films on glass substrates were removed by fitting using the Cauchy exponential model.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Naokazu Idota: 0000-0001-6913-1787 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was financially supported in part by a Grant-in-Aid for Scientific research on innovative Areas “New Polymeric Materials Based on Element-Blocks (No.2401)” (JSPS KAKENHI Grant No. JP24102002). Notes

The authors declare no competing financial interest.

Figure 7. Variation in refractive indices of 8h_OP_TiO2/COP hybrid films as a function of TiO2 content. Refractive indices were determined by light interference method, and the least-square method was adopted with an algorithm of curve fittings.

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ACKNOWLEDGMENTS The authors thank Sakai Chemical Industry Co., Ltd., for donating the aqueous TiO2 dispersion and oleyl phosphate and Zeon Corporation for providing COP.

indices.22 The refractive indices of PMMA-based hybrid films increased linearly with increases in the TiO2 volume fraction, and rise to 1.86 at 20 vol % TiO2 content (n = 1.49 for neat PMMA). Although the refractive indices of polymer matrixes are quite different between PMMA and COP, the magnitude of increasing in the refractive index in this study was very similar to that in the previous study. It is therefore likely that the increment of refractive index in polymer-based hybrids with well-dispersed TiO2 nanoparticles depends primarily on the TiO2 content.

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REFERENCES

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CONCLUSIONS The present study demonstrates that surface modification of TiO2 nanoparticles with OP can be achieved using a phasetransfer process between the aqueous and toluene layers. Essentially all the TiO2 nanoparticles in the aqueous layer were transferred to the toluene layer by reacting for at least 8 h with appropriate stirring. The IR and solid-state 31P MAS NMR analyses demonstrated that mono-OP was preferentially bound to the TiO2 nanoparticle surfaces via Ti−O−P bond formation, leading mainly to a tridentate environment as a binding state. It was thus concluded that the surfaces of TiO2 nanoparticles were modified with OP at the liquid−liquid two-phase interface. DLS measurement of OP_TiO2 showed that the aggregation of TiO2 nanoparticles was effectively suppressed via E

DOI: 10.1021/acsami.6b13208 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b13208 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX